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Bioenergy crops are expected to provide biomass as a replacement for fossil resources, but their impact on the water cycle is still under question. This study aimed at both quantifying the ability of bioenergy crops to use soil water and analysing the relationship between their root systems and soil water uptake. Water content was monitored continuously for 7 years (2007 – 2013) under perennial (Miscanthus × giganteus and Panicum virgatum), semi-perennial (Festuca arundinacea and Medicago sativa) and annual (Sorghum bicolor and × Triticosecale) bioenergy crops. Root distribution was characterized in 2010 down to 3 m depth. Soil water deficit (SWD) was calculated as the difference between field capacity and actual water content. Maximal SWD (0 – 210 cm) during the growing season was higher for semi-perennials, despite a lower biomass production than perennials. Water capture in deep soil layers was greater under perennials and semi-perennials than under annual crops. A curvilinear asymptotic relationship was found between water capture and root density and described by a model the parameters of which varied between crops, indicating a variable soil water capture for a given root density. This study provides quantitative information required to simulate the impact of bioenergy crops on drainage and aquifer loading.
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REGULAR ARTICLE
Soil water uptake and root distribution of different perennial
and annual bioenergy crops
Fabien Ferchaud &Guillaume Vitte &Frédéric Bornet &
Loïc Strullu &Bruno Mary
Received: 20 May 2014 /Accepted: 12 November 2014 /Published online: 23 November 2014
#Springer International Publishing Switzerland 2014
Abstract
Background and aims Bioenergy crops are expected to
provide biomass as a replacement for fossil resources, but
their impact on the water cycle is still under question. This
study aimed at both quantifying the ability of bioenergy
crops to use soil water and analysing the relationship
between their root systems and soil water uptake.
Methods Water content was monitored continuously for
7 years (20072013) under perennial (Miscanthus ×
giganteus and Panicum virgatum), semi-perennial
(Festuca arundinacea and Medicago sativa) and annual
(Sorghum bicolor and × Triticosecale) bioenergy crops.
Root distribution was characterized in 2010 down to 3 m
depth. Soil water deficit (SWD) was calculated as the
difference between field capacity and actual water content.
Results Maximal SWD (0210 cm) during the growing
season was higher for semi-perennials, despite a lower
biomass production than perennials. Water capture in
deep soil layers was greater under perennials and semi-
perennials than under annual crops. A curvilinear as-
ymptotic relationship was found between water capture
and root density and described by a model the parame-
ters of which varied between crops, indicating a variable
soil water capture for a given root density.
Conclusions This study provides quantitative informa-
tion required to simulate the impact of bioenergy crops
on drainage and aquifer loading.
Keywords Bioenergy.Energy crops .Soil water.Root
system .Miscanthus .Switchgrass
Abbreviations
IIrrigation
P Precipitation
PET Potential evapotranspiration
PWC Proportional water capture
RID Root intersection density
RLD Root length density
RMSE Root mean square error
SWC Soil water content
SWD Soil water deficit
Introduction
In response to the challenges of climate change and
depletion of fossil resources, biomass is expected to
contribute significantly to the energy transition by pro-
viding renewable carbon for bioenergy, biomaterials and
Plant Soil (2015) 388:307322
DOI 10.1007/s11104-014-2335-y
Responsible Editor: Peter J. Gregory .
F. Ferchaud (*):G. Vitte:F. B orn et :L. Strullu :B. Mary
INRA, UPR1158 AgroImpact, Site de Laon, Pôle du Griffon,
180 rue Pierre-Gilles de Gennes, 02000 Barenton-Bugny,
France
e-mail: fabien.ferchaud@laon.inra.fr
G. Vitte
e-mail: vitteg@gmail.com
F. Bo rnet
e-mail: frederic.bornet@laon.inra.fr
L. Strullu
e-mail: loic.strullu@laon.inra.fr
B. Mary
e-mail: bruno.mary@laon.inra.fr
biochemicals (IPCC 2011; Ragauskas et al. 2006).
Among biomass resources, dedicated bioenergy crops
have large technical potential and will probably be a
major player in the increase of bioenergy production
(Bentsen and Felby 2012;Chumetal.2011). The wide
range of conversion technologies leads to a large diver-
sity of candidate crops: short rotation coppices, peren-
nial crops, semi-perennial forage crops and annual crops
(Karp and Shield 2008; Lewandowski et al. 2003;
Sanderson and Adler 2008; van der Weijde et al. 2013;
Zegada-Lizarazu and Monti 2011).
Perennial C4 rhizomatous crops like Miscanthus ×
giganteus (hereafter referred to as miscanthus) or Pan-
icum virgatum (hereafter referred to as switchgrass) are
considered as promising energy crops because of their
high biomass production, low nutrient requirements and
low greenhouse gas emissions (Cadoux et al. 2014;Don
et al. 2011; Somerville et al. 2010). However, large
deployment of these crops could modify their regional
environment. One particular concern is the effect on the
water cycle through modifications in evapotranspiration
(McIsaac et al. 2010; Vanloocke et al. 2010). Several
authors have suggested that perennial bioenergy crops
consume more water than annual crops, because of their
higher biomass production, longer growing period and
deeper root system (Heaton et al. 2010; Powlson et al.
2005;Roweetal.2009). Using soil moisture measure-
ments over four growing seasons in central Illinois,
McIsaac et al. (2010) estimated that miscanthus had
higher evapotranspiration than switchgrass and maize-
soybean rotation. Hickman et al. (2010) found similar
results at the same site during one growing season by
using a micrometeorological method. Using a model-
based approach, Le et al. (2011) predicted an average 58
and 36 % increase of total seasonal evapotranspiration
for miscanthus and switchgrass respectively, compared
to maize in the Midwest United States. This higher
water consumption during crop growth will reduce the
amount of water drained during winter. Vanloocke et al.
(2010) predicted with a dynamic global vegetation mod-
el a decrease in drainage ranging from 50 to
250 mm year
1
with miscanthus cultivation instead of
current land cover for the Midwest United States. This
decrease of drainage is likely to impact aquifers in case
of large-scale land conversions.
Among factors influencing crop water use, morphol-
ogy and distribution of roots within the soil profile are of
prime importance because they define the amount of
water that can be supplied to the crop (Jackson et al.
2000). Deep rooting (>2 m) has been reported by several
authors for miscanthus and switchgrass (Ma et al. 2000;
Neukirchen et al. 1999;RicheandChristian2001). This
extensive root system may allow these crops to maintain
their growth in case of drought period but is also likely
to lead to a greater soil water deficit (SWD) than annual
crops at the end of the growing season (Riche and
Christian 2001). To our knowledge, only one study has
compared soil water uptake and root distribution of
different bioenergy crops (Monti and Zatta 2009). How-
ever this study was restricted to one growing season and
the soil sampling depth was only 120 cm. There is a
need to compare a wide range of candidate bioenergy
crops over multiple seasons to take into account climate
variability and in a deep soil to maximize the differences
in root distribution between crops.
We hypothesized that perennial C4 crops use deep
soil water resources because of their extensive root
system and high biomass production, leading to a higher
SWD than with other crops. The aim of this study was
(1) to quantify soil water utilization for perennial, semi-
perennial and annual bioenergy crops using a long term
and continuous monitoring of soil water, and (2) to
study the relationship between the root system of the
crops and soil water uptake.
Materials and methods
Study site and experimental design
This study is based on an ongoing long-term experiment
carried out by INRA at the experimental station of
Estrées-Mons, northern France (49.872°N, 3.013°E),
on a Haplic Luvisol (IUSS Working Group WRB
2006). The experiment was initiated in 2006 and six
crops were compared, representing a wide range of
bioenergy crop types: two perennial C4 crops, two
semi-perennial forage crops and two annual crops. The
chosen crops were miscanthus (Miscanthus × giganteus
Greef & Deuter ex Hodkinson & Renvoize), switch-
grass (Panicum virgatum cv. Kanlow),fescue (Festuca
arundinacea cv. Dulcia from 2006 to 2008, Noria from
2009 to 2010 and Bariane after 2010), alfalfa (Medicago
sativa cv. Alpha from 2006 to 2008, Orca from 2009 to
2010 and Salsa after 2010), fibre sorghum (Sorghum
bicolor (L.) Moench cv. H133) and triticale (×
Triticosecale Wittmack cv. Triskell from 2006 to 2008,
Amarillo from 2009 to 2011 and Tarzan after 2011). The
308 Plant Soil (2015) 388:307322
annual crops were grown in rotation (triticale grown
after sorghum and vice-versa) as well as the semi-
perennial crops (alfalfa grown after fescue and vice-
versa) and all crops were present each year. A catch
crop was sown every year in August or early September
between triticale and sorghum (rye in 2007, mustard in
2008, oat-vetch mixture in 2009 and mustard-clover
mixture from 2010 to 2013). The perennial crops were
established in 2006 and the semi-perennial crops were
sown in 2006 (first rotation), 2009 (second rotation) and
2011 (third rotation). Two harvest dates were compared
for miscanthus and switchgrass: an early harvest in
October and a late harvest in February. The experiment
also included two nitrogen treatments for each crop
except alfalfa, with a plot size of 360 m
2
and three
replicates per treatment. Details about crop management
and experimental treatments are given by Cadoux et al.
