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This study used the stable 15N isotope to quantitatively examine the effects of cutting on vegetative buffer uptake of NO3(-)-N based on the theory that regular cutting would increase N demand and sequestration by encouraging new plant growth. During the summer of 2002, 10 buffer plots were established within a flood-irrigated pasture. In 2003, 15N-labeled KNO3 was applied to the pasture area at a rate of 5 kg N ha(-1) and 99.7 atom % 15N. One-half of the buffer plots were trimmed monthly. In the buffers, the cutting effect was not significant in the first few weeks following 15N application, with both the cut and uncut buffers sequestering 15N. Over the irrigation season, however, cut buffers sequestered 2.3 times the 15N of uncut buffers, corresponding to an increase in aboveground biomass following cutting. Cutting and removing vegetation allowed the standing biomass to take advantage of soil 15N as it was released by microbial mineralization. In contrast, the uncut buffers showed very little change in 15N sequestration or biomass, suggesting senescence and a corresponding decrease in N demand. Overall, cutting significantly improved 15N attenuation from both surface and subsurface water. However, the effect was temporally related, and only became significant 21 to 42 d after 15N application. The dominant influence on runoff water quality from irrigated pasture remains irrigation rate, as reducing the rate by 75% relative to the typical rate resulted in a 50% decrease in total runoff losses and a sevenfold decrease in 15N concentration.
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Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
Quantifying the Impact of Regular Cutting on Vegetative Buffer Efficacy
for Nitrogen-15 Sequestration
A. Bedard-Haughn,* K. W. Tate, and C. van Kessel
centrations in runoff from irrigated pasture range from
0.2 to 5 mg L
(Bedard-Haughn, unpublished data, 2002).
This study used the stable
N isotope to quantitatively examine
Buffer strips are broadly defined as strips of vegeta-
the effects of cutting on vegetative buffer uptake of NO
–N based
tion that improve or maintain water quality downslope
on the theory that regular cutting would increase N demand and
of an agriculture or forestry operation (Barling and
sequestration by encouraging new plant growth. During the summer
of 2002, 10 buffer plots were established within a flood-irrigated pas-
Moore, 1994). Buffers function to remove pollutants by
ture. In 2003,
N-labeled KNO
was applied to the pasture area at a
reducing or filtering surface runoff and/or by filtering
rate of 5 kg N ha
and 99.7 atom %
N. One-half of the buffer plots
ground water and stream water (Dosskey, 2001). Atten-
were trimmed monthly. In the buffers, the cutting effect was not
uation of NO
by buffers is attributed to a combination
significant in the first few weeks following
N application, with both
of factors, including denitrification, infiltration, and
the cut and uncut buffers sequestering
N. Over the irrigation season,
plant uptake (Hill, 1996). The relative importance of
however, cut buffers sequestered 2.3 times the
N of uncut buffers,
each factor varies according to buffer characteristics
corresponding to an increase in aboveground biomass following cut-
such as hydrology, vegetation type (grass vs. forest), soil
ting. Cutting and removing vegetation allowed the standing biomass
type (coarse vs. fine), buffer width, and pollutant type
to take advantage of soil
N as it was released by microbial mineraliza-
(Bharati et al., 2002; Schmitt et al., 1999). In irrigated
tion. In contrast, the uncut buffers showed very little change in
sequestration or biomass, suggesting senescence and a corresponding
pasture, infiltration and plant uptake appear to have a
decrease in N demand. Overall, cutting significantly improved
greater impact on NO
attenuation than denitrification
attenuation from both surface and subsurface water. However, the
(Verchot et al., 1997). A recent field study in California
effect was temporally related, and only became significant 21 to 42 d
N-enriched NO
tracers in an irrigated pasture
N application. The dominant influence on runoff water quality
system found that up to 50% of applied
N was removed
from irrigated pasture remains irrigation rate, as reducing the rate by
by plant uptake the first 10 d following application,
75% relative to the typical rate resulted in a 50% decrease in total
making uptake the dominant mechanism for N attenua-
runoff losses and a sevenfold decrease in
N concentration.
tion (Bedard-Haughn et al., 2004). However, minimal
uptake occurred over the remainder of the growing sea-
son, even with available N in the soil. Consequently,
n California, irrigated pasture provides a relatively
N continued to be lost throughout the irrigation season
low-cost source of green forage during the summer
via runoff, despite the presence of vegetative buffers. In
months when surrounding rangelands are dry and dor-
examining grass buffer trapping efficiency for sediment
mant. Irrigation rates vary by irrigation method, but for
and nutrients, Dillaha et al. (1989) reported higher levels
flood irrigation, rates are as high as 70 L s
at the top
of soluble nutrients leaving buffers than entering them,
of the slope, applied continuously over an 8- to 14-h
which they attributed to low trapping efficiency for solu-
period (up to 12 cm). In the Sierra Nevada foothills,
ble nutrients and to release of nutrients previously
with slopes from 5 to 30%, this can generate runoff
stored in the buffer. This contributes to concern that
losses of up to 70% of the applied water (Tate et al.,
buffer efficiency may decrease over time, and that buff-
2000b). Given that irrigated pasture is both fertilized
ers will ultimately become a source of N (and other
and grazed, there is concern that runoff water contains
nutrients) rather than a sink (Mendez et al., 1999).
toxic levels of pathogens and nutrients.
Plant N demand and uptake can be key factors in
Nitrate N is a soluble nutrient commonly cited as a
controlling N losses in many ecosystems (Mulholland
source of ground- and surface-water contamination. In
et al., 2000). Demand and uptake vary with the N status
the United States, the legal drinking water limit for
of the vegetation, NO
availability, plant growth rate,
–N is 10 mg L
, but concentrations as low as 1 mg
and plant age or phenology. All other factors remaining
can contribute to algal blooms (Mendez et al., 1999).
constant, plant N uptake during growth will be greater
Nitrate has been implicated in eutrophication in sea-
if the vegetation is N deficient or if there is an abundance
water and fresh water (Cole et al., 2004). Measured con-
of available N. Maximum N uptake occurs during the
vegetative growth phase when roots are actively growing
and soil moisture is high (Jackson et al., 1988); increas-
Department of Plant Sciences, University of California, Davis, CA
95616. A. Bedard-Haughn, current address: Department of Soil Sci-
ing plant age tends to decrease N uptake (Schenk, 1996).
ence, University of Saskatchewan, 51 Campus Drive, Saskatoon, SK,
As Jackson et al. (1988) observed in the Sierra Nevada
S7N 5A8, Canada. Received 28 Jan. 2005. *Corresponding author
foothills, even well-watered grasses can senesce within
weeks of anthesis, decreasing N demand. When new
Published in J. Environ. Qual. 34:1651–1664 (2005).
sources of N are introduced, microbial immobilization
Technical Reports: Surface Water Quality
© ASA, CSSA, SSSA Abbreviations: DON, dissolved organic nitrogen; LME, linear mixed
effects; SFREC, Sierra Foothill Research and Extension Center.677 S. Segoe Rd., Madison, WI 53711 USA
Published online August 9, 2005
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
may compete effectively with plants for N (Jackson et regular cutting and removal of vegetation in buffer strips
al., 1989). Subsequent turnover and mineralization re-
would increase plant
N uptake and retention, (ii) mea-
leases this previously immobilized N. In annual grass-
sure the impact of regular cutting on attenuation of
lands in the Sierra Nevada foothills, turnover of the
N, and (iii) determine whether there was a cor-
microbial N pool can occur rapidly (less than one day)
responding impact on attenuation of
N in the soil solu-
and continuously (Davidson et al., 1990; Jackson et al.,
tion and on
N sequestration in the soil. In addition,
1989), necessitating continual plant demand for N to
we considered whether decreasing irrigation rate affects
minimize nutrient losses.
buffer efficiency by comparison with results on an adja-
It may be possible to increase plant N uptake via regular
cent site. By examining the water quality measurements
cutting, which would increase N demand by encouraging
in conjunction with the soil and vegetation results, a
compensatory regrowth. Within two weeks after shoot
N recovery budget was developed, providing
harvest, uptake of N increases (Ourry et al., 1990).
insight into the relative importance of the different N
Matheson et al. (2002) found that although regular cut-
sinks and pools in the function of vegetative buffers in
ting of vegetation decreased new shoot production, it
irrigated pasture.
increased the NO
assimilation capacity of shoots by a
factor of 5 compared with shoots that were not cut,
suggesting that even when total plant biomass is reduced
by cutting, the positive effects on N sequestration might
Site Description
offset this.
The University of California Sierra Foothill Research and
The role of plant uptake in attenuating nutrients is
Extension Center (SFREC), located 100 km northeast of Sac-
diminished when nutrients are returned to the soil via
ramento, CA, has a xeric climate and hilly terrain. During the
decomposition, therefore periodic harvesting of buffer
summer of 2002, 10 adjacent plots were established within an
vegetation might improve the long-term effectiveness
existing flood-irrigated pasture at SFREC. Each plot consisted
of buffers (Dosskey, 2001). Mowing alone will increase
of a 5-m-wide by 16-m-long (80 m
) buffer area immediately
plant N uptake, but removal of the cut vegetation is
downslope of a 25-m
pasture area (Fig. 1). The pasture-buffer
required to prevent nutrient release via decomposition
areas were dominated by orchard grass (Dactylis glomerata
(Barling and Moore, 1994). Although grazing also re-
L.), velvet grass (Holcus lanatus L.; also known as Yorkshire
moves vegetation, up to 60 to 90% of the ingested N
fog), and dallis grass (Paspalum dilatatum Poir.). Soils
can be returned to the pasture system, mostly as urine
(Table 1) were fairly uniform throughout the site and were
classified as fine-loamy, mixed, thermic, Mollic Haploxeralfs
(Di and Cameron, 2002).
of the Auburn–Las Posas–Argonaut rocky loam association
N-enriched techniques in the field pro-
(Herbert and Begg, 1969) and site slope ranged from 15 to
vides a powerful insight into plant–soil N dynamics
18%. The entire study area was fenced to prevent disturbance
(Powlson and Barraclough, 1993), commonly within a
by the cattle grazing the surrounding pasture.
single growing season (Bardgett et al., 2003; Di et al.,
1999; Jackson et al., 1989; Mulholland et al., 2000). For
this study,
N-enriched isotopes allowed new NO
Cutting and Irrigation
be distinguished from NO
already present in the sys-
Beginning in June 2003, a cutting treatment was randomly
tem and to be quantitatively traced through the buffers
allocated to 5 of the 10 buffer areas. Adjacent pasture-buffer
(Bedard-Haughn et al., 2003).
areas were separated by landscape edging, which effectively
Given the previously observed abatement in plant N
prevented runoff crossover between buffers. Preferential flow
uptake in mature buffers in irrigated pasture (Bedard-
along the edging was minimized by typical irrigation manage-
Haughn et al., 2004) and the potential for increasing
ment techniques. For the duration of the 2003 irrigation season
plant N uptake via vegetation management, this study
(June–October), vegetation in the five cut buffers was trimmed
monthly using nylon-line trimmers to levels corresponding
was designed to: (i) quantitatively determine whether
Fig. 1. Schematic of pasture-buffer plot layout. Not to scale. “Biomass” buffers received no
N and were used to get quantitative estimates of
aboveground biomass. Soil samples were taken at the same downslope distances as the soil solution samples.
