Soil Freezing Research Articles

Projected changes in soil freeze depth and their eco-hydrological impacts over the Tibetan Plateau during the 21st century

In the context of global warming, the soil freeze depth (SFD) over the Tibetan Plateau (TP) has undergone significant changes, with a series of profound impacts on the hydrological cycle and ecosystem. The complex terrains and high elevations of the TP pose great challenges in data acquisition, presenting difficulties for studying SFD in this region. This study employs Stefan's solution and downscaled datasets from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to simulate the future SFDs over the TP. The changing trends of the projected SFDs under different Shared Socio-economic Pathways (SSP) scenarios are investigated, and; the responses of SFDs to potential climatic factors, such as temperature and precipitation, are analyzed. The potential impacts of SFD changes on eco-hydrological processes are analyzed based on the relationships between SFDs, the distribution of frozen ground, soil moisture, and the Normalized Difference Vegetation Index (NDVI). Results show that the projected SFDs of the TP are estimated to decrease at rates of 0.100 cm/yr under the SSP126, 0.330 cm/yr under the SSP245, 0.565 cm/yr under the SSP370, and 0.750 cm/yr under the SSP585. Additionally, the SFD decreased at a rate of 0.160 cm/yr during the historical period from 1950 to 2014, which was between the decreasing rates of the SSP126 and SSP245 scenarios. The projected SFDs are negatively correlated with air temperature and precipitation, more significant under the higher emissions scenario. The projected decrease in SFDs will significantly impact eco-hydrological processes. A rapid decrease in SFD may lead to a decline in soil moisture content and have adverse impacts on vegetation growth. This research provides valuable insights into the future changes in SFD on the TP and their impacts on eco-hydrological processes.

Effects of phase change material inclusion on reducing greenhouse gas emissions from soil in cold region.

Increased gas emissions from soil into the atmosphere are one form of ecosystem feedback in response to climate change. Soil temperature plays a critical role in the soil emission of carbon dioxide (CO2) and nitrous oxide (N2O) suggesting that the release of gases can be reduced by regulating soil temperature. This study proposes a green microencapsulated phase-change material (mPCM) as a soil temperature regulator due to its ability to absorb and release heat during temperature phase transition. The objective is to test how mPCM in soil mixtures influences CO2 and N2O fluxes under laboratory-controlled conditions. For this purpose, a series of soil incubations were carried out with different temperature regimes and soil moisture. The test results revealed that at 20% soil moisture mPCM reduced cumulative CO2 emissions from the soil by 16.4% during the thawing stage and by 20.5% during the freezing stage. At 25% soil moisture, mPCM showed a greater effect reducing cumulative CO2 emissions by 23.9% during the thawing stage and by 24.2% during the freezing stage. At below-zero temperatures, mPCM reduced the total N2O flux by 11.6% at 20% soil moisture and by 26.0% at 25% soil moisture, compared to soil without mPCM. As soil moisture increased, the effects of mPCM on CO2 and N2O fluxes became more pronounced. Cyclic freezing and thawing of soil led to an increase in gas flux. This variation was reduced by the mPCM due to its ability to mitigate the change of soil temperature. Inhibition of the rise in soil temperature due to the inclusion of mPCM reduced the rate of activation of soil mineralization, which reduced gas fluxes. This study demonstrates the potential of mPCM application to reduce greenhouse gas emissions from soil through thermoregulation.

