Soil Frost Research Articles

Soil frost effects on streamflow recessions in a subarctic catchment

AbstractThe Arctic is warming rapidly. Changing seasonal freezing and thawing cycles of the soil are expected to affect river run‐off substantially, but how soil frost influences river run‐off at catchment scales is still largely unknown. We hypothesize that soil frost alters flow paths and therefore affects storage–discharge relations in subarctic catchments. To test this hypothesis, we used an approach that combines meteorological records and recession analysis. We studied streamflow data (1986–2015) of Abiskojokka, a river that drains a mountainous catchment (560 km2) in the north of Sweden (68° latitude). Recessions were separated into frost periods (spring) and no‐frost periods (summer) and then compared. We observed a significant difference between recessions of the two periods: During spring, discharge was linearly related to storage, whereas storage–discharge relationships in summer were less linear. An analysis of explanatory factors showed that after winters with cold soil temperatures and low snowpack, storage–discharge relations approached linearity. On the other hand, relatively warm winter soil conditions resulted in storage–discharge relationships that were less linear. Even in summer, relatively cold antecedent winter soils and low snowpack levels had a propagating effect on streamflow. This could be an indication that soil frost controls recharge of deep groundwater flow paths, which affects storage–discharge relationships in summer. We interpret these findings as evidence for soil frost to have an important control over river run‐off dynamics. To our knowledge, this is the first study showing significant catchment‐integrated effects of soil frost on this spatiotemporal scale.

Seasonal dynamics and controls of deep soil water infiltration in the seasonally-frozen region of the Qinghai-Tibet plateau

Deep soil water infiltration is a key hydrological process in seasonally frozen regions, and displays a distinct behavior from that in other unfrozen regions; however, the seasonal dynamics and key controls of deep soil water infiltration in the seasonally frozen region of the Qinghai-Tibet plateau (QTP) are still poorly understood. In this study, we examine the seasonal dynamics of deep soil water infiltration, and attempt to quantitatively assess the relative influences of several driving factors by applying a random forests statistical analysis. The following key results were obtained. Deep soil water infiltration shows marked seasonal variation. Rainfall intensity exerts little effect on the seasonal dynamics of deep soil water infiltration as almost all rainfall returns to the atmosphere via evapotranspiration. No infiltration occurred when the depth of lower layer frozen soil reached 29 cm, whereas deep infiltration increased during the thawing of soil frost, suggesting that deep infiltration is impeded by soil freezing. Overall, the observed seasonal dynamics of deep soil water infiltration and volumetric soil moisture content were similar during the freeze-thaw process. Furthermore, deep soil water infiltration was more strongly influenced by deep volumetric soil moisture than by shallower volumetric soil moisture. Our results suggest that the topsoil Mattic Epipedon, soil organic matter content, and root systems’ role in vertical moisture movement should be taken into consideration when modeling hydrological processes in alpine meadows.

Effect of snowmelt infiltration on groundwater recharge in a seasonal soil frost area: a case study in Northeast China.

The effect of spring snowmelt infiltration in a seasonal soil frost area on groundwater recharge was evaluated by systematically monitoring meteorological factors, soil temperature and humidity, groundwater table and temperature, electrical conductivity, and the value of δ18O in a small field site over a 2-year period. The variation of soil temperature and humidity, groundwater table during the freezing period, and the snowmelt period respectively, as well as their correspondence to the relevant environmental factors, and the influencing factors of the permeability of frozen layer were analyzed. The results showed that the evaluation of precipitation infiltration in seasonal soil frost areas should be divided into three stages: a non-freezing period, a freezing period, and a snowmelt period. Snow is the main form of precipitation during the freezing period, and groundwater cannot be recharged. During the snowmelt period of spring, the snow cover that accumulated during the freezing period infiltrates together with rainfall and has a significant effect on groundwater recharge. The general precipitation infiltration process occurs after the frozen soil thaws completely. These research results can improve the accuracy of groundwater recharge calculations for snowmelt infiltration in the seasonal soil frost area of Northeast China and provide a scientific basis for the evaluation and management of regional water resources.

