Freeze-thaw Period Research Articles

Fall-applied manure with cover crop did not increase nitrous oxide emissions during spring freeze-thaw periods

In cold humid temperate regions, peak nitrous oxide (N2O) fluxes from agricultural soils occur during spring freeze-thaw periods. Fall-applied manure that adds water-soluble nutrients to the soil prior to freeze-up could contribute to spring N2O emissions. The objective of our field experiment was to evaluate the relationship between water-soluble nutrients and N2O emissions during the spring freeze-thaw period in a sandy-loam soil with fall-applied manure and fall-sown cover crops in Québec, Canada. Dairy cattle manure (solid or liquid form) was applied in September, then 100% ryegrass (Lolium multiflorum Lam.) and 50% ryegrass and 50% hairy vetch (Vicia villosa Roth) were sown. In the following spring (March–April), soil N2O fluxes were measured in non-steady-state closed chambers, and soil samples were analyzed for soil reactive nitrogen (ammonium, NH4+ and nitrate NO3−), dissolved organic carbon and nitrogen, soil moisture (water-filled pore space), and denitrification activity. Spring N2O fluxes were not related to the fall-applied manure and cover crop treatments, possibly due to nitrogen transformations or loss from soil after manure application. The NH4+ concentration, water-filled pore space, and denitrification activity were significant predictors of soil N2O emissions during the spring freeze-thaw period, but only explained about 24% of the variation in these N2O emissions. This suggests that N2O was produced by biological processes such as nitrifier denitrification. N2O emission may be the result of stochastic diffusion processes through pores with ice-water mixtures, as well as through cracks that form during freezing-thawing processes. Therefore, water-soluble soil nutrients may be poor predictors of spring N2O emissions from manure-amended soils in our region.

Assessment of future climate change impacts on water-heat-salt migration in unsaturated frozen soil using CoupModel

The transport mechanisms of water, heat, and salt in unsaturated frozen soil, as well as its response to future climate change are in urgent need of study. In this study, western Jilin Province in north-eastern China was studied to produce a model of coupled water-heat-salt in unsaturated frozen soil using CoupModel. The water, heat, and salt dynamics of unsaturated frozen soil under three representative concentration pathway (RCP) scenarios were simulated to analyze the effects of future climate change on unsaturated frozen soil. The results show that water, heat, and salt migration are tightly coupled, and the soil salt concentration in the surface layer (10 cm) exhibits explosive growth after freezing and thawing. The future (2020–2099) meteorological factors in the study area were predicted using the Statistical Downscaling Model (SDSM). For RCP2.6, RCP4.5, and RCP8.5 scenarios, future temperatures during the freeze-thaw period increased by 2.68°C, 3.18°C, and 4.28°C, respectively; precipitation increased by 30.28 mm, 28.41 mm, and 32.17 mm, respectively; and evaporation increased by 93.57 mm, 106.95 mm, and 130.57 mm, respectively. Climate change will shorten the freeze-thaw period, advance the soil melting time from April to March, and enhance water and salt transport. Compared to the baseline period (1961–2005), future soil salt concentrations at 10 cm increased by 1547.54 mg/L, 1762.86 mg/L, and 1713.66 mg/L under RCP2.6, RCP4.5, and RCP8.5, respectively. The explosive salt accumulation is more obvious. Effective measures should be taken to prevent the salinization of unsaturated frozen soils and address climate change.

The Effect of a Sand Interlayer on Soil Evaporation during the Seasonal Freeze–Thaw Period in the Middle Reaches of the Yellow River

Reducing soil evaporation in arid and semi-arid areas of the Yellow River Basin greatly benefits the efficient utilization of water resources in winter and spring, particularly during the seasonal freeze–thaw period. We conducted a field experiment in winter to understand the influences of different sand interlayers (depths of 5, 10, and 15 cm and particle sizes of 0.5–1.5 mm and 2.0–2.5 mm) on soil evaporation during the seasonal freeze–thaw period. The results show that the sand interlayer reduced soil evaporation during the seasonal freeze–thaw period. Decreasing the depth of the sand layer was more effective at reducing the evaporation than increasing the grain size. Soil evaporation reduced as the sand interlayer approached the surface. With constant particle size, total soil evaporation decreased by 40%, 20%, and 18% for sand interlayer depths of 5, 10, and 15 cm, respectively, compared to the homogeneous soil column. With a constant sand interlayer depth, the inhibition of soil evaporation for a particle size of 0.5–1.5 mm was clear. That is significant for improving the efficient utilization of water resources and sustainable development of agriculture in the Yellow River Basin.

