Winter Soil Research Articles

Winter–spring transition induces changes in nutrients and microbial biomass in mid-alpine forest soils

Active microbial biomass beneath seasonal snowpacks may influence biogeochemical processes in mountain soils. Here we show that soil microbial biomass in temperate forest ecosystems reaches its annual peak under snow, and declines rapidly as snow melts in the spring. This decline was associated with changes in temperature regime and resource availability. However, in contrast to alpine ecosystems at higher elevations, fungal/bacterial ratios, evaluated by amino sugar analysis, were lower in snow-covered winter soils and increased during snowmelt. We associated this shift to a faster decline in bacterial with respect to fungal biomass during snowmelt.

Soil erosion in an avalanche release site (Valle d'Aosta: Italy): towards a winter factor for RUSLE in the Alps

Abstract. Soil erosion in Alpine areas is mainly related to extreme topographic and weather conditions. Although different methods of assessing soil erosion exist, the knowledge of erosive forces of the snow cover needs more investigation in order to allow soil erosion modeling in areas where the snow lays on the ground for several months. This study aims to assess whether the RUSLE (Revised Universal Soil Loss Equation) empirical prediction model, which gives an estimation of water erosion in t ha yr−1 obtained from a combination of five factors (rainfall erosivity, soil erodibility, topography, soil cover, protection practices) can be applied to mountain areas by introducing a winter factor (W), which should account for the soil erosion occurring in winter time by the snow cover. The W factor is calculated from the ratio of Ceasium-137 (137Cs) to RUSLE erosion rates. Ceasium-137 is another possible way of assessing soil erosion rates in the field. In contrast to RUSLE, it not only provides water-induced erosion but integrates all erosion agents involved. Thus, we hypothesize that in mountain areas the difference between the two approaches is related to the soil erosion by snow. In this study we compared 137Cs-based measurement of soil redistribution and soil loss estimated with RUSLE in a mountain slope affected by avalanches, in order to assess the relative importance of winter erosion processes such as snow gliding and full-depth avalanches. Three subareas were considered: DS, avalanche defense structures, RA, release area, and TA, track area, characterized by different prevalent winter processes. The RUSLE estimates and the 137Cs redistribution gave significantly different results. The resulting ranges of W evidenced relevant differences in the role of winter erosion in the considered subareas, and the application of an avalanche simulation model corroborated these findings. Thus, the higher rates obtained with the 137Cs method confirmed the relevant role of winter soil erosion. Despite the limited sample size (11 points), the inclusion of a W factor in RUSLE seems promising for the improvement of soil erosion estimates in Alpine environments affected by snow movements.

On the Arctic near-surface permafrost and climate sensitivities to soil and snow model formulations in climate models

This study investigates the sensitivity of the Canadian Regional Climate Model (CRCM5) simulated near surface permafrost and its climate interactions to soil and snow formulations. In particular, sensitivities to the depth of the soil column, inclusion of organic soils and modified snow conductivity formulation are investigated. The impact of these modifications are first assessed in offline simulations performed with the Canadian Land Surface Scheme (CLASS), which is the land surface scheme used in CRCM5, when driven by ERA-40/ERA-Interim for the 1957–2008 period. Analysis of CLASS simulations shows major improvements in the simulated permafrost extent, particularly with a deeper soil column. Inclusion of organic soil decreased the summer ground heat flux and therefore the summer soil temperatures, leading to improvements in the simulated active layer thickness (ALT). The impact of the new snow thermal conductivity formulation is moderate compared to the effect of organic soils, but reduces the cold biases in winter soil temperatures. CRCM5 experiments revealed similar sensitivities to soil depth, organic soil and snow conductivity changes as with the offline simulations. Significant changes are noted in the land–atmosphere interactions, through modified energy and moisture partitioning at the surface resulting from the inclusion of the organic soils. The inter-annual variability of the ALT shows larger sensitivities to summer temperatures for mineral soil while experiments including organic soils show increased sensitivities to annual temperatures. The ALT trends in the CRCM5 are similar to the observed values, despite the overestimation of ALT associated with a warm bias in the CRCM5 climate.

