Simulating soil freeze/thaw cycles typical of winter alpine conditions: Implications for N and P availability

Seasonally snow-covered alpine soils may be subjected to freeze/thaw cycles, particularly during years having little snow and during the late winter and early spring periods. Freeze/thaw cycles can stimulate soil mineralization and could therefore be one factor regulating nitrogen (N) and phosphorus (P) availability and cycling. In this study laboratory incubation experiments using four soils having contrasting properties have been used to characterize the change in N and P forms (microbial and soluble inorganic/organic) that occur after simulated freeze/thaw cycles.Soil samples were collected from locations representing extreme examples of either direct human management (grazed meadow (site M) and extensive grazing beneath larch (site L)) or those disturbed by more natural events (recent avalanche and colonisation by alder (site A)) and from beneath the expected forest climax vegetation beneath fir (site F). Topsoil from these sites, maintained at two different water contents (20 and 30%, w/w), were exposed to either a single (SF) or four sequential (4SF) freeze/thaw cycles. Each cycle consisted of 12 h at −9 °C and 12 h at +4 °C mimicking a diurnal pattern.A SF cycle reduced microbial N for soils from sites F and A and was accompanied by a significant increase in dissolved organic nitrogen (DON) at both moisture contents. In contrast, the microbial N of soils from M and L was not affected by the freeze/thaw cycles, suggesting a particular adaptation of soil microbes to these extremes in temperature. Freeze/thaw cycles resulted in a significant increase in the net ammonification in all soils.Extractable total dissolved N (TDN) and total dissolved P (TDP) increased in all soils after a SF cycle, however, the relative importance of the different N and P forms differed. At the lower soil moisture content, NO₃⁻ concentrations remained constant or slightly decreased in all soils, except that from site M. In all other soils DON appeared to replace NO₃⁻ as the potentially mobile N source after the freeze/thaw cycles. The relative contribution of dissolved organic P to TDP after freeze/thaw remained significant, and greater than 50% in all soils.Freeze/thaw cycles, in seasonally snow covered soils, are likely to have a selective effect on the microbial biomass. Freezing and thawing resulted in a pulse of net ammonification and DON release, which represent an important influence upon N cycling in these alpine systems.     

Effects of Freeze–Thaw Cycles on Brown Forest Soil Available Phosphorus in Northeastern China

Frequently occurring freeze–thaw events can affect soil nutrients. Available phosphorus (AP) is one of the most important plant nutrients in a brown forest soil. Initial soil moisture (15%, 20%, 25%, and 30%), fertilizer concentrations [0, 20, 40, and 60 ppm monopotassium phosphate (KH₂PO₄)], and freeze–thaw cycles (0, 1, 3, and 6) were considered to determine the effects of freeze–thaw events on AP, and the results indicated that (1) when variables initial soil moisture and fertilizer concentrations were defined, AP concentrations decreased with the increase of freeze–thaw cycles; (2) when variables freeze–thaw cycles and fertilizer concentrations were defined, AP concentrations presented decreased trend with the increase of initial soil moisture, especially initial soil moisture increase from 20% to 30%; and (3) when variables freeze–thaw cycles and initial soil moisture were defined, freeze–thaw events has more obvious influence on AP concentrations in soil samples with no fertilization added than mixed monopotassium phosphate (KH₂PO₄) before experiment.     

Synergetic variations of active layer soil water and salt in a permafrost-affected meadow in the headwater area of the Yellow River,northeastern Qinghai–Tibet plateau