Simulation of Soil Water Evaporation during Freeze–Thaw Periods under Different Straw Mulch Thickness Conditions

Straw mulching is an effective agricultural technology to reduce soil water loss in arid and semi-arid areas. Herein, the soil temperature and soil water content of bare land (LD) and 5 cm (JG5), 10 cm (JG10), 15 cm (JG15), 20 cm (JG20) and 30 cm (JG30) straw mulch thicknesses were measured through field experiments performed to assess the soil water evaporation using the simultaneous heat and water model during a freeze–thaw period. The results showed that the inhibiting effect of straw mulching on soil water evaporation during the freeze-thaw period reached 24–56.7%, and straw mulch reduced the range of daily soil water evaporation by 2.02–2.48 mm, the effects of random factors on the daily soil water evaporation were significantly decreased. The highest soil water evaporation rate occurs during the unstable freezing stage, and the lowest occurs during the stable freezing stage. When the straw mulch thickness exceeded 10 cm, the effect of increasing straw mulch thickness on daily soil water evaporation was reduced. The straw mulch layer could not completely inhibit the effect of the external environment on soil water evaporation even when the straw mulch thickness was increased to 30 cm. This research results can provide a basis for the scientific evaluation and prevention of soil water evaporation in arid and semi-arid areas.

Study on the variation in evapotranspiration in different period of the Genhe River Basin in China

Evapotranspiration is an important component and key link of river basin water cycles and plant hydrological processes, and is a core issue in global climate change research. It is not only an important way to understand the energy and water consumption of permafrost regions, but also is an important channel to master the water cycle and energy balance in cold regions. In this paper, multiple linear regression analysis method and weighted comprehensive analysis of major factors method were used to investigate the variation characteristics and impact factors of evapotranspiration in the Genhe River Basin. The results showed the following: (1) The monthly average evapotranspiration in the Genhe River Basin during the freezing-thawing periods in 1980–2017 was 28.29 mm. Compared with the freezing-thawing periods, the total evapotranspiration in the growing seasons was much higher than that in the freezing-thawing periods, with monthly average evapotranspiration of 67.71 mm; (2) The main factors affecting evapotranspiration in the Genhe River Basin were precipitation and temperature. During the freezing-thawing periods, the variation in evapotranspiration in May was mainly determined by temperature. In the growing season, precipitation was the main factors affecting evapotranspiration in June. This will lay a foundation for clarifying the relationship between permafrost–climate change–hydrologic cycle in the permafrost active layer during the land surface process, so as to provide some basic data and important scientific basis for the comprehensive study of the hydrologic process and its impact on climate, ecology, water resources and environment in the permafrost area.

Responses of available phosphorus in different slope aspects to seasonal freeze-thaw cycles in Northeast China

Freeze-thaw cycles (FTCs) are one of the main factors that alter soil physicochemical properties, consequently affecting soil productivity. However, there is limited research regarding the responses of available phosphorus (AP) in different slope aspects to seasonal FTCs. In this study, two slopes with different aspects, sunny (Ssn) and shady (Ssd), were selected in Meihekou, Northeast of China, located at a high latitude with the seasonal monsoonal climate. Spatio-temporal changes in AP, soil organic matter (SOM), and soil moisture content (SMC) on Ssn and Ssd were analyzed during the seasonal freeze-thaw period at seven sampling times from October 2015 to March 2016. Each sampling included different slope positions and soil depths (5 [D5], 10 [D10], 15 [D15], and 20 cm [D20]). Seasonal FTCs significantly (p < 0.01) affected AP content on both Ssn and Ssd. The AP trends over time were similar between Ssn and Ssd, i.e., initial increase and subsequent decrease, peaking in February. Ssn was considerably more sensitive to FTCs, especially in the surface soil. Overall, after seasonal FTCs, the AP content increased to 464.9 % and 68.2 % on Ssn and Ssd, respectively. Regarding soil depth, AP content increased the maximum at soil depth D5 on Ssn and Ssd with an increment rate of 563.2 % and 122.0 %, respectively. Regarding slope positions, AP content of the down slope increased the maximum (64.9 mg kg−1), with a 774.6 % increment rate on Ssn, while on Ssd, it increased the maximum on the middle slope (23.4 mg kg−1), with an increment rate of 114.3 %. SOM content of Ssd was significantly higher than that of Ssn (p < 0.01), and SOM content of the lower slope was significantly higher than that of the upper slope, which was more apparent on Ssd. For the entire slope, good power function relationships existed between AP and SOM on both Ssn (r2 = 0.59) and Ssd (r2 = 0.80). However, because of the continuous change in SMC, there were no clear relationships between AP and SMC in the field. Our findings demonstrate that slope aspect is a key driver of the different responses of AP during the seasonal freeze-thaw process, and that slope information could be useful for understanding the functional mechanisms of freeze–thaw cycles on nutrient loss processes in this region.