Winter climate change affects growing‐season soil microbial biomass and activity in northern hardwood forests

Understanding the responses of terrestrial ecosystems to global change remains a major challenge of ecological research. We exploited a natural elevation gradient in a northern hardwood forest to determine how reductions in snow accumulation, expected with climate change, directly affect dynamics of soil winter frost, and indirectly soil microbial biomass and activity during the growing season. Soils from lower elevation plots, which accumulated less snow and experienced more soil temperature variability during the winter (and likely more freeze/thaw events), had less extractable inorganic nitrogen (N), lower rates of microbial N production via potential net N mineralization and nitrification, and higher potential microbial respiration during the growing season. Potential nitrate production rates during the growing season were particularly sensitive to changes in winter snow pack accumulation and winter soil temperature variability, especially in spring. Effects of elevation and winter conditions on N transformation rates differed from those on potential microbial respiration, suggesting that N-related processes might respond differently to winter climate change in northern hardwood forests than C-related processes.

Desert shrub responses to experimental modification of precipitation seasonality and soil depth: relationship to the two‐layer hypothesis and ecohydrological niche

Summary Ecohydrological niches are important for understanding plant community responses to climate shifts, particularly in dry lands. According to the two‐layer hypothesis, selective use of deep‐soil water increases growth or persistence of woody species during warm and dry summer periods and thereby contributes to their coexistence with shallow‐rooted herbs in dry ecosystems. The resource‐pool hypothesis further suggests that shallow‐soil water benefits growth of all plants while deep‐soil water primarily enhances physiological maintenance and survival of woody species. Few studies have directly tested these by manipulating deep‐soil water availability and observing the long‐term outcomes. We predicted that factors promoting infiltration and storage of water in deep soils, specifically greater winter precipitation and soil depth, would enhance Artemisia tridentata (big sagebrush) in cold, winter‐wet/summer‐dry desert. Sagebrush responses to 20 years of winter irrigation were compared to summer‐ or no irrigation, on plots having relatively deep or shallow soils (2 m vs. 1 m depths). Winter irrigation increased sagebrush cover, and crown and canopy volumes, but not density (individuals/plot) compared to summer or no irrigation, on deep‐soil plots. On shallow‐soil plots, winter irrigation surprisingly decreased shrub cover and size, and summer irrigation had no effect. Furthermore, multiple regression suggested that the variations in growth were related (i) firstly to water in shallow soils (0–0.2 m) and secondly to deeper soils (> 1 m deep) and (ii) more by springtime than by midsummer soil water. Water‐use efficiency increased considerably on shallow soils without irrigation and was lowest with winter irrigation. Synthesis. Sagebrush was more responsive to the seasonal timing of precipitation than to total annual precipitation. Factors that enhanced deep‐water storage (deeper soils plus more winter precipitation) led to increases in Artemisia tridentata that were consistent with the two‐layer hypothesis, and the contribution of shallow water to growth on these plots was consistent with the resource‐pool hypothesis. However, shallow‐soil water also had negative effects on sagebrush, suggesting an ecohydrological trade‐off not considered in these or related theories. The interaction between precipitation timing and soil depth indicates that increased winter precipitation could lead to a mosaic of increases and decreases in A. tridentata across landscapes having variable soil depth.

Effect of Climate Change on Soil Temperature in Swedish Boreal Forests

Complex non-linear relationships exist between air and soil temperature responses to climate change. Despite its influence on hydrological and biogeochemical processes, soil temperature has received less attention in climate impact studies. Here we present and apply an empirical soil temperature model to four forest sites along a climatic gradient of Sweden. Future air and soil temperature were projected using an ensemble of regional climate models. Annual average air and soil temperatures were projected to increase, but complex dynamics were projected on a seasonal scale. Future changes in winter soil temperature were strongly dependent on projected snow cover. At the northernmost site, winter soil temperatures changed very little due to insulating effects of snow cover but southern sites with little or no snow cover showed the largest projected winter soil warming. Projected soil warming was greatest in the spring (up to 4°C) in the north, suggesting earlier snowmelt, extension of growing season length and possible northward shifts in the boreal biome. This showed that the projected effects of climate change on soil temperature in snow dominated regions are complex and general assumptions of future soil temperature responses to climate change based on air temperature alone are inadequate and should be avoided in boreal regions.

