-
[show abstract]
[hide abstract]
ABSTRACT: Increases in soil freezing associated with decreases in snow cover have been identified as a significant disturbance to nitrogen
(N) cycling in northern hardwood forests. We created a range of soil freezing intensity through snow manipulation experiments
along an elevation gradient at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains, NH USA in order to improve
understanding of the factors regulating freeze effects on nitrate (NO3
−) leaching, nitrous oxide (N2O) flux, potential and in situ net N mineralization and nitrification, microbial biomass carbon (C) and N content and respiration,
and denitrification. While the snow manipulation treatment produced deep and persistent soil freezing at all sites, effects
on hydrologic and gaseous losses of N were less than expected and less than values observed in previous studies at the HBEF.
There was no relationship between frost depth, frost heaving and NO3
− leaching, and a weak relationship between frost depth and winter N2O flux. There was a significant positive relationship between dissolved organic carbon (DOC) and NO3
− concentrations in treatment plots but not in reference plots, suggesting that the snow manipulation treatment mobilized available
C, which may have stimulated retention of N and prevented treatment effects on N losses. While the results support the hypothesis
that climate change resulting in less snow and more soil freezing will increase N losses from northern hardwood forests, they
also suggest that ecosystem response to soil freezing disturbance is affected by multiple factors that must be reconciled
in future research.
KeywordsClimate change-Dissolved organic matter-Methane-Microbial biomass-Nitrate-Nitrous oxide
Biogeochemistry 04/2012; 102(1):223-238. · 3.07 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Climate change is predicted to reduce or delay annual wintertime snow pack formation in the forests of the northeastern US.
Any delay in snowpack formation could increase soil freezing in winter and, thereby, alter soil characteristics and processes.
We examined the hypothesis that delayed snowpack would disrupt soil structure and change organic matter bioavailability in
an experimental snow removal study at the Hubbard Brook Experimental Forest (HBEF), NH, USA. Pairs of reference and snow removal
treatment plots were studied in four different sites at HBEF. Snow was removed from November–January of two winters, inducing
soil freezing throughout both winters. Size class distribution and organic matter concentration and content of aggregates,
and carbon and nitrogen mineralization potential of size fractions were quantified for surface mineral soils in the spring
of both years immediately after snowmelt. In the first year of sampling, the only significant effect of snow removal was an
increase in the smallest (<53μm) size fraction of mineral soil. In the second year, snow removal increased organic matter
concentrations of macroaggregate (250–2,000μm) and microaggregate (53–250μm) size fractions. This change corresponded to
an increase in net N mineralization potential and the ratio of N to C mineralized in the macroaggregate fraction, but there
were no effects of snow removal on C mineralization. We propose that soil freezing increases the movement of organic matter
from organic to mineral soil horizons and increases the N content of mineralizable substrates in mineral soil following years
with delayed snowpack formation.
Biology and Fertility of Soils 04/2012; 45(1):1-10. · 2.32 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We exploited the natural climate gradient in the northern hardwood forest at the Hubbard Brook Experimental Forest (HBEF)
to evaluate the effects of climate variation similar to what is predicted to occur with global warming over the next 50–100years
for northeastern North America on soil carbon (C) and nitrogen (N) cycle processes. Our objectives were to (1) characterize
differences in soil temperature, moisture and frost associated with elevation at the HBEF and (2) evaluate variation in total
soil (TSR) and microbial respiration, N mineralization, nitrification, denitrification, nitrous oxide (N2O) flux, and methane (CH4) uptake along this gradient. Low elevation sites were consistently warmer (1.5–2.5°C) and drier than high elevation sites.
Despite higher temperatures, low elevation plots had less snow and more soil frost than high elevation plots. Net N mineralization
and nitrification were slower in warmer, low elevation plots, in both summer and winter. In summer, this pattern was driven
by lower soil moisture in warmer soils and in winter the pattern was linked to less snow and more soil freezing in warmer
soils. These data suggest that N cycling and supply to plants in northern hardwood ecosystems will be reduced in a warmer
climate due to changes in both winter and summer conditions. TSR was consistently faster in the warmer, low elevation plots.