Effects of biochar and straw on greenhouse gas emission and its response mechanism in seasonally frozen farmland ecosystems

Freeze-thaw cycle promotes the decomposition of soil organic matter in cold regions, causing carbon and nitrogen to be emitted in the forms of CO2, CH4 and N2O, resulting in positive feedback to climate warming. To effectively regulate greenhouse gas emissions, four different regulation modes, namely, biochar addition (BA), straw addition (SA), combined biochar and straw (CBS) and a natural control (BL), were established. The characteristics of soil greenhouse gas emissions under different treatments and their response relationships to soil water, heat, carbon and nitrogen were explored. The results revealed that the SA and CBS treatments effectively inhibited the substantial reduction in soil temperature, moisture content, inorganic nitrogen and dissolved organic carbon during the freezing period; among them, the average soil inorganic nitrogen under the SA and CBS treatments increased by 15.36 and 11.62 mg·kg−1 compared to that in the BL treatment, respectively. Simultaneously, both N2O and CO2 emission fluxes were low, and the difference was small under the various treatments. However, the soil showed an absorption trend with respect to CH4, and the BA and CBS treatments promoted this effect; furthermore, the response relationships between CH4 and soil water, heat and carbon were enhanced. During the thawing period, the CBS treatment most effectively promoted the increase in soil water, heat, carbon and nitrogen, while it inhibited the flux of CH4 and N2O in soils, and the average CH4 emission flux under the CBS treatment decreased by 8.25 ~ 30.75 μg∙kg−1 relative to that under the other treatments. Concurrently, the responses of CH4 and N2O emission fluxes to soil water, heat, carbon and nitrogen were weakened under this treatment. Although the CBS treatment increased the CO2 emissions flux during this period, in view of the overall effect of the entire freeze–thaw period, the CBS treatment most effectively reduced the global warming potential (GWP) of the soil. Therefore, it is suggested that the joint application of biochar and straw is the most effective strategy for greenhouse gas budget management and soil nutrient restoration in seasonally frozen areas.

Dinitrogen (N2) pulse emissions during freeze-thaw cycles from montane grassland soil

Short-lived pulses of soil nitrous oxide (N2O) emissions during freeze-thaw periods can dominate annual cumulative N2O fluxes from temperate managed and natural soils. However, the effects of freeze thaw cycles (FTCs) on dinitrogen (N2) emissions, i.e., the dominant terminal product of the denitrification process, and ratios of N2/N2O emissions have remained largely unknown because methodological difficulties were so far hampering detailed studies. Here, we quantified both N2 and N2O emissions of montane grassland soils exposed to three subsequent FTCs under two different soil moisture levels (40 and 80% WFPS) and under manure addition at 80% WFPS. In addition, we also quantified abundance and expression of functional genes involved in nitrification and denitrification to better understand microbial drivers of gaseous N losses. Our study shows that each freeze thaw cycle was associated with pulse emissions of both N2O and N2, with soil N2 emissions exceeding N2O emissions by a factor of 5–30. Increasing soil moisture from 40 to 80% WFPS and addition of cow slurry increased the cumulative FTC N2 emissions by 102% and 77%, respectively. For N2O, increasing soil moisture from 40 to 80% WFPS and addition of slurry increased the cumulative emissions by 147% and 42%, respectively. Denitrification gene cnorB and nosZ clade I transcript levels showed high explanatory power for N2O and N2 emissions, thereby reflecting both N gas flux dynamics due to FTC and effects of different water availability and fertilizer addition. In agreement with several other studies for various ecosystems, we show here for mountainous grassland soils that pulse emissions of N2O were observed during freeze-thaw. More importantly, this study shows that the freeze-thaw N2 pulse emissions strongly exceeded those of N2O in magnitude, which indicates that N2 emissions during FTCs could represent an important N loss pathway within the grassland N mass balances. However, their actual significance needs to be assessed under field conditions using intact plant-soil systems.