Persistence of E. coli O157:H7 in Frozen Soils: Role of Freezing Temperature

Soilborne pathogen infections are increasingly reported globally in recent years. Infectious agents have contaminated most of seasonal frozen zone and have been found in permafrost due to the effects of intensified human activities on global warming. Therefore, in regard to sustainable agriculture, it is particularly important to assess the environmental behavior of those pathogens in frozen soils. Due to high pathogenicity and low infection threshold, E. coli O157:H7 (EcO157) is a worldwide public health concern, and recent studies have focused more on its fate in the environment. The survival of this serotype in a large variety of environmental media under temperature above 0 °C has been investigated, while its persistence in frozen soils has received less attention. In this study, we collected soils with different textures from a seasonally frozen zone in northeast China and investigated the persistence of EcO157 in soils at freezing temperatures (−5 °C and −15 °C) and moisture content (30% and 60% water holding capacity (WHC)) of the soils. By fitting the survival data with a Weibull model, we obtained three parameters: first log reduction time (δ in days), survival curve shape parameters (p), and monthly average reduction in EcO157 (MAR, log·gdw−1·mon−1). The results showed that temperature has a major impact on persistence, while moisture content has less effect on the survival of EcO157. Further multi-variable analysis revealed that the physical and chemical properties (e.g., sand fraction) of soil play an important role in survival. Certain bacterial groups are significantly correlated with the survival of EcO157 in frozen soils at −5 °C but not for the ones incubated at −15 °C. Our data could provide background data to evaluate the health risk associated with EcO157. The results could be helpful to improve sustainable soil practices and to develop regulations and policies aiming to achieve sustainable agriculture.

Study on the Supercooling Characteristics of Freezing Soil Based On Nucleation Theory

AbstractThe clarification of freezing in a soil‐water system is critical for assessing the formation of a freezing zone and liquid water flow. The supercooling phenomenon of soil pore solutions has been found during the freezing process, but the mechanism remains poorly understood. In this study, we propose a free energy function of soil‐water systems based on the Classical Nucleation Theory. The analytical solution of the critical nucleation problem of saline soil‐water system is obtained by combining the initial freezing temperature model and Pitzer activity coefficients model in electrolyte solutions. Then, the freezing‐thawing experiments of saline soil with various salt contents were conducted for verifying the analytical solution. The derived boundary nucleation rate is the quantitative solution of the critical condition for the supercooling. The findings suggested that the theory results agreed well with the experiment results. For the salt‐free soil‐water system, the critical maximum radius of supercooling was 7.15 nm. We compared eight classical ice‐water interfacial tension models, and the “Reinhardt & Doye” and “DeMott & Rogers” models showed excellent performance when using the new free energy theoretical framework to predict the crystallization nucleation rate of soil‐water systems. A positive correlation between the boundary nucleation rate and soil‐water potential is detected. According to the influencing factors, the boundary nucleation rate of soil‐water system can be divided into three zones: salt nature control zone (R > 100 μm), salt‐pore mixed control zone (100 μm > R > 100 nm), and pore size control zone (R < 100 nm).

Light grazing reduces gaseous nitrogen emissions from temperate grassland soils during freeze‒thaw cycles: An intact core incubation study

Livestock grazing and soil freeze‒thaw cycles (FTCs) can affect the biogeochemical processes of nitrogen (N) and gaseous N (N2O, NO, and N2) emissions from grassland soils. However, the effect of grazing intensity on soil gaseous N emissions during FTCs and the underlying mechanisms are not clearly understood. In this intact core incubation study, soil gaseous N emissions during two FTCs were simultaneously quantified from temperate grasslands that included grazing exclusion (GE), light grazing (LG), and heavy grazing (HG) in Inner Mongolia. Additionally, the abundance of N cycle-related functional genes and the main soil characteristics were determined to better understand the drivers of gaseous N emissions. The results showed that N2 emissions dominate the gaseous N loss from all investigated soils during FTCs, with cumulative N2 exceeding NO and N2O emissions by factors of 47–135 and 71–161, respectively. Increased soil moisture during thawing promoted N2 and N2O emissions from all three sites, except for N2O at the LG site. However, no obvious NO emission peak was observed from all investigated soils during FTCs. Soil C and N availability and aeration changed by grazing regulated soil N2O fluxes, while the abundances of key functional genes generally did not show significant correlations with gaseous N emissions. Moreover, compared to the GE and HG sites, LG substantially decreased the soil N2O and total gaseous N emissions during FTCs, suggesting that light grazing rather than long-term grazing exclusion could be a promising measure to reduce gaseous N losses during spring thaw. Our results highlighted the importance of the simultaneous determination of all kinds of gaseous N emissions during FTCs for closing the ecosystem N balance and developing appropriate strategies for grassland management.