N cycling processes appeared to be more sensitive to variation in soil moisture induced by climate variation, whereas C cycling
processes appeared to be more strongly influenced by temperature.
Ecosystems 04/2012; 12(6):927-943. · 3.49 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Soil–atmosphere fluxes of trace gases (especially nitrous oxide (N2O)) can be significant during winter and at snowmelt. We investigated the effects of decreases in snow cover on soil freezing and trace gas fluxes at the Hubbard Brook Experimental Forest, a northern hardwood forest in New Hampshire, USA. We manipulated snow depth by shoveling to induce soil freezing, and measured fluxes of N2O, methane (CH4) and carbon dioxide (CO2) in field chambers monthly (bi-weekly at snowmelt) in stands dominated by sugar maple or yellow birch. The snow manipulation and measurements were carried out in two winters (1997/1998 and 1998/1999) and measurements continued through 2000. Fluxes of CO2 and CH4 showed a strong seasonal pattern, with low rates in winter, but N2O fluxes did not show strong seasonal variation. The snow manipulation induced soil freezing, increased N2O flux and decreased CH4 uptake in both treatment years, especially during winter. Annual N2O fluxes in sugar maple treatment plots were 207 and 99 mg N m−2 yr−1 in 1998 and 1999 vs. 105 and 42 in reference plots. Tree species had no effect on N2O or CO2 fluxes, but CH4 uptake was higher in plots dominated by yellow birch than in plots dominated by sugar maple. Our results suggest that winter fluxes of N2O are important and that winter climate change that decreases snow cover will increase soil:atmosphere N2O fluxes from northern hardwood forests.
Global Change Biology 08/2006; 12(9):1748 - 1760. · 6.86 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Understanding how exogenous and endogenous factors control the distribution, production and mortality of fine roots is fundamental to assessing the implications of global change, yet our knowledge of control over fine root dynamics remains rudimentary. To improve understanding of these processes, the present study developed regression relationships between environmental variables and fine root dynamics within a northern hardwood forest in New Hampshire, USA, which was experimentally manipulated with a snow removal treatment. Fine roots (< 1 mm diameter) were observed using minirhizotrons for 2 years in sugar maple and yellow birch stands and analyzed in relation to temperature, water and nutrient availability. Fine root dynamics at this site fluctuated seasonally, with growth and mortality peaking during warmer months. Monthly fine root production was strongly associated with mean monthly air temperature and neither soil moisture nor nutrient availability added additional predictive power to this relationship. This relationship exhibited a seasonal temperature hysteresis, which was altered by snow removal treatment. These results suggest that both exogenous and endogenous cues may be important in controlling fine root growth in this system. Proportional fine root mortality was directly associated with mean monthly soil temperature, and proportional fine root mortality during the over-winter interval was strongly related to whether the soil froze. The strong relationship between fine root production and air temperature reported herein contrasts with findings from some hardwood forest sites and indicates that controls on fine root dynamics vary geographically. Future research must more clearly distinguish between endogenous and exogenous control over fine root dynamics in various ecosystems.
Global Change Biology 05/2003; 9(5):670 - 679. · 6.86 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Soil freezing is a disturbance of the below ground environment, potentially resulting in increased losses of NO3- and surface water acidification. Here, we report the effects of soil freezing on interannual variation in stream chemistry at the Hubbard Brook Experimental Forest, New Hampshire. Data from 1970 to 1997 of soil frost depth, snow cover, precipitation, air temperature, and stream discharge and chemistry were used in a stepwise linear regression model to select the variables that best predicted deviations of annual stream concentrations from 4-year running averages. Variables quantifying soil freezing severity were selected as significant predictors of short-term fluctuations in stream K+, NO3-, Ca2+, and Mg2+ concentrations from 1970 to 1989, explaining 59 and 47% of the short-term variability in K+ and NO3-, respectively. Fine-root mortality and disturbance of root-soil-microbe interactions, with subsequent effects on decomposition and nutrient uptake, likely contributed to the mobilization of K+ and NO3- to streamwater following severe soil freezing events. The relationship between soil freezing and stream chemistry, however, weakened during the period 1990-1997. Because soil freezing has had inconsistent effects on stream chemistry during the period 1970-1997, it is unclear whether future changes in the frequency, duration, and depth of soil freezing events as the result of changes in the snow cover regime under a warmer climate will have significant impacts on the losses of NO3- and nutrient-base cations from temperate northern ecosystems.