Optimization of SWAT-Paddy for modeling hydrology and diffuse pollution of large rice paddy fields

The Soil and Water Assessment Tool (SWAT) simplified the hydrological processes in large patches of paddy fields. Based on the original SWAT code, the main equations are modified and validated with the long-term field observations in freezing-thawing watershed. The original SWAT and SWAT with paddy module (SWAT-P) both perform well in the simulation of stream flow at the basin scale (NSE ≥ 0.7). SWAT-P performs better in the soil water simulation (RMSE = 24.22 mm, smaller than 104.63 mm of SWAT). The monthly diffuse nitrogen loadings simulated by SWAT-P are within a reasonable range and close to the monitored nitrate loads in 2011 and 2012. SWAT-P represents the ponding water depth dynamics during the rice growing period. The original SWAT overestimates the diffuse pollution loads in the freeze-thaw period. SWAT-P has better performance for water cycles and diffuse pollution load simulations in watersheds with large rice paddy fields.

Effects of sand‐mulch thickness on soil evaporation during the freeze–thaw period

AbstractControl of evaporation from seasonally frozen soil is an important method for alleviating water shortages in arid and semi‐arid areas. To investigate the inhibition of soil evaporation by sand and the major factors that influence soil evaporation, a series of field experiments with five sand‐mulch thicknesses (0 cm, bare soil [BS], 1 cm [T1], 2 cm [T2], 3 cm [T3] and 4 cm [T4], with an average diameter of 1 mm) were conducted during the freeze–thaw period in Northern China. Soil evaporation characteristics in the three freeze–thaw stages were revealed and the major factors influencing soil evaporation were analysed using grey correlation analysis. The results showed that the cumulative soil evaporation decreased with increasing sand‐mulch thickness during the freeze–thaw period, and only small differences in soil evaporation were observed between the T3 and T4 treatments. The reduction in soil evaporation under different sand‐mulch thicknesses was 19.2–62.6% in the unstable freezing stage (P1), 2.0–28.3% in the stable freezing stage (P2) and 4.8–20.4% in the thawing stage (P3). In P1, solar radiation was a major factor influencing soil evaporation in all treatments and vapour pressure was a major factor in the sand‐mulch treatments, and the influence of relative humidity on soil evaporation decreased in the T4 treatment. During the coldest P2, solar radiation was lowest so that relative humidity and wind speed became the more dominant influence factors on soil evaporation in all treatments, and surface soil water content was a major factor in the sand‐mulch treatments. In P3, average air temperature and solar radiation were major factor influencing soil evaporation in all treatments and vapour pressure was a major factor in the BS and T1 treatments, whereas water surface evaporation was the major factor in the T2, T3 and T4 treatments. The results suggest that the addition of sand mulch in agricultural fields may be a beneficial practice to reduce water stress in arid and semi‐arid areas.

Biochar application for the improvement of water-soil environments and carbon emissions under freeze-thaw conditions: An in-situ field trial

There are few studies about biochar application in seasonally frozen soil areas. The regulatory mechanism of biochar on the water-soil environment and carbon emissions in seasonally frozen soil areas is unclear, which affects the study of nutrient migration and spring cropping systems under the control of biochar. For this purpose, we monitored the soil temperature (Ts), soil liquid moisture content (Ms) and soil respiration (Rs) rate during the freeze-thaw period under different application amounts of corn stover biochar (0 t∙ha−1, 15 t∙ha−1, 30 t∙ha−1, 45 t∙ha−1 and 60 t∙ha−1). The results showed that biochar can reduce the thermal conductivity of soil, thus improving the thermal insulation effect of frozen thawed soil, and Ts increased by 1.8–5.7 °C. The Ts and Ms were more sensitive to the high biochar application amount than to the low application amount. At the same time, biochar changed the soil aggregate distribution, and Pearson correlation analysis indicated that the soil water retention capacity increased by increasing the macroaggregate content (>0.25 mm), and the Ms increased by 3.7–6.1%. Principal component regression (PCR) analysis showed that biochar can promote soil carbon emission, and Rs of soil treated with biochar was 0.01–0.58 μmol m−2 s−1 higher than that of the control. The Ms and Ts were the most important factors promoting the carbon emissions of freeze-thaw soil under the synergistic effect of biochar and freeze-thaw conditions. However, biochar may promote soil CO2 emissions by affecting the water-soil environment. Considering the soil moisture, seed germination and growth conditions in spring, the suitable biochar application amount was determined to be 44–51 t∙ha−1. This study provides theoretical support for determining reasonable and effective biochar control measures and improving the soil productivity of farmland soil in seasonally frozen soil areas.