The coupling effects and mechanisms of water, heat, and salt in frozen soils are considered to be one of the core scientific issues in frozen soil studies. This study was based on in situ observation data of active layer soil volumetric water content (VWC), temperature, and bulk electrical conductivity (EC) obtained at an alpine meadow site from October 2016 to November 2019. The site is located in the headwater area of the Yellow River (HAYR). We analyzed the synergetic variations of active layer soil VWC, temperature, and bulk EC during the freeze and thaw processes and discussed the underlying mechanisms. When the thaw process occurred from 10 to 80 cm depths, the VWC and bulk EC at a 10 cm depth showed synchronous high-frequency fluctuations and both increased linearly. The linear decreasing rate of the VWC (bulk EC) at an 80 cm depth in the freeze depths between 0 and 40 cm was 2 (1.6–2.3) times that of the VWC (bulk EC) at an 80 cm depth in the freeze depths occurring 0–10 cm. As soil temperature decreased in the frozen layer, unfrozen water content (bulk EC) decreased nonlinearly along with the absolute value of soil temperature (|T|), following a power (logarithmic) function. This study provided data that partly elucidate the interactions among permafrost, meadow, and ecohydrological processes in the HAYR. Also, our results can be used as a scientific basis for decision making on the protection and restoration of alpine grasslands, as well as for soil salinization studies.     

不同温度下的土壤热导率模拟

土壤热导率是研究陆地表层水热盐耦合运动的基本物理参数。由于水汽潜热传热在高温下的显著作用,高温下的土壤热导率显著高于常温值。该研究的目的是建立能够有效预测高温下土壤热导率的模型。在气体扩散定律的基础上,该文结合常温土壤热导率模型,提出了一个计算高温土壤热导率的新方法。并利用热脉冲技术实际测定了不同温度、不同含水率下的土壤热导率,对新模型进行了测试验证。结果表明,Cass等的水汽运移促进因子参数依赖于土壤质地,且存在较大的不确定性。经过对该参数修正后,建立的热导率模型均能够较好地模拟出高温下的土壤热导率。     

蒙古高原中部草地土壤冻融过程及土壤含水量分布

利用土壤剖面的温度、湿度观测数据,结合气象资料初步分析了蒙古高原中部典型针茅草原在季节转变过程中(2003～2004年)的土壤冻融过程和土壤含水量分布动态。研究表明,0～150cm深度范围的土壤完全冻结天数为154～160d。冻融日循环主要发生在表层0～5cm。0～30cm土层的土壤含水量变化剧烈,与地温有较好的一致性。0～10cm深度土壤含水量高于其他土层。随着深度的增加,土壤含水量季节波动性变小。冻结过程有利于保持土壤水分,有利于春季草地植被返青。     

Brief and vigorous N2O production by soil at spring thaw

In an acid sandy loam soil (pH 3.8), field production of N²O was two orders of magnitude higher at thaw in the spring than at any time during the rest of the year. Soil thaw in midwinter did not result in any increase in N²O flux. Soil water content remained at, or above field capacity during measurements; nitrate was added in excess. This effect could be reproduced in the laboratory: thawing soil cores at controlled temperature, nitrate and moisture yielded a large flush of N²O compared to an unfrozen control. The results indicate the importance of microbial N²O production during thaw for total annual N²O-emission.     

The effects of freeze‐and‐thaw cycles on phosphorus availability in highland soils in Turkey

Ongoing global warming may result in colder soil and thawing cycles and will increase the frequency of soil freezing‐and‐thawing‐treated cycles (FTCs) during winter in the cool‐temperate and high‐latitude regions. The purpose of this study was to determine the effects of repeated freeze–thaw cycles on the solubility and adsorption of P in lab and field experiments on Pellustert, Argiustoll, Haplustept, Fluvaquent, and Calciorthid soils, the major soil groups in E Turkey. The results demonstrated that, depending on the soil type, the freeze–thaw cycle could increase the adsorption and desorption of P within a certain temperature range. Repeated freezing and thawing decreased equilibrium P concentration (EPC) and increased P adsorption. EPC and P adsorption were strongly correlated with the number of FTCs. The highest P adsorption and the lowest P desorption was found in Pellustert followed by Argiustoll, Calciorthid, Haplustept, Fluvaquent when refrozen at –10°C for 15 d, then thawed at +2.5°C for 18 h, and 9 times FTC. However, in the field study, the adsorption value was lower than the value obtained from the laboratory condition. It appears that increasing the frequency of freeze–thaw processes depending on increase in temperature that leads to decreased plant‐available soil P pools, thus requires more P fertilizer in soil solution to supply adequate P during the plant‐growth period.     