(2014). In this study, we selected experimental treat-
ments maximizing plant growth and thus water con-
sumption: the late harvest for miscanthus and switch-
grass and the highest nitrogen treatment for all crops.
During the period 20072013, the mean N fertilization
rates for the selected treatments were 120 kg ha
1
year
1
for miscanthus, switchgrass, sorghum and triticale,
170 kg ha
1
year
1
for fescue and 0 for alfalfa. The
experiment did not receive irrigation, except in May
2011 for fescue, alfalfa, sorghum and triticale (58 mm
in total) to facilitate crop establishment during a drought
period.
Climatic data
Climatic data were obtained from an automatic weather
station situated on the experimental site. Over the period
20072013, mean temperature was 10.6 °C, annual
rainfall (P) and Penman potential evapotranspiration
(PET) were 686 and 714 mm respectively and annual
global radiation was 4154 MJ m
2
. The water balance
(P-PET) during the growing season displayed a rather
large variability between years (Table 1). We considered
March 1 as the beginning of the growing season because
it corresponds approximately to the time when winter
crops like triticale start growing again and PET begins
running higher than 1 mm day
1
.WeconsideredNo-
vember 1 as the end of the growing season because all
annual and semi-perennial crops have been harvested
and perennials are close to total senescence. The wettest
year was 2008 and 2009 was the driest, with only
116 mm of precipitation from June to September.
Springs 2010 and 2011 were drier than the 7-year
average.
Biomass production
The aboveground biomass at harvest was estimated for
each crop. At each harvest date, plants were harvested
manually and weighed. Miscanthus and switchgrass
were harvested in February or early March. Fescue and
alfalfa were harvested in two or three cuttings depending
on the years, with the last cut in October. Sorghum was
harvested in late September and triticale in late July or
early August. Details about sampling methodologies are
given by Cadoux et al. (2014). The dry matter content
was determined after drying representative subsamples
at 65 °C for 96 h. The biomass production was
expressed in tons of dry matter per hectare and per year
for all crops.
Soil and soil water monitoring
Measurements
We used water content reflectometers (Campbell Scien-
tific CS616) to monitor the soil moisture profile contin-
uously from July 2007 to November 2013. Probes were
installed in May 2007 in six plots (one plot per crop),
inserted horizontally into the soil at 15, 45, 75, 105, 135,
165 and 195 cm depth (three replicates at 15 cm depth
and two replicates at the other depths). Temperature
sensors (Campbell Scientific 107) were also placed at
15 and 195 cm depth. Data were recorded at an hourly
time step using CR1000 Campbell Scientific data log-
gers. Probes placed at 15 cm depth were removed for
soil tillage and reinserted as soon as possible.
Soil cores taken down to 210 cm from the six plots in
2006 were analysed to determine soil characteristics
which were veryhomogeneous in the six plots (Table 2).
Bulk density was measured at each CS616 depth in May
2007 using steel cylinders of 98 cm
3
(5 cm diameter,
three replicates) and measurements were repeated for
the upper depth (15 cm) in 2010 and 2011 or 2012 with
six replicates. Gravimetric water content was measured
in the three blocks twice a year (in mid-March and early
November) from 2007 to 2013. At each date of mea-
surement, soil cores were collected down to a depth of
150 cm with a driller 18 mm in diameter. The cores were
divided into five layers (030, 3060, 6090, 90120
and 120150 cm). In each soil layer, one soil sample
Plant Soil (2015) 388:307322 309
was formed by mixing five soil cores for each plot. In
the instrumented plots, additional measurements were
made during summer 2009 and from November 2011 to
November 2013 down to 210 cm, with three individual
cores per plot divided into seven layers. Only gravimet-
ric measurements made in the instrumented plots were
used to calibrate CS616 probes.
Data processing and calculations
Data from CS616 probes need proper correction and
calibration in order to obtain accurate soil moisture
measurements (Rudiger et al. 2010). First of all, data
were regularly collected in a database managed with
PostgreSQL and eventual outliers were eliminated. Sec-
ondly, the soil temperature was simulated at each CS616
depth using a script developed with R software (R Core
Tea m 2014). We used Fouriers law to simulate heat
conduction transfer through the soil profile. Soil tem-
perature at 15 and 195 cm depth were taken as boundary
conditions and the initial temperature along the soil
profile was determined by linear interpolation between
the two depths. Depth and time increments as well as
thermal diffusivity (alpha) were optimized using two
Tabl e 1 Meteorological data: P = precipitation (mm), PET = Penman potential evapotranspiration (mm) recorded at Estrées-Mons over the
period 20072013
Year MarchJune JulyOctober MarchOctober
P PET P-PET P PET P-PET P PET P-PET
2007 290 364 73 216 327 110 507 690 184
2008 281 350 69 317 318 1 598 668 70
2009 200 341 142 145 432 288 344 773 429
2010 136 374 239 270 346 76 406 720 315
2011 108 328 219 243 294 52 351 622 271
2012 282 294 13 219 346 127 501 640 139
2013 202 293 91 321 345 24 523 637 114
Mean 214 335 121 247 344 97 461 679 217
Tabl e 2 Soil characteristics measured in the experimental plots (mean±standard deviation)
Soil layers (cm)
030 3060 6090 90120 120150 150180 180210
Clay (g kg
1
) 169 ±25 227 ± 33 267 ± 22 243±11 222 ±12 190 ± 13 234 ± 21
Fine silt (g kg
1
) 320 ± 17 305 ± 21 283±44 277 ± 20 275 ± 13 272±29 305 ± 10
Coarse silt (g kg
1
) 459 ±16 418 ± 19 410 ± 22 440±16 467 ± 23 48 28 404±5
Fine sand (g kg
1
) 38±9 43±18 37 ± 21 36± 15 35±13 46 ±13 52± 15
Coarse sand (g kg
1
) 13±3 6±2 2±1 2±1 1±1 1±1 3±1
Organic C
a
(g kg
1
) 9.8± 0.3 6.0±0.3 3.0±0.2 2.3±0.2 2.1±0.3 1.7±0.3 2.6±0.7
pH in water 7.7± 0.2 7.8± 0.2 7.8±0.1 8.0± 0.1 8.0±0 8.2± 0.1 8.2± 0.1
CaCO
3
(g kg
1
) 2±1 2±2 1±1 2±2 1±1 1±2 3±4
WFC
b
(g kg
1
) 243±3 221 ±3 219 ± 3 217 ± 4 222 ± 2 228 ± 6 238 ± 3
WWP
c
(g kg
1
) 90± 12 102± 18 107±11 104 ±9 95±13 89± 6 104±5
Bulk density (g cm
3
) 1.47± 0.05 1.55±0.02 1.57±0.03 1.58±0.01 1.55±0.03 1.49± 0.03 1.51± 0.03
a
Anne Method (AFNOR X 31-109)
b
Water content at field capacity (median of field measurements made in March over the period 20072013), corresponding to ca. 20 kPa
water potential
c
Water content at permanent wilting point (measured with Richards pressure plates at 1.5 MPa water potential)
310 Plant Soil (2015) 388:307322
other plots of the same experiment with supplementary
soil temperature measurements at 75 and 135 cm depth.
The optimized value for alpha was 24 cm
2
h
1
and the
root mean square error (RMSE) was 0.2 °C over a
period of 434 days. Thirdly, period measurements from
the CS616 probes were corrected for measured or sim-
ulated soil temperature, using the temperature correction
equation provided by Rudiger et al. (2010) with the
slope coefficient for silt loam. The fourth step consisted
in deriving a relationship between corrected period mea-
surements and volumetric soil water contents, obtained
from gravimetric water contents and bulk density mea-
surements. A covariance analysis was applied with R
software for the two or three replicates of each plot and
depth in order to choose (with a 95 % confidence level)
between an individual calibration with a specific linear
regression equation for each replicate, a common cali-
bration or an individual calibration of the intercept with
a common slope. The mean coefficient of determination
was 0.86 for the 90 probes (n=17) and the mean RMSE
was 0.016 cm
3
cm
3
. Finally, all corrected period mea-
surements were converted into volumetric and gravi-
metric water content and the two or three replicates were
averaged. Missing data were filled in by linear interpo-
lation and data were aggregated to obtain daily mea-
surements. Standard deviation between replicates was,
on average over the period 20072013, 0.019 cm
3
cm
3
for the first layer (three replicates) and 0.007 cm
3
cm
3
for the other layers (two replicates).