Experimental study on mass transfer behavior and mechanism in nuclide-containing sandy soil under temperature gradient

The level of nuclear waste emergency disposal is an important factor restricting the rapid development of the nuclear energy industry under the trend of global energy revolution. To promote the effective application of artificial ground freezing (AGF) for the treatment of nuclear wastewater leakage, the mass transfer and migration mechanism of key nuclides (137Cs, 90Sr, and 129I) during unidirectional freezing of saturated silty sand were elucidated in this study. Temperature sensors and a nuclear magnetic resonance (NMR) were used to explore the internal temperature and moisture variation characteristics of nuclide-containing sandy soil during the freezing. Furthermore, using the designed device, a unidirectional freezing of different nuclide-containing sandy soils was performed to establish a relationship between the development characteristics of the freezing front and the mass transfer of nuclide. The results show that the T2 spectrum of nuclide-containing sandy soils has the most significant change from 0 °C to -1 °C, and the bound water content of I-containing sandy soil remains evident change when the temperature decreases below -1 °C. During unidirectional freezing, owing to the influence of the freezing front on nuclide migration, the resistivity curve exhibits a V-shaped jump, and the concentration peaks appear in turn along the freezing direction and show a stepwise increase. Nuclide mobility is closely related to the freezing rate of the corresponding location. With an increase in freezing thickness, the freezing rate decreases and the nuclide mobility increases. Owing to the combined influence of water-ice phase transition, concentration gradient and matrix suction in sandy soil, the concentration only increases within a small area at the face of the freezing front. The differences in migration behavior of nuclides in sandy soil driven by temperature gradient is mainly due to the different degrees of compression of nuclide ion on the diffusion double electric layer of soil particles, which results in different forms of unfrozen water within the sandy soils. The overall mobility of the three nuclides shows the following sequence: Cs>I>Sr. In the practical application of AGF, attention should be paid to the effects caused by the rapid migration of low-valence cationic nuclides and the retention of high valence cationic nuclides in the frozen wall.

Investigating the thermal state of permafrost with Bayesian inverse modeling of heat transfer

Abstract. Long-term measurements of permafrost temperatures do not provide a complete picture of the Arctic subsurface thermal regime. Regions with warmer permafrost often show little to no long-term change in ground temperature due to the uptake and release of latent heat during freezing and thawing. Thus, regions where the least warming is observed may also be the most vulnerable to permafrost degradation. Since direct measurements of ice and liquid water contents in the permafrost layer are not widely available, thermal modeling of the subsurface plays a crucial role in understanding how permafrost responds to changes in the local energy balance. In this work, we first analyze trends in observed air and permafrost temperatures at four sites within the continuous permafrost zone, where we find substantial variation in the apparent relationship between long-term changes in permafrost temperatures (0.02–0.16 K yr−1) and air temperature (0.09–0.11 K yr−1). We then apply recently developed Bayesian inversion methods to link observed changes in borehole temperatures to unobserved changes in latent heat and active layer thickness using a transient model of heat conduction with phase change. Our results suggest that the degree to which recent warming trends correlate with permafrost thaw depends strongly on both soil freezing characteristics and historical climatology. At the warmest site, a 9 m borehole near Ny-Ålesund, Svalbard, modeled active layer thickness increases by an average of 13 ± 1 cm K−1 rise in mean annual ground temperature. In stark contrast, modeled rates of thaw at one of the colder sites, a borehole on Samoylov Island in the Lena River delta, appear far less sensitive to temperature change, with a negligible effect of 1 ± 1 cm K−1. Although our study is limited to just four sites, the results urge caution in the interpretation and comparison of warming trends in Arctic boreholes, indicating significant uncertainty in their implications for the current and future thermal state of permafrost.