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
to post-grazing height (5–10 cm) in the surrounding pasture. only minimal percolation. To ensure uniform distribution of
both the
N fertilizer solution and the additional water, theCutting frequency was based on observed recovery time re-
quired between grazing events in the surrounding pasture. All application area was subdivided into m
plots. Natural abun-
dance background levels of
N in all N pools were measured10 pasture areas were trimmed at the same intervals as the
cut buffers. Cut residues were collected and removed from before application of
N-labeled fertilizer to account for natu-
ral variability and dilution of the applied
N fertilizer by
N.the site. Uncut buffers were not trimmed.
Plot irrigation was by gated pipe, which delivered water Within a given plant species, the standard deviation of natural
abundance atom % was within 0.0002 atom %.separately to each pasture-buffer area. Irrigation rate was
controlled by a valve and monitored by flow meters (Model Isotopic levels are reported as atom %
N excess, which
refers to the amount of
N present relative to the averageWT; Netafim, Tel Aviv, Israel) that allowed measurement of
both rate and quantity of water applied. Water was applied naturally occurring background
N levels for that particular
N pool. Atom %
N excess amounts were extrapolated to obtain5 m upslope from the buffer–pasture interface to maximize
control of water distribution within the study area. During the total amount of
N in a given pool by weight and/or volume
and thus to determine a
N budget.this project, the irrigation rate was calibrated to 1 L s
buffer for approximately 3 h every 9 d. This irrigation rate is
75% lower than the rate applied in Bedard-Haughn et al.
Vegetation Sampling and Analysis
(2004) study on an adjacent set of plots (Table 1). A lower
irrigation rate was used in an effort to reduce runoff losses
Grab samples of vegetation were collected 3, 11, 21, 42, 60,
and improve irrigation efficiency. On average, 29% of the
79, 98, and 114 d after
N application. To determine how far
applied irrigation water was lost as runoff. Total duration of
N fertilizer had moved into the buffers, vegetation sam-
each irrigation event varied according to the volume needed
ples were collected along a cross-slope transect within the
to restore soil water content, which was determined using
zone of
N application and at downslope distances of 1, 4, 8,
evapotranspiration data from the California Irrigation Man-
12, and 16 m from the application area. The uncut buffer
agement Information System weather station located at
vegetation samples were separated by the three dominant
SFREC. Climate and plant growth conditions during the 2003
grass species, whereas cut buffer vegetation samples repre-
growing season were normal for the region.
sented composites of all species present due to identification
Collection troughs installed across the bottom of each
obstacles associated with newly clipped vegetation. All plant
buffer collected surface water runoff. Three pairs of ceramic
samples were oven-dried at 65C and analyzed for
N isotopic
soil solution samplers (Soilmoisture Equipment, Santa Bar-
composition via mass spectrometry (Integra Integrated Stable
bara, CA) were installed in each buffer area at 1, 4, and
Isotope Analyzer; Europa Scientific, Crewe, UK) at the Uni-
12 m downslope of the
N application (Fig. 1). Samplers were
versity of California, Davis, Stable Isotope Facility (van Kessel
installed to depths of 15 and 45 cm, the average depths to the
et al., 1994). The current sensitivity of our stable isotope ratio
bottom of the A horizon and the top of the heavy clay Bt
mass spectrometers is 0.0002 atom %
horizon, respectively.
Of the two plots that did not receive
N, one received the
same regular cutting as the cut buffers whereas the other was
left to mature the same as the uncut buffers. The species
Nitrogen-15 Application
composition, vegetation age, and irrigation rates of these two
In June 2003, four days after the first cutting and three days
nonlabeled buffers were equivalent to the labeled buffers.
before the first irrigation,
N-labeled KNO
was applied in
Accurate biomass measurements could not be taken from
solution at a rate of 5 kg N ha
and 99.7 atom %
N. The
the labeled buffers without compromising results, so on each
rate and atom %
N concentration were selected to provide
sampling day, representative biomass measurements were
an approximation of post-irrigation fertilizer N levels while
taken from the two nonlabeled buffers (Fig. 1). All living
allowing the tracer to be detectable in all N pools throughout
biomass within a randomly placed 0.1-m
quadrat was col-
the duration of the experiment. The
N solution was applied
lected, dried, and weighed. For the cut buffer, three composite
across 8 of the 10 plots (4 cut, 4 uncut). The area labeled was
quadrat measurements were collected on each day. For the
1 m wide across the width of each plot and located 0.75 m
uncut buffer, one representative measurement was taken for
above the buffer areas (Fig. 1). Application rate and area
each of the three dominant species. Although this lower num-
were based on Bedard-Haughn et al. (2004). Following appli-
ber contributed to greater variability for uncut biomass values,
cation, the
N fertilizer was watered in with 18 L of water
it allowed for regular sampling over the season without eradi-
per m
; under field conditions, this volume was sufficient to
cating the less prevalent species. Cover measurements for the
rinse the
N solution off of the foliar surfaces but allowed
uncut buffers were taken on Days 11, 42, and 114 using the line
intercept method (Canfield, 1941) to determine the relative
dominance of each of the three dominant grass species.
Table 1. Values (mean SD) for field site properties averaged
Vegetation N content was multiplied by atom %
N excess
across all buffers. Soil properties are average values for the
0- to 15-cm layer across all buffers.
values to get the mass (mg) of
N in each g of vegetation.
The total mass (mg) of
N sequestered in vegetation in a
given buffer area was determined by multiplying the mg
Property Current study et al. (2004)
vegetation values times biomass values (g m
) and extrap-
C, % 3.3 0.6 3.0 0.4
olating to the whole area using cover data.
N, % 0.3 0.1 0.3 0.04
C to N ratio 10.4 0.6 10.4 0.4
Sand, %† 36.8 4.9 31.7 3.8
Runoff Sampling and Analysis
Silt, %† 52.5 4.9 50.6 1.6
Clay, %† 10.6 1.0 11.0 0.9
Runoff samples were collected on the same dates as vegeta-
Slope, % 16.9 1.1 10.9 0.8
tion samples. Samples were taken from the collection troughs
Runoff losses, %‡ 29.2 12.7 56.8 16.4
15 min following the leading edge of runoff and again just
Particle size expressed on measured volume % basis for 2003, calculated
before the end of the irrigation event and were stored frozen
volume % basis for 2002 (Eshel et al., 2004).
Runoff volume/irrigation volume.
until analysis. Based on results from an adjacent irrigated site,
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
these two measurements captured the maximum variability
Soil Solution Sampling and Analysis
during the irrigation period (Bedard-Haughn et al., 2004). The
Immediately before each irrigation event, vacuum was ap-
15-min interval provided a measurement of maximum
plied to the soil solution sampling tubes and allowed to draw
concentration, whereas the event-end sample reflects the mini-
moisture from the soil for 10 d before sample collection
N concentration, but the maximum
N load. Sample
(Bedard-Haughn et al., 2004). Although vacuum was not ap-
collection (500 mL) was as a “grab” sample from the runoff
plied continuously over the 10-d period, suction was still pres-
collection trough. Runoff rates were determined at regular
ent at sampling in most sampling tubes. After Day 42, the
intervals by measuring the volume of runoff in a 5-s period.
time between irrigation and sample collection was shortened
Runoff rate data were used to determine runoff losses (Table 1).
from 9 to 3 d, which substantially improved the reliability of the
N isotope analyses were performed on three N
suction in the tubes and the volume of sample collected. Soil
pools: NO
, and total N. Samples were filtered to re-
solution samples were stored frozen until analysis for NO
move sediment and vegetation residues from runoff. The
via the TiCl
diffusion (25-mL aliquots) as outlined in Bedard-
N and NO
N were determined by NH
diffusion of
Haughn et al. (2004).
a 100-mL aliquot onto polytetrafluoroethylene-encased acid
traps (Stark and Hart, 1996). To measure NO
N, the Stark
Nitrogen-15 Recovery Budget
and Hart (1996) method was modified using TiCl
N recovery budget illustrates the mass of
N seques-
Chloride Solution, 20%; Fisher, Hampton, NH) to reduce
tered and/or measured in runoff relative to the mass of
to NH
as outlined in Bedard-Haughn et al. (2004). Total
applied at the beginning of the study. For soil and vegetation
N was determined on a separate 20-mL aliquot by performing
samples, atom %
N excess amounts were extrapolated using
a persulfate digestion (American Public Health Association,
total N content, soil bulk density (mass/volume), and vegeta-
1989) to convert the dissolved organic nitrogen (DON) and
tion biomass (mass/area) to obtain the total amount of
to NO
, and samples were then diffused for NO
a given sink by mass and thus to determine the total amount
above. Following diffusion, acid disks were removed from
N stored in the pasture-buffer areas. The total amount of
polytetrafluoroethylene packets and analyzed via mass spec-
N lost via runoff (
N load) during a given irrigation event
trometry. The DON–
N for each sample was calculated using
was determined by multiplying runoff volume by
N concen-
an isotope mixing model via difference from total
N (Shearer
trations for each measured interval and integrating over time.
and Kohl, 1993):
Summing these values for the measured irrigation events, to-
gether with quantitative estimates for intervening irrigation
events where runoff was not measured, provided a value for
the total amount of
N lost as runoff from the pasture-buffer
areas. There were no total flux measurements for soil solution,
refers to the atom %
N value for a given N form
so total subsurface
N load could not be calculated.