土壤温度预报方程研究进展

土壤温度(尤其是地表温度)是陆地和大气之间相互作用中关键的物理量,在地球系统中扮演了十分重要的角色。土壤温度预报技术一直是陆面模式、数值天气预报和气候预测中核心科学问题。本文系统回顾了土壤温度预报方程的研究进展,从经典的热传导方程到考虑了土壤水分垂直运动物理过程的热传导-对流方程,从用单一正弦波逼近到用傅里叶级数逼近地表温度日变化,从假设对流参数无日变化为常数到考虑其日变化,着重概述了土壤热传导-对流方程的创建、改进及求解。最后,本文对热传导-对流方程在地表能量平衡、土壤水分垂直运动、水通量和地震、冻土热传输研究中的应用进行了回顾。同时指出,全相态的土壤水和植物根系对热传导-对流方程的影响是土壤温度预报方程未来的研究方向。     

土壤水氮动态及作物生长耦合EPIC-Nitrogen2D模型

为计算农业区不同作物生长条件下土壤水氮迁移转化过程,该文基于Erosion/Productivity Impact Calculator(EPIC)作物模型建立了作物根系生长子模块,将其进行有限元数值离散,与土壤氮素迁移转化模型Nitrogen2D耦合,使模型能计算作物生长条件下土壤水氮迁移转化过程。该作物生长模块可计算多种胁迫下作物根系对土壤水分和氮素的动态吸收速率,及作物收获时的生物量和吸氮量。采用武汉大学灌溉排水试验场冬小麦生长条件下土壤水氮试验数据对模型进行了率定,并用于土壤水氮分布和作物生物量预测,土壤含水率、氮素的模拟值与实测值的一致性系数分别为0.86~0.97、0.52~0.98,Nash效率系数为0.59~0.90(含水率)、0.44~0.93(土壤氮素),说明模拟结果与实测值吻合度较高。同时,分别采用该文的作物生长模块和简单根系吸收模块计算根系吸氮过程,结果显示,简单根系吸收模型会显著高估作物吸氮量,而作物生长模型则由于考虑了根系生长和各环境因子的胁迫作用,计算结果更符合作物实际吸氮过程,计算的根系吸氮量相对均方根误差为3.4%~46%。     

Soil nitrogen leaching losses in response to freeze–thaw cycles and pulsed warming in a temperate old field

Climate warming and increased climate variability are both predicted to increase the frequency of soil freeze–thaw cycles in temperate regions. We exposed intact soil-plant mesocosms to freeze–thaw cycles and examined the effects on nitrogen leaching losses. Freezing treatments were performed by incubating the mesocosms in the soil with their tops exposed to air to impose freezing from the top down, such that realistic freezing rates and cycle amplitudes were experienced across the soil profile. Leaching events were then initiated by water addition the following day for both the freezing treatment and control mesocosms. While water addition alone explained the major part of soluble organic nitrogen leaching, nitrate leaching approximately doubled in response to freeze–thaw cycles, and nitrogen leaching remained high after 11 freeze–thaw cycles. In a second experiment, pulses of warming were applied in situ to mesocosms over fall, winter or spring, in order to melt snow, and thereby increase freeze–thaw cycling by exposing soils to diurnal fluctuations in air temperature. Warming pulses had little effect on sub-surface soil temperatures and no effect on soil nitrogen leaching. However, warming pulses over spring severely reduced the abundance of the legume Coronilla varia in the following growing season. Overall, the results of these experiments indicate that while increased soil freeze–thaw cycles combined with leaching events are capable of increasing soil nitrogen losses, warming pulses will only promote increased freeze–thaw cycles if they are followed by cold, snow-free weather. The strong effect of warming on the N-fixer C. varia highlights that changes in plant species composition in response to warming may have stronger implications for soil nitrogen dynamics than the direct effects of freeze–thaw cycles on soil nitrogen leaching losses.     