A Modeling-Based Approach for Soil Frost Detection in the Northern Boreal Forest Region With C-Band SAR

This paper presents a new approach for monitoring soil frost in the northern boreal forest region using co-polarized C-band synthetic aperture radar (SAR) data. Due to the high sensitivity of the C-band signal to vegetation, estimating the soil freeze/thaw (F/T) state directly from the measured backscatter is not feasible over dense vegetation, such as boreal forests. The presented method is based on applying a simple zeroth-order model to estimate the contribution of the ground and the forest canopy on the observed total backscatter. The retrieved ground and canopy backscatter values were compared with in situ information on soil F/T state. By using a linear least sum of square errors classification algorithm, the retrieved ground and canopy backscatter values representing frozen and thawed ground were successfully separated. The method was tested for various soil types and incidence angles. For soil types with higher water holding capacities and lower infiltration rates such as fine Haplic Podzol and Umbric Gleysol, the estimation accuracy of the F/T state was over 97%, whereas for drier, well-drained soil types such as Haplic Arenosol and Coarse Haplic Podzol it was over 94%. Estimation accuracy slightly increased with higher incidence angle. The method is not feasible in rocky terrain due to very low water content, or in wet snow conditions due to lack of penetration of the C-band SAR signal through wet snow. With low ancillary data and computational requirements, the proposed method is applicable for continuous near real-time monitoring of soil F/T state.

The effect of a freeze–thaw cycle on dissolved nitrogen dynamics and its relation to dissolved organic matter and soil microbial biomass in the soil of a northern hardwood forest

Recent global warming models project a significant change in winter climate over the next few decades. The decrease in snowpack in the winter will decrease the heat insulation function of the snowpack, resulting in increased soil freeze–thaw cycles. Here, we examined the impact of winter freeze–thaw cycles on year-round dissolved nitrogen (N) and carbon (C) dynamics and their relationship with dissolved organic matter and microbial biomass in soil by conducting an in situ experimental reduction in snowpack. We investigated dissolved inorganic N (NH4+ and NO3−), dissolved organic N (DON), dissolved organic carbon (DOC), inorganic N leaching, soil microbial biomass, and microbial activities (mineralization and nitrification) in the surface soil of a northern hardwood forest located in Japan. Experimental snowpack reduction significantly increased the number of soil freeze–thaw cycles and soil frost depth. The NH4+ content of the surface soil was significantly increased by the amplified soil freeze–thaw cycles due to decreased snowpack, while the soil NO3– content was unchanged or decreased slightly. The gravimetric soil moisture, DON and DOC contents in soil and soil microbial biomass significantly increased by the snowpack removal in winter. Our results suggest that the amplified freeze–thaw cycles in soil increase the availability of DON and DOC for soil microbes due to an increase in soil freezing. The increases in both DON and DOC in winter contributed to the enhanced growth of soil microbes, resulting in the increased availability of NH4+ in winter from net mineralization following an increase in soil freeze–thaw cycles. Our study clearly indicated that snow reduction significantly increased the availability of dissolved nitrogen and carbon during winter, caused by increased soil water content due to freeze–thaw cycles in winter.

Recent progress in basic research of soil frost and its application to agricultural management in Hokkaido, Japan — Developments of several observational methods and results under cold condition —

Recent progress in basic research of soil frost and its application to agricultural management in Hokkaido, Japan — Developments of several observational methods and results under cold condition —

Frost Heave of Irrigation Canals in Seasonal Frozen Regions

Soil frost heave acts as a driver of the emerged fracture in the concrete lining of irrigation canals and subsequent water leakage in seasonal frozen ground. A model test was carried out on the frost heave of a U‐shaped canal with concrete lining. The heat and water migration during freezing, and frost heave‐induced deformation, and force in normal direction were live monitored by high‐precision transducers. The results prove that the freezing front descends downward over time at a specified thermal boundary, with considerable migration of water within the scope of 0–40 cm. The maximum deformation occurred at the bottom of the lining and decreased upward with the rate of frost heave lowering over time while the normal force showing little change in the monitoring points, implying that stress concentration does not show up during freezing. Besides, the layered settlement observation reveals that frost heave dominates the total deformation while creep, the universal source of deformation, accounts for a negligible proportion. A practical model was proposed based on a simple theoretical model for heat‐water coupled transfer in a partially saturated medium and was numerically implemented in COMSOL. The computed results were compared with the monitored data including frozen depth, water content, normal displacement, and frost heave force. Finally, the rational thickness of the insulation board was determined based on the partial insulation method.