(NT total dissolved
N) and m
refers to the quantity of N
in g. For irrigations where samples were not collected (Days
30, 51, 70, 80, and 106), runoff, concentration, and
N values
Statistical Analysis
were estimated by calculating the linear relationship between
The results were analyzed using linear mixed effects model
adjacent sampling dates.
analysis (S-PLUS; Insightful Corporation, 2001). Linear mixed
effects analysis can be applied to both structured and observa-
Soil Sampling and Analysis
tional studies (Pinheiro and Bates, 2000) and was used here
to account for the influence of fixed (cutting) effects on buffer
Soil samples were taken at 0, 1, 4, and 12 m from the
N uptake levels and for the repeated measures (group effect–
application at 3 and 114 d following
N application. On both
plot identity) embedded in the data structure. Treating time
dates, samples were taken to a 15-cm depth in two increments
as a fixed effect provided a test of how response varied over
(0–7 and 7–15 cm) using a slack hammer (Ben Meadows Com-
the duration of the study. The magnitude and direction ()
pany, Janesville, WI), corresponding to the depth of the A
of the coefficient for treatment and time effects was used to
horizon. Soil texture was determined on the 114-d samples
define the relationship between
N uptake and runoff
N load
using laser diffraction and reported in volume percent (Eshel
and cutting effects. This flexible model also allowed within-
et al., 2004). On Day 114, soil samples were also taken to a
group variance and correlation structures for handling within-
1-m depth in two increments (0–40 and 40–100 cm) to allow
group (plot) heteroscedasticity and temporally correlated er-
for a more complete estimate of the final
N budget. Soil
rors (irrigation series within year) (Pinheiro and Bates, 2000).
samples were oven-dried at 40C and analyzed for total N
This approach has been used in modeling other complex longi-
N via mass spectrometry. Soil C was analyzed by mass
tudinal datasets (Atwill et al., 2002; Tate et al., 2000a, 2003).
spectrometry in conjunction with soil N. Bulk density measure-
The soil and soil solution data were analyzed using the nonpar-
ments for all depth increments were done on oven-dried intact
ametric Wilcoxon rank sum test (S-PLUS; Insightful Corpora-
cores and were within the range of values measured by Dahl-
tion, 2001), which is not restricted by assumptions of normality.
gren et al. (1997) in SFREC grazed pasture.
Soil microbial
N was measured using fumigation–extrac-
tion method (Brookes et al., 1985), with fumigation for 48 h
with chloroform vapor and extraction with 0.5 M K
Aboveground Vegetation
N was determined by persulfate digestion (American
Cut or uncut, there was a general decrease in atom
Public Health Association, 1989) to convert the DON and
N excess (i.e., %
N present in excess of background
to NO
, and diffusion using a modification of the Stark
N levels) with increasing distance from the
N applica-
and Hart (1996) method, as outlined in Bedard-Haughn et al.
tion zone. However, there was
N present in vegetation
(2004). Microbial
N was determined by difference between
at the 16-m distance even after a single irrigation event
fumigated and nonfumigated samples for both dates (0–15 cm
only) for the 0- and 1-m distances.
(11 d; Fig. 2). At the first vegetation sampling following
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
Fig. 2. Atom %
N excess in buffer vegetation by distance from
N application (averages, standard error bars). From top to bottom, days after
N application 11 d, 60 d, 114 d. Note log y axis.
N application (11 d), vegetation atom %
N excess was had the lowest. The total biomass values for a given
uncut buffer varied according to the cover distributionhigher for uncut buffers within 1 m of the
N application
zone, whereas further downslope, vegetation atom % of the species within that area.
When biomass (Fig. 3) and percent cover distribution
N excess was higher for cut buffers (Fig. 2). As the
irrigation season progressed (60 d, 114 d), atom %
N data were used to determine mass of
N for each domi-
nant species in a given uncut buffer, orchard grassexcess values remained higher in uncut buffers for the
1-m sampling distance, but there were no downslope tended to sequester the majority of the
N, whereas
velvet grass sequestered the least (Table 2). Althoughdifferences between cut and uncut buffers.
There were differences in seasonal biomass trends dallis grass had the highest biomass per m
(Fig. 3), it
was intermediate in its
N storage. The mass of
Nbetween the cut and uncut buffers (Fig. 3). The cut buffer
biomass values reflect the effects of regular cutting, with sequestered by a given species did not change signifi-
cantly over the course of the season.increasing biomass values between cuttings and sharp
drops in biomass on the actual cutting dates. For the The amount of
N sequestered by each species was
summed to get the total mass (mg) of
N sequestereduncut buffers, biomass values varied nonlinearly through-
out the season, but generally, of the three species, dallis per uncut buffer (Fig. 4). Values for uncut buffers reflect
N in the standing biomass on a given date, whereasgrass had the highest biomass (per m
) and velvet grass
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
Fig. 3. Vegetation biomass (g m
) by time after
N application for each of the three dominant species within the uncut buffers and for a
composite of all species present per m
within the cut buffers.
values for cut buffers are cumulative, reflecting the
N an order of magnitude, despite a much smaller reference
in the standing biomass as well as the
N removed from
area. Unlike the cut buffers, the total cumulative mass
the plots by cutting. Overall, the uncut buffers had a
N sequestered in the standing vegetation of the
constant mass of
N sequestered over the course of the
application zone did not increase over the season; it
season, regardless of biomass fluctuations. In contrast,
increased from Day 11 to Day 42 and then decreased
the cut buffers had a lower mass of
N sequestered
to a new lower level, suggesting
N losses from the
immediately following
N application, indicative of the
standing vegetation (Fig. 4).
lower biomass in these buffers on Day 11. Over the
A similar decrease in
N mass within the zone of
course of the season, however, there was a linear in-
application was observed in the soil microbial biomass
crease in the mass of
N in the cut buffers such that by
(Fig. 5). In both the 0- to 7- and 7- to 15-cm depth
the end of the season there was nearly double the
increments, the amount of microbial
N decreased be-
amount of
N sequestered in the cut buffers compared
tween Days 3 and 114. In contrast, just 1 m downslope,
with the uncut.
the amount of microbial
N increased between Days
The linear mixed effects (LME) model confirms that
3 and 114 in both depth increments. There were no
cutting effect on
N uptake was time dependent (Table 3).
significant differences in microbial
N content between
Cutting alone resulted in a decrease in
N uptake by
the cut and uncut buffers, regardless of date.
buffer vegetation (coefficient ⫽⫺13.2, P 0.1); how-
Within 114 d of
N application, 14 to 16% of the total
ever, if the interaction with time is taken into consider-
amount of
N applied was taken up by the pasture
ation, cutting substantially increased the amount of
vegetation within the zone of
N application (Table 2).
sequestered, with the most significant differences be-
However, the observed differences in recovery between
tween cut and uncut buffers occurring at the end of the
the cut and uncut buffers were most notable in the buffer
season (coefficient ⫽⫹46.6, P ⫽⬍0.0001).
vegetation, where the cut buffers recovered an average
The majority of
N sequestration by vegetation oc-
of 59 mg (2.4%) of the applied tracer compared with
curred within the
N application zone (Fig. 4). As for
26 mg (1%) in the uncut buffers.
the cut buffers, the
N application zone was cut regularly
and the
N contained in the vegetation was removed,
Surface Runoff
so there was a steady increase over the season of
Runoff rates averaged 0.4 L s
(SD 0.1)
removed. The difference in mass of
N removed from
the application zone versus the cut buffers was nearly within 15 min of the start of runoff and leveled off at
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
Table 2. The
N budget for runoff, soil, and vegetation after final
time was taken into consideration, with the cut buffers
irrigation (Day 114). Percent recovery refers to the mass of
having less
N load in runoff as shown by the negative
N recovered in a given pasture-buffer area relative to the
regression coefficients. For the NO
and NH
total mass applied (2500 mg) in the zone of
N application.
the effect of cutting only became statistically significant
N recovery per
(LME P 0.05) on Day 42, whereas for total dissolved N,
pasture-buffer area (SD)
cutting had a significant effect by Day 21 (LME P 0.05).
Cut buffer Uncut buffer
For the DON pool, cutting reduced the
N load (LME
P 0.08) regardless of time since
N application; adding
time as a fixed effect improved the significance slightly,
but not enough to warrant its inclusion in the model.
1 0.2 1 0.2
3 0.6 4 1.3
Dissolved organic nitrogen (DON) 3 0.3 4 0.7
Subsurface: Soil Solution and Soil
Total dissolved N 7 0.6 9 2.5
The NO
N concentration of the soil solution
N zone
(Fig. 7) was similar in range to the
N concentration of
0–7 cm 571 223 419 102
the NO
N in runoff (Fig. 6), but the soil solution
7–15 cm 79 31 77 37
15–40 cm 513 460 384 349
N concentrations tended to be much more vari-
40–100 cm 52 35 81 35
able. This was particularly true in the first 42 d after
Total (0–100 cm) 1215 429 961 293
N application during which time the samples were col-
lected 10 d after irrigation, versus after 3 d.