Annual ecosystem respiration is resistant to changes in freeze-thaw periods in semi-arid permafrost.

Warming in cold regions alters freezing and thawing (F-T) of soil in winter, exposing soil organic carbon to decomposition. Carbon-rich permafrost is expected to release more CO2 to the atmosphere through ecosystem respiration (Re) under future climate scenarios. However, the mechanisms of the responses of freeze-thaw periods to climate change and their coupling with Re in situ are poorly understood. Here, using 2years of continuous data, we test how changes in F-T events relate to annual Re under four warming levels and precipitation addition in a semi-arid grassland with discontinuous alpine permafrost. Warming shortened the entire F-T period because the frozen period shortened more than the extended freezing period. It decreased total Re during the F-T period mainly due to decrease in mean Re rate. However, warming did not alter annual Re because of reduced soil water content and the small contribution of total Re during the F-T period to annual Re. Although there were no effects of precipitation addition alone or interactions with warming on F-T events, precipitation addition increased total Re during the F-T period and the whole year. This decoupling between changes in soil freeze-thaw events and annual Re could result from their different driving factors. Our results suggest that annual Re could be mainly determined by soil water content rather than by change in freeze-thaw periods induced by warming in semi-arid alpine permafrost.

Occurrence, distribution, and ecological risk of pharmaceuticals in a seasonally ice-sealed river: From ice formation to melting

Occurrence, distribution, and ecological risk of 21 pharmaceuticals in the Jilin Songhua River were investigated during its freeze–thaw periods, including ice formation, sealed, and breakup. Florfenicol was the most abundant pharmaceutical, with mean concentrations of 123.4 ± 61.1 ng L−1 in water and 73.8 ± 66.3 ng kg−1 in ice. Sulfadiazine occurred at a higher mean concentration in downstream areas (45.6 ± 7.4 ng L−1) than in upstream areas (0.7 ± 0.7 ng L−1). Most pharmaceuticals appeared in relatively high concentrations in water during the ice-breakup period. Complex factors including pharmaceutical usage patterns, ice-regulated photodegradation, biodegradation, water flow, and freeze-concentration effects, as well as the release of pharmaceuticals from ice, were responsible for the temporal variation of pharmaceuticals. Pseudo-ice/water distribution coefficients showed the distribution of pharmaceuticals in ice and demonstrated the effects of their release from the ice on their temporal variations. Most pharmaceuticals posed a risk to algae; of these, amoxicillin exhibited the highest risk. In addition, thawing increased the concentration of thiamphenicol in water, which elevated its ecological risk level. The findings suggest that the pharmaceuticals retained in ice should be considered with regard to regulating pharmaceuticals’ temporal variations in seasonal ice-covered rivers during the freeze–thaw process.