Two contrasting years of continuous N[formula omitted]O and CO[formula omitted] fluxes on a shallow-peated drained agricultural boreal peatland

Drained agricultural boreal peatlands comprise a large source of nitrous oxide (N2O) and carbon dioxide (CO2) but a small sink or source of methane (CH4). N2O fluxes have high spatial and temporal variability and are often measured with the chamber technique. Therefore, continuous measurements of N2O fluxes are needed to better understand how N2O emissions are triggered and to reduce the uncertainty of annual N2O budget estimations. Here we present a two-year-long time series of continuous measurements of CO2 and N2O fluxes of a shallow-peated drained agricultural boreal peatland cultivated for grass silage. The fluxes were measured with the area-averaging eddy covariance technique. Several N2O peak events were observed throughout all seasons. The peaks were associated with meteorological or management events, such as soil thawing or freezing, precipitation, fertilization and glyphosate application. The annual N2O budget was 4.74 ±0.47 and 6.08 ±0.49 kg N2O-N ha−1 y−1 in 2020 and 2021, respectively. The annual CO2 budget, comprising the sum of net ecosystem exchange and biomass export, was 3.70 ±0.22 and 5.54 ±0.33 t CO2-C ha−1 y−1 in 2020 and 2021, respectively. The N2O budget during the first, warmer winter was 106% higher than during the second, meteorologically more typical winter, due to the higher frequency of soil freezing–thawing cycles. The average annual N2O budget was 36%–50% lower than the IPCC Emission Factor (EF) while the CO2 budget was in accordance with the IPCC EF. CO2 emissions dominated the total CO2-eq emissions of our site but N2O also had a significant contribution of 12%. Our results also suggest that glyphosate application enhanced N2O emissions in the last quarter of 2021. However, the full rotation should be measured to confirm whether there is a need to re-evaluate the N2O IPCC EF for ‘grassland drained boreal’ land-use class.

Experimental Study on the Effect of Fines Content on the Frost Swelling Characteristics of Coarse-Grained Soil in Canal Base Under Open System

Abstract Engineering usually considers coarse-grained soils as non-frost swelling soils, but serious frost swelling still occurs in coarse-grained canal bases, which is directly related to the recharge conditions and the fine particle content in the soil. Little attention is currently paid to the effect of different fine particle contents on coarse-grained soil frost swelling, especially after the fine particle admixture content exceeds 16%. This paper considers the characteristics of coarse-grained soils in water conservancy projects with fines content between 0% and 50%. The coarse-grained soils with 5%, 15%, 25%, 35%, and 45% fines content were designed for freezing and swelling tests. The evolution of temperature and moisture fields and the amount of freezing and swelling of coarse-grained soils during the freezing process were studied by using servo-type freezing and swelling and thawing tester. The experimental results show that the cooling process of soil samples can be divided into a rapid cooling stage, a slow cooling stage, and a freezing stabilization stage. The cooling rate and the frost heave amount with increasing fines content showed a trend of first increasing and then decreasing.

Impacts of seasonally frozen soil hydrothermal dynamics on the watershed hydrological processes inferred from a spatially distributed numerical modelling approach

The freeze–thaw cycle over the surface seasonally frozen soil is an important soil hydrothermal dynamic process linking land surface processes and climatic changes in the cold regions. With the advancement of frozen soil hydrothermal dynamic studies and remote sensing technology, the simulation of frozen soil hydrological processes based on the distributed numerical model has become a hotspot to better understand the impact of frozen soil hydrothermal dynamics on the watershed hydrological processes in the cold regions over a large spatial scale. However, the quantitative analysis of the impact of seasonally frozen soil hydrothermal processes on watershed runoff at long-term time scales remained an unsolved issue in the field of frozen soil hydrology. Under the framework of the watershed distributed eco-hydrological model ESSI-3, a fully distributed frozen soil hydro-thermal processes integrated modeling system (FFIMS model) was established based on the coupled water and heat transferring mechanism for frozen soil hydro-thermal process simulations in the frozen surface or at a certain depth of a watershed. By coupling the FFIMS model with the distributed eco-hydrological model ESSI-3, the impacts of seasonally frozen soil hydrothermal processes on hydrological processes were investigated from the perspective of temporal-spatial domain with the simulated hydrothermal and hydrological processes for a long-term period from 2008 to 2016 over a watershed located in the cold region of Northeastern China. The results suggested that the soil freeze–thaw cycling posed different impacts with limited significance throughout the whole hydrological processes of the watershed in different seasons. Significant impacts on the hydrological processes were particularly observed in the thawing period of a year, when soil ice meltwater contributing to the discharge of the study watershed reached to about 35% in average in this period. ESSI-3 coupled with the FFIMS modelling system obviously improved the performance of the original ESSI-3 in cold region watershed simulations, and the averaged Nash efficiency coefficients obtained increased from almost 0 to 0.77 in the thawing period of a year. The study demonstrated the importance of application of spatially distributed numerical model with physical mechanism for seasonally frozen soil water and heat transfer process simulations.