0–7 cm 91 18 95 25
7–15 cm 23 14 18 2
In the cut buffers, the solution samplers at the 15-cm
15–40 cm 157 61 224 33
depth tended to have decreasing NO
N concentra-
40–100 cm 260 115 220 94
tions with increasing distance from the zone of
N appli-
Total (0–100 cm) 531 144 557 135
cation (Fig. 7). In contrast, those samplers at the same
Vegetation type
N zone
depth in the uncut buffers developed a pattern of in-
Grass 395 52 338 50
creasing NO
N concentration with increasing dis-
tance by Days 101 and 116. The samplers at the 45-cm
Grass (composite) 59 10 26 5
depth did not demonstrate any clear patterns associated
Orchard grass NA† 17 6
Velvet grass NA 1 2
with distance from the zone of
N application. Regard-
Dallis grass NA 7 5
less of sampler depth and distance from the zone of
Total recovery
application, the NO
N concentrations were signifi-
N recovered 2207 522 1891 444
cantly higher for soil solution in the uncut buffer than
Total recovery, %† 88 20 76 18
in the cut buffer (P 0.002, Wilcoxon rank sum test).
Cut buffers have composite values only.
This difference between the cut and uncut buffers for
N concentrations in the subsurface water was
approximately 0.7 L s
(SD 0.2) by the end
not reflected in the 0- to 15-cm soil atom %
N excess
of the 3-h irrigation event. When
N concentrations
(Fig. 8). There was no significant difference in soil atom
were multiplied by runoff volume to calculate the total
N excess between the cut and uncut buffers on
load of
N in runoff over a given irrigation event, the
either sampling date (P 0.7, Wilcoxon rank sum test).
N load in runoff in all N pools was greater from the
There was also no difference between sampling dates.
uncut buffer than from the cut buffer after Day 42
The only general pattern was a decrease in atom %
(Fig. 6). The NO
N load decreased to a steady state
excess with increasing distance from the zone of
N appli-
by Day 42, NH
N load increased to a steady state
by Day 42, and DON–
N load remained relatively level
throughout the study. Maximum NO
N was lost in
Nitrogen-15 Recovery Budget
the first 21 d after
N application, and maximum differ-
ences in NO
N load between the cut and uncut buff- The
N lost via runoff was relatively small compared
with the amount applied: 0.3% of the applied
N wasers appeared after Day 60. For the NH
and DON
pools, significant differences between the cut and uncut lost in runoff from the cut buffers and 0.4% of the
N was lost in runoff from the uncut buffersbuffers started to appear as early as Day 42. The data
gap on Day 60 is due to the occurrence of an isolated (Table 2). Maximum recovery occurred in the soil, where
approximately 38 to 49% of the applied
N was mea-precipitation event on that sampling day; the total vol-
ume of precipitation was comparable with the volume sured as total soil
N within the zone of
N application.
A further 21 to 22% was measured in the soil withinduring a typical irrigation event. Over one-half of the
precipitation fell within 1 h; the total duration of the the buffers. The vegetation within the zone of
N appli-
cation recovered 14 to 16% of the applied
N over theevent was 8 h. For Day 60, vegetation and soil solution
samples could be collected, but there was no measur- course of the study. Only a small amount was recovered
by the buffer vegetation itself: 2% in the cut buffers andable runoff.
According to the runoff LME model, for the NO
–, 1% in the uncut buffers. The difference in
N recovery
between the cut and uncut buffers was not significantNH
–, and total dissolved
N pools, cutting alone did
not have a significant effect on the
N load. There was, for any pool except for the within-buffer vegetation
(Table 2). Total
N recovery from the cut buffers washowever, a significant effect when the interaction with
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
Fig. 4. Total mass (mg) of
N sequestered in aboveground vegetation within a given area by time after
N application for
N application zone,
uncut buffer areas, and cut buffer areas, where
N application zone and cut buffer areas are cumulative, including
N removed by clipping
during the irrigation season.
88% (SD 20) and from the uncut buffers was 76% tion (Table 3); higher
N sequestration in the uncut
(SD 18).
vegetation reflects the greater initial biomass available
for N uptake. Soon thereafter, however,
N sequestra-
tion increased in the cut buffers as regular cutting main-
tained plant N demand.
Effects of Cutting on Plant Nitrogen-15 Uptake
In comparing the N capacity of cut and uncut buffers
over the irrigation season, the cut buffers sequestered
The effect of cutting on plant
N uptake was not
twice the
N of the uncut buffers (Fig. 4). Given that
significant in the first few weeks following
N applica-
the cut and uncut buffers had very similar atom %
Table 3. Linear mixed effects model estimating
N uptake by
N excess values for much of the irrigation season, the
buffer vegetation over time by treatment (uncut versus cut).
difference in sequestration can be attributed primarily to
Coefficients quantify the expected effect of cutting and time
increases in biomass in the cut buffers. The increase in
on mg
N sequestered per buffer area relative to the refer-
ence level.
biomass following each cutting (Fig. 3) was a typical com-
pensatory response to defoliation (Ferraro and Oester-
Model term Coefficient 95% CI† P
held, 2002). This period of growth should be a period
Intercept 20.2 10.2, 30.2 0.0002
of high N demand (Jackson et al., 1988). Cutting of
Uncut‡ 0.0
aboveground vegetation can increase shoot N assimila-
Cut 13.2 30.3, 3.9 0.1
tion by more than 5 times (Matheson et al., 2002). Cut-
Days since
N application
11 d‡ 0.0
ting and removing vegetation from the buffers allowed
21 d 0.3 10.7, 10.2 1.0
the standing biomass to take advantage of immobilized
42 d 9.1 1.3, 19.6 0.1
N as it was released by microbial mineralization
60 d 5.1 5.4, 15.6 0.3
79 d 9.5 1.0, 19.9 0.1
(Bardgett et al., 2003). Removal of the cut vegetation
98 d 6.1 4.3, 16.6 0.2
is essential, otherwise decomposition will simply return
114 d 5.6 4.8, 16.1 0.3
nutrients to the system, increasing the potential for
Treatment days after
Cut 11 d‡ 0.0
losses via runoff or leaching (Dosskey, 2001). In con-
Cut 21 d 6.4 8.4, 21.2 0.4
trast, the uncut buffers showed very little change in
Cut 42 d 3.5 11.3, 18.3 0.6
Cut 60 d 19.9 5.1, 34.7 0.01
sequestration throughout the irrigation season (Fig. 4),
Cut 79 d 26.7 11.9, 41.5 0.0008
suggesting the occurrence of senescence and a corre-
Cut 98 d 38.8 24.0, 53.6 0.0001
sponding decrease in N demand (Jackson et al., 1988)
Cut 114 d 46.6 31.8, 61.4 0.0001
or the absence of net growth during the study.
95% confidence interval for coefficient (lower, upper).
Reference category for variable.
Examining interspecific differences in
N was ex-
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
Fig. 5. Soil microbial biomass
soil) by depth, distance from
N application, and time after
N application. Note log y axis.
pected to provide insight into the functioning of the buffers had greater root biomass, the vegetation imme-
uncut buffers, and to help determine whether one spe-
diately downslope of the zone of application may have
cies might be better suited for buffers in irrigated pas-
been better able to take advantage of
N moving down-
ture systems; however, the primary determinant of
slope via lateral movement, or, particularly if there was
sequestration was total aboveground biomass within the
significant lateral root development, drawn on the much
buffer. Dallis grass had greater biomass per m
higher concentrations of
N available within the zone
orchard grass (Fig. 3), but orchard grass was by far more
of application itself.
prevalent with the buffers (averaging 60% of buffer
area, compared with 20% of buffer area for dallis grass),
Effects of Cutting on Surface Runoff Nitrogen-15
and so served to sequester the most
N of the three
For the first 21 d following application of the
N tracer,
species. Although the cut buffers were not examined
there were no differences in surface runoff NO
by species, orchard grass appeared to be the dominant
between the cut and uncut buffers (Fig. 6). Regardless
species within the cut buffers (and in the surrounding
of cutting treatment, there was excess
N measured in
pasture), attributable in large part to its rapid regrowth
the surface runoff during the first irrigation event after
following grazing or cutting, compared with moderate
N was applied. Similarly, there was
N measured
and slow regrowth for dallis grass and velvet grass, re-
in the soil solution (Fig. 7), soil (Fig. 8), and vegetation
spectively (USDA, 2005). Rapid regrowth following cut-
(Fig. 2) at the furthest distance from the zone of
ting and extent of ground cover are likely the best pre-
application following the first irrigation event. This sug-
dictors of plant uptake ability in managed buffers.
gests that both the cut and uncut buffers were attenuat-
Cut or uncut, maximum plant uptake of
N occurred
ing some
N, but during this period, the NO
N tracer
within the first 4 m of the buffer (Fig. 2). The higher
was extremely mobile and its redistribution via surface
vegetation atom %
N excess observed in the first meter
runoff was identical for both the cut and uncut buffers.
downslope of the application area for the uncut buffers
As Di and Cameron (2002) observed, maximum NO
compared with the cut buffers may be attributable in
leaching tends to occur whenever NO
is present in the
part to differences in root biomass. Although soil mois-
soil profile during periods of significant drainage, as
ture is a major factor controlling fine root production
would be associated with irrigation events. It is interest-
in annual grasslands (Cheng and Bledsoe, 2002), cutting
ing to note, however, that the sharp decrease in surface
may have reduced the root biomass (Williams et al.,
runoff NO
N between Days 21 and 42 (Fig. 6) corre-
2003), increasing root turnover, or inhibited the produc-
tion of new roots (Matheson et al., 2002). If the uncut sponds to the first post–
N application cutting of the
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
Fig. 6. Runoff
N load over the course of the irrigation season. Values are averaged by buffer treatment and time; error bars represent standard
error. Note log y axis.
buffer vegetation, suggesting a very strong initial cutting cycled into other N pools, as shown by the parallel
decrease in runoff NO
N load and increases in runoffeffect on runoff water quality.