Comparison of Seasonal Soil Microbial Process in Snow-Covered Temperate Ecosystems of Northern China

More than half of the earth's terrestrial surface currently experiences seasonal snow cover and soil frost. Winter compositional and functional investigations in soil microbial community are frequently conducted in alpine tundra and boreal forest ecosystems. However, little information on winter microbial biogeochemistry is known from seasonally snow-covered temperate ecosystems. As decomposer microbes may differ in their ability/strategy to efficiently use soil organic carbon (SOC) within different phases of the year, understanding seasonal microbial process will increase our knowledge of biogeochemical cycling from the aspect of decomposition rates and corresponding nutrient dynamics. In this study, we measured soil microbial biomass, community composition and potential SOC mineralization rates in winter and summer, from six temperate ecosystems in northern China. Our results showed a clear pattern of increased microbial biomass C to nitrogen (N) ratio in most winter soils. Concurrently, a shift in soil microbial community composition occurred with higher fungal to bacterial biomass ratio and gram negative (G-) to gram positive (G+) bacterial biomass ratio in winter than in summer. Furthermore, potential SOC mineralization rate was higher in winter than in summer. Our study demonstrated a distinct transition of microbial community structure and function from winter to summer in temperate snow-covered ecosystems. Microbial N immobilization in winter may not be the major contributor for plant growth in the following spring.

Winter Soil CO2 Flux from Different Mid-Latitude Sites from Middle Taihang Mountain in North China

Winter soil respiration is a very important component of the annual soil carbon flux in some ecosystems. We hypothesized that, with all other factors being equal, shorter winter SR result in reduced contribution to annual soil C flux. In this study, the contribution of winter soil respiration to annual soil respiration was measured for three sites (grassland: dominated by Artemisia sacrorum, Bothriochloa ischaemum and Themeda japonica; shrubland: dominated by Vitex negundo var. heterophylla; plantation: dominated by Populus tomatosa) in a mountainous area of north China. Diurnal and intra-annual soil CO2 flux patterns were consistent among different sites, with the maximum soil respiration rates at 12∶00 or 14∶00, and in July or August. The lowest respiration rates were seen in February. Mean soil respiration rates ranged from 0.26 to 0.45 µmol m−2 s−1 in the winter (December to February), and between 2.38 to 3.16 µmol m−2 s−1 during the growing season (May-September). The winter soil carbon flux was 24.6 to 42.8 g C m−2, which contributed 4.8 to 7.1% of the annual soil carbon flux. Based on exponential functions, soil temperature explained 73.8 to 91.8% of the within year variability in soil respiration rates. The Q10 values of SR against ST at 10 cm ranged from 3.60 to 4.90 among different sites. In addition, the equation between soil respiration and soil temperature for the growing season was used to calculate the “modeled” annual soil carbon flux based on the actual measured soil temperature. The “measured” annual value was significantly higher than the “modeled” annual value. Our results suggest that winter soil respiration plays a significant role in annual soil carbon balance, and should not be neglected when soil ecosystems are assessed as either sinks or sources of atmospheric CO2.

The overwintering biology of the acorn weevil, Curculio glandium in southwestern Ontario

The acorn weevil, Curculio glandium, is a widespread predator of acorns in eastern North America that overwinters in the soil as a larva. It is possible that low temperatures limit its northern geographic range, so we determined the cold tolerance strategy, seasonal variation in cold tolerance, and explored the physiological plasticity of overwintering larvae. Weevil larvae were collected from acorns of red and bur oak from Pelee Island, southwestern Ontario in fall 2010 and 2011. C. glandium larvae are freeze avoidant and larvae collected from bur oak acorns had lower supercooling points (SCPs: −7.6±0.36°C, LT50: −7.2°C) than those collected from red oak acorns (SCPs: −6.1±0.40°C, LT50: −6.1°C). In the winter of 2010–2011, SCPs and water content decreased, however these changes did not occur in 2011–2012, when winter soil temperatures fluctuated greatly in the absence of the buffering effect of snow. To examine whether larvae utilize cryoprotective dehydration, larvae from red oak acorns were exposed to −5°C in the presence of ice for seven days. These conditions decreased the SCP without affecting water content, suggesting that SCP and water content are not directly coupled. Finally, long-term acclimation at 0°C for six weeks slightly increased cold tolerance but also did not affect water content. Thus, although larval diet affects cold tolerance, there is limited plasticity after other treatments. The soil temperatures we observed were not close to lethal limits, although we speculate that soil temperatures in northerly habitats, or in years of reduced snow cover, has the potential to cause mortality in the field.