Discrimination and mapping ground surface freeze and thaw states over Northeastern China based on the improved dual-index algorithm

Accurate delineation of spatiotemporal variations in ground surface soil freeze and thaw (F/T) states is essential to appraise many geoscience issues, such as the hydrological circulation and land surface-atmosphere feedbacks. Recently, an Improved Dual-index algorithm (DIA) method was proposed by accounting for the influence of soil moisture variations on the discrimination accuracy with passive microwave remote sensing (RS) data products. Compared with the original DIA, the Improved DIA method has proven to be a more practical approach on surface soil F/T states discrimination. However, the method has only been applied and verified in cold regions of high-altitude (e.g., Tibetan Plateau), it's applicability and effectiveness in the cold areas in mid-high latitudes, where the geographic and climatic conditions are quite different, yet remained to be further explored. The present study investigated the feasibility of using AMSR-E (the Advanced Microwave Scanning Radiometer-EOS) and AMSR2 (the second Advance Microwave Scanning Radiometer) passive microwave RS data products to discriminate the F/T states of the ground surface for a long period from 2002 to 2019 by means of the Improved DIA method over a typical mid-high latitude cold region of Northeastern China. Seasonal variation characteristics of soil moisture in mid-high latitude areas were similar with those in high-altitude areas, even though the spatial heterogeneity of soil water content was significant in different regions. Discriminating surface soil F/T states with the Improved DIA method derived overall discriminating accuracy of about 91.6% in the study area, which demonstrated excellent feasibility of the Improved DIA method in mid-high latitude cold regions. The mapping results shown surface soil F/T cycle in Northeastern China responding to climate change was examined from the perspective of regional average, both the proportion of frozen soil area and frozen days showed significant decreasing trends continuously with differed quite spatially. The discriminating accuracy of the Improved DIA method was found to be lower in plain areas with dense populations and large farmland areas compared to mountainous areas when human activities were not taken into consideration, as quantifying human activities can be challenging. The Improved DIA method has been well verified in both high-altitude and high-latitude regions; it has great potential in global scale research.

Gas storage of peat in autumn and early winter in permafrost peatland

The carbon (C) balance of permafrost peatlands in autumn and winter, which affects the annual C budget estimation, has become a hotspot of studies on peatlands C cycle. This study combined the static chamber method, in situ soil profile measurements, and incubation experiments to investigate release and storage of C during autumn and early winter in a permafrost peatland in the Da Xing'an Mountains, Northeast China. Our results showed that the peak values of CH4 fluxes (30 August 2016) lagged behind those of CO2 fluxes (24 July 2016). At the onset of soil freezing, CH4 fluxes slightly increased, while CO2 fluxes decreased. During soil freezing in autumn, gases were found to be mainly stored in the soil as dissolved CH4 and CO2 and dissolved C concentrations (CH4, CO2, and DOC (dissolved organic carbon)) increased with depth. DOC concentrations were closely related to dissolved C gases, implying that the stored dissolved C gases might be derived from DOC decomposition. The CO2: CH4 ratio decreased sharply from the freezing of the surface layer to the total freezing of the soil, indicating larger CH4 storage in totally frozen soil. The incubation experiments also showed larger CH4 storage in the frozen soils and the stored C gases could influence the assessment of C emissions during thawing. These findings have important implications for clarifying the gas storage of permafrost peatland in autumn and early winter. The results may also clarify the key link of C emissions between the growing season and the nongrowing season.