The soil water content (SWC, in mm) was calculated
in each 30 cm soil layer and summed up over the
monitored soil profile (0210cm).Foreachcrop,
SWC calculated over the three replicated plots with
gravimetric measurements were compared to SWC cal-
culated in the single instrumented plot using CS616
probes. We found a good, unbiased relationship between
the two estimates (y=1.005 x; R
2
=0.94; n=65), which
indicated that SWC assessed with CS616 probes were
representative of the whole field. The soil water deficit
(SWD, in mm) was defined as the difference between
SWC at field capacity and the measured SWC (Beale
et al. 1999) for each soil layer or over the monitored soil
profile. For each soil layer, the proportional water cap-
ture (PWC, in %) was calculated as the fraction of
potentially available water that had been captured by
plant roots (Monti and Zatta 2009):
PWC ¼SWCFCSWC
SWC FC SWCWP
100 ð1Þ
where SWC
FC
is the water content at field capacity (in
mm) and SWC
WP
the water content at permanent wilting
point (in mm). SWC
FC
was calculated as the median of
the gravimetric measurements made in winter (March)
over the period 20072013. SWC
WP
was measured with
Richards pressure plates at 1.5 MPa water potential.
Root mapping
We collected data on root distribution during the year
2010 for each plot in which soil water was monitored
using a modified trench profile method (Tardieu 1988).
First of all, a trench 300 cm deep was dug into the plot.
The observed vertical profile (180 cm wide, 300 cm
deep) which was perpendicular to the crop row was then
prepared. After the working surface had been smooth-
ened, roots were made visible by removing approxi-
mately 1 cm of soil with a knife. Next, roots were
mapped on three adjacent 60 × 300 cm grids with cells
of 1.9 × 1.9 cm. Since root counting was a very time-
intensive operation, the number of root impacts in each
cell was measured only on 20 % of the cells for each
60 cm wide grid (the seven cells at the right of the grid),
and the presence or absence of root impact was noted for
the other cells. The distribution of roots was recorded on
11 and 25 June 2010 for miscanthus and switchgrass
respectively (4-year-old crops), on 14 and 21 September
2010 for fescue and alfalfa respectively (1.5-year-old
crops), on September 9, 2010 for sorghum (at the be-
ginning of anthesis) and on July 13, 2010 for triticale
(10 days before physiological maturity).
Relationship between root density and proportional
water capture
In studies dealing with roots and water uptake, the root
distribution is often described using the root length
density (RLD), i.e. the total root length per unit of soil
volume (Gregory 2006). Experimental measurement of
RLD by extracting soil cores or soil monoliths can be
extremely labour-intensive. Mapping and counting root
impacts on a vertical soil profile has the advantage of
being easier to do in the field but provides no direct
measurement of RLD. However, the root intersection
density (RID), i.e. the mean number of root impacts per
cm
2
(Chopart et al. 2008), can be calculated from such
measurements and linear relationships between RID and
RLD have been found for various crops (Chopart et al.
2008; Chopart and Siband 1999;Dusserreetal.2009).
Plant Soil (2015) 388:307322 311
We therefore assumed that RID could be used as an
indicator of RLD. Indeed, we also determined RLD for
miscanthus and switchgrass (Ferchaud et al. 2012)and
verified the linear relationship between RLD and RID
for these two crops.
RID was calculated in each 60 × 300 cm grid for each
layer of 30 cm thickness.
The relationship between RID and water capture was
studied in 2010 using PWC calculated at the date of the
maximal soil water deficit (over 0210 cm). The rela-
tionship was described for each crop with a model
derived from King et al. (2003):
PWC ¼ay0
ðÞ1ek:RID

þy0ð2Þ
where kis a resource capture coefficientwhich sum-
marizes details of water uptake physiology and soil
water transport. A higher kvalue leads to a faster in-
crease in water extraction when root density increases.
Compared to the original model of King et al. (2003),
we added two parameters: awhich is the highest PWC
achievable by the crop (a=100 % in the original model)
and y
0
which is the PWC obtained in free root soil layers
due to possible water capillary rise (y
0
=0 in the original
model). A common value for all crops was chosen for
y
0
. The parameter optimization for aand kwas realised
with the Excel solver using the GRG nonlinear method.
The minimized criterion was the RMSE.
Statistical analysis
All statistical analyses were performed with R (R Core
Tea m 2014). The effect of the crop on highest and
lowest SWC of each year was evaluated by one-factor
analysis of variance (ANOVA), using the different
probes as replicates. We used a two-factor ANOVA
without replication with crop and year as factors in order
to test the crop effect on mean SWD for each soil layer
and for 0210 cm. For PWC, we included the soil layer
as third factor. For RID, the effect of the crop was
evaluated in each layer by one-factor ANOVA, using
the three adjacent grids as replicates. Differences be-
tween crops were evaluated with Tukeys HSD (honest
significant difference) test for all variables. The assump-
tions of ANOVA were checked by visual examination of
the residuals against predicted values and using Shapiro-
Wilk and Levenes tests. If necessary, we used square
root transformation or arcsine square root transforma-
tion to satisfy these assumptions.
Results
Soil water content
SWC over the soil profile (0210 cm, measured with
CS616 probes) fluctuated over the 7-year period with a
regular pattern for all crops (Fig. 1). It was at its highest
level in winter (above 700 mm), decreased every year
during spring and summer and rose in autumn. SWC in
winter was rather stable between years and crops. It was
close to the estimated SWC at field capacity (mean=
730 mm) and peaked from time to time at approximately
750 mm. Differences in maximum SWC between crops
were only significant 3 years out of 6. In 20092010 and
20102011, SWC did not reach field capacity for sev-
eral crops (miscanthus and switchgrass in 20092010,
miscanthus, switchgrass, fescue and alfalfa in 2010
2011). Indeed, in these cases, autumn and winter pre-
cipitations were not sufficient for the deeper layers to
reach field capacity. The timing of SWC decrease
depended on the crops, with an earlier decline for triti-
cale, fescue and alfalfa (except in 2009 and 2011 when
these two crops were newly sown) than for the other
crops. The lowest SWC over a year were observed on
average over the period 20072013 in mid-October for
miscanthus, in mid-September for switchgrass, fescue,
alfalfa and sorghum and in early July for triticale. Min-
imal SWC were more variable between years and crops
than maximal SWC. Differences in minimal SWC be-
tween crops were significant every year, with a range of
variation between crops higher than 100 mm 5 years out
of 7. The lowest SWC over the period 20072013 were
observed in 2009 for all crops except for triticale, for
which the lowest SWC was observed in 2013. These
values (433 mm for alfalfa, 451 mm for fescue, 495 mm
for switchgrass, 507 mm for sorghum, 526 mm for
miscanthus and 528 mm for triticale) were significantly
higher than the estimated SWC at permanent wilting
point (mean=318 mm). The timing of SWC increase
after this minimal point was more stable between crops
than the timing of SWC decrease in spring, but generally
faster for annual crops than for the other crops.
Maximal soil water deficit
In order to quantify soil water utilization by the crops,
we calculated the maximal SWD each year over the soil
profile (0210 cm), corresponding to the minimal SWC
for the year. Maximal SWD ranged from 61 mm for
312 Plant Soil (2015) 388:307322
triticale in 2007 to 294 mm for alfalfa in 2009. For each
crop, the variability of the maximal SWD between years
was large, with standard deviations of 41 to 59 mm.
Nevertheless, alfalfa had the largest maximal SWD 6
years out of 7 and fescue always had the second largest
maximal SWD. The ranking of other crops was more
variable between years. On an average, over the period
20072013, the differences between crops were signif-
icant (Fig. 2). Alfalfa had a higher maximal SWD
(218 mm) than the other crops except fescue. Maximal
SWD was lowest for sorghum (142 mm) and interme-
diate for triticale, miscanthus and switchgrass (156, 157
and 171 mm respectively).
At maximal SWD, SWD calculated in each soil
layer was higher in the topsoil (Fig. 2). On an aver-
age, 47 % of the total SWD was located in the upper
two layers (060 cm), 38 % in the three intermediate
layers (60150 cm) and only 15 % in the two deeper
layers (150210 cm). However, this distribution was
crop-dependent: annual crops had a higher propor-
tion of the total SWD in the 060 cm layer (56 %)
and a lower proportion in the 150210 cm layer
(11 %) than the other crops. SWD in the 150
210 cm layer was 16 mm for sorghum and triticale,
significantly lower than alfalfa (46 mm). SWD in the
first layer (030 cm) was significantly lower for
miscanthus and switchgrass (44 and 35 mm respec-
tively) than for fescue, alfalfa and triticale (66, 56
and56mmrespectively).