During Days 3 to 42, some of the NO
N appears DON– and NH
N (Fig. 6). One possible pathway for
the movement between the NO
and NH
N poolsto have been immobilized by microbial biomass and
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
Fig. 7. Soil solution NO
N concentrations by time and distance from
N application. Values are averaged by buffer treatment, time, and
distance; error bars represent standard error. Note log y axis.
is dissimilatory nitrate reduction to ammonium. Al- out the experiment is indicative of a rapid N turnover;
Davidson et al. (1990) observed a turnover time of 0.3though it is unlikely here given the inherently low soil
levels, this could not be confirmed with our field to 1.6 d in grassland soils at SFREC.
tracer study design. In a microcosm study, Matheson et
al. (2002) found that within 32 d, up to 49% of applied
Effects of Severe Cutting on
N was reduced to NH
N and up to 25% was
Nitrogen-15 Retention
immobilized in the microbial biomass. After Day 42,
The buffer areas that were cut regularly exhibited
there were significantly lower
N loads in runoff from
N retention due to the continual plant demand
the cut buffers compared with the uncut buffers for both
for N as it was released by the microbial biomass (Fig. 4).
the NO
and NH
N pools (Fig. 6). This corresponds
The uncut buffers also had good N retention within
with the observed increase in aboveground plant bio-
the time frame of the irrigation season, but previous
mass and plant
N storage in the cut buffers (Fig. 4).
research in irrigated pasture (Bedard-Haughn et al.,
However, the difference between cut and uncut buffers
2004) suggests that plant decomposition during the win-
did not increase substantially after 42 d despite contin-
ter months would ultimately contribute to N losses from
ued growth in the cut buffers, indicating that even though
the uncut buffers. The rate and amount of new growth
there was a continual demand for
N, mineral N was
and hence new N demand within uncut buffers will
available for
N runoff losses, albeit at extremely low
determine how much of the recycled
N will be retained
concentrations (1 g total dissolved N L
over the long term.
Unlike the runoff NH
N pool, there was no lag
Even during the course of the irrigation season, there
time between the application of
N and the leveling off
N losses observed within the application zone
in runoff DON–
N load. This may reflect the observa-
vegetation (Fig. 4), despite regular cutting and removal
tion of Davidson et al. (1990) that these grassland soils
of vegetation
N. This may be due to unintended severe
have a significant heterotrophic microbial sink for
cutting (i.e., too short) of the vegetation in the applica-
, particularly when NH
availability is low. There
tion zone compared with the buffer areas; vegetative
was also minimal temporal effect on the efficacy of
growth and vigor in this zone after 42 d was limited.
cutting for reducing runoff DON–
N load: cut buffers
Severe cutting has been found to contribute to elevated
had consistently lower runoff DON–
N load than uncut
rates of root death (Jarvis and Macduff, 1989). Cutting
buffers throughout the experiment (P 0.08; Fig. 6).
can also give rise to increased partitioning of N to the
The higher DON–
N in runoff from the uncut buffers
belowground biomass and increased rhizodeposition
may reflect slightly greater partitioning of mineral
(Paterson and Sim, 2000). In a study using
to the microbial pool in the absence of significant plant
synthetic sheep urine, Williams et al. (2003) observed
demand. For example, Jackson et al. (1989) observed
N in the soil when vegetation was subject to
that microbial immobilization of NO
and NH
regular cutting. This belowground partitioning associ-
greater than plant uptake regardless of plant growth
ated with aboveground cutting and the increased poten-
stage, and for NH
, the relative dominance of immobili-
tial for root death, coupled with the high C levels already
zation was even more pronounced after plant senes-
present in the rhizosphere, provide optimum conditions
cence. In this study, however, the partitioning was not
for microbial
N uptake (Jackson et al., 1989). As ob-
significant enough to be reflected in microbial biomass
N (Fig. 5). The constancy of the DON–
N load through- served, soil microbes can compete effectively for both
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
Fig. 8. Atom %
N excess in soils by distance from
N application at 3 d (top) and 114 d (bottom) after
N application. Values are averaged
by treatment and distance; error bars represent standard error.
and NO
(Bardgett et al., 2003; Davidson et al., buffers established the spatial pattern of decreasing
1990). This immobilization of inorganic N can be an
N with increasing distance from the zone of
effective mechanism for minimizing leaching losses in
application; at the same time, the difference in soil solu-
agroecosystems (Di et al., 1999), but given the rapid
tion NO
N concentration between the cut and uncut
turnover of the microbial N pool, the positive effects
buffers became much more pronounced (Fig. 7). These
may be temporary (Jackson et al., 1989). This may be
patterns complement the decrease in
N load in the
of particular importance where frequent wet–dry cycles
surface runoff from the cut buffers after 42 d (Fig. 6).
occur, as is the case in irrigated pasture, because wetting
Given that maximum root concentrations in California
cycles can cause significant pulses of N mineralization
grasslands tend to occur in the top 10 to 20 cm of the
(Fierer and Schimel, 2002).
soil profile (Cheng and Bledsoe, 2002; Jackson et al.,
Analysis of the microbial biomass
N (Fig. 5) does
1988), these soil solution patterns likely reflect the in-
show high microbial immobilization of the applied
creased root uptake associated with increased vegeta-
in the first few days following application. There is then
tion growth in the cut buffers (Ourry et al., 1990).
a decrease in microbial
N in the soil within the area
In the cut buffers, the 15-cm soil solution NO
of application over the course of the irrigation season,
concentrations remained at a maximum closest to the
but just 1 m downslope, the microbial
N increases. If
zone of
N application, but decrease with distance due
this increase were due to decomposition of
to vegetative buffer uptake (Fig. 7). In the uncut buffers,
vegetation within the buffers themselves, there would
15-cm soil solution NO
N concentrations were vari-
likely be a difference between the cut and uncut buffers
able or increased with distance due to increased down-
because the cut buffers did not show any evidence of
slope movement via surface runoff and subsurface lat-
senescence during the irrigation season. Instead, the
eral flow (Bedard-Haughn et al., 2004) and due to lower
increase in microbial
N at the 1-m distance may be
plant N demand associated with senescence of the ma-
attributable to losses via root exudation and/or decom-
ture vegetation (Jackson et al., 1988). In the 45-cm soil
position in the rhizosphere of the zone of
N application,
solution samples (Fig. 7), the differences between the
uptake by microbial biomass, and subsequent mineral-
cut and uncut buffers likely reflect leaching from the
ization and lateral movement of inorganic N.
root zone because the lack of significant root density at
The differences in
N retention between the regularly
this depth (Cheng and Bledsoe, 2002) makes it unlikely
cut buffers and the severely cut application zone (Fig. 4)
that differences in plant uptake are the cause of differ-
highlight the importance of responsible buffer manage-
ences in concentration.
ment; cutting must be managed to allow for maximum
A similar pattern of higher
N levels in the uncut
compensatory regrowth, otherwise any benefits associ-
buffers was expected for the 0- to 15-cm soil atom %
ated with cutting may be lost.
excess (Fig. 8), but there were no significant differences
between the cut and uncut buffers on either sampling
Effects of Cutting on Subsurface Nitrogen-15
date. However, the soil atom %
N excess values re-
flected a combination of the soil and the root biomass;
The primary effect of cutting in the subsurface envi-
roots were not analyzed separately. The cut buffers are
ronment is lower NO
N concentrations in the soil
likely to have greater belowground partitioning of
solution within the cut buffers. Within 45 d of
N appli-
cation, the 15-cm soil solution samples from the cut into the root biomass due to stress effects of cutting
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
(Paterson and Sim, 2000; Williams et al., 2003). Given to 7 to 9 mg N buffer
in this study (Table 2). Vegetative
that the uncut buffers have higher NO
N concentra-
growth and vigor were comparable with surrounding
tions in the 15-cm soil solution samples (Fig. 7), uncut
pasture irrigated at the typical higher rate. This empha-
N and cut root
N may balance each other
sizes the critical importance of managing irrigation rates
out, resulting in similar total soil
N values.
to minimize runoff as a primary method for reducing
nutrient loading in surface water (Tate et al., 2000b).
Vegetative buffers still have a significant impact on nu-
Nitrogen-15 Recovery Budget
trient loading, but must remain secondary measures
The majority of the applied
N (59–71%) was recov-
(Barling and Moore, 1994).
ered in the soil beneath the pasture and buffer areas,
Reducing the irrigation rates by 75% also appears to
indicating that for this study, infiltration was the domi-
have substantially increased the relative importance of
nant mechanism for minimizing
N losses in surface run-
infiltration, particularly within the zone of
N applica-
off. A further 17 to 19% of the applied
N was recovered
tion. Verchot et al. (1997) also found infiltration to be
by vegetation uptake. Although the majority of the infil-
a major mechanism for minimizing N losses in surface
tration and uptake occurred within the zone of
N appli-
runoff under unsaturated conditions. At the end of this
cation itself, the buffers attenuated approximately 25%
study, the amount of soil
N stored in the A horizon
of the applied
N, mostly within the soil, indicating
(0–15 cm) within the zone of
N application (Table 2)
that the buffers themselves were effective, regardless of
was approximately 10 times that stored when the higher
cutting treatment. Runoff losses represented less than 1%
irrigation rate was applied (Bedard-Haughn et al., 2004).
of the applied
N (Table 2). Note that the only permanent
However, some of this greater soil storage is related to
sink for the applied
N was that removed in the cut vege-
N losses from the vegetation within the
tation; all other
N could potentially be re-released at a
zone of application (Fig. 4).
later point and become available for leaching and runoff.