Environmental Science and Technology 05/2003; 37(8):1575-80. · 5.23 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Reductions in snow cover undera warmer climate may cause soil freezing eventsto become more common in northern temperateecosystems. In this experiment, snow cover wasmanipulated to simulate the late development ofsnowpack and to induce soil freezing. Thismanipulation was used to examine the effects ofsoil freezing disturbance on soil solutionnitrogen (N), phosphorus (P), and carbon (C)chemistry in four experimental stands (twosugar maple and two yellow birch) at theHubbard Brook Experimental Forest (HBEF) in theWhite Mountains of New Hampshire. Soilfreezing enhanced soil solution Nconcentrations and transport from the forestfloor. Nitrate (NO3
–) was thedominant N species mobilized in the forestfloor of sugar maple stands after soilfreezing, while ammonium (NH4
+) anddissolved organic nitrogen (DON) were thedominant forms of N leaching from the forestfloor of treated yellow birch stands. Rates ofN leaching at stands subjected to soil freezingranged from 490 to 4,600 mol ha–1yr–1, significant in comparison to wet Ndeposition (530 mol ha–1 yr–1) andstream NO3
– export (25 mol ha–1yr–1) in this northern forest ecosystem. Soil solution fluxes of Pi from the forestfloor of sugar maple stands after soil freezingranged from 15 to 32 mol ha–1 yr–1;this elevated mobilization of Pi coincidedwith heightened NO3
– leaching. Elevated leaching of Pi from the forestfloor was coupled with enhanced retention ofPi in the mineral soil Bs horizon. Thequantities of Pi mobilized from the forestfloor were significant relative to theavailable P pool (22 mol ha–1) as well asnet P mineralization rates in the forest floor(180 mol ha–1 yr–1). Increased fineroot mortality was likely an important sourceof mobile N and Pi from the forest floor,but other factors (decreased N and P uptake byroots and increased physical disruption of soilaggregates) may also have contributed to theenhanced leaching of nutrients. Microbialmortality did not contribute to the acceleratedN and P leaching after soil freezing. Resultssuggest that soil freezing events may increaserates of N and P loss, with potential effectson soil N and P availability, ecosystemproductivity, as well as surface wateracidification and eutrophication.
Biogeochemistry 10/2001; 56(2):215-238. · 3.07 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Overwinter and snowmelt processes are thought to be critical to controllersof nitrogen (N) cycling and retention in northern forests. However, therehave been few measurements of basic N cycle processes (e.g.mineralization, nitrification, denitrification) during winter and littleanalysis of the influence of winter climate on growing season N dynamics.In this study, we manipulated snow cover to assess the effects of soilfreezing on in situ rates of N mineralization, nitrification and soilrespiration, denitrification (intact core, C2H2 – based method),microbial biomass C and N content and potential net N mineralization andnitrification in two sugar maple and two yellow birch stands with referenceand snow manipulation treatment plots over a two year period at theHubbard Brook Experimental Forest, New Hampshire, U.S.A. The snowmanipulation treatment, which simulated the late development of snowpackas may occur in a warmer climate, induced mild (temperatures >–5 C) soil freezing that lasted until snowmelt. The treatmentcaused significant increases in soil nitrate (NO3
–)concentrations in sugar maple stands, but did not affect mineralization,nitrification, denitrification or microbial biomass, and had no significanteffects in yellow birch stands. Annual N mineralization and nitrificationrates varied significantly from year to year. Net mineralization increasedfrom 12.0 g N m–2 y–1 in 1998 to 22 g N m–2 y–1 in 1999 and nitrification increased from 8 g N m–2 y–1 in 1998 to 13 g N m–2 y–1 in 1999.Denitrification rates ranged from 0 to 0.65 g N m–2 y–1. Ourresults suggest that mild soil freezing must increase soil NO3
– levels by physical disruption of the soil ecosystem and not by direct stimulation of mineralization and nitrification. Physical disruption canincrease fine root mortality, reduce plant N uptake and reduce competitionfor inorganic N, allowing soil NO3
– levels to increase evenwith no increase in net mineralization or nitrification.