We examined the influence of climate conditions
on maximal SWD. Using data of precipitation (P),
irrigation (I) and potential evapotranspiration (PET),
Fig. 1 Evolution of the soil water content (SWC, 0210 cm) over
time (from July 2007 to November 2013) for each crop (Mis
miscanthus, Swi switchgrass, Fes fescue, Alf alfalfa, Sor sorghum,
Tri triticale). For fescue, alfalfa, sorghum and triticale, changes
between plots were made the first day of March. Asterisks indicate
significant differences between crops for the highest and lowest
SWC of each year (° = NS; * = p<0.05; ** = p<0.01; *** =
p<0.001)
Fig. 2 Soil water deficit (SWD)
calculated for each crop over the
period 20072013. SWD is the
mean (over 7 years) of the
maximal values of SWD (0
210 cm) encountered during each
year. Different letters indicate
significant differences (p<0.05)
between crops. The seven soil
layers (from 030 cm to 180
210 cm) are represented with a
grey gradient from white to dark
grey
Plant Soil (2015) 388:307322 313
the water balance (P + I - PET) was calculated each
year and for each crop from March 1 to the date of
maximal SWD. The water balance was 238 mm on
an average and was very similar for most crops (256
±5 mm), except for triticale (148 mm). It was neg-
atively correlated with maximal SWD (r=0.63;
p<0.001; Fig. 3).
The influence of aboveground biomass produc-
tion on maximal SWD was also investigated. No
significant correlation was found between biomass
production and maximal SWD (Fig. 4). This was
true not only when all crops were grouped together
but also for each crop independently, despite large
differences in biomass production between crops
and years for some crops. Miscanthus, switchgrass
and triticale had a rather stable biomass production
over the period 20072013 but fescue, alfalfa and
sorghum displayed a higher variability with low
biomass production (<10 t ha
1
) some years. Sur-
prisingly, the highest maximal SWD (294 and
275 mm for alfalfa and fescue respectively) was
observed for low biomass production (3.2 and
6.4 t ha
1
) during the year 2009, which was the
driest year and when fescue and alfalfa were newly
sown. The biomass production of catch crops grown
between triticale and sorghum was not taken into
account because it was very low (0.5±0.2 t DM ha
1
on average over the period 20072013).
Proportional water capture
PWC was calculated for each year and each soil layer at
the date of the maximal SWD. Crops, soil depth and their
interaction significantly affected PWC. On average, PWC
decreased with depth from 74 % in the 030 cm layer to
19 % in the 180210 cm layer, and never reached 100 %
below 30 cm. However, differences between crops were
significant for all soil layers (Fig. 5). For the first soil layer
(030 cm), PWC was significantly lower for miscanthus
and switchgrass than for other crops. The highest PWC
observed in the 030 cm layer over the period 20072013
was only 76 and 57 % for miscanthus and switchgrass
respectively, compared to 100 % for the other crops. The
differences between crops were smaller in the 3060 and
6090 cm layers, with a higher PWC for fescue and
alfalfa. PWC was smaller for sorghum and triticale below
90 cm. Alfalfa had the highest PWC (42 and 33 % on
average) in the 150180 and 180210 cm layers respec-
tively. It was significantly higher than for sorghum and
triticale (15 and 16 % in the 150180 cm layer; 12 and
11 % in the 180210 cm layer, respectively). The variabil-
ity of PWC between years was smaller in deeper than in
upper layers for annual crops, but not for other crops.
PWC calculated at the date of maximal SWD were
compared to PWC calculated at the end of the growing
season, i.e. November 1 (data not shown). PWC changed
very slightly for the three deeper layers (only 1 % on
Fig. 3 Relationship between the maximal soil water deficit and
the water balance (P + I - PET) observed each year for each crop
Fig. 4 Relationship between the maximal soil water deficit and
the aboveground biomass production observed each year for each
crop
314 Plant Soil (2015) 388:307322
average) but was reduced in the upper layers. This means
that there was no or very little additional water retrieval in
the deeper layers after the date of maximal SWD, even
for crops for which maximal SWD was observed on
average more than 1 month before the end of their
growing period (switchgrass, fescue, alfalfa).
Root distribution and root intersection density
Figure 6shows the root distribution of each crop ob-
served in 2010 on the trench profiles. Miscanthus,
switchgrass and alfalfa had a particularly deep root
system: the maximum rooting depth was 300, 288 and
276 cm respectively. Sorghum had a more superficial
root system (maximum rooting depth = 128 cm) and
fescue and triticale were intermediate with a maximum
rooting depth of 200 cm. The proportion of cells includ-
ing roots decreased with depth, more or less according
to the crops. This proportion decreased from 79 %
(sorghum) to 100 % (fescue) in the 030 cm layer down
to 0 % (sorghum) to 10 % (miscanthus) in the 180
210 cm layer. For the crops with a maximum rooting
depth exceeding 210 cm, the proportion of cells includ-
ing roots in the 210300 cm layer was 7, 1 and 4 %
respectively for miscanthus, switchgrass and alfalfa.
Root intersection density (RID) decreased drastically
with depth for all crops (Table 3). It varied from 0.57
(sorghum) to 1.17 roots cm
2
(fescue) in the 030 cm
layer, and became lower than 0.04 roots cm
2
for all
crops in the 180210 cm layer. The crop effect was
significant in all soil layers. RID was significantly
higher for fescue than sorghum, alfalfa and miscanthus
in the first layer and higher for fescue, alfalfa and
switchgrass than for miscanthus and sorghum in the
3060 and 6090 cm layers. In the 150180 and 180
210 cm layers, miscanthus, switchgrass and alfalfa had a
significantly higher RID than sorghum and triticale.
Over 0210 cm, the mean RID was 0.15 and 0.18 roots
cm
2
respectively for sorghum and miscanthus, 0.24
and 0.26 roots cm
2
respectively for triticale and alfalfa
and 0.30 and 0.32 roots cm
2
respectively for switch-
grass and fescue. Only miscanthus, switchgrass and
alfalfa produced roots below 210 cm, with a significant-
ly lower RID for switchgrass.
Relationship between root distribution and water uptake
We studied the relationship between RID measured in
2010 and PWC calculated for each soil layer at the date
of the maximal SWD in 2010. Maximal SWD occurred
on July 11 for triticale, on August 15 for switchgrass,
fescue, alfalfa and sorghum and on October 31 for
miscanthus. The year 2010 was the second driest year
during the growing season, after 2009. PWC were there-
fore higher during year 2010 than the average for all
crops at almost all depths.
A positive linear correlation was found between RID
and PWC for all species (r=0.72; p<0.001). However,
the relationship was not strictly linear but rather curvi-
linear asymptotic and differences appeared between
crops (Fig. 7). The highest PWC found in the 030 cm
layer varied widely between crops, from 53 % for
Fig. 5 Proportional water capture (PWC) versus depth for each
crop (Mis miscanthus, Swi switchgrass, Fes fescue, Alf alfalfa, Sor
sorghum, Tri triticale). PWC is calculated at the date of the
maximal soil water deficit (mean value over the period 2007
2013). Horizontal bars represent the range between the minimal
and the maximal values found over the 7-year period. Asterisks
indicate significant differences between crops in each soil layer (*
=p<0.05; ** = p<0.01; *** = p<0.001)
Plant Soil (2015) 388:307322 315
Fig. 6 Spatial distribution of roots observed on the vertical trench wall (180 × 300 cm) for each crop in 2010. Each black dot represents a
grid cell (1.9 × 1.9 cm) containing at least one root
Tabl e 3 Root intersection density (mean number of root impacts per cm
2
) measured for each soil layer and crop (mean± standard deviation)
Soil layer (cm) Miscanthus Switchgrass Fescue Alfalfa Sorghum Triticale
030 0.800±0.159 bc 1.038±0.107 ab 1.173 ±0.169 a 0.669± 0.066 c 0.571±0.056 c 0.867±0.140 abc
3060 0.155± 0.037 b 0.513±0.082 a 0.546± 0.009 a 0.510±0.113 a 0.194± 0.030 b 0.295±0.084 b
6090 0.134± 0.055 b 0.314±0.018 a 0.352± 0.095 a 0.379±0.035 a 0.123± 0.013 b 0.278±0.057 a
90120 0.043±0.016 bc 0.101±0.035 ab 0.127±0.071 ab 0.126 ±0.042 ab 0.016±0.010 c 0.160±0.058 a
120150 0.043 ± 0.034 a 0.072 ± 0.039 a 0.027 ± 0.013 a 0.067 ± 0.039 a 0.001 ±0.001 b 0.042±0.017 a
150180 0.050 ±0.006 ab 0.057±0.017 a 0.020± 0.012 bc 0.051 ±0.014 ab 0 ±0 d 0.014±0.011 c
180210 0.031 ±0.022 a 0.020± 0.004 ab 0.005±0.002 bc 0.022±0.004 ab 0 ±0 c 0.003±0.004 c
210240 0.023 ± 0.010 a 0.010 ± 0.004 b 0 ± 0 c 0.020± 0.002 a 0±0 c 0 ±0 c
240270 0.020 ± 0.012 a 0.003 ± 0.002 b 0 ± 0 b 0.016±0.009 a 0 ± 0 b 0±0 b
270300 0.022 ± 0.016 a 0.001 ± 0.001 b 0 ± 0 b 0.001±0.000 b 0 ± 0 b 0 ± 0 b
Different letters indicate significant differences (p< 0.05) between crops in each soil layer
316 Plant Soil (2015) 388:307322
switchgrass to 100 % for fescue and triticale whereas the
corresponding RID was similar for most crops. This was
also true for the deeper soil layers where a large
variability in PWC was observed for identical RID.