Rainfall and temperatures changes have confounding impacts on Phytophthora cinnamomi occurrence risk in the southwestern USA under climate change scenarios

Global change will simultaneously impact many aspects of climate, with the potential to exacerbate the risks posed by plant pathogens to agriculture and the natural environment; yet, most studies that explore climate impacts on plant pathogen ranges consider individual climatic factors separately. In this study, we adopt a stochastic modeling approach to address multiple pathways by which climate can constrain the range of the generalist plant pathogen Phytophthora cinnamomi (Pc): through changing winter soil temperatures affecting pathogen survival; spring soil temperatures and thus pathogen metabolic rates; and changing spring soil moisture conditions and thus pathogen growth rates through host root systems. We apply this model to the southwestern USA for contemporary and plausible future climate scenarios and evaluate the changes in the potential range of Pc. The results indicate that the plausible range of this pathogen in the southwestern USA extends over approximately 200,000 km(2) under contemporary conditions. While warming temperatures as projected by the IPCC A2 and B1 emissions scenarios greatly expand the range over which the pathogen can survive winter, projected reductions in spring rainfall reduce its feasible habitat, leading to spatially complex patterns of changing risk. The study demonstrates that temperature and rainfall changes associated with possible climate futures in the southwestern USA have confounding impacts on the range of Pc, suggesting that projections of future pathogen dynamics and ranges should account for multiple pathways of climate-pathogen interaction.

Soil CO2 emissions from five different types of land use on the semiarid Loess Plateau of China, with emphasis on the contribution of winter soil respiration

Many studies have investigated the response of soil respiration to environmental factors. However, there are few studies estimating soil CO2 emissions in the land use of the Loess Plateau, China. Five different types of land use including a natural oak (Quercus liaotungensis) forest, a natural oriental arborvitae (Platycladus orientalis) forest, a black locust (Robinia pseudoacacia) plantation, a natural shrubland, and bare land were investigated from April 2010 to April 2012 in the semiarid Loess Plateau region. Total and winter season soil CO2 emissions were estimated using integration and interpolation methods based on periodic measurements of soil respiration and environmental factors. The integrated average annual and winter soil CO2 emissions (555.73–937.53 g C m−2 and 96.57–146.70 g C m−2) were higher than the interpolated values (480.52–805.83 g C m−2 and 82.83–102.31 g C m−2). The mean soil CO2 efflux and mean winter soil CO2 efflux during the 2 years ranged from 2.03 to 3.23 μmol m−2 s−1 and 0.52–0.80 μmol m−2 s−1 among different types of land use. The mean winter soil CO2 efflux was 11–25% of that of the mean growing season. Q10 values for the five types were negatively correlated with average soil temperature and moisture. Soil organic carbon content, soil total nitrogen content and C/N ratio were not correlated with the amount of winter soil CO2 emission, but correlated with annual total CO2 emission and the contribution rate from winter period in positive and negative trends, respectively. Model improvement may improve the estimation accuracy of soil CO2 emissions using the integration method, and increasing the frequency of soil respiration measurements is important for the interpolation method. It is inferred that the annual carbon sequestration, CO2 emission, and the economical conditions of carbon budget follow a descending sequence as oak forest > shrubland > oriental arborvitae forest > black locust plantation > bare land.

