冻融交替对土壤氮素转化及相关微生物学特性的影响

土壤冻融是发生在中、高纬度及高海拔地区的一种常见的自然现象。冻融作用通过影响土壤物理性质及生物学性状进而对土壤氮素转化过程产生重要影响,但目前关于冻融作用对土壤氮素转化过程影响的研究结果还不尽一致,对于冻融作用下土壤微生物学特性的研究相对较少。本文着重论述了冻融作用对土壤氮素转化过程(有效氮素含量变化、氮素净矿化速率、氮素损失途径等)的影响,并对冻融作用下土壤微生物生理和代谢特性进行了归纳和总结,简要指出目前研究过程中存在的问题,并对未来研究方向提出展望。     

Soil freeze-thaw cycle experiments: Trends, methodological weaknesses and suggested improvements

Although freeze-thaw cycles can alter soil physical properties and microbial activity, their overall impact on soil functioning remains unclear. This review addresses the effects of freeze-thaw cycles on soil physical properties, microorganisms, carbon and nutrient dynamics, trace gas losses and higher organisms associated with soil. I discuss how the controlled manipulation of freeze-thaw cycles has varied widely among studies and propose that, despite their value in demonstrating the mechanisms of freeze-thaw action in soils, many studies of soil freeze-thaw cycles have used cycle amplitudes, freezing rates and minimum temperatures that are not relevant to temperature changes across much of the soil profile in situ. The lack of coordination between the timing of soil collection and the season for which freeze-thaw cycles are being simulated is also discussed. Suggested improvements to future studies of soil freeze-thaw cycles include the maintenance of realistic temperature fluctuations across the soil profile, soil collection in the appropriate season and the inclusion of relevant surface factors such as plant litter in the fall or excess water in the spring. The implications of climate change for soil freeze-thaw cycles are addressed, along with the need to directly assess how changes in soil freeze-thaw cycle dynamics alter primary production.     

农田黑土季节性冻融过程及其水分分布特征

秋冬、冬春季节转换过程中0~10 cm农田黑土温度日较差较大,土壤经历着反复的冻融交替过程,大于20 cm的土壤温度日较差逐渐减小,冻融作用对深层土壤的影响逐渐减弱。融冻期土壤融化过程是由地表向下和由季节冻结层底面向上两个方向同时进行;而冻融期土壤冻结过程由土壤表面向下单方向进行。冻融过程中土壤水分发生迁移而重新分布,冻结期上层土壤首先冻结并聚集水分;融化期冻层融化水分向冻结锋面迁移,越靠近冻结层水分含量越大。     

冻融对粉质黏土强度指标的影响

依托同三高速公路扩建工程方正—哈尔滨段加宽一侧粉质黏土路基,采用室内三轴试验方法,研究了不同压实度、不同含水率下不同冻融次数后土体强度指标的变化规律。试验结果表明粉质黏土冻融后强度指标发生规律性变化:土的黏聚力损失随着冻融次数增多而增大,同一压实度下含水率高的土体黏聚力损失幅度较大;土的内摩擦角随着冻融次数的增多有逐渐增长的趋势,但增长幅度较小。运用Matlab对试验数据进行回归分析,建立了同一压实度下粉质黏土黏聚力、内摩擦角与含水率和冻融次数的回归方程。该方程能够直观模拟冻融对粉质黏土强度指标的影响规律,计算结果与实测结果吻合良好,可用于冻融后粉质黏土黏聚力和内摩擦角的计算。     

反复冻融对肉制品品质的影响

冷冻肉制品在现代肉及肉制品加工工业中起着很重要的作用,是原料肉最常用的保存方法。储存和运输时温度的波动,以及一些销售环节温度控制不当,会引起肉制品反复冷冻-解冻(反复冻融),这会引起肉制品内部的一系列生理生化反应,进而导致肉制品品质的下降。就反复冻融对肉制品品质方面的影响展开论述,以期为原料肉的合理冷冻储存提供参考。     

Is the decline of soil microbial biomass in late winter coupled to changes in the physical state of cold soils?

During winter when the active layer of Arctic and alpine soils is below 0 °C, soil microbes are alive but metabolizing slowly, presumably in contact with unfrozen water. This unfrozen water is at the same negative chemical potential as the ice. While both the hydrostatic and the osmotic components of the chemical potential will contribute to this negative value, we argue that the osmotic component (osmotic potential) is the significant contributor. Hence, the soil microorganisms need to be at least halotolerant and psychrotolerant to survive in seasonally frozen soils. The low osmotic potential of unfrozen soil water will lead to the withdrawal of cell water, unless balanced by accumulation of compatible solutes. Many microbes appear to survive this dehydration, since microbial biomass in some situations is high, and rising, in winter. In late winter however, before the soil temperature rises above zero, there can be a considerable decline in soil microbial biomass due to the loss of compatible solutes from viable cells or to cell rupture. This decline may be caused by changes in the physical state of the system, specifically by sudden fluxes of melt water down channels in frozen soil, rapidly raising the chemical potential. The dehydrated cells may be unable to accommodate a rapid rise in osmotic potential so that cell membranes rupture and cells lyse. The exhaustion of soluble substrates released from senescing plant and microbial tissues in autumn and winter may also limit microbial growth, while in addition the rising temperatures may terminate a winter bloom of psychrophiles.Climate change is predicted to cause a decline in plant production in these northern soils, due to summer drought and to an increase in freeze-thaw cycles. Both of these may be expected to reduce soil microbial biomass in late winter. After lysis of microbial cells this biomass provides nutrients for plant growth in early spring. These feedbacks, in turn, could affect herbivory and production at higher trophic levels.     

南极半岛海洋气候区的土壤Ⅲ.冻-融作用与水分状况

自由水活动在南极海洋性气候区土壤形成与演化过程中发挥极为重要的作用。本文对土壤自由水的来源、冻-融作用、永冻层和活动层动态以及自由水参与下的物质迁移过程等几个方面进行了论述,讨论了上述不同过程的影响因素以及土壤发生学意义,指出以土壤水形态转化和自由水活动为基础的土壤过程具有显著的微域性特点,是决定本区土壤发生类型与分布模式的重要因素之一。     

冻融作用对旱地土壤中不同吸附形态铵根离子的影响

[目的]探讨冻融作用对土壤中不同吸附形式铵根离子的影响。[方法]通过室内模拟试验,研究了冻融作用对三江平原旱地土壤吸附铵根离子总量(水浸提)、强吸附态量(0.01 mol/L KCl溶液浸提)的影响。[结果]冻融处理和非冻融对照处理下,相比线性方程,旱地土壤吸附的铵根离子总量能更好的由Freundlich方程拟合(R20.99,SE1.69)。冻融作用对铵根离子的总吸附量基本没有影响:当土壤中加入的NH4+初始浓度从0 mg/L升高到200 mg/L时,冻融条件下NH4+总吸附量从-0.52 mg/kg升高到39.0 mg/kg;非冻融条件下NH4+总吸附量从-0.70 mg/kg升高到38.5 mg/kg。土壤中强吸附态NH4+的吸附等温线呈线性(R20.99,SE0.54),强吸附态NH4+的吸附量经过冻融过程后有明显增加,当土壤中加入的NH4+初始浓度从0 mg/L升高到200 mg/L时,冻融条件下NH4+强吸附态含量从-2.36 mg/kg呈线性升高到28.81 mg/kg;非