冻融循环对川西亚高山森林土壤酶活性的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 冻融循环对川西亚高山森林土壤酶活性的影响 DOI: 10.5846/stxb201902250355 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金青年项目（31700542）；重点实验室开放基金（四川省科技厅）（ESP1704） Effects of the freeze-thaw cycle on soil enzyme activities in a sub-alpine forest in western Sichuan Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:土壤酶活性的研究有助于认识植物-微生物-土壤有机质之间的相互关系，土壤酶的生产遵循"经济法则"，冻融循环将会改变生产土壤酶的资源分配，最终影响土壤的有机质分解过程。亚高山森林土壤具有明显的季节性冻融循环，为深入研究亚高山森林冬季土壤生态过程，以川西亚高山针阔混交林、针叶林以及阔叶林土壤为研究对象，通过室内培养研究冻融循环对6种与碳（C）、氮（N）、磷（P）相关土壤酶活性的影响。结果表明，与N和P相关的土壤β-N-乙酰葡萄糖苷酶、磷酸酶活性受冻融循环影响显著，其中，β-N-乙酰葡萄糖苷酶活性在整个冻融处理过程中表现出先升高后降低并趋于平稳的趋势，磷酸酶活性在后期明显增加。与C相关的土壤纤维素酶、β-葡萄糖苷酶、过氧化物酶、多酚氧化酶在冻融循环过程中没有显著变化。林型对土壤酶活性产生了显著影响。其中，阔叶林的土壤纤维素酶活性显著低于针叶林；阔叶林的土壤过氧化物酶活性显著低于针阔混交林和针叶林；针叶林的土壤磷酸酶活性显著高于针阔混交林和阔叶林。冻融循环和林型的交互作用对土壤酶活性没有显著影响。结果表明，该地区土壤酶活性对冻融循环响应存在差异，气候变化引起的冻融循环将进一步影响川西亚高山森林土壤生态系统的有机质分解过程。 Abstract:Research on soil enzyme activities is beneficial to identify relationships between plants, microbes, and soil organic matter. Freeze-thaw cycles will change resource allocations for the production of soil extracellular enzymes, which affects organic matter decomposition processes in the soil. Based on the pre-recognition of sub-alpine seasonal freeze-thaw cycles and considering typical coniferous-broadleaf mixed forest, coniferous forest, and broadleaf forest soil in the sub-alpine region of western Sichuan, we controlled the freeze-thaw environment and analyzed the effects of freeze-thaw cycles on enzyme activities in the soil, including carbon, nitrogen, and phosphorus. According to the results, the freeze-thaw cycles had a significant impact on β-N-acetylglucosidase and phosphatase activities associated with soil N and P. The activity of β-N-acetylglucosidase was elevated at the beginning of freeze-thaw period and then decreased with an increase in freeze-thaw frequency. The phosphatase activity increased significantly in the later portion of the freeze-thaw cycle. However, the freeze-thaw cycle had no significant effect on C-related soil enzyme activities. The changes in C-related soil enzyme activities in this study were more determined by litter decomposition processes and forest types. Among them, cellulase and β-glucosidase activities were significantly correlated with the decomposition process of organic matter. The limiting factors of peroxidase and polyphenol oxidase and low temperature and low oxygen were still present in this test, so the activities of these two enzymes did not change significantly. On the other hand, the effect of forest types on cellulase, peroxidase and phosphatase activities was significant. The soil cellulase activity of broadleaf forest was significantly lower than that of coniferous forest. The peroxidase activity of broadleaf forest soil was significantly lower than that of coniferous-broadleaf mixed forest and coniferous forest. The soil phosphatase activity of coniferous forest soil was significantly higher than that of coniferous-broadleaf mixed forest and broadleaf forest. Our results indicate that the effects of the freeze-thaw cycles on extracellular enzyme activities in the soil vary in this area. Freeze-thaw cycles that result from climate change will further influence the organic matter decomposition processes of the soil ecology system in the sub-alpine forest in western Sichuan. 参考文献 相似文献 引证文献

Countermeasures combined with thermosyphons against the thermal instability of high-grade highways in permafrost regions

Permafrost degradation caused by climate warming and human activities would result in the thermal instability of embankments in permafrost regions, which would increase the maintenance cost. Two-phase closed thermosyphon (TPCT) is a widely-accepted green countermeasure against the problem in permafrost regions. However, the combination of TPCT with other countermeasures is usually proposed when it comes to a high-grade highway with a wide pavement. In order to explore the ideal combination, four combinations are compared by the instrumented physical embankment models: 1) inclined TPCT and insulation; 2) inclined TPCT, insulation and crushed-rock revetment; 3) L-shaped TPCT and insulation and 4) L-shaped TPCT, insulation and crushed-rock revetment. Under this experimental condition, the experimental results show that the TPCTs can effectively cool down the embankment center, the crushed-rock revetments can keep the soil slope and slope toe frozen during the whole freeze-thaw period after the 5th cycle, and the insulation can effectively prevent heat from entering into the embankment in warm seasons. After a thorough comparison of the thermal distribution and heat flux during seven freeze-thaw cycles, the embankment combined with L-shaped TPCT, insulation and crushed-rock revetment is proved to be the best way to keep the thermal stability of the wide-paved embankment. The results have potential to use for the future optimal construction against the thermal instability of high-grade highways in permafrost regions.