Moderate Warming in Microcosm Experiment Does Not Affect Microbial Communities in Temperate Vineyard Soils

Changes in the soil microbial community structure can lead to dramatic changes in the soil ecosystem. Temperature, which is projected to increase with climate change, is commonly assumed to affect microbial communities, but its effects on agricultural soils are not fully understood. We collected soil samples from six vineyards characterised by a difference of about 2 °C in daily soil temperature over the year and simulated in a microcosm experiment different temperature regimes over a period of 1 year: seasonal fluctuations in soil temperature based on the average daily soil temperature measured in the field; soil temperature warming (2 °C above the normal seasonal temperatures); and constant temperatures normally registered in these temperate soils in winter (3 °C) and in summer (20 °C). Changes in the soil bacterial and fungal community structures were analysed by automated ribosomal intergenic spacer analysis (ARISA). We did not find any effect of warming on soil bacterial and fungal communities, while stable temperatures affected the fungal more than the bacterial communities, although this effect was soil dependent. The soil bacterial community exhibited soil-dependent seasonal fluctuations, while the fungal community was mainly stable. Each soil harbours different microbial communities that respond differently to seasonal temperature fluctuations; therefore, any generalization regarding the effect of climate change on soil communities should be made carefully.

Effects of Different Water and Nitrogen Control Schemes on Yield of Winter Wheat and Soil Enzyme Activity in Dryland

A two-year field experiment located in rain-fed growing areas in Xiangfen County,Shanxi Province,was conducted to study the effects of different water and nitrogen control schemes on the yield formation of winter wheat and soil enzyme activity during the growth period in dryland.The aim of this project was to provide a scientific basis for high yield of rain-fed wheat and efficient management of water and fertilizer in China.The results showed that the urease activity had the tendency of ascending first and descending later with the progress of growth period since revival stage,with the maximum in the jointing stage in dryland.The catalase and alkaline phosphatase activity gradually increased from revival stage to maturity stage,with the maximum occurring in the grain filling stage or maturity.The activity of all the three soil enzymes of the N treatments were significantly higher than that of the control treatment without N application.In a certain N range(0-195 kg/hm2),the soil urease activity was higher with higher N level and more times of N application.The catalase and alkaline phosphatase activity for different amount of nitrogen were not significantly different.The activity of the three soil enzymes was significantly increased by ridge filming-furrow sowing cultivation in dryland,which also improved the maximum tillering number and spike number,with the yield 13% higher than that of the same N application treatment.Therefore,ridge filming-furrow sowing cultivation was an important scheme to improve the yield of dryland winter wheat.

Digging deep to open the white black box of snow root phenology

Snow roots are specialized structures recently discovered in the Caucasian alpine snow-bed plant Corydalis conorhiza. They form extensive networks that grow into snow packs against gravity, most probably to gather nitrogen from snow. Here we test the hypothesis that snow roots are true winter organs, i.e., they should already start growth early in winter to lay down the infrastructure for N capture from snow packs well before their melt-out. This would require winter surface and soil temperatures continuously close to or above freezing. Excavations of snow roots from snow packs in January and May, accompanied by temperature recordings and anatomical observations, supported our hypothesis. These findings complete the annual cycle of snow root phenology. They also emphasize the evolutionary and ecological significance of these specialized winter organs. Moreover, their likely association with a particular abiotic temperature and snow regime will facilitate the search for snow roots in other species.

Nitrogen isotope tracer acquisition in low and tall birch tundra plant communities: a 2 year test of the snow–shrub hypothesis

Deciduous shrub density and landcover are increasing across many areas of the Arctic. Shrub growth may be promoted by a snow-shrub feedback whereby relatively tall shrubs accumulate deeper snow, raising winter soil temperature minima, increas- ing microbial activity, and enhancing soil solution nitrogen (N). Although there is good evidence for the above components of the hypothesis, it has not yet been determined if shrubs can access the elevated N pool generated by deepened snow. We added isotopic N tracer ( 15 N) in late summer to control and snowfenced low birch hummock tundra to test the influence of deepened snow on N cycling. Further- more, tracer was added to tall birch tundra to compare N cycling in low and tall shrub ecosystems that have the same species composition. Experimentally deep- ened snow in low birch tundra did not significantly affect 15 N uptake by shrubs or any other species 2 years after the tracer addition. However, there were strong differences between the low and tall birch ecosystems, with the deciduous shrubs and graminoids accumulating more 15 N than the evergreen shrubs in the relatively productive tall shrub site, and vice versa in the relatively infertile low birch site. The greater 15 N acquisition by birch in the more fertile site, together with the absence of a deepened snow effect on 15 N acquisition by any species in the low birch hummock ecosystem, suggest that climate-change induced increases in birch shrub growth and expansion across the landscape will tend to occur most rapidly in and around existing tall birch shrub patches.