Although the amount of
N recovered within the
buffer vegetation was low compared with the overall N
pool, there was a significant difference in vegetation
Although both the cut and uncut buffers served to
recovery between the cut and uncut buffers, with the
N in surface runoff, regular cutting of vegeta-
cut buffers recovering approximately twice as much
tion in upland buffer areas contributed to a significant
as the uncut buffers (Table 2). The absence of a signifi-
increase in plant
N uptake and a corresponding de-
cant difference in total
N recovery between the cut
and uncut buffers does not reflect the temporal improve- crease in
N concentration of both the surface runoff
ment in runoff water quality or vegetative uptake (Ta-
and the subsurface water, indicating that cutting is a
ble 3). It does, however, reflect the absence of signific ant
viable management technique for improving both the
differences in vegetation, runoff, soil solution, and soil
capacity and effectiveness of vegetative buffers in irri-
N concentrations between the cut and uncut buffers
gated pasture. Monthly cutting of buffer vegetation dou-
in the first 21 d of the experiment, when
N concentra-
N uptake compared with uncut buffers. Although
tions in all N pools were at their highest.
mineralization of microbially immobilized
N provided
The applied
N that was not recovered in the runoff,
an ongoing source of
N over the course of the irrigation
soil, or vegetation likely reflects losses due to denitrifica-
season, vegetation in the cut buffers had greater N de-
tion, volatilization, or leaching within the soil profile to
mand due to increased growth and potential for shoot
depths greater than 1 m. Note that runoff losses may
be higher under the more typical granular fertilizer ap-
The dominant factor affecting
N concentration in
surface runoff from irrigated pasture is the irrigation
rate itself. Reducing the irrigation rate by 75% substan-
Runoff and Nitrogen Losses
tially decreased both the volume of runoff and the con-
centration of
N within the runoff. This appears to be
Reducing the irrigation rate from 4 to1Ls
primarily due to greater infiltration within the zone of
decreased the runoff losses by approximately 50% (Ta-
N application. Given this increase in infiltration with
ble 1) compared with Bedard-Haughn et al. (2004), a
the lower irrigation rate, consideration must be given
buffer study on an adjacent set of plots that used the
to the long-term effectiveness of infiltration as a mecha-
more typical irrigation rates for the region. The initial
nism for attenuating
N. Nitrogen storage within the
runoff rate of 0.4 L s
was identical to that ob-
soil may be ephemeral and could eventually be leached
served in Bedard-Haughn et al. (2004), but the maxi-
to ground water unless removed by plant uptake and
mum level of 0.7 L s
was considerably lower
cutting or denitrification.
than the previously measured 3 L s
. This smaller
However, total buffer vegetation uptake was rela-
range of runoff rates was reflected in a smaller range
tively small in this irrigated pasture, so the importance
N loads between the beginning and the end of a
of the cutting effect needs to be considered under a
given irrigation event.
broader range of N inputs and in other agroecosystems.
By reducing the irrigation rate by 75%, the total
Within-pasture fertilizer timing and irrigation manage-
amount of dissolved N lost from a given buffer de-
ment must still be considered the primary techniques
creased by six- to eightfold, from 55 mg N buffer
the 4 L s
irrigation rate (Bedard-Haughn et al., 2004) for minimizing NO
losses in irrigated pasture.
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
Ferraro, D.O., and M. Oesterheld. 2002. Effect of defoliation on grass
growth. A quantitative review. Oikos 98:125–133.
This research was funded by the UC Water Resource Cen-
Fierer, N., and J.P. Schimel. 2002. Effects of drying-rewetting fre-
ter. The authors acknowledge the support of the Sierra Foot-
quency on soil carbon and nitrogen transformations. Soil Biol.
Biochem. 34:777–787.
hill Research and Extension Center and the UC Davis Stable
Herbert, F.W., and E.L. Begg. 1969. Soils of the Yuba area, California.
Isotope Facility. A. Bedard-Haughn also wishes to recognize
Dep. of Soils and Plant Nutr., Univ. of California, Davis, and
diligent field and laboratory assistance from D. Bedard-
County of Yuba, California.
Haughn, C. Peterson, and D. Sok and fellowship/scholarship
Hill, A.R. 1996. Nitrate removal in stream riparian zones. J. Environ.
support from the UC Davis Department of Agronomy and
Qual. 25:743–755.
Range Science, Natural Sciences and Engineering Research
Insightful Corporation. 2001. S-PLUS Version 6.0. Insightful Corp.,
Council of Canada, UC Davis Graduate Studies, UC Davis
Jackson, L.E., J.P. Schimel, and M.K. Firestone. 1989. Short-term
Soil Science Graduate Group, and Jastro-Shields Graduate
partitioning of ammonium and nitrate between plants and microbes
Research Awards.
in an annual grassland. Soil Biol. Biochem. 21:409–415.
Jackson, L.E., R.B. Strauss, M.K. Firestone, and J.W. Bartolome.
1988. Plant and soil-nitrogen dynamics in California annual grass-
land. Plant Soil 110:9–17.
American Public Health Association. 1989. Standard methods for the
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... At the beginning of the 7th growing season, the lowest soil NO 3 availability was observed in the hayfields and not in the poplar buffers, despite hayfields had been fertilized. However, the hayfields were harvested twice a year resulting in a net N exportation annually, which can lower soil NO 3 (Bedard-Haughn et al., 2005). Furthermore, topsoil N enrichment typically occurs in hybrid poplar agroforestry systems because of nutrient-rich leaf litter inputs (Singh, 2007;Singh et al., 1989). ...
... Annual harvests of herbaceous vegetation can decrease soil P and NO 3 in riparian buffers (Bedard-Haughn et al., 2005;Hille et al., 2019;Uusi-Kämppä, 2005). In poplar buffers, soil nutrient removal could be maximized using a high planting density, short rotation cycles and whole-tree harvesting (Adegbidi et al., 2001;Fortier et al., 2015;Truax et al., 2018b), but also by selecting genotypes exhibiting high growth rates and low nutrient use efficiency (i.e. ...
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Bioenergy buffers planted with fast-growing woody crops such as hybrid poplars (Populus spp.) are increasingly integrated into agricultural land to enhance the provision of ecosystem services. Genotype and planting stock type selection may influence buffer establishment success and soil properties, which may affect buffer functions such as soil nutrient removal, hydrological regulation (i.e. soil dewatering) or local climate regulation (i.e. temperature reduction). This farm-scale study tested the effects of two unrelated hybrid poplar genotypes (P. deltoides × P. nigra, genotype D × N-3570 and P. maximowiczii × P. balsamifera, genotype M × B-915311) and two planting stock types (unrooted whips and bare-root stocks) on biomass growth, soil nutrient availability and soil microclimate in 15 m wide bioenergy buffers located downslope of hayfields in the cold temperate region of southern Québec, Canada. Poplar genotype had an important effect on biomass growth, soil microclimate and the supply rates of several macronutrients and micronutrients, while planting stock type had little effect on these variables. The P. maximowiczii × P. balsamifera genotype produced the highest biomass and had generally lower nutrient availability in the soil, but not for soil NO3, which was less available under the P. deltoides × P. nigra genotype. Compared to the hayfield soils, only the P. maximowiczii × P. balsamifera genotype maintained a lower supply rate of available P in the buffer soil. Compared to the hayfield soils, NH4 supply rates were lower in the buffers for both genotypes, while NO3 supply rates were similar or higher. Both genotypes had the capacity to maintain cooler soil temperatures and lower moisture content in the soil of the buffer compared to the adjacent hayfields, but this capacity was greater for the P. maximowiczii × P. balsamifera genotype. Because plant-soil interactions are strongly affected by hybrid poplar genetics, genotype selection is an important consideration in the design of multifunctional agricultural buffers. In temperate agroecosystems, future studies should try to identify productive poplar genotypes that would decrease topsoil NO3 and P availability and reduce soil water content in order to minimize potential N and P losses from buffer soils during runoff events.
... coppices, in Platanus hybrida Brot. coppices or in herbaceous systems [1,11,12]. No studies have evaluated harvest effects in more widely spaced hybrid poplar (Populus × spp.) buffers planted to produce firewood, a common energy source for heating in rural areas of the northern temperate zone [13,14]. ...
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Linear bioenergy buffers planted with fast-growing trees along field edges are increasingly used to address challenges related to sustainable biomass production, climate change mitigation (i.e., carbon storage and microclimate regulation), water quality protection, and forest habitat connectivity in agricultural landscapes. This study assessed: (1) the extent to which 15 m wide hybrid poplar bioenergy buffers (1666 stems/ha) with closed canopy responded to thinning (diamond pattern of tree removal); (2) the regrowth of poplars from cut stumps following gap harvesting; (3) the effects of harvesting treatments on soil microclimate and nutrient availability; and (4) the spatiotemporal pattern of tree growth in unthinned plots. After three post-thinning years, results showed a strong growth response of seven-year-old hybrid poplar trees to thinning (12% increase in diameter and 30% increase in individual stem volume), accompanied by a slight decline in stand productivity. Gap harvesting was not an effective treatment to regenerate the stand from shoots growing from cut stumps because of the high deer browsing. Overall, thinning had marginal effects on soil nutrients and microclimate, compared with gap harvesting, which increased soil temperature, soil moisture, and the availability of several macro and micronutrients. However, harvest effects on soil nutrients were mostly observed during the first postharvest year, with the exception of soil nitrate, which was lowest in the gap treatment during the second postharvest year. Finally, the spatial pattern observed in tree growth between the buffer rows suggests that other more operational thinning patterns (row or corridor thinning) need to be evaluated in linear buffers.
... Biomass harvesting in agricultural buffers allows N and P to be exported out of the system as a strategy to prevent soil nutrient saturation and leaching losses (Bedard-Haughn et al., 2005;Kelly et al., 2007). There is also growing evidence that agricultural buffer soils tend to have improved N and P bioavailability compared to upland cultivated soils as perennial vegetation cover restoration along field margins increases soil organic matter inputs, soil microbial activity and element cycling rates (Tufekcioglu et al., 2001;Stutter and Richards, 2012;Satchithanantham et al., 2019). ...