A modularized parallel distributed High–Performance computing framework for simulating seasonal frost dynamics in Canadian croplands

The winter/spring season in cold climate regions has been recognized as a critical period for cropland nutrient loss and greenhouse gas emissions and has been predicted to be vulnerable to climate change. Agricultural system models serve as essential tools in anticipating potential outcomes. The RZ-SHAW model, combining the Simultaneous Heat and Water Model (SHAW) with the Root Zone Water Quality Model 2 (RZWQM2), comprehensively describes snow and soil freezing dynamics. Showing promising results in simulating winter frost dynamics it, however, remains computationally intensive. High-performance computing has been recognized as a promising approach to reducing simulation runtimes. A modularized parallel distributed high-performance computing (HPC) framework (RS-DPCF) with both multi-threading (MT) and multi-processing (MP) capability was developed for RZ–SHAW calibration and simulation. Two case studies demonstrated that the developed RS-DPCF program could significantly reduce RZ–SHAW calibration time, allow improved computation resource scalability, and perform field-scale simulations of overwintering conditions for multiple croplands across Canada. For the RZ–SHAW model, MT provided a parallel-computing efficiency superior to that of MP.

Effect of Soil Texture on Water and Salt Transport in Freeze—Thaw Soil in the Shallow Groundwater Area

Research on the variation in soil water, heat, and salt in unsaturated zones during the freeze–thaw process has great significance in efficiently utilizing water resources and preventing soil salinization. The freeze–thaw field experiment was carried out with the lysimeter as the test equipment to analyze characteristics of the soil freeze–thaw process, profile water content, main ion content, and salt content of three textured soils with the groundwater table depth of 0.5 m. The results showed that the soil temperature gradient and freezing depth were greater as the average soil particle size increased. The increment of water content at the depth of 0 to 30 cm in sandy loam and loamy sand decreased by 40.20~93.10% and 28.14~65.52% compared with that in sandy soil, and the average increment of salt content at the depth of 0 to 30 cm decreased as the average soil particle size increased during the freeze–thaw period. The average content of Ca2+, Na+, Cl−, and SO42− in loamy sand and sandy soil decreased by 4.37~45.50% and 22.60~70.42% compared with that in sandy loam at the end of the freeze–thaw period, and the correlation between soil salt content and water content decreased with the increase in the average soil particle size. The research results can provide a theoretical basis for soil salinization prevention and crop production in shallow groundwater areas.

Assessing hydrological sensitivity to future climate change over the Canadian southern boreal forest

This study develops a physically based hydrological process model using the Cold Region Hydrological Modelling (CRHM) platform to simulate the water budget storages and fluxes over the Canadian southern boreal forest (SBF). Evaluation of the CRHM-based model in a well-gauged SBF basin, White Gull Creek (WGC), Saskatchewan, Canada, indicated quite good performance in reproducing historical observations of streamflow, snow water equivalent (SWE), evapotranspiration (ET) and soil moisture without parameter calibration from streamflow. The entire SBF was then evenly divided into 2243 virtual basins, each of which was structured and parameterized with the same land cover, soil and hydrological parameters as the WGC basin, but with local latitude and topography in order to examine the sensitivity of governing hydrological processes to future climate variability and perturbation. Hydrological sensitivity in the virtual basins was assessed by examining the differences between hydrological simulations driven by 4-km gridded convection-permitting Weather Research and Forecasting (WRF) outputs in the current period (ctrl, 2001–2013) and a Pseudo Global Warming period (pgw, 2087–2099). The WRF simulation in the pgw period was forced by a perturbation of the same boundary conditions from ERA reanalysis data as for the ctrl period, and the perturbation was based on the ensemble-mean of projected changes from the CMIP5 RCP 8.5 emission scenario. Results showed that temperature would increase by 4.5℃ to 7℃ over the SBF but increases in annual precipitation of 15–24% would more than compensate for the effects of warming on runoff generation and result in greater streamflow volumes. Annual streamflow volumes would increase by 64 mm (35%) and 95 mm (16%) in the west and east, and by 48 mm (17%) in the central SBF. Annual snowfall and maximum SWE would decrease by 89–109 mm (∼29%) in the east, 3–8 mm (∼6%) in the west, and 31–50 mm (∼20%) in the central SBF. Annual mean soil moisture storage would decrease by 54–56 mm (27%) in the west and central, and by only 37 mm (14%) in the east SBF. Decreases in soil moisture would be caused by reduced soil freezing and enhanced thawing under future warming which would increase soil water loss from ET, subsurface runoff and percolation into groundwater storage. The larger sensitivity of streamflow and snow processes in the east SBF is partly due to the wetter climate and the larger increase in annual precipitation, the later also buffered the sensitivity of soil moisture to warming. These results show that the SBF would switch to a higher water yield region, dominated by rainfall-runoff fed streamflow over a longer snow-free season, and provide first-order guidance for sustainable water management of the SBF in the future.