Dissolved organic matter and inorganic N jointly regulate greenhouse gases fluxes from forest soils with different moistures during a freeze-thaw period

ABSTRACT Antecedent soil moisture before freezing can affect greenhouse gases (GHG) fluxes from soils during thaw, but their critical threshold values for GHG fluxes and the underlying mechanisms are still not clear. By using packed soil-core incubation experiments, we have studied nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4) fluxes from a mature broadleaf and Korean pine-mixed forest soil and an adjacent white birch forest soil with nine levels of soil moisture ranging from 10 to 90% water-filled pore space (WFPS) during a 2-month freezing at −8°C and the following 10-day thaw at 10°C. The threshold values of soil moisture ranged from 50 to 70% WFPS for CH4 uptake and from 70 to 90% WFPS for N2O and CO2 emissions from the two soils during the freeze-thaw period. Under the optimum soil moisture condition, fulvic-like compounds with high bioavailability contributed more than 60% of dissolved organic matter (DOM) in the soil. Cumulative N2O emissions from forest soils during the freeze-thaw period were greatest when the concentration ratio of nitrate-N to dissolved organic carbon (DOC) was 0.04 g N g−1 C. Cumulative soil CO2 emissions and CH4 uptake during the freeze-thaw period were both regulated by the interaction between soil DOC and net N mineralization. The activities of β-1,4-glucosidase and β-1,4-N-acetyl-glucosaminidase, microbial biomass C and N, and the microbial biomass C-to-N ratios, were all significantly correlated to the soil N2O, CO2, and CH4 fluxes. Overall, upon a freeze-thaw period with different soil moistures, GHG fluxes from forest soils were jointly regulated by inorganic N and DOC concentrations, and related to the labile components of DOM released into the soil, which could be strictly controlled by the related microbial properties.

Annual dynamics of soil gross nitrogen turnover and nitrous oxide emissions in an alpine shrub meadow

Soil nitrogen (N) transformations play a vital role in maintaining grassland productivity and influence nitrous oxide (N2O) emissions. To evaluate annual dynamics of soil gross N turnover and its effects on N2O emissions in typical grasslands, we conducted year-round measurements of soil gross N turnover rates, inorganic N pool sizes and N2O fluxes in an alpine shrub meadow of the Qinghai–Tibet Plateau. Gross ammonium (NH4+) and nitrate (NO3−) immobilization rates were calculated by the “isotope dilution method” (i.e., based on the consumption rates) and the “reformed difference method” (i.e., based on the differences between gross and net turnover rates). The “reformed difference method” avoids additional measurements of net N turnover rates in unlabeled soils compared to the traditional “difference method”, and provides more reliable estimates of NH4+ immobilization in soils with low ambient NH4+ pool sizes than the “isotope dilution method”. The annual gross rates of mineralization, nitrification, NH4+ and NO3− immobilization amounted to 606 ± 43, 236 ± 27, 389 ± 24 and 82 ± 19 mg N kg−1 dry soil yr−1, respectively, in a topsoil of 10 cm. The soil gross N turnover rates during freezing and freeze–thaw periods contributed considerably to the annual totals (22–44%). Thus, the freezing and freeze–thaw periods were not dormant seasons for microbial N transformations, even under a frigid alpine climate. The annual N2O emission was 0.20 ± 0.03 kg N ha−1 yr−1, 35% of which originated from a short-lived emission pulse during the freeze–thaw period (52 d). The promotion of gross mineralization increased soil NH4+ pool sizes, whereas the increase in gross nitrification did not enhance soil NO3− pool sizes during the freeze–thaw period. Coupled nitrification–denitrification dominated NO3− consumption and freeze–thaw related N2O production and hence, the low NO3− pool sizes were not indicative of the potential of pulsed N2O emissions. Year-round measurements with intensive observations during both the non-freezing and freeze–thaw periods are indispensable to fully understand soil N cycling and its response to climate change in alpine ecosystems.

Effects of freeze-thaw and soil moisture on content and spectral structure properties of dissolved organic matter in forest soil leachates.