The model derived from King et al. (2003) was fitted
for each crop independently (Fig. 7). Water capture was
observed for sorghum in the layers 150180 cm and
180210 cm (PWC=14 and 10 % respectively), al-
though no visible root was found in these layers. We
used the mean of these two values (12 %) for the y
0
parameter for all crops. Simulated PWC were in good
agreement with observed data, with a mean RMSE of
7 % (Table 4). The goodness of fit was equivalent for all
crops except for fescue, which had the highest RMSE.
The parameter values obtained after optimization were
rather variable between crops (Table 4). The resource
capture coefficientkwas highest for fescue, followed
by switchgrass and alfalfa and smaller for sorghum and
triticale. Annual crops were characterized by an avalue
(highest PWC achievable by the crop) greater than for
other crops, particularly perennials.
Discussion
Soil water deficit and water capture
Few studies have compared the evolution of soil water
content during the growing season for different
bioenergy crops. For example, alfalfa has been com-
pared to annual crops (Entz et al. 2001) but not to fescue
or perennial C4 crops. Our study is the first one com-
paring perennial, semi-perennial and annual bioenergy
crops. Entz et al. (2001) found that soil water content
during summer (over 0150 cm) was always lower for
alfalfa than for annual crops during 5 years of cultiva-
tion. This is consistent with our study showing that the
highest maximal SWD was observed for alfalfa 6 years
out of 7. The differences in maximal SWD over 0
210 cm between miscanthus, switchgrass and annual
crops were small and not significant in our study with
a different ranking according to the year. McIsaac et al.
(2010), who monitored soil moisture under miscanthus,
switchgrass (cv. Cave-In-Rock) and a maize-soybean
Fig. 7 Relationship between proportional water capture (PWC)
and root intersection density (RID) for each crop. PWC was
calculated at the date of maximal soil water deficit in 2010: a
RID expressed in linear scale; bRID expressed in logarithmic
scale. Symbols are experimental data and lines are modelled data.
Miscanthus (Mis) is represented by a dark continuous line,switch-
grass (Swi)byalight continuousline,fescue(Fes)byadark dotted
line, alfalfa (Alf)byalight dotted line, sorghum (Sor)byadark
broken line and triticale (Tri)byalight broken line
Tabl e 4 Parameter and statistical criterion values obtained for each crop after fitting the model describing the relationship between root
intersection density and proportional water capture (see Eq. 2in text)
Miscanthus Switchgrass Fescue Alfalfa Sorghum Triticale
a(%) 63.5 55.9 74.2 75.1 93.2 100.0
k(cm
2
) 12.6 24.4 39.6 23.9 7.3 3.1
RMSE (%) 5.4 4.9 13.8 7.3 4.6 5.7
Plant Soil (2015) 388:307322 317
rotation, also found contrasting results between growing
seasons. They found that the minimal soil moisture
under miscanthus and switchgrass was either equal
(2 years out of 4) or lower (the other 2 years) than under
annual crops. However, in contrast to our results, they
observed that miscanthus resulted in lower minimal soil
moisture than switchgrass during the four growing
seasons.
We found that maximal SWD was correlated to the
water balance (P + I - PET) but not to the aboveground
biomass production. The biomass production of the
semi-perennial crops was only 57 % of the miscanthus
production and 64 % of the switchgrass production but
their maximal SWD was higher. In fact, maximal above-
ground biomass of perennial crops (in October) was
even higher than the biomass at harvest (in February)
due to leaf fall during winter (only for miscanthus) and
carbon transfer from aboveground to belowground parts
in autumn (Dohleman et al. 2012; Strullu et al. 2011).
This suggests that miscanthus and switchgrass had
higher water use efficiency than fescue and alfalfa.
Beale et al. (1999) calculated water use efficiencies for
miscanthus in UK with the maximal aboveground bio-
mass reached by the crop during the growing season.
They found comparable values to other C4 crops, such
as maize, and higher values than C3 crops such as
willow. Furthermore, it is possible that the timing of
the growing periods of the crops affects water use effi-
ciency due to differences in climate conditions. The lack
of correlation between maximal SWD and aboveground
biomass production observed during the 7 years for each
crop independently indicates that SWD was much more
sensitive to the water balance than biomass production.
Our results also indicate that perennial C4 crops and
semi-perennial forage crops, and particularly alfalfa,
have the ability to take up significant amounts of water
in deep soil layers (150210 cm). Water uptake is even
likely to occur below 210 cm for miscanthus,
switchgrass and alfalfa since these crops have roots
deeper than 210 cm. Our results are in agreement with
Campbell et al. (1994) and Dardanelli et al. (1997)who
showed that alfalfa growing in deep soil can withdraw
water to a depth of 250 cm, and Finch and Riche (2008)
who found significant soil water depletion down to
170 cm with miscanthus at two sites in England.
However, the ability of miscanthus and switchgrass
to take up deep soil water did not lead to a significantly
higher maximal SWD than annual crops because it was
compensated by a lower SWD in the 030 cm layer.
This lower SWD near the soil surface, which had not
been emphasized in previous studies, was at least partly
due to lower soil evaporation. Indeed, these two crops
have a high and dense canopy during summer, when
PET is maximal, which is likely to limit soil evapora-
tion. Furthermore, the fallen leaves of miscanthus accu-
mulating at the soil surface form a 24 cm thick mulch
(Amougou et al. 2012) which enhances the reduction of
soil evaporation.
Root distribution
We assumed that our protocol was appropriate to com-
pare the root systems of the different crops. The root
systems of annual crops measured at anthesis or post-
anthesis were probably close to maximal development
(Hoad et al. 2001; Zhang et al. 2004). Root distribution
of perennial and semi-perennial crops was recorded at
two periods during the year 2010 (June and September
respectively). Root extension of perennial crops had
probably reached steady state since the crops were
4 years old. Neukirchen et al. (1999)didnotfindany
effect of the sampling date on root density when com-
paring three dates of measurements in a 5-year-old
miscanthus. Semi-perennials were at the end of their
second year of growth at the time of measurement: the
recorded root distribution corresponded to well-
established crops and was probably close to the maximal
root development achieved during the 2010 growing
season although we do not know if it was at steady state.
The climate conditions in 2010, with a first part of the
growing season (MarchJune) drier than the average,
might have affected root development of annual and
semi-perennial crops. However, this is unlikely because
the available soil water was high in March (240 mm
over 0120 cm) and the biomass of these crops in 2010
was similar to the average (104 % of the 20072013
mean biomass production).
In our conditions, i.e. in a deep loamy soil with no
obstacles to rooting, crops exhibited large differences in
rooting depth. Miscanthus, switchgrass and alfalfa had a
particularly deep root system. The maximal rooting
depth of miscanthus (300 cm) was deeper than that
recorded in other field experiments: 200 cm for a 6-year-
old crop in a silty clay loam in England (Riche and
Christian 2001) and 250 cm for a 3-year-old crop in a
sandy loam in Germany (Neukirchen et al. 1999). The
difference with our study might result from differences
in soil characteristics or maximum depth of observation.
318 Plant Soil (2015) 388:307322
The maximum rooting depth of switchgrass measured in
our experiment (288 cm) was intermediate between that
observed by Riche and Christian (2001) for a 6-year-old
crop in England (240 cm) and that reported by Ma et al.
(2000) in a sandy loam in Alabama for a 7-year-old crop
(330 cm). Evidence of a deep root system for alfalfa was
also found in several studies (Campbell et al. 1994;
Dardanelli et al. 1997). The maximum rooting depth of
200 cm observed for triticale was consistent with the
highest values reported for winter cereals such as winter
wheat (Hoad et al. 2001;Kingetal.2003; Zhang et al.
2004). Sorghum had the shallowest root system in our
conditions, with only 128 cm depth. This value was
lower than that found by Robertson et al. (1993)for
various grain sorghum cultivars in a sub-tropical envi-
ronment in Australia (190 cm). Monti and Zatta (2009)
found roots of fibre sorghum in Italy down to 120 cm
depth. In our conditions, sorghum had a short growing
period because it was sown after mid-May (May 21 in
2010) due to its susceptibility to low temperatures and
this may have limited root development.