Работа свай винтовых конусно-спиральных в условиях морозного пучения грунтов

Работа свай винтовых конусно-спиральных в условиях морозного пучения грунтов

Modeling the Cryogenic Texture Formation Process in the Soil Around the Pipeline

This article is concerned with the study and analysis of heat exchange processes, moisture transfer and cryogenic texture formation in fine dispersed soils when laying and operating underground pipelines. The segregation frost heaving around cold big-diameter pipelines buried in the soil has been investigated. The method of modeling the secondary frost heaving is presented. The conditions under which this model is applicable as a result of assumptions made during its construction are described. The model is presented in a stringent mathematical formulation. The checking numerical case has been calculated. The process of ice layers formation graph is given. Under certain conditions one layer can be formed and under the other conditions two layers can be formed. Mostly it depends on the pipe temperature. For this case the guidelines for soil frost heaving prevention are given. The guidelines are general and applicable to other cases.

Declines in northern forest tree growth following snowpack decline and soil freezing.

Changes in growing season climate are often the foci of research exploring forest response to climate change. By contrast, little is known about tree growth response to projected declines in winter snowpack and increases in soil freezing in seasonally snow-covered forest ecosystems, despite extensive documentation of the importance of winter climate in mediating ecological processes. We conducted a 5-year snow-removal experiment whereby snow was removed for the first 4-5weeks of winter in a northern hardwood forest at the Hubbard Brook Experimental Forest in New Hampshire, USA. Our results indicate that adverse impacts of reduced snowpack and increased soil freezing on the physiology of Acer saccharum (sugar maple), a dominant species across northern temperate forests, are accompanied by a 40±3% reduction in aboveground woody biomass increment, averaged across the 6years following the start of the experiment. Further, we find no indication of growth recovery 1year after cessation of the experiment. Based on these findings, we integrate spatial modeling of snowpack depth with forest inventory data to develop a spatially explicit, regional-scale assessment of the vulnerability of forest aboveground growth to projected declines in snowpack depth and increased soil frost. These analyses indicate that nearly 65% of sugar maple basal area in the northeastern United States resides in areas that typically experience insulating snowpack. However, under the RCP 4.5 and 8.5 emissions scenarios, we project a 49%-95% reduction in forest area experiencing insulating snowpack by the year 2099 in the northeastern United States, leaving large areas of northern forest vulnerable to these changes in winter climate, particularly along the northern edge of the region. Our study demonstrates that research focusing on growing season climate alone overestimates the stimulatory effect of warming temperatures on tree and forest growth in seasonally snow-covered forests.

Research on the distribution of saline soil along the Chaerhan-Golmud Highway, northwestern China

In order to assess and improve the engineering stability of saline soil with high chloride content along a 40-km section of the Chaerhan-Golmud Highway in northwestern China, more than 200 monitoring points for saline soil soluble salt chemical tests were established. The total salt content distribution, the chloride ion content distribution, and the sulfate ion content distribution along both sides of road were determined, and the chlorine-sulfur ratio distribution and the PH value distribution along both sides of road were also determined. These chemical contents can significantly affect soil freezing temperatures, frost heave, and salt expansion, which can undermine subgrade stability, ultimately, cause subgrade damage. Therefore, when subgrade filler is selected for this highway, attention must be paid to its chemical indicators as well as its physical properties.

The effect of freezing and thawing on water flow and MCPA leaching in partially frozen soil

Limited knowledge and experimental data exist on pesticide leaching through partially frozen soil. The objective of this study was to better understand the complex processes of freezing and thawing and the effects these processes have on water flow and pesticide transport through soil. To achieve this we conducted a soil column irrigation experiment to quantify the transport of a non-reactive tracer and the herbicide MCPA in partially frozen soil. In total 40 intact topsoil and subsoil columns from two agricultural fields with contrasting soil types (silt and loam) in South-East Norway were used in this experiment. MCPA and bromide were applied on top of all columns. Half the columns were then frozen at −3 °C while the other half of the columns were stored at +4 °C. Columns were then subjected to repeated irrigation events at a rate of 5 mm artificial rainwater for 5 h at each event. Each irrigation was followed by 14-day periods of freezing or refrigeration. Percolate was collected and analysed for MCPA and bromide. The results show that nearly 100% more MCPA leached from frozen than unfrozen topsoil columns of Hov silt and Kroer loam soils. Leaching patterns of bromide and MCPA were very similar in frozen columns with high concentrations and clear peaks early in the irrigation process, and with lower concentrations leaching at later stages. Hardly any MCPA leached from unfrozen topsoil columns (0.4–0.5% of applied amount) and concentrations were very low. Bromide showed a different flow pattern indicating a more uniform advective-dispersive transport process in the unfrozen columns with higher concentrations leaching but without clear concentration peaks. This study documents that pesticides can be preferentially transported through soil macropores at relatively high concentrations in partially frozen soil. These findings indicate, that monitoring programs should include sampling during snow melt or early spring in areas were soil frost is common as this period could imply exposure peaks in groundwater or surface water.