The contents and stability of soil dissolved organic matter (DOM) can affect key processes of soil carbon and nitrogen cycle. The responses of DOM content and its spectral structure pro-perties in forest soils to climate change remain unclear. We collected soil samples from two temperate forests, i.e., the broadleaf and Korean pine mixed forest (BKPF) and adjacent secondary white birch forest (WBF), in Changbai Mountains, northeastern China. Using a combination of three-dimensional fluorescence spectrum and parallel factor analysis, a simulated freeze-thaw experiment was conducted in the laboratory. We examined the effects of freeze-thaw intensity, freeze-thaw cycle and their interaction on the content, components and spectral properties of DOM leached from the two forest surface soils with different moisture levels. The results showed that DOM content and components of soil leachates varied with forest types, soil moisture, freeze-thaw intensity and freeze-thaw cycle. The DOM content in the leachates was lowest at medium moisture level and was significantly affected by the high freeze-thaw intensity. In addition, the DOM content increased first and then decreased with the increases of freeze-thaw cycles. Three fluorescence components of DOM in the forest soil leachates were identified as humic acid-like DOM, fulvic acid-like DOM and protein-like DOM. The DOM components of BKPF soil leachates were mainly consisted of fulvic acid-like substances with a high humification index. However, the DOM from WBF soil leachates was dominated by humic acid-like substances with low stability, and the three fluorescence components were significantly affected by the freeze-thaw intensity. Results from the redundancy analysis showed that under the experimental conditions, forest type played a leading role in changing DOM properties. The DOM content and its three fluorescence intensities of WBF soil leachates were higher than those of BKPF. Soil moisture significantly affected the aromaticity of DOM in the forest soil leachates, and the DOM aromaticity of soil leachates from the two forest stands ranked as medium moisture > high moisture > low moisture. With the increases of freeze-thaw intensity, the DOM aromaticity of BKPF soil leachates significantly decreased. Furthermore, the increases of freeze-thaw cycles significantly increased the humification degree of DOM in the forest soil leachates. Therefore, upon different freeze-thaw disturbance, the DOM content and bioavailability of soil leachates with low moisture tended to increase, particularly in the WBF soil leachates, which may result in an increased lea-ching of DOM in temperate forest soils during spring freeze-thaw periods. The results provide a refe-rence for further investigating DOM turnover in temperate forest soils during spring freeze-thaw periods.

Effect of Straw Mulch on Soil Evaporation during Freeze–Thaw Periods

Reducing soil evaporation is important to alleviate water shortages in arid and semi-arid regions. The objective of this work was to reveal the effect of straw mulch on soil evaporation based on field experiments during a freeze–thaw period in Northern China. Four soil surface mulch treatment modes were investigated: Bare soil (BS), 1 cm thick straw mulch with 100% coverage rate (J1), 2 cm thick straw mulch with 100% coverage rate (J2), and 2 cm thick straw mulch with 50% coverage rate (J3). Principal component analysis was used to analyze the major factors influencing soil evaporation in three freeze–thaw stages. The results show that cumulative soil evaporation decreased with increased straw mulch thickness and coverage rate. The effect of straw mulching on soil evaporation was obvious during the stable freezing period, and soil evaporation with straw mulch treatments was reduced by 49.0% to 58.8% compared to BS treatment, while there was little difference for straw mulch treatments in the thawing stage. The relationship between cumulative soil evaporation under different straw mulch modes and time was well fitted by the power function. In the unstable freezing stage, the major factors for all treatments influencing soil evaporation were surface soil temperature and water surface evaporation; in the stable stage, they were solar radiation and relative humidity, and in the thawing stage, they were solar radiation and air temperature. The research results can provide a basis for addressing soil water storage and moisture conservation and restraining ineffective soil evaporation in arid and semi-arid areas.

Machine-Learning Methods for Water Table Depth Prediction in Seasonal Freezing-Thawing Areas.

Long-term and accurate predictions of regional groundwater hydrology are important for maintaining environmental sustainability in arid agricultural areas that experience seasonal freezing and thawing where serious water-saving measurements are used. In this study, we firstly developed a machine-learning method by integrating a multivariate time series controlled auto-regressive method and the ridge regression method (CAR-RR) for water table depth modeling. We applied and evaluated this model in the Hetao Irrigation District, located in northwest China where the freezing-thawing period is 5 months long. To train and validate the model, we used monthly data of water diversion, precipitation, evaporation, and drainage from 1995 to 2013. The CAR-RR model yielded more accurate results than the support vector regression (SVR) and multiple linear regression (MLR) models did in the validation period. To extend the model applicability during freezing-thawing periods, we included additional temperature information. We compared results obtained using temperature only during the freezing-thawing period with results obtained without temperature, which showed that the input data of the temperature during the freezing-thawing period significantly improved the model accuracy. To resolve the problem of capturing the peaks and troughs of CAR-RR, we further developed an integrated CAR-SVR model to consider the nonlinearity. The optimal model (CAR-SVR) was then used to predict the water table depth under future water-saving measurements. It demonstrated that water diversion was the most important factor affecting the water table depth. A water table depth with less than 3.64 billion m3 water diversion will result in risks of environment problems.