Biogeochemistry 10/2001; 56(2):191-213. · 3.07 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this special section of Biogeochemistry, we present results from asnow manipulation experiment in the northernhardwood forest ecosystem at the Hubbard BrookExperimental Forest in the White Mountains ofNew Hampshire, U.S.A. Snow is important as aninsulator of forest soils. Later developmentof snowpacks, as may occur in a warmer climate,may result in increases in soil freezing (i.e.colder soils in a warmer world) and could causechanges in fine root and microbial mortality,hydrologic and gaseous losses of nitrogen (N),and the acid-base status of drainage water. Inour study, we kept soils snow free by shovelinguntil early February during the mild winters of1997/1998 and 1998/1999. The treatment producedmild, but persistent soil freezing and inducedsurprisingly significant effects on rootmortality, soil nitrate (NO3
–) levelsand hydrologic fluxes of C, N and P. In thisspecial section we present four papersaddressing, (1) soil temperature and moistureresponse to our snow manipulation treatment(Hardy et al.), (2) theresponse of fine root dynamics to treatment(Tierney et al.), (3) theresponse of soil inorganic N levels, insitu N mineralization and nitrification,denitrification and microbial biomass to thetreatment (Groffman et al.)and (4) soil solution concentrations and fluxesof C, N and P (Fitzhugh et al.). In this introductory paper we: (1)review the literature on snow effects on forestbiogeochemistry, (2) introduce our manipulationexperiment and (3) summarize the resultspresented in the other papers in this issue.
Biogeochemistry 10/2001; 56(2):135-150. · 3.07 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Climate change will likelyresult in warmer winter temperatures leading toless snowfall in temperate forests. Thesechanges may lead to increases in soil freezingbecause of lack of an insulating snow cover andchanges in soil water dynamics during theimportant snowmelt period. In this study, wemanipulated snow depth by removing snow for twowinters, simulating the late development of thesnowpack as may occur with global warming, toexplore the relationships between snow depth,soil freezing, soil moisture, and infiltration.We established four sites, each with two pairedplots, at the Hubbard Brook Experimental Forest(HBEF) in New Hampshire, U.S.A. and instrumentedall eight plots with soil and snow thermistors,frost tubes, soil moisture probes, and soillysimeters. For two winters, we removed snowfrom the designated treatment plots untilFebruary. Snow in the reference plots wasundisturbed. The treatment winters (1997/1998 and1998/1999) were relatively mild, withtemperatures above the seasonal norm and snowdepths below average. Results show the treatedplots accumulated significantly less snow andhad more extensive soil frost than referenceplots. Snow depth was a strong regulator ofsoil temperature and frost depth at all sites.Soil moisture measured by time domainreflectometry probes and leaching volumescollected in lysimeters were lower in thetreatment plots in March and April compared tothe rest of the year. The ratio of leachatevolumes collected in the treatment plots tothat in the reference plots decreased as thesnow ablation seasons progressed. Our data showthat even mild winters with low snowfall,simulated by snow removal, will result inincreased soil freezing in the forests at theHBEF. Our results suggest that a climate shifttoward less snowfall or a shorter duration ofsnow on the ground will produce increases insoil freezing in northern hardwood forests.Increases in soil freezing will haveimplications for changes in soil biogeochemicalprocesses.