Effects of a snow-compaction treatment on soil freezing, snowmelt runoff, and soil nitrate movement: A field-scale paired-plot experiment

A frozen soil layer may impede snowmelt infiltration, resulting in a large amount of runoff that influences the soil water balance and anion movement in the soil profile. To examine the relationships among soil frost depth, snowmelt runoff, and nitrate leaching in agricultural fields, we measured both the snowmelt runoff for three winters and other environmental factors including the soil frost depth and nitrate content in a ∼10,000 m2 field. We divided the field into two subplots: one was maintained in a natural snow cover condition (the control plot), and the snow cover was compacted on the other plot (the treated plot) to enhance the development of the soil frost depth. In all three winters, soil frost depths in the control plot were <0.2 m and very little runoff was observed during the snowmelt period. In contrast, the soil frost depth became >0.4 m and a large amount of snowmelt runoff was observed in the treated plot. The depth of the peak nitrate concentration after the snowmelt period was shallower in the treated plot compared to the control plot. Moreover, a significant linear relationship was observed between (1) the amount of nitrate in the 0–0.3 m depth after the snowmelt period and (2) the total amount of snowmelt infiltration calculated by subtracting the amount of snowmelt runoff from the amount of snowmelt water. These results suggest that snow compaction can be a promising technique to develop a uniform soil frost depth in large-scale fields, which consequently controls the soil water and nutrient movement in the soil layer.

Water-vapor-heat behavior in a freezing unsaturated coarse-grained soil with a closed top

A series of one-side freezing experiments with closed tops were performed to illustrate moisture migration and frost heave mechanisms of the coarse-grained soil for high speed railway subgrades in cold regions, as well as silty clay and standard sand as controls. The experimental results show that the pot effect can be formed in the soils with different unsaturated porosities when their tops are closed. The vapor migration and frost heave of unsaturated soils become more significant under larger temperature gradient. Water supply can not determine the formation of the pot effect, and only influence its intensity. Vapor migration is prominent when the initial water contents of soils are low (e.g. saturation of 30%), however, the water migration is dominant when the initial moistures of soils are high (e.g. saturation of 70%). It is also found that the initial water contents of the soils only influence the in-situ frost heave amounts; however, the total frost heave amounts are related to the combined actions of soil frost susceptibility and water migration, etc.

Planned snow compaction approach (yuki-fumi) contributes toward balancing wheat yield and the frost-kill of unharvested potato tubers

Snow compaction implements the thermal conditions compatible with both killing volunteer potatoes and good wheat growth. To reveal how soil freezing regulates volunteer potatoes without preventing the growth of winter wheat, we conducted experiments in a wheat field in Hokkaido, Japan (42°53′N, 143°05′E), from winter 2013 to summer 2017. Owing to the enhanced thermal conductivity of snowpack during cold periods, different timings and frequencies of snow compaction allowed us to target different soil temperatures and frost depths. In most years and blocks, snow compaction achieved soil frost depth >0.3 m and killed all potato tubers, but in 2016–17, soil frost depth was <0.3 m in DC, and 7% of potatoes sprouted. These findings indicate that snow compaction before a heavy snowpack can effectively kill tubers. Wheat grain yield did not differ among treatments except in 2013–14. Deep soil freezing did not always reduce the wheat yield, but delayed snowmelt and thus delayed growth enhanced the likelihood of plants experiencing higher air temperature at the early spike development stage. Although changes in yield components such as culm length and spikelet density were associated with varietal responses to temperature, warm conditions in 2014 reduced the dry matter and shortened the grain-filling period, resulting in a greater decrease in yield. Furthermore, rapid hard freezing and direct physical impact may have caused much greater injury to wheat under a shallow snowpack in 2013–14. Thus, our findings indicate that proper snow compaction can balance wheat production and kill unharvested potatoes in crop rotation, and except for hot summer, gentle procedure of grain-filling could compensate the grain weight even in less spring growth.