Root density has frequently been found to decrease
exponentially with depth (Gregory 2006). This expo-
nential decrease is more or less verified for most crops,
but not for alfalfa or miscanthus (Table 3). The root
distribution observed for miscanthus, i.e. a rather low
density in the upper layers with a drastic decrease below
30 cm and a constant and rather high density in the
deeper layers, is consistent with the results of
Neukirchen et al. (1999) and Riche and Christian
(2001). However, it differs from the study of Monti
and Zatta (2009) who found a surprisingly very low root
density for miscanthus below 90 cm.
Relationship between root distribution and water
capture
PWC was correlated to RID, meaning that root density
was a limiting factor for extracting water, at least in deep
layers. Indeed, soil-root water transfer occurs mainly in
the few centimetres surrounding the roots, due to limi-
tations in soil and/or root hydraulic conductivity
(Garrigues et al. 2006). However, the shape of the
relationship (curvilinear asymptotic) showed that deep
roots with a low density were relatively more efficient
for recovering water than shallower, denser roots. Rob-
ertson et al. (1993) found the same type of relationship
between water consumption and RLD for grain
sorghum with a plateau above an RLD threshold.
Zhang et al. (2004) also reported higher water uptake
per unit of root length in depth than near the soil surface
for rain-fed and irrigated winter wheat. When root den-
sity increases, competition between neighbouring roots
is enhanced, reducing their relative efficiency.
Water capture equivalent to 15 mm in the 150
210 cm layer was observed for sorghum which had no
roots in this layer. The deepest roots of sorghum might
have not been observed due to spatial variability. It is
also possible that upward soil water transfer occurred in
these free root layers due to a hydraulic gradient caused
by root uptake in the upper layers. Adding the y
0
pa-
rameter (PWC in free root layers) to the model allowed
us to take this observation into account and increase the
goodness of fit for annual crops (RMSE was divided by
2.5 on an average) with no impact for the other crops.
We chose to take a common value for y
0
although it is
likely that the upward water transfer depends on soil
characteristics and decreases with depth when the dis-
tance to the deepest root increases.
The optimized parameters were rather variable be-
tween crops. The differences observed for a(highest
PWC achievable by the crop), with lower values for
perennial crops, could be partly explained by differences
in soil evaporation between crops. However, the same
tendency was observed when the model was fitted on
data excluding the first soil layer. Very different values
obtained for the resource capture coefficientksuggest
large differences in water capture efficiency between
crops. Monti and Zatta (2009) also found differences
between crops but their ranking (miscanthus > sorghum
>switchgrass)wasdifferentfromours(switchgrass>
miscanthus > sorghum). In fact, their data on root den-
sity were very different from ours: low root density of
miscanthus in depth compared to switchgrass and sor-
ghum. We hypothesize that this discrepancy was due to
a warmer climate, favourable to sorghum, in Italy and to
the presence of a water table close to the soil surface
which could penalize root growth of miscanthus.
Differences between crops for kcan result from fac-
tors relative to root system or crop evaporative demand
(growing period, morphological factors, etc.). The root
system of annual crops like cereals grows simultaneous-
ly to the canopy and achieves its maximal development
at anthesis (King et al. 2003), contrary to perennial
crops. Consequently, the deep roots of annual crops
have less time than shallower roots to take up soil water.
Robertson et al. (1993) hypothesized that incomplete
water extraction in depth for grain sorghum under severe
Plant Soil (2015) 388:307322 319
drought was due not only to lower root density but also
to the lack of time for deeper roots to extract water
between the arrival of roots in the soil layer and crop
maturity. An extraction front travelling down the soil
profile with time has been observed for annual crops
(Dardanelli et al. 1997,2004; Robertson et al. 1993)and
is generally attributed to the growth of the root system.
In 2009, the driest year of our experiment, an extraction
front clearly appeared for sorghum: water depletion
started approximately late June in the 3060 cm layer
(1 month after sowing), late July in the 6090 cm layer,
mid-August in the 90120 and 120150 cm layers and
late August for the two deeper layers. This difference in
the timing of water extraction with depth could explain
low values of kand high values of aobserved for annual
crops. Among root characteristics, root spatial arrange-
ment could also explain differences between crops. For
example, the degree of root clustering can significantly
change soil water uptake for a given root density
(Beudez et al. 2013). Physiological properties such as
root hydraulic conductivity could also influence the
ability of roots to extract soil water (Nippert et al. 2012).
Conclusions
This study provides an original monitoring of soil water
utilization by different perennial and annual energy
crops during 7 years. As expected, perennial and semi-
perennial crops were characterized by proportional wa-
ter capture in deep soil layers higher than annual crops.
Conversely, PWC was lower in the upper soil layer for
miscanthus and switchgrass than for the other crops.
Semi-perennial crops lead to a greater soil water deficit
than the other crops, due to an important water uptake
both in surface and deep layers. Contrary to our initial
hypothesis, perennial C4 crops resulted in similar SWD
than annual crops whereas perennials were more pro-
ductive and had a deeper root system.
Our study also highlights the relationship between
water uptake and root distribution of the crops. PWC
was correlated to root density with a curvilinear asymp-
totic function but its parameters were crop-dependent.
Therefore root density was not the only factor determin-
ing maximal water uptake. Since aboveground biomass
was not correlated to SWD, other factors such as the
timing and length of the growing period are likely to
affect water use.
In the perspective of predicting the effect of
bioenergy crops on water drainage, a complete water
balance will have to be made in order to quantify the
ratio between evapotranspiration and drainage for each
crop. Our results already indicate that the risk of drain-
age reduction compared to annual crops is probably
higher with semi-perennial crops like alfalfa than with
perennial C4 crops. The impact of the crops on drainage
will also depend on soil and climate characteristics.
Areas with deep soils and low winter rainfall are prob-
ably more susceptible to exhibit large differences in
drainage between crops. Further studies are needed to
explore this effect of soil and climate variability. Finally,
our dataset will be useful to test and improve soil-crop
models in order to simulate the impact of bioenergy
crops on drainage and aquifer loading under various
environmental conditions.
Acknowledgments We are grateful to F. Mahu for the mainte-
nance of the soil moisture and temperature probes, J. Duval for the
database development, T. Laemmel for his help in data processing
and L. Le Guen, E. Mignot, C. Demay and F. Millon for their
technical assistance. This work was supported by the French
National Research Agency (ANR) under the project Regixand
by BPI-France under the project Futurol.
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... Solar radiation, also known as sunlight, as the key factor in photosynthesis that converts carbon dioxide and water to life-sustaining hydrocarbons, is considered one of the major energy sources for plant survival [36]. Hence, it is regarded as one of the major variables in determining the distribution of bioenergy plants [37][38][39]. Accordingly, the amount of sunlight is deemed to be a critical constraint on growing switchgrass. The average global solar radiation dataset was obtained from WorldClim Version 2 database as well [40]. ...
... It is supported by evidence that indicators of soil have been considered an important factor for switchgrass production [41]. Soil quality is constrained by various limitations including soil type, effective soil depth, and soil moisture [38,39,42]. We acquired the datasets of soil type and effective soil depth from the World Soil Information website [43]. ...
... = 10, cv.folds = 10, max.trees = 1000), and the other parameters of the BRT model were held at their default values. A detailed description of the BRT model can be found elsewhere [37][38][39]. For the splendid simulation performance, we obtained the final predicted value by calculating the mean prediction across 25 simulation processes. ...
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... Perennial grain crops may differentially affect N₂O emissions compared to annual grain crops. Earlier studies suggest that increases in below-ground biomass via deeper, denser root systems, increased mineral ) and ammonium (NH 4 + )] uptake and longer growing seasons associated with perennial systems may reduce N₂O emissions relative to annual systems Ferchaud et al., 2015;Gregorich et al., 2005;Rochette et al., 2018). Conversely, conflicting research has shown that N 2 O emissions from soil may increase with the implementation of a perennial cropping system. ...
... We hypothesize that this comparative reduction in emissions by the perennial grain relative to the spring grain is in large part due to differences in the length of the growing season. Perennials begin utilizing water and mineral N immediately after snowmelt, several weeks prior to seeding of an annual crop, thus preventing the formation of N 2 O via denitrification in perennial fields Ferchaud et al., 2015;Gregorich et al., 2005;Rochette et al., 2018). While the perennial grain reduced N 2 O emissions during the spring thaw period, it did not do so to the extent of the perennial forage. ...