Biogeochemistry 10/2001; 56(2):151-174. · 3.07 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The retention of nutrients within an ecosystem depends on temporal andspatial synchrony between nutrient availability and nutrient uptake, anddisruption of fine root processes can have dramatic impacts on nutrientretention within forest ecosystems. There is increasing evidence thatoverwinter climate can influence biogeochemical cycling belowground,perhaps by disrupting this synchrony. In this study, we experimentallyreduced snow accumulation in northern hardwood forest plots to examinethe effects of soil freezing on the dynamics of fine roots (< 1="" mm="" diameter)measured="" using="" minirhizotrons.="" snow="" removal="" treatment="" during="" therelatively="" mild="" winters="" of="" 1997–1998="" and="" 1998–1999="" induced="" mild="" freezingtemperatures="" (to="" –4="" c)="" lasting="" approximately="" three="" months="" atshallow="" soil="" depths="" (to="" –30="" cm)="" in="" sugar="" maple="" and="" yellow="" birch="" stands.this="" treatment="" resulted="" in="" elevated="" overwinter="" fine="" root="" mortality="" in="" treatedcompared="" to="" reference="" plots="" of="" both="" species,="" and="" led="" to="" an="" earlier="" peak="" infine="" root="" production="" during="" the="" subsequent="" growing="" season.="" these="" shiftsin="" fine="" root="" dynamics="" increased="" fine="" root="" turnover="" but="" were="" not="" largeenough="" to="" significantly="" alter="" fine="" root="" biomass.="" no="" differences="" inmorality="" response="" were="" found="" between="" species.="" laboratory="" tests="" on="" pottedtree="" seedlings="" exposed="" to="" controlled="" freezing="" regimes="" confirmed="" that="" mildfreezing="" temperatures="" (to="" –5="" c)="" were="" insufficient="" to="" directlyinjure="" winter-hardened="" fine="" roots="" of="" these="" species,="" suggesting="" that="" themarked="" response="" recorded="" in="" our="" forest="" plots="" was="" caused="" indirectly="" bymechanical="" damage="" to="" roots="" in="" frozen="" soil.="" elevated="" fine="" root="" necromass="" intreated="" plots="" decomposed="" quickly,="" and="" may="" have="" contributed="" an="" excess="" fluxof="" about="" 0.5="" g="">2yr, which is substantial relative tomeasurements of N fluxes from these plots. Our results suggest elevatedoverwinter mortality temporarily reduced fine root length in treatmentplots and reduced plant uptake, thereby disrupting the temporalsynchrony between nutrient availability and uptake and enhancing ratesof nitrification. Increased frequency of soil freezing events, as may occurwith global change, could alter fine root dynamics within the northernhardwood forest disrupting the normally tight coupling between nutrientmineralization and uptake.
Biogeochemistry 01/2001; 56(2):175-190. · 3.07 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Investigations of snowcover dynamics beneath vegetation canopies require either measured or estimated solar and thermal radiation values at the snow surface. A deterministic method is presented that uses portable arrays of pyranometers and pyrgeometers to quantify the amount of incoming radiation at the snow surface. Example solar and thermal radiation datasets are presented from boreal deciduous, boreal coniferous and temperate coniferous forest stands. The data indicate that the canopies transmitted 33% (4-8 March), 15% (6-10 February), and 3% (22-24 September) of the above-canopy radiation. In the boreal deciduous and temperate conifer stands, thermal radiation is increased by 25% and 34% respectively. Thermal gains partially offset solar reduction, such that incoming all-wave radiation is decreased by 22% and 25% respectively for each of these stands. When recorded at a high temporal resolution, array data can estimate below-canopy diffuse solar radiation values for estimation techniques that treat direct and diffuse transmission independently. We provide examples of how radiometer array data are used to derive simple canopy radiation transmissivity parameters for global, beam and diffuse radiation. Radiometer arrays also provide data for detailed investigations to assess within-stand radiation variability, or to investigate radiation variations across land cover discontinuities, to advance our understanding of snowcover energetics in complex environments.