A High‐Resolution Land Model With Groundwater Lateral Flow, Water Use, and Soil Freeze‐Thaw Front Dynamics and its Applications in an Endorheic Basin

AbstractHuman water regulation, groundwater lateral flow, and the movement of frost and thaw fronts (FTFs) affect soil water and thermal processes, as well as energy and water exchanges between the land surface and atmosphere. Reasonable representation of these processes in land surface models is very important to improving the understanding of land‐atmosphere interactions. In this study, mathematical descriptions of groundwater lateral flow, human water regulation, and FTFs were synchronously incorporated into a high‐resolution community land model, which is then named the Land Surface Model for Chinese Academy of Sciences (CAS‐LSM). With a series of atmospheric forcings and high‐resolution land surface data from the Heihe Watershed Allied Telemetry Experimental Research (HiWATER) program, numerical simulations of the period 1981–2013 using CAS‐LSM with 1‐km resolution were conducted for an endorheic basin, the Heihe River Basin in China. Compared with observations, CAS‐LSM reproduced the distributions of groundwater, evapotranspiration, and permafrost reasonably and well matched the temporal changes in ground temperature, heat fluxes, and FTFs. Results illuminate the temporal and spatial characteristics of frozen soil and the changes in the land‐atmosphere exchange of carbon, water, and energy. The permafrost and seasonally frozen soil were distinguished. In the seasonally frozen areas, the maximum soil frost depth increased by 0.65 mm/year within natural areas and decreased by 2.12 mm/year in human‐dominated areas. The active layer thickness increased 8.63 mm/year for permafrost. In the permafrost zone evapotranspiration and latent heat flux increased, and the sensible heat flux declined. In the human‐dominated areas water use raised the latent heat flux and reduced the sensible heat flux, net ecosystem exchange, and streamflow recharging to the eco‐fragile region in the lower reaches. Results suggested that the land surface model CAS‐LSM is a potential tool for studying land surface processes, especially in cold and arid regions experiencing human interventions.

High-frequency monitoring reveals seasonal and event-scale water quality variation in a temporally frozen river

Potential influences of climate change on water quality, riverine suspended sediments, nitrogen and organic matter loads in temporally frozen rivers, which have ice-covered flow and snow-affected basins, are poorly understood. However, before being able to understand potential future changes, the impact of ice and snow needs to be investigated more thoroughly for years which were hydrologically different. We investigated seasonal and event scale concentration-discharge (C-Q) dynamics of total suspended solids/turbidity, nitrate-N (NO3-N) and chemical oxygen demand (COD), which is indicative of the amount of organic matter in river water. In particular, the influence of ice cover, contrasting spring thaw, and soil frost conditions on intra-annual fluxes and the C-Q response of the three solutes are detected based on over four years of hourly data. Seasonally flow-weighted suspended solids and NO3-N concentrations were at their highest in either the autumn or spring thaw, but COD concentrations were the highest each year in autumn. NO3-N and COD levels typically decreased during winter. The ice-covered river water was less turbid compared to open-channel water at an equivalent river discharge likely due to in-stream factors. Storms during the freshet period introduced flushing of organic matter and suspended solids. The ratio of organic matter yield to water yield was similar each freshet and was independent of the amount of precipitation as snow or soil frost status. The freshet NO3-N yield per water yield was higher during the years with a thick snowpack and the consequent thawed soil compared to a year with soil frost and minor snowpack. 91 storm events studied revealed differences and similarities in storm dynamics in between the three variables. Anti-clockwise hysteresis was most common for the variables, with turbidity peaking faster than the COD or NO3-N concentration in most of the storms. Snowmelt storms showed highly variable C-Q responses inbetween the variables. However, spring thaw-related COD concentration peaks abated more slowly compared to turbidity or NO3-N. NO3-N showed a strong dilution pattern during several autumn storms during an extremely wet year, indicating limited N sources for flushing from the catchment. As a result, the flow-weighted mean NO3-N concentration was not the largest during the year of largest water yield instead which was true for suspended solids and COD. We found no evidence that warmer winters with precipitation as rain instead of snow would increase suspended sediment, organic matter and NO3-N load at entire winter-spring season or annual timescales.

Bacterial and fungal communities in boreal forest soil are insensitive to changes in snow cover conditions.