Bayesian methods for predicting LAI and soil water content

LAI of winter wheat (Triticum aestivum L.) and soil water content of the topsoil (200 mm) and of the subsoil (500 mm) were considered as state variables of a dynamic soil-crop system. This system was assumed to progress according to a Bayesian probabilistic state space model, in which real values of LAI and soil water content were daily introduced in order to correct the model trajectory and reach better future evolution. The chosen crop model was mini STICS which can reduce the computing and execution times while ensuring the robustness of data processing and estimation. To predict simultaneously state variables and model parameters in this non-linear environment, three techniques were used: extended Kalman filtering (EKF), particle filtering (PF), and variational filtering (VF). The significantly improved performance of the VF method when compared to EKF and PF is demonstrated. The variational filter has a low computational complexity and the convergence speed of states and parameters estimation can be adjusted independently. Detailed case studies demonstrated that the root mean square error of the three estimated states (LAI and soil water content of two soil layers) was smaller and that the convergence of all considered parameters was ensured when using VF. Assimilating measurements in a crop model allows accurate prediction of LAI and soil water content at a local scale. As these biophysical properties are key parameters in the crop-plant system characterization, the system has the potential to be used in precision farming to aid farmers and decision makers in developing strategies for site-specific management of inputs, such as fertilizers and water irrigation.

Shrub canopies influence soil temperatures but not nutrient dynamics: An experimental test of tundra snow–shrub interactions

Shrubs are the largest plant life form in tundra ecosystems; therefore, any changes in the abundance of shrubs will feedback to influence biodiversity, ecosystem function, and climate. The snow–shrub hypothesis asserts that shrub canopies trap snow and insulate soils in winter, increasing the rates of nutrient cycling to create a positive feedback to shrub expansion. However, previous work has not been able to separate the abiotic from the biotic influences of shrub canopies. We conducted a 3-year factorial experiment to determine the influences of canopies on soil temperatures and nutrient cycling parameters by removing ∼0.5 m high willow (Salix spp.) and birch (Betula glandulosa) shrubs, creating artificial shrub canopies and comparing these manipulations to nearby open tundra and shrub patches. Soil temperatures were 4–5°C warmer in January, and 2°C cooler in July under shrub cover. Natural shrub plots had 14–33 cm more snow in January than adjacent open tundra plots. Snow cover and soil temperatures were similar in the manipulated plots when compared with the respective unmanipulated treatments, indicating that shrub canopy cover was a dominant factor influencing the soil thermal regime. Conversely, we found no strong evidence of increased soil decomposition, CO2 fluxes, or nitrate or ammonia adsorbtion under artificial shrub canopy treatments when compared with unmanipulated open tundra. Our results suggest that the abiotic influences of shrub canopy cover alone on nutrient dynamics are weaker than previously asserted.

Snow removal reduces annual cellulose decomposition in a riparian boreal forest

Kreyling, J., Haei, M. and Laudon, H. 2013. Snow removal reduces annual cellulose decomposition in a riparian boreal forest. Can. J. Soil Sci. 93: 427–433. Decomposition is a key process in carbon and nutrient cycling. However, little is known about its response to altered winter soil temperature regimes in boreal forests. Here, the impact of soil frost on cellulose decomposition over 1 yr and soil biotic activity (bait-lamina sticks) over winter, in spring, and in summer was investigated using a long-term (9-yr) snow-cover manipulation experiment in a boreal Picea abies forest. The experiment consisted of the treatments: snow removal, increased insulation, and ambient control. The snow removal treatment caused longer and deeper soil frost (minimum temperature −8.6°C versus −1.4°C) at 10 cm soil depth in comparison with control, while the increased insulation treatment resulted in nearly no soil frost during winter. Annual cellulose decomposition rates were reduced by 46% in the snow removal manipulation in comparison with control conditions. Increased insulation had no significant effect on decomposition. The decomposition was mainly driven by microorganisms, as no significant difference was observed for containers enclosed with a 44-µm and a 1-mm mesh. Soil biotic activity was slightly increased by both the snow removal and the increased insulation treatment in comparison with control conditions over winter. However, this effect disappeared over spring and summer. We conclude that soil frost can have strong effects on decomposition in boreal ecosystems. Further studies should investigate to which degree the observed reduction in decomposition due to reduced snow cover in winter slows or even offsets the expected increase in decomposition rates with global warming.