基于改进水云模型和Radarsat-2数据的农田土壤含水量估算

为了直接将雷达遥感中"水云模型"进行反演应用,该研究将"水云模型"中植被参数改为雷达植被指数,利用全极化数据直接支持遥感反演土壤含水量,无需遥感反演植被参数输入。改进模型为利用雷达遥感结合"水云模型"进行土壤含水量监测提供了一种高效便捷方法。基于Radarsat-2全极化数据对冬小麦覆盖的农田土壤含水量进行估算,利用2014年在陕西杨凌区获取的4个生育期内Radarsat-2卫星数据及同步田间测量108组冬小麦农田土壤含水量地面测量数据进行模型参数校正和精度验证。验证结果精度为:改进的雷达植被指数模型原叶面积指数模型(实测叶面积指数验证)原叶面积指数模型(光学遥感反演叶面积指数验证),且改进的雷达植被指数模型可以在多个生育期内对农田土壤含水量进行监测。     

植烟黄壤氮素矿化动态模拟研究

研究不同温度和水分条件下植烟土壤有机氮的矿化动态,为田间土壤氮素矿化的预测提供依据。采用好气间歇淋洗方法,探求不同温度培养模式[恒温20℃,恒温35℃,变温(5、10、15、20、25、30、35、30、25、20℃)],不同温度(5~40℃)与不同土壤含水量(风干土~53%)交互作用下的土壤有机氮矿化动态,并建立回归方程。运用田间土壤氮素矿化数据,进行模型验证。结果显示,变温培养下土壤氮素矿化动态与恒温培养显著不同,变温下土壤矿化氮的累积动态以积温模型的拟合效果最好;指数模型能够较好描述土壤有机氮矿化对土壤水分含量的反应。在土壤氮素矿化积温模型和水分函数的基础上,建立了变化温度与水分条件下的土壤氮素矿化模型。田间实测矿化数据验证了此模型的可行性。因此,可以利用有效积温和土壤含水量来估测田间土壤氮素矿化量。     

关中平原农田土壤水力参数空间分异与模拟

土壤水力参数是土壤水分和污染物迁移等陆面过程数值模拟的重要基础参数。为探明关中平原农田土壤水力参数空间分异特征,建立空间分布预测模型,在关中平原网格布设124个样点,采集根层0-20 cm原状和扰动土壤样品,利用van Genuchten模型拟合获取土壤水分特征曲线,获得残余含水量(θr)、饱和含水量(θs)以及系数α和n等土壤水力参数。采用经典统计学、地统计学和结构方程方法分析了θr、θs、α和n的空间变异特征及影响因子,建立了水力参数传递函数预测模型。结果表明:θr和α为强变异,θs为中等变异,n为弱变异。θr、θs、α和n半方差函数最佳拟合模型分别为球状模型、指数模型、指数模型和球状模型。θs和n具有强烈的空间依赖性,变程分别为32.7,54.3 km;θr和α具有中等程度空间依赖性,变程均为52.8 km。土壤质地、容重、pH、有机质和海拔是影响土壤水力参数空间分布的主要因子。基于土壤理化性质和海拔建立的水力参数传递函数模型具有较好的模拟效果,可用于关中地区大尺度农田生态系统土壤水力参数的模拟预测。     

川西北高寒区冻融交替作用后土壤水热运移模拟研究

川西北高寒区在黄河和长江上游的水源涵养与补给、生态平衡中发挥着重要作用,该区土壤长期受到昼夜及季节性冻融作用影响,但土壤水分和热量在受到冻融交替作用后的变化规律,及其在草地退化及沙化过程中的作用还不明确。通过野外采集土样进行室内土柱模拟水热运移实验,利用有限元软件HYDRUS建立了一维土柱模型,开展基于冻融循环作用后的非饱和沙化草地和天然草地土壤水热迁移过程的数值计算研究,揭示不同冻融界面条件下沙化草地和天然草地土壤的水热分布特征、空间运移特性。结果表明：HYDRUS软件模拟沙化草地和天然草地土壤体积含水率的模拟结果R²>0.98,其平均值分别为0.997,0.996,模拟效果较理想;在土壤温度特性方面,模拟结果R²>0.98,平均值0.999,HYDRUS软件较好地模拟了川西北高寒区冻融交替作用后天然草地和沙化草地土壤温度和体积含水量的变化特征。通过模拟预测发现,反复冻融作用后,天然草地和沙化草地土壤体积含水率随时间变化均呈现上下波动的趋势,天然草地各土层体积含水率总体均高于沙化草地含水率;土壤温度模拟值呈现先波动上升后波动下降...     