The northern regions are experiencing considerable changes in winter climate leading to more frequent warm periods, rain-on-snow events and reduced snow pack diminishing the insulation properties of snow cover and increasing soil frost and freeze-thaw cycles. In this study, we investigated how the lack of snow cover, formation of ice encasement and snow compaction affect the size, structure and activities of soil bacterial and fungal communities. Contrary to our hypotheses, snow manipulation treatments over one winter had limited influence on microbial community structure, bacterial or fungal copy numbers or enzyme activities. However, microbial community structure and activities shifted seasonally among soils sampled before snow melt, in early and late growing season and seemed driven by substrate availability. Bacterial and fungal communities were dominated by stress-resistant taxa such as the orders Acidobacteriales, Chaetothyriales and Helotiales that are likely adapted to adverse winter conditions. This study indicated that microbial communities in acidic northern boreal forest soil may be insensitive to direct effects of changing snow cover. However, in long term, the detrimental effects of increased ice and frost to plant roots may alter plant derived carbon and nutrient pools to the soil likely leading to stronger microbial responses.

Land cover controls on depression‐focused recharge on the Oak Ridges Moraine, southern Ontario, Canada

AbstractThe Oak Ridges Moraine (ORM) is a key hydrogeologic feature in southern Ontario. Previous research has emphasized the importance of depression‐focused recharge (DFR) for the timing and location of water recharge to the ORM's aquifers. However, the significance of DFR has not been empirically demonstrated, and the ORM's permeable surficial deposits imply that rainfall and snowmelt will largely recharge vertically rather than move laterally to topographic depressions. The exception may be during winter and spring, when concrete soil frost limits infiltration and encourages overland flow. The potential for DFR was examined for closed depressions under forest and agricultural land covers with similar soils and surficial geology. Air temperatures, precipitation, snow depth and water equivalent, soil water contents, soil freezing, and depression surface‐water levels were monitored during the winter and spring of 2012–2013 and 2013–2014. Recharge (R) was estimated at the crest and base of each depression using a 1‐dimensional water balance approach and surface‐applied Br− tracing. Both forest and agricultural land covers experienced soil freezing; however, forest soils did not develop concrete frost. Conversely, agricultural fields saw concrete frost, overland flow, episodic ponding, and subsequent drainage of rain‐on‐snow and snowmelt inputs in open depressions. Recharge at the base of open depressions exceeded that in surrounding areas by an order of magnitude, suggesting that DFR is a significant hydrologic process during winter and spring under agricultural land cover on the ORM. Closed topographic depressions under agricultural land cover on the ORM crest may serve as critical recharge “hot spots” during winter and spring, and the ability of the unsaturated zone beneath these depressions to modify the chemistry of recharging water deserves further attention.

Two contrasting seasonal patterns in microbial nitrogen immobilization from temperate ecosystems

Ecosystems with seasonal snow cover and soil frost comprise more than half of the terrestrial ecosystems. Recent studies have reported peak microbial biomass carbon (MBC) and nitrogen (MBN) in late winter in alpine and arctic regions, followed by a rapid decline when soil temperatures rise to 0 °C. In these snow-covered ecosystems, the N incorporated into microbes from soil during winter is considered vital for plant nutrient uptake in the following growing seasons. However, evidence from temperate ecosystems has rarely been reported. We studied 24 ecosystems from two forest-steppe ecotones (Saihanba and Saihanwula) with similar climatic conditions, but different soil properties, to comprehensively analyze the seasonal dynamics of microbial biomass and its controlling factors. MBC consistently reached its annual peak in winter, and this seasonal trend was not influenced by different vegetation types or study areas. In contrast, the seasonal dynamics of MBN were strongly influenced by the study area. There were minimum values of winter MBN observed in the Saihanba area while maximum values of winter MBC observed in the Saihanwula area. Soil properties showed poor nutrient status in the Saihanba area compared with the Saihanwula area. Our results suggest winter microbes in the Saihanba area invest N to produce extracellular enzymes, rather than retaining in biomass. We argue that it is a possible strategy to cope with nutrient stress caused by plant competition in the following spring. However, the peak of MBN occurred in the winter in the Saihanwula area, suggesting a potential resource for spring plant N uptake in this area. We also found the magnitude of MBN was positively related to the soil water content under different vegetation types in the Saihanwula area. Our study demonstrates two contrasting seasonal patterns of soil MBN, suggesting that the general conceptual model of N cycle based on the studies from alpine and arctic areas may not be applicable to temperate ecosystems.