Remote Sensing Soil Freeze‐Thaw Status and North American N2O Emissions From a Regional Inversion

AbstractNorth American nitrous oxide (N2O) emissions over 2011–2018 are estimated using the CarbonTracker‐Lagrange regional inversion framework. Emissions are strongest in the Midwestern corn/soybean belt and display a distinct dual maxima seasonal pattern. The first maximum occurs in late winter/early spring, suggestive of freeze‐thaw (FT) effects on denitrification rates and associated N2O emissions. The second maximum occurs in late spring/early summer, consistent with a growing season nitrogen fertilizer‐driven source, although fertilizer applied in late fall may contribute to the FT pulse as well. Interannual variability in the first maximum correlates significantly to soil freeze thaw status derived from remote sensing data. A requisite frozen period in the preceding early winter appears necessary to create conditions for the N2O pulse after thawing. The FT pulse is a prominent feature of the annual cycle in Canadian cropland, where it may be of comparable magnitude to growing season emissions. In contrast, the growing season peak in N2O dominates the FT peak in the Midwestern Corn‐Soybean region of the United States.

Substantiation of the Escalator Tunnel Construction Technology Based on the Results of Field Research

Purpose. The authors aim to substantiate a new technology for the construction of an escalator tunnel in Ukraine based on the results of field studies performed as part of surveying measurements during the construction of the inclined run of the Dnipro metro. Methodology. The article analyzes the main technology of the escalator tunnel construction, which was based on the development of an inclined working from top to bottom with preliminary preparation of the massif by means of artificial soil freezing. Several cases of application of the bottom-up escalator tunnel construction method using the New Austrian Tunneling Method (NATM), which is successfully used in metro of other countries, are considered. The main principle of the method is that a shotcrete system is used as a temporary (primary) support, and a reinforcement system with monolithic concrete is used as a permanent support. Findings. The authors analyzed the peculiarities of the escalator tunnel excavation of the Dnipro metro by the Turkish company «Limak» in 2017–2018. It was noted that the excavation was carried out from the bottom up in conditions of strong rock (plagiogranite) using drilling and blasting operations. The characteristic results of field studies during drilling and blasting operations in the process of developing the calotte are analyzed. On the basis of these data, it is possible to trace the tendency to the appearance of minor overburden during sinking. The value of the deformed state (displacement of the contour of the designed casing) is within the normative limits, which indicates the effective use of these works within the framework of NATM. It is proved that in the conditions of military operations, the technology of building an escalator tunnel from the bottom up is a guarantee of the safety of workers and equipment. Originality. The results of the scientific work presented in the article make it possible to determine the level of deformed state during the excavation of the escalator tunnel of the Dnipro metro in hard rocks. During the field studies, the dependences of deformation of plagiogranite in the inclined bore were obtained. Practical value. An alternative technology for the construction of a bottom-up escalator tunnel based on NATM with the provision of normative deformation of the surrounding rock mass is substantiated.

The influence of tunnel insulation measures on the temperature spatiotemporal variation of frozen soil during artificial ground freezing

Artificial ground freezing (AGF) has been widely used in the construction of subway connection channels to reinforce the soil around the tunnel, especially the water-rich soft soil, to ensure the stability of the tunnel and the overlying buildings. However, the direct contact between the frozen soil curtain and the tunnel segment would cause the loss of cooling capacity, which is not conducive to ensuring the freezing effect. The indoor freezing and thawing model of silty soft soil was established to explore the influence of tunnel insulation measures with different thermal conductivity on the temperature spatiotemporal variation of frozen soil curtain through indoor simulation tests. The effects of thermal insulation materials with different thermal conductivity on the stable temperature and development speed of frozen soil curtains were also compared. The results indicated that the interface of tunnel segments has a significant impact on the freezing speed and stable temperature of frozen soil during the freezing process, as well as the thawing time during the natural thawing process. Compared with steel segment tunnel, the use of thermal insulation coating with low thermal conductivity increased the temperature of the soil at the same distance by 5.88% to 42.5%, and prolonged the freezing time of the soil far from the tunnel by 38.8%. On the contrary, the insulation foam with higher thermal conductivity not only reduced the temperature of frozen soil by 13.8% at most, but also shortened the freezing time by 42.1% at most, which can significantly improve the construction efficiency. For the natural thawing process, the thermal insulation coating would reduce the thawing time of frozen soil by more than 30%, while the thermal insulation foam can increase the thawing time of frozen soil far away from the tunnel by at least 23%, which was more conducive to extending the allowable duration of frozen soil excavation. The results can provide a reference for the selection of segment insulation measures in the AGF construction of subway cross channels in silty soft soil.

A model of segregation frost heave for saturated soil freezing under overburden pressure

To study the inhibition mechanism of overburden pressure on water migration in freezing soil, the pressure-suction coexistence state of film water is analyzed, and it is found that the main function of ice pressure is to offset and balance the theoretical suction. Under the combined action of ice pressure and theoretical suction, the surface adsorption force that drives water migration is produced. Based on this, the mechanical model of the pressure-suction coexistence state is introduced into the frozen fringe theory, and it is found that as the overburden pressure increases, the surface adsorption force gradually increases, while the total permeability coefficient of the frozen fringe decreases according to a high-order power function. Given this, a theoretical model of segregation frost heave under pressure is established and an explicit equation for the water migration velocity is given. Finally, a frost heave loading test is performed on saturated silt under open-system conditions, and the main parameters of the test process are substituted into the explicit equation for water migration velocity, the results showed that the calculated value of the model is completely consistent with the experimental value, thus verifying the correctness of the model. In summary, the proposed model has important theoretical value and practical significance for improving the existing frost heave theory.

Yield tradeoffs of early corn harvest to enhance pennycress establishment

AbstractOilseed pennycress (Thlaspi arvense L.) establishment following grain corn (Zea mays L.) harvest in northern latitudes is challenging because of the short duration between corn harvest and soil freeze. The use of shorter season corn hybrids for a region may partly solve this challenge, but tradeoffs in reduced corn yield may be a concern. This study evaluated corn relative maturity (CRM) hybrids on pennycress establishment, seed and oil yields, and corn grain yield tradeoffs. The CRM hybrids ranged from 76‐ to 95‐day for northern sites (Morris and Rosemount, MN), and from 95‐ to 113‐day for the southern sites (Custar, OH, and Lexington, IL). The checks were full‐season corn (113‐ and 95‐day corn for the southern and the northern sites, respectively) harvested for silage. Pennycress plant density, green cover, and yield were generally greater when pennycress was seeded following silage corn due to early seeding and reduced corn stover. Pennycress seed yields ranged from 388 to 778, 641 to 834, 362 to 784, and 834 to 2038 kg ha−1 at Custar, Lexington, Morris, and Rosemount, respectively. The results showed that growing 86‐ and 105‐day grain corn in the northern and southern sites, respectively, facilitated timely pennycress establishment with little or no corn grain yield loss compared with full‐season hybrids. Developing pennycress varieties better suited for emergence under high corn stover or employing stover management practices to increase autumn emergence is suggested. Additionally, an economic analysis of the entire cropping sequence (corn–pennycress–soybean) is needed.