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Populus hybrids are increasingly planted in multifunctional bioenergy buffers bordering crop fields. However, such small agroforestry systems are vulnerable to damage caused by overabundant deer populations. We measured after 8 years the effects of white-tailed deer (Odocoileus virginianus), genotype and planting stock type on tree growth and ecosystem services provision (biomass production; energy, nutrient and carbon storage; and phytoaccumulation of soil nitrate and phosphorus) in 15-m wide buffers located downslopes of hayfields in southern Québec (Canada). Two deer protection treatments (fenced and unfenced), two genotypes (P. deltoides × P. nigra, genotype D×N-3570 and P. maximowiczii × P. balsamifera, genotype M×B-915311) and two stock types (bare-root stocks: ±1.8 m in height and whips: 2.5 m in height) were studied. In unfenced plots, deer heavily browsed poplars (increase in the height of the first branch by 59 cm), repeatedly rubbed tree bark (36% of trees with rubs for genotype D×N-3570, and 59% for genotype M×B-915311), slightly decreased survival (by 2.9%), which reduced tree-level and stand-level productivity. Proportion of trees with rubs, first branch height and survival were correlated (p < 0.001) with woody biomass yields. Fenced poplars increased wood volume, biomass and energetic content by 20-21%, and C, N and P stocks in aboveground biomass by 13-20%. Higher soil phosphorus bioavailability was also found in unfenced plots. Genotype D×N-3570, which allocates less biomass to branches, grows its first branches farther from the ground and rapidly develops a rough and thick bark, was the least affected by deer, despite its high palatability. Genotype M×B-915311, which had higher biomass allocation to branches, a smooth and thin bark, but low palatability, was the most affected by deer. Soils in plots of genotype M×B-915311 had the lowest macronutrients availability, except for nitrate. Biomass feed-stock quality (i.e. low nutrient concentrations and high heating value of woody biomass) was highest for genotype M×B-915311. In both fenced and unfenced plots, whips were more productive than bare-root stocks, especially for genotype D×N-3570. Optimizing ecosystem services provision in bioenergy buffers can be achieved by fencing, and by genotype and stock type selection.
... Wetlands are capable of retaining several metals, of both human and natural origin, which in high concentrations harm aquatic life (especially fish) in downstream areas. Typical sources of copper, lead, and other potentially toxic metals were described in section of Chapter 4. As runoff increases, some potential elements are diluted (e.g., calcium, silica), but most toxic metals (e.g., copper, lead, zinc) and some nutrients such as nitrate ( Bedard et al. 2005) are mobilized from sediments more effectively and their concentration in surface water increases ( Kerr et al. 2008). Numeric standards have been legally adopted for many toxic metals. ...
Technical Report
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2.1 Wetlands Overview 1 2.1.1 What Are Wetlands? 1 2.1.2 Delineating Wetland Boundaries 4 2.1.3 Wetland Maps 6 2.1.4 Wetland Studies in San Juan County 8 Prior Studies 8 The 2010 Wetland Study 8 2.1.5 Characteristics and Distribution of Wetlands in San Juan County 10 2.1.6 What’s Different About San Juan County 14 2.2 Why SJC Wetlands Are Important 15 2.2.1 Basics of Wetland Functioning 15 2.2.2 Purifying Water and Protecting Water Quality 17 Importance in SJC 17 Key Factors Affecting Water Purification Functions of Wetlands 21 2.2.3 Supporting Habitat and Species 21 Importance of Habitat Functions of SJC Wetlands 21 Key Factors for Predicting Habitat Functions 23 2.2.4 Supporting Other Natural Functions and Values 24 Hydrologic Functions and Values 24 Open Space Values 25 2.3 Potential Impacts to SJC Wetlands 26 2.3.1 Effects of Removing Water from Wetlands 27 2.3.2 Effects of Constructing Ponds or Otherwise Adding Water 28 2.3.3 Effects of Degrading the Water Quality of Wetlands 29 2.3.4 Effects of Removing Vegetation In or Around Wetlands 29 2.3.5 Effects of Human Presence 29 2.3.6 Development Intensity 30 2.4 Strategies for Protecting Wetland Functions 30 2.4.1 Enforcement of Regulations 30 2.4.2 Protective Purchasing 31 2.4.3 Prioritizing Wetlands 31 2.4.4 Establishing Minimum Wetland Size for Regulation 41 2.4.5 Wetland Buffers 43 Introduction to Buffers 43 Applying Best Available Science to Buffer Width Requirements 45 Buffers for Protecting Wetland Water Quality 49 Buffer Widths for Protecting Habitat and Wetland Species 59 2.4.6 Restoration, Enhancement, Establishment of Wetlands and Their Functions 63 2.5 Data Gaps and the Need to Expand the Knowledge Base 67 2.6 Synopsis and Options 72 2.7 Literature Cited 76 Appendix 2A. Procedures Used to Prepare This BAS Document: 2A-1. Procedures used to improve previous map of SJC wetlands 2A-2. Procedures used during on-site assessments of statistical sample of SJC possible wetlands Appendix 2B. List of Wetland Species 2B-1. Wetland Plants of San Juan County 2B-2. Wetland Wildlife of San Juan County (by Island) Appendix 2C. Results From On-site Assessment of SJC Wetlands 2C-1. Scores from the WDOE Rating Method, by SJC wetland visited in 2010 2C-2. Summary of conditions in statistical sample of SJC wetlands, summer 2010, numeric indicators 2C-3. Summary of conditions in statistical sample of SJC wetlands and associated surroundings, summer 2010, categorical indicators 2C-4. Summary of conditions in uplands immediately surrounding each wetland in the statistical sample of SJC wetlands, summer 2010, numeric indicators 2C-5. Wetland Prevalence Index and native and invasive plant species percent cover in quadrats sampled during summer 2010 visit to SJC wetlands 2C-6. Frequencies of vascular plant taxa among sites and quadrats sampled during summer 2010 visit to SJC wetlands Appendix 2D. Summary Tables from GIS Compilation of Existing Spatial Data for Possible Wetlands of SJC. Appendix E (electronic only). Wetlands Geodatabase and other data files.
... Buffer effectiveness depends on buffer characteristics such as surface hydraulic properties, vegetation species, soil type, slope morphology, and buffer width (Balestrini et al., 2011;Bharati et al., 2002;Dunn et al., 2011;Schmitt et al., 1999). Buffer strip efficacy is also affected by the agricultural system (land management and crop) and the management practices used in the buffered area (Bedard-Haughn et al., 2005). ...
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When soil nitrate levels are low, plants suffer nitrogen (N) deficiency but when the levels are excessive, soil nitrates can pollute surface and subsurface waters. Strategies to reduce the nitrate pollution are necessary to reach a sustainable use of resources such as soil, water and plant. Buffer strips and cover crops can contribute to the management of soil nitrates, but little is known of their effectiveness in semiarid vineyards plantations. The research was carried out in the south coast of Sicily (Italy) to evaluate nitrate trends in a vineyard managed both conventionally and using two different cover crops (Triticum durum and Vicia sativa cover crop). A 10 m-wide buffer strip was seeded with Lolium perenne at the bottom of the vineyard. Soil nitrate was measured monthly and nitrate movement was monitored by application of a 15N tracer to a narrow strip between the bottom of vineyard and the buffer and non-buffer strips. Lolium perenne biomass yield in the buffer strips and its isotopic nitrogen content were monitored. Vicia sativa cover crop management contributed with an excess of nitrogen, and the soil management determined the nitrogen content at the buffer areas. A 6 m buffer strip reduced the nitrate by 42% with and by 46% with a 9 m buffer strip. Thanks to catch crops, farmers can manage the N content and its distribution into the soil over the year, can reduced fertilizer wastage and reduce N pollution of surface and groundwater.
... Although non harvested herbaceous buffers offer little longterm N and P storage potential (Bedard-Haughn et al., 2005;Kelly et al., 2007;R€ aty et al., 2011), numerous studies have found no difference between tree buffers and herbaceous buffers at reducing N loads in agricultural riparian zones (Lyons et al., 2000;Mayer et al., 2007;Sabater et al., 2003). This may be related to the fact that nutrient storage in woody biomass may be relatively low in some mature riparian forest buffers (Peterjohn and Correll, 1984). ...
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In many temperate agricultural areas, riparian forests have been converted to cultivated land, and only narrow strips of herbaceous vegetation now buffer many farm streams. The afforestation of these riparian zones has the potential to increase carbon (C) storage in agricultural landscapes by creating a new biomass sink for atmospheric CO2. Occurring at the same time, the storage of nitrogen (N) and phosphorus (P) in plant biomass, is an important water quality function that may greatly vary with types of riparian vegetation. The objectives of this study were (1) to compare C, N and P storage in aboveground, belowground and detrital biomass for three types of riparian vegetation cover (9-year-old hybrid poplar buffers, herbaceous buffers and natural woodlots) across four agricultural sites and (2) to determine potential vegetation cover effects on soil nutrient supply rate in the riparian zone. Site level comparisons suggest that 9-year-old poplar buffers have stored 9-31 times more biomass C, 4-10 times more biomass N, and 3-7 times more biomass P than adjacent non managed herbaceous buffers, with the largest differences observed on the more fertile sites. The conversion of these herbaceous buffers to poplar buffers could respectively increase C, N and P storage in biomass by 3.2-11.9 t/ha/yr, 32-124 kg/ha/yr and 3.2-15.6 kg/ha/yr, over 9 years. Soil NO3 and P supply rates during the summer were respectively 57% and 66% lower in poplar buffers than in adjacent herbaceous buffers, potentially reflecting differences in nutrient storage and cycling between the two buffer types. Biomass C ranged 49-160 t/ha in woodlots, 33-110 t/ha in poplar buffers and 3-4 t/ha in herbaceous buffers. Similar biomass C stocks were found in the most productive poplar buffer and three of the four woodlots studied. Given their large and varied biomass C stocks, conservation of older riparian woodlots is equally important for C balance management in farmland. In addition, the establishment of poplar buffers, in replacement of non managed herbaceous buffers, could rapidly increase biomass C, N and P storage along farm streams, which would be beneficial for water quality protection and global change mitigation. Copyright © 2015 The Authors. Published by Elsevier Ltd.. All rights reserved.
This manual explains background and use of the spreadsheet posted separately here and which can be used to rapidly assess ecosystem services of an individual wetland.
The function of wetlands on non-point source pollution is clear; thus to conduct wetland sustainable management is significant. As typical wetland plants, the removal effect and height distribution of N and P of reed , cattail , and giant reed were determined in Yixing. Results showed that all these plants could reduce N and P concentrations in wetlands, and a “backflow” rule of nitrogen and phosphorus was observed in the plants. In spring and summer, the nitrogen and phosphorus contents were high, but these nitrogen and phosphorus returned to the lower part of the plants during autumn and winter. Therefore, this rule is important in wetland management , specifically for determining plant harvesting time. This study presents the wetland restoration and sustainable management mode , which not only can protect farmers’ interests but also help promote wetland restoration work steady and orderly forward. Hence, the principle of “positive publicity, financial compensation, community participation , technology support, market leading, model advancing, and common management” can be used as guidelines of wetland restoration work for large-scale wetland restoration and construction in the overall Taihu Lake watershed .