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Perennial grain crops represent a novel hybrid between annually harvested grain crops and perennial forage crops, which are seeded once and grow for multiple subsequent seasons. Previous research has shown comparatively reduced nitrous oxide (N2O) emissions from perennial forage crops relative to annual grain crops; however, the effect of perennial grain cropping on N2O emissions is unclear. We quantified field N2O emissions along an experimental continuum of perenniality (perennial forage, perennial grain, fall grain, spring grain and fallow) established at two sites within Alberta, Canada with contrasting soils: luvisolic at the Breton site and chernozemic at the Edmonton site. We used static chambers and a micrometeorological technique based on an open-path Fourier-transform infrared gas sensor (OP-FTIR). Perennial grain crops reduced cumulative N2O emissions at the Breton site by 60% and 94% in years two and three of the study, respectively (Ps < 0.0001). Conversely, no reduction in N2O emissions by the perennial grain crop relative to the annual crop was evident at the Edmonton site. Correlation analyses encompassing both sites revealed that the average root density from 0 to 60 cm was negatively correlated with soil available nitrogen (N) (0–15 cm depth) in years one (Ps < 0.01) and two (Ps < 0.05). Moreover, in year two, root density was negatively correlated with cumulative N2O emissions, specifically at the Breton site (P < 0.01). Results suggest that the enhanced root density of perennial crops reduced soil N availability at the Breton site, which translated into reduced cumulative N2O emissions in year two. Notably, increased root density did not correlate with reduced N2O emissions at the Edmonton site, suggesting that factors such as increased soil clay and carbon content in the Chernozemic soil overrode crop controls on N2O emission. Further, OP-FTIR measurements at the Breton site were in general agreement with static chamber measurements, which collectively informed that the bulk reduction in cumulative N2O emissions from the perennial grain plots occurred during spring thaw. Overall, the ability for perennial cereal grain crops to reduce N2O emissions relative to annual crops was site-specific.
... Ruan et al. [28] tested eight N fertilizer rates from 0 to 196 kg N ha −1 year −1 during three years and observed an exponential increase in annual N 2 O emissions from switchgrass with increasing N fertilization. Beside fertilization, the length and timing of the growing season may also vary between annual and perennial crops, affecting N and water uptake and thus mineral N availability and soil water content [34]. ...
... Data were recorded at an hourly time step using Campbell Scientific CR1000 data loggers and regularly collected in a database managed with PostgreSQL. Measurements from CS616 probes were then corrected for temperature variations using the equation provided by Rudiger et al. [42] and calibrated using gravimetric water content measurements and bulk densities obtained from previous measurements in the same experiment [34,43]. These data were aggregated to obtain daily values of soil temperature (at 15 cm depth), water content (0-30 cm) and water-filled pore space (WFPS), calculated assuming a solid density of 2.65 g cm −3 . ...
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Field N2O emissions are a key point in the evaluation of the greenhouse gas benefits of bioenergy crops. The aim of this study was to investigate N2O fluxes from perennial (miscanthus and switchgrass), semi-perennial (fescue and alfalfa) and annual (sorghum and triticale) bioenergy crops and to analyze the effect of the management of perennials (nitrogen fertilization and/or harvest date). Daily N2O emissions were measured quasi-continuously during at least two years in a long-term experiment, using automated chambers, with 2-5 treatments monitored simultaneously. Cumulative N2O emissions from perennials were strongly affected by management practices: fertilized miscanthus harvested early and unfertilized miscanthus harvested late had systematically much lower emissions than fertilized miscanthus harvested late (50, 160 and 1470 g N2O-N ha−1 year−1 , respectively). Fertilized perennials often had similar or higher cumulative emissions than semi-perennial or annual crops. Fluxes from perennial and semi-perennial crops were characterized by long periods with low emissions interspersed with short periods with high emissions. Temperature, water-filled pore space and soil nitrates affected daily emissions but their influence varied between crop types. This study shows the complex interaction between crop type, crop management and climate, which results in large variations in N2O fluxes for a given site.
... Precipitation is the only resource for soil water recharge, and the depth of soil water recharge by rainwater is limited under rain-fed agricultural conditions (Guo, 2021). The maximum recharge depth of enrichment by rainwater reached 180 cm in a 1-year-old alfalfa stand (Ferchaud et al., 2015). The existence of soil erosion and water shortage is the main factor limiting sustainable agriculture production and is the principal cause of ecological fragility in this region (Fu et al., 2017). ...
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Ridge-furrow rainwater-harvesting (RFRH) has emerged as an effective technology to mitigate drought stress, control soil erosion, and increase crop yield in semiarid regions of China. However, the use of plastic film mulch in RFRH makes this technology impractical. A field experiment was conducted for three consecutive years in a randomized complete block design to determine 1) the runoff coefficient for ridges compacted with soil mixed with two types of biochar (rice straw biochar and cow dung biochar) at two application rates (single application rate i.e., 3 × 10⁴ kg ha⁻¹; and double application rate i.e., 6 × 10⁴ kg ha⁻¹; and 2) the effects of the ridges compacted with soil mixed with two types of biochar at two application rates on soil physico-chemical properties, economic benefit, fodder yield and water use efficiency (WUE) of alfalfa, and ridges compacted with soil (no biochar) as control. The average runoff coefficient for NB, SRSB, DRSB, SCDB, and DCDB (NB, SRSB, DRSB, SCDB, and DCDB were ridges compacted with soil, compacted with soil mixed with single rice straw biochar application, double rice straw biochar application, single cow dung biochar application, and double cow dung biochar application, respectively) over these three years was 31%, 28%, 27%, 22%, and 21%, respectively. Ridges compacted with soil-biochar crust had lower runoff coefficients, soil water storage, net income, and higher soil nutrients, when compared to ridges compacted with soil. The topsoil temperature at ridge tops was affected by the ridges compacted with soil-biochar crust, but the topsoil temperature at furrow bottoms was not affected. Compared to ridges compacted with soil mixed with cow dung biochar, ridges compacted with soil mixed with rice straw biochar had lower soil nutrients, and higher soil water storage resulting in higher fodder yield and WUE of alfalfa. With runoff, the nutrients in biochar flowed from ridges to furrows, becoming usable for plants. Ridges compacted with soil-biochar crust increased topsoil nutrients, especially soil organic matter ranging from 15% to 34%, resulting in high fodder yield and WUE of alfalfa. Compared to NB, annual fodder yield for SRSB, DRSB, SCDB, and DCDB increased by 9.0%, 5.9%, 4.4%, and 3.8%, respectively, over three years, while WUE for the same treatments increased by 2.81, 1.95, 0.65, and 0.45 kg ha⁻¹ mm⁻¹. Rice straw biochar at an application rate of 3 × 10⁴ kg ha⁻¹ was found to be suitable type of biochar for increase in fodder yield and WUE of alfalfa in RFRH. Future studies should be conducted in the form of long-term field study to determine economic benefits of biochar application.
... Estimates for the annual rotation fall within the ranges of temperate agricultural crops summarized in Fan et al. (2016) . The estimated root depths for miscanthus agree with the shallow root distribution for rainfed miscanthus found by Mann et al. (2013); however, this is shallower than the roots observed to depths of 2.7 m by Ferchaud et al. (2015) and 1.8 m by (Neukirchen et al., 1999). Note that the root depth shown in Figure 5b is much shallower than the parameterized R D values shown in Table 2 and Supplemental Figures 3 and S3, which represent the depth where root fraction becomes 0. The exponential decay of root density (Equation 2) produces a long tail of nominal root fraction values which we chose to truncate at 1%. ...
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Full-text available
Abstract Biofuel crops, including annuals such as maize (Zea mays L.), soybean [Glycine max (L.) Merr.], and canola (Brassica napus L.), as well as high‐biomass perennial grasses such as miscanthus (Miscanthus × giganteus J.M. Greef & Deuter ex Hodkinson & Renvoiz), are candidates for sustainable alternative energy sources. However, large‐scale conversion of croplands to perennial biofuel crops could have substantial impacts on regional water, nutrient, and C cycles due to the longer growing seasons and differences in rooting systems compared with most annual crops. However, due to the limited tools available to nondestructively study the spatiotemporal patterns of root water uptake in situ at field scales, these differences in crop water use are not well known. Geophysical imaging tools such as electrical resistivity (ER) reveal changes in water content in the soil profile. In this study, we demonstrate the use of a novel coupled hydrogeophysical approach with both time domain reflectometry soil water content and ER measurements to compare root water uptake and soil properties of an annual crop rotation with the perennial grass miscanthus, across three growing seasons (2009–2011) in southwest Michigan, USA. We estimated maximum root depths to be between 1.2 and 2.2 m, with the vertical distribution of roots being notably deeper in 2009 relative to 2010 and 2011, likely due to the drought conditions during that first year. Modeled cumulative ET of both crops was underestimated (2–34%) relative to estimates obtained from soil water drawdown in prior studies but was found to be greater in the perennial grass than the annual crops, despite shallower modeled rooting depths in 2010 and 2011.