Effects of freeze–thaw on aggregate stability and the organic carbon and nitrogen enrichment ratios in aggregate fractions

Soil samples from the Hexi Corridor located in the arid regions of Northwestern China were collected from a site that had received fertilizer applications for 23 years. Effects of freeze–thaw on aggregate stability and the organic carbon (OC) and nitrogen (N) enrichment ratios in water stable aggregate (WSA) fractions were investigated. In treatments combining the application of N fertilizer with green manure (GN) or straw (SN), the percentage of >0.25 mm WSA fraction was not significantly different from the control soil that received no fertilizer or organic amendment. After a freeze–thaw cycle, the percentages of the >0.25 mm WSA fraction in the GN and SN treatments showed no change, but the size of this fraction in the other treatments decreased. In addition, the organic carbon (OC) and N enrichment ratios in the >0.25 mm WSA fraction in GN and SN treatments increased after a freeze–thaw cycle. However, in this size fraction, the OC and N enrichment ratios decreased in other treatments. Both the changes of the percentages of the >0.25 mm WSA fraction and the OC and N enrichment ratios in this fraction under freeze–thaw in the GN and SN treatments exhibited the most significant increases compared with other treatments (P < 0.05). The results indicated that the GN and SN treatments could prevent the damaging effects of freeze–thaw on aggregate stability and protect soil from erosion.     

陆面数据同化方法在绿洲农田土壤温湿度模拟中的应用

为准确预测绿洲农田土壤水分的变化以利于合理分配有限的水资源,该文利用耦合于中尺度大气数值模式MM5和WRF中的Noah Lsm与集合卡尔曼滤波方法建立了一维的陆面数据同化系统,将其应用到绿洲农田土壤的水热模拟研究。模拟结果表明：在陆面数据同化系统中同化土壤湿度后,不仅提高了绿洲农田土壤湿度的模拟精度,还可在一定程度上提高土壤温度和潜热的模拟性能;在同化土壤湿度的基础上进一步同化土壤温度,可显著提高绿洲农田土壤温度的模拟精度。若将研究结果进一步应用到中尺度大气数值模式MM5和WRF,可以较好地监测大面积的土壤温湿度变化以便做出合理的水资源调配。     

Do freeze‐thaw events enhance C and N losses from soils of different ecosystems? A review

Freezing and thawing of soils may affect the turnover of soil organic matter and thus the losses of C and N from soils. Here we review the literature with special focus on: (i) the mechanisms involved, (ii) the effects of freezing temperature and frequency, (iii) the differences between arable soils and soils under natural vegetation, and (iv) the hypothesis that freeze‐thaw events lead to significant C and N losses from soils at the annual scale. Changes in microbial biomass and populations, root turnover and soil structure might explain increased gaseous and solute fluxes of C and N following freeze‐thaw events, but these mechanisms have seldom been addressed in detail. Effects of freeze‐thaw events appear to increase with colder frost temperatures below 0°C, but a threshold value for specific soils and processes cannot be defined. The pool of C and N susceptible to freeze‐thaw events is rather limited, as indicated by decreasing losses with short‐term repeated events. Elevated nitrate losses from soils under alpine and/or arctic and forest vegetation occurred only in the year following exceptional soil frost, with greatest reported losses of about 13 kg N ha^?1. Nitrate losses are more likely caused by reduced root uptake rather than by increased N net mineralization. N₂O emissions from forest soils often increased after thawing, but this lasted only for a relatively short time (days to 1–2 months), with the greatest reported cumulative N₂O emissions of about 2 kg N₂O‐N ha^?1. The emissions of N₂O after freeze‐thaw events were in some cases substantially greater from arable soils than from forest soils. Thus, freeze‐thaw events might induce gaseous and/or solute losses of N from soils that are relevant at the annual time scale. While a burst of CO₂ after thawing of frozen soils is often found, there is strong evidence that, at the annual time scale, freeze‐thaw cycles either have little effect or will even reduce soil C losses as compared with unfrozen conditions. On the contrary, a milder winter climate with fewer periods of soil frost may result in greater losses of C from soils that are presently influenced by extended frost periods.     