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Non grazed, vegetated buffer strips are often recommended as best management practices to protect waterbodies from sediments and nutrients in runoff from grazed pastures. The objectives of this study were to characterize levels of nitrate/nitrogen (NO3-N), total phosphorus (Total P), and total suspended solids (TSS) in runoff and evaluate the potential water quality improvements from 10 m buffer strips on irrigated Sierra Nevada foothill pastures. We found that 15% and 69% of irrigation water applied to sprinkler and flood irrigated pastures became runoff, respectively. There were distinct temporal patterns of constituent concentration in runoff during irrigation events having ramifications for effective water quality monitoring and study design. The 10 m buffer did not significantly reduce concentrations and loads of NO3-N in runoff from sprinkler and flood irrigated pastures. The buffer also failed to reduce Total P concentration under either irrigation schemes, or Total P and TSS load under sprinkler irrigation. The buffer did reduce TSS concentration under both irrigation schemes, TSS load under flood irrigation, and Total P load under flood irrigation. These results reflect the effectiveness of buffers during the first year following buffer establishment. Improved irrigation efficiency to reduce runoff generation h perhaps the most readily acceptable and practical first step for reducing the potential for negative water quality impacts from these systems.
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The objective of this study was to identify and model environmental and management factors associated with cattle feces deposition patterns across annual rangeland watersheds in the Sierra Nevada foothills. Daily cattle fecal load accumulation rates were calculated from seasonal fecal loads measured biannually on 40 m(2) permanent transects distributed across a 150.5 ha pasture in Madera County, Calif. during the 4 year period from 1995 through 1998. Associations between daily fecal load per season, livestock management, and environmental factors measured for each transect were determined using a linear mixed effects model. Cattle feces distribution patterns were significantly associated with location of livestock attractants, slope percentage, slope aspect, hydrologic position, and season. Transects located in livestock concentration areas experienced a significantly higher daily fecal load compared to transects outside of these concentration areas (P < 0.001). Percent slope was negatively associated with daily fecal load, but this association had a significant interaction with slope aspect (P = 0.02). Daily fecal load was significantly lower during the wet season compared to the dry season (P = 0.002). Daily fecal loading rates across hydrologic positions were dependent upon season. Our results illustrate the opportunities to reduce the risk of water quality contamination by strategic placement of cattle attractants, and provide a means to predict cattle feces deposition based upon inherent watershed characteristics and management factors.
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When diffusion techniques are used to prepare samples for 15N analyses, low or variable N recovery is sometimes observed. The effect of low recovery on estimates of 15N enrichment is unknown. Also, the suitability of diffusion techniques for use with a variety of salt solutions and digests containing very low N concentrations (<0.8 mg L-1) has not been determined. We evaluated a diffusion technique where NH3-N is trapped on acidified filter paper disks that are sealed between two strips of Teflon [polytetrafluoroethylene (PTFE)] tape. Because the PTFE protects the acid trap from neutralization, large volumes of solution can be diffused in plastic specimen containers and the sample can be vigorously mixed during the diffusion period. Six days of diffusion at 22°C resulted in >92% recovery of 50 μg NH4/+-N from 75 mL of 2 M NaCl, total Kjeldahl digests, or 2 M KCl preserved with acid. Leaving sample containers open for S d after diffusion for NH4/+ completely eliminated contamination of NO3/--N by residual NH4/+; however, recovery of NO3/- was inversely related to the length of the open period. Recovery of NO3/--N from 0.S M K2SO4 and alkaline persulfate digests after a 3-d open period was 90 and 77%, respectively. Complete recovery of NH4/+ or NO3/- was not required to obtain accurate estimates of 15N enrichment; however, when recovery was <100%, the method of blank correction was critical. When the mass of N contamination measured in diffusion blanks was used to blank-correct 15N enrichments, diffused standards often had 15N enrichments that were significantly different from nondiffused standards; however, when the mass of N in blanks was estimated using an isotope dilution equation, there was excellent agreement between diffused and nondiffused standards, regardless of the degree of recovery.
Spatial variability in the 13C and 15N natural abundance of plant residues (δ13Cresidue and δ13Nresidue) and soil (δ13Csoil and δ13Nsoil) at the landscape scale remains largely unexplored. This 2-yr study assessed landscape-scale patterns and investigated topographic and soil factors that control spatial variability. Low values of δ13Cresidue were associated primarily with the lower level and footslope elements, with significantly (P ≤ 0.001) higher (less negative) values in the upper level and shoulder elements. Strong similarities in the spatial patterns observed for δ13Cresidue and δ13Csoil indicate that the 13C content of soil organic matter is largely dependent on δ13Cresidue. Although the δ13C of the crop residues exhibited marked year-to-year variability, which reflected genetic differences and variations in growing season precipitation and perhaps N-fertilization regime, landscape-scale patterns for δ13Cresidue were similar in both years. These data support the hypothesis that δ13Cresidue varies in response to environmental conditions and also suggest that topographic or hydrologic controls are at work. The presence of a landscape-scale pattern in δ13Cresidue and δ13Csoil may be expected to have a confounding effect on studies that employ the 13C natural abundance method to detect changes in plant species composition or to estimate the rate of soil organic matter turnover. Spatial patterns for δ15Nresidue and δ15Nsoil indicated that 15N was distributed in an essentially random pattern.
Gross nitrification rates ranged from 12-46% of gross mineralization rates during the growing season of annual grasses. Pools of NH4+ and NO3- remained below 7 and 4 μg/g soil, respectively, but turned over about once a day. Microbial assimilation of NO3- occurred at rates similar to previous estimates of plant uptake. Hence 2 common assumptions, that nitrifying bacterial are poor competitors for NH4+ and that microbial immobilization of NO3- is insignificant, are not correct for this grassland system. Soil heterogeneity probably results in NH4+ availability to NH4+ oxidizers at some microsites, while NO3- assimilation by heterotrophic microorganisms occurs at other microsites where NH4+ is not available. Relatively high rates of NO3- production and consumption in an ecosystem with an annual mean hydrologic loss of NO3--N of only 3.3 kg/ha indicate the importance of NO3- in the internal N cycle of this ecosystem. Nitrification potential rates, which are an index of population size, declined during the dry season, but a significant population remained viable when soil water potential was below -9 MPa, indicating that nitrifying bacteria can tolerate severe desiccation. A simple diffusion model demonstrates the dependence of NH4+ availability on soil moisture. Population decline during the dry season may result from both desiccation stress and a lack of substrate availability for maintenance energy of the population. Spatial compartmentalization of sites of production and consumption of inorganic-N, along with diffusional constraints among such microsites, appear to be critical factors affecting N-cycling characteristics of this California ecosystem. -from Authors
An experiment was carried out to test the hypothesis that defoliation of a semi-natural grassland species influenced the fate of the urea nitrogen (N) from sheep urine. The distribution of the 15N added as urea in a synthetic sheep urine (SSU) at 50 g N m-2, to Agrostis capillaris and fallow soil was followed over a period of 56 days in the plant and soil in a glasshouse microplot experiment. The grass was subject to three regimes; regular twice weekly defoliation, a single defoliation (both to 40 mm), and uncut. Regular cutting increased the total N concentration of shoots and reduced the biomass of roots, though 15N recovery in the shoots was not appreciably different from the uncut grass. A single defoliation immediately before SSU addition decreased 15N recovery. More 15N was recovered in the surface 50 mm of soil beneath the regularly cut than in the single and uncut treatments. The soil mineral N pool was the largest sink for SSU 15N. Microbial N increased threefold with the addition of SSU, but effects of the grass treatments on the proportion of SSU 15N in the microbial biomass could not be detected. It was concluded that regular defoliation of grass compared with no cutting could lead to greater losses of NO3- by leaching and denitrification.
The objective of this study was to compare the N leaching loss and pasture N uptake from autumn-applied dairy shed effluent and ammonium fertilizer (NH4Cl) labeled with 15N, using intact soil lysimeters (80 cm diameter, 120 cm depth). The soil used was a sandy loam, and the pasture was a mixture of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens). The DSE and NH4Cl were applied twice annually in autumn (May) and late spring (November), each at 200 kg N ha-1. The N applied in May 1996 was labeled with 15N. The lysimeters were either spray or flood irrigated during the summer. The autumn-applied DSE resulted in lower N leaching losses compared with NH4Cl. However, the N applied in the autumn had a higher potential for leaching than N applied in late spring. Between 4.5–8.1% of the 15N-labeled mineral N in the DSE and 15.1–18.8% of the 15N-labeled NH4Cl applied in the autumn were leached within a year of application. Of the annual N leaching losses in the DSE treatments (16.0–26.9 kg N ha-1), a fifth (20.3–22.9%) was from the mineral N fraction of the DSE applied in the autumn, with the remaining larger proportion from the organic fraction of the DSE, soil N and N applied in spring. In the NH4Cl treatments, more than half (53.8–64.8%) of the annual N leaching loss (55.9–57.6 kg N ha-1) was derived from the autumn-applied NH4Cl. DSE was as effective as NH4Cl in stimulating pasture production. Since only 4.4–4.5% of the annual herbage N uptake in the DSE treatment and 12.3–13.3% in the NH4Cl treatment were derived from the autumn-applied mineral N, large proportions of the annual herbage N uptake must have been derived from the N applied in spring, the organic N fraction in the DSE, soil N and N fixed by clover. The recoveries of 15N in the herbage were similar between the DSE and the NH4Cl treatments, but those in the leachate were over 50% less from the DSE than from the NH4Cl treatment. The lower leaching loss of 15N in the DSE treatment was attributed to the stimulated microbial activities and increased immobilization following the application of DSE.