... However, both TB and FRB were lower in April than in October ( Table 2). The decrease of FRB was mostly due to the decline of fine roots on the row, which might be caused by lower yearly and seasonal rainfall; thinner roots production was closely related with rainfall course, and secondly, the switchgrass stands were 6-7 years old (Peek et al. 2005;Meier and Leuschner 2008;Carrillo et al. 2014;Ferchaud et al. 2014). ...
Article
To change row spacing is a common agricultural practice to adjust plant density to enhance plant growth and development. Here we examined the fine root (≤1.0 mm in diameter) distribution and morphological characteristics of switchgrass (Panicum virgatum L.) in the 7th growth year under three row spacing settings (20 cm, 40 cm and 60 cm) on a terrace field in the semi-arid Loess Plateau region. Roots from 0 to 150 cm soil layers were utilized to evaluate the root biomass, distribution and morphological traits in April and October 2016. Results showed that fine root biomass (FRB) mainly distributed in the 0–40 cm soil layer, and it was higher under the 20 cm and 40 cm than that under the 60 cm row spacing treatment. FRB in October was lower than in April under all three row spacings. Compared with 20 cm and 60 cm row spacing, root surface area, root length density and specific root length under 40 cm row spacing were higher, while root diameter was smaller. These conferred switchgrass with larger soil exploration and root-soil interface. It implied that switchgrass grown under 40 cm row spacing had stronger ability on exploiting soil resources in the semi-arid area.
... Therefore, it is critical to characterize root traits responsible for high efficiency of water and nutrient uptake, consequently reducing nutrient loss to the environment. Root assessments of other perennial and annual bioenergy crops have demonstrated that deep rooting confers the ability to tap into water and nutrient patches present in deep soil layers (Ferchaud, Vitte, Bornet, Strullu, & Mary, 2015). Nevertheless, available information about root traits of elephantgrass mainly focused on total biomass production and how it compares to other alternative bioenergy crops (Erickson, Soikaew, Sollenberger, & Bennett, 2012;Liang et al., 2019), without evaluating the return of bioenergy residuals as soil amendments. ...
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Root morphology and production are important for soil nutrient acquisition and C sequestration, but these traits are poorly understood in the bioenergy crop elephantgrass [Pennisetum purpureum (L.) Schum.]. Our objective was to characterize root traits of elephantgrass receiving different nutrient management practices in comparison with bahiagrass (Paspalum notatum Flüggé) pasture grown in the southeastern U.S. Treatments were bahiagrass + 50 kg N ha−1 (BHG), and elephantgrass receiving either: 50 kg N ha−1 (E50), 50 kg N ha−1 + biochar (E50BC), 50 kg N ha−1 + lignocellulosic fermentation residual (E50FR), or 250 kg N ha−1 (E250). Roots were sampled annually for 4 yr (2013‐2016). Root C and N concentration were measured at termination (2016) of the study. Both crop species exhibited similar root length density (RLD) and root mass density (RMD) across all depths in 2014 and 2015. BHG root diameter was 55% greater than all elephantgrass treatments. By 2016, E50FR increased elephantgrass RLD and RMD in the shallow soil layers (< 0.2 m). Root N content was 15‐39% lower for all elephantgrass treatments than BHG in the 0‐0.1 m depth, and 22‐25% lower for E50 and E50BC in the 0.1‐ 0.2 m depth compared with BHG. Additionally, roots C content was 6% higher in the 0‐0.1 m compared with the 0.1‐0.2 m soil depth irrespective of treatment. Application of biochar and lignocellulosic fermentation residual as amendments produced a stimulatory effect on elephantgrass root growth in soil shallow layers, which could affect nutrient and water acquisition. This article is protected by copyright. All rights reserved
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The growing loss of soil functionality due to contamination by metal(loid)s, alone or in combination with organic pollutants, is a global environmental issue that entails major risks to ecosystems and human health. Consequently, the management and restructuring of large metal(loid)-polluted areas through sustainable nature-based solutions is currently a priority in research programs and legislation worldwide. Over the last few years, phytomanagement has emerged as a promising phytotechnology, focused on the use of plants and associated microorganisms, together with ad hoc site management practices, for an economically viable and ecologically sustainable recovery of contaminated sites. It promotes simultaneously the recovery of soil ecological functions and the decrease of pollutant linkages, while providing economic revenues, e.g. by producing non-food crops for biomass-processing technologies (biofuel and bioenergy sector, ecomaterials, biosourced-chemistry, etc.), thus contributing to the international demand for sustainable and renewable sources of energy and raw materials for the bioeconomy. Potential environmental benefits also include the provision of valuable ecosystem services such as water drainage management, soil erosion deterrence, C sequestration, regulation of nutrient cycles, xenobiotic biodegradation, and metal(loid) stabilization. Phytomanagement relies on the proper selection of (i) plants and (ii) microbial inoculants with the capacity to behave as powerful plant allies, e.g ., PGPB: plant growth-promoting bacteria and AMF: arbuscular mycorrhizal fungi. This review gives an up-to-date overview of the main annual, perennial, and woody crops, as well as the most adequate cropping systems, presently used to phytomanage metal(loid)-contaminated soils, and the relevant products and ecosystems services provided by the various phytomanagement options. Suitable bioaugmentation practices with PGPB and AMF are also discussed. Furthermore, we identify the potential interest of phytomanagement for stakeholders and end-users and highlight future opportunities boosted by an effective engagement between environmental protection and economic development. We conclude by presenting the legal and regulatory framework of soil remediation and by discussing prospects for phytotechnologies applications in the future.
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Long‐term management of croplands influences the fluxes and sources of nitrous oxide (N2O). We examined this premise in a greenhouse study by using soils collected from a 38‐year‐old field experiment. The sampled treatments were continuous barley (CB), continuous fescue (CF), and two phases of an 8‐year rotation: faba bean (FB) and alfalfa–bromegrass hay. Barley was grown as a test crop in the greenhouse in each soil. The ranking of N2O emissions was hay> FB> CB> CF (P<0.001). We quantified the 15N‐site preference to assess the N2O‐producing processes. Denitrification was the predominant source, contributing with 77.4% of the N2O production. We also evaluated nitrogen additions: urea alone or urea with a nitrification inhibitor (nitrapyrin or DMPSA). Compared with urea alone, nitrapyrin and DMPSA reduced N2O emissions by 16% and 25%, respectively. We used urea labeled with 15N to trace N to N2O emissions, aboveground plant N uptake, and N retention by soils. Total 15N‐recovery (N2O+plant+soil) was highest under FB (86%) and lowest under CB (29%). We further separated the N2O derived from urea versus N2O from soil organic matter (SOM). The inhibitor DMPSA reduced the N2O derived specifically from added urea‐N by more than half (P<0.001). With the addition of urea, N2O production from mineralization of SOM‐N accelerated over the control (without urea), termed the priming effect. This priming of SOM‐N contributed with 13% of the total N2O production when averaged across the four management legacies. The CB soil had the highest proportion of priming‐derived N2O, with 24%. Management legacies clearly differed in soil carbon and N, which governed N2O production from denitrification and SOM priming. This article is protected by copyright. All rights reserved
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Biomass from dedicated crops is expected to contribute significantly to the replacement of fossil resources. However, sustainable bioenergy cropping systems must provide high biomass production and low environmental impacts. This study aimed at quantifying biomass production, nutrient removal, expected ethanol production, and greenhouse gas (GHG) balance of six bioenergy crops: Miscanthus × giganteus, switchgrass, fescue, alfalfa, triticale, and fiber sorghum. Biomass production and N, P, K balances (input-output) were measured during 4 years in a long-term experiment, which included two nitrogen fertilization treatments. These results were used to calculate a posteriori ‘optimized’ fertilization practices, which would ensure a sustainable production with a nil balance of nutrients. A modified version of the cost/benefit approach proposed by Crutzen et al. (2008), comparing the GHG emissions resulting from N-P-K fertilization of bioenergy crops and the GHG emissions saved by replacing fossil fuel, was applied to these ‘optimized’ situations. Biomass production varied among crops between 10.0 (fescue) and 26.9 t DM ha−1 yr−1 (miscanthus harvested early) and the expected ethanol production between 1.3 (alfalfa) and 6.1 t ha−1 yr−1 (miscanthus harvested early). The cost/benefit ratio ranged from 0.10 (miscanthus harvested late) to 0.71 (fescue); it was closely correlated with the N/C ratio of the harvested biomass, except for alfalfa. The amount of saved CO2 emissions varied from 1.0 (fescue) to 8.6 t CO2eq ha−1 yr−1 (miscanthus harvested early or late). Due to its high biomass production, miscanthus was able to combine a high production of ethanol and a large saving of CO2 emissions. Miscanthus and switchgrass harvested late gave the best compromise between low N-P-K requirements, high GHG saving per unit of biomass, and high productivity per hectare.
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