Snow removal reduces annual cellulose decomposition in a riparian boreal forest

Kreyling, J., Haei, M. and Laudon, H. 2013. Snow removal reduces annual cellulose decomposition in a riparian boreal forest. Can. J. Soil Sci. 93: 427-433. Decomposition is a key process in carbon and nutrient cycling. However, little is known about its response to altered winter soil temperature regimes in boreal forests. Here, the impact of soil frost on cellulose decomposition over 1 yr and soil biotic activity (bait-lamina sticks) over winter, in spring, and in summer was investigated using a long-term (9-yr) snow-cover manipulation experiment in a boreal Picea abies forest. The experiment consisted of the treatments: snow removal, increased insulation, and ambient control. The snow removal treatment caused longer and deeper soil frost (minimum temperature -8.6°C versus -1.4°C) at 10 cm soil depth in comparison with control, while the increased insulation treatment resulted in nearly no soil frost during winter. Annual cellulose decomposition rates were reduced by 46% in the snow removal manipulation in comparison with control conditions. Increased insulation had no significant effect on decomposition. The decomposition was mainly driven by microorganisms, as no significant difference was observed for containers enclosed with a 44-µm and a 1-mm mesh. Soil biotic activity was slightly increased by both the snow removal and the increased insulation treatment in comparison with control conditions over winter. However, this effect disappeared over spring and summer. We conclude that soil frost can have strong effects on decomposition in boreal ecosystems. Further studies should investigate to which degree the observed reduction in decomposition due to reduced snow cover in winter slows or even offsets the expected increase in decomposition rates with global warming.

Seasonal patterns of microbial extracellular enzyme activities in an arctic tundra soil: Identifying direct and indirect effects of long-term summer warming

Arctic systems, which store ∼50% of global soil carbon, are undergoing rapid climatic warming that may drive significant carbon release to the atmosphere. To better understand how warming impacts arctic decomposition, we characterized the effects of a twenty-two year long tundra greenhouse warming experiment on decomposer-produced extracellular enzymes, nutrients, and microbial biomass across a year. This experiment, which is the longest running tundra ecosystem warming study in existence, was previously shown to have altered the plant and soil communities. The greenhouse treatment has also changed the seasonal soil temperature regime by indirectly increasing winter soil temperature, an effect that was likely facilitated through an increase in snow-trapping shrub biomass. Irrespective of the warming treatment, we observed that peak nutrient pools, microbial biomass, and hydrolytic enzyme activities all occurred from the late winter through thaw. This pattern was decoupled from peak oxidative enzyme activities, which occurred during the summer. The greenhouse treatment amplified the natural seasonal cycle of extracellular enzyme activities, suggesting that tundra decomposer communities maintain a temporal niche space which is critical to understanding how arctic biogeochemical cycling will respond to warming. A spatial separation was also observed; extracellular enzyme activities in the deeper soil horizons were more sensitive to warming than at the surface. Direct greenhouse warming did not strongly stimulate decomposition: only oxidative enzyme activities in the surface horizon increased during the summer. Unexpectedly, the strongest treatment effect observed was a stimulation of hydrolytic enzyme activities at depth in the mineral horizon from the late winter through thaw (which also affected extracellular enzyme stoichiometry, increasing C:N and C:P acquisition activities), before the greenhouse treatment was directly active. This effect declined during senescence and was reversed in early winter, suggesting that negative biotic-abiotic feedbacks may curtail increased decomposer activity in warming arctic systems.