黑河中游农田荒漠过渡带土壤冻融过程中水热动态

以黑河中游典型农田荒漠过渡带为例,对过渡带3种景观单元冻融期土壤水热动态进行了野外定位监测。结果表明:(1)土壤温度随气温剧烈变化,变幅随土壤深度的增加而减小,3种景观单元土壤温度变幅由剧烈到平缓的顺序为:荒漠农田防护林,并依次形成60,100和80cm深的冻土层;(2)受土壤性质和地表覆盖的影响,冻融过程中,农田、防护林土壤含水量变化明显,且农田土壤水分含量4月初达到最大值,而荒漠土壤含水量则基本保持不变;(3)土壤水分变化滞后于土壤温度的变化,防护林土壤水和温度变化较农田缓慢;(4)浅层地下水位在冻结期下降,融化期回升,且回升速率大于下降速率。冻融过程可有效减小土壤水分的蒸发和渗漏,冻后聚墒明显,利于下层土壤水分的保持,对于来年植物生长具有一定的意义。     

基于模型平均法的表层土壤含水率多模型综合反演

土壤水分是农作物和牧草生长的关键因素,也是影响全球气候变化和水循环的重要因子,因此准确监测土壤水分对促进农牧业可持续管理至关重要。基于Landsat 8 OLI多光谱影像,在表面生物物理特性和地形参数的基础上,结合野外实测土壤含水率,采用经验模型法分别构建了反距离加权法（Inverse Distance Weighting,IDW）、多元线性回归（Multivariable Linear Regression,MLR）、随机森林（Random Forest,RF）的土壤含水率反演模型,并采用模型平均法（Granger-Ramanathan,GR）进行组合反演,结果显示,通过模型平均法组合单一模型后在该研究区的土壤含水率反演中表现出了更好的适用性,模型平均法GR对于土壤含水率的估算效果佳,建模集与验证集R2≥0.88,均方根误差均不大于1.42%,且模型性能远优于反距离加权法、多元线性回归和随机森林。     

Evaluation of the Advanced Canopy-Atmosphere-Soil Algorithm (ACASA) model performance over Mediterranean maquis ecosystem

The Advanced Canopy-Atmosphere-Soil Algorithm (ACASA) model is used to predict energy, water and carbon fluxes over a Mediterranean maquis site located in North-Western Sardinia (Italy) and the model performance is evaluated. Flux simulations are compared with Eddy Covariance field measurements collected from 2004 to 2007. The site experiences a drought season during the summer months in which the vegetation becomes water stressed. Results from the months of January, April, and July are analyzed to demonstrate the model behavior in different environmental conditions. In general, simulated and observed fluxes matched when both the thermal and moisture regime are optimal. During the July water stress period the model underestimated latent heat and carbon fluxes due to a strong stress response linked to soil properties and plant physiological characteristics. The selection of values for key parameters, e.g. maximum ideal photosynthetic capacity (RUBISCO), wilting point, soil water content, and root and leaf area ratio, is crucial to obtain close agreement between simulated and observed fluxes. The model was designed so that the most sensitive parameters are measurable quantities. Using the ACASA model to predict energy and mass fluxes between the vegetation and atmosphere appears promising in this context, and it could significantly improve our ability to estimate fluxes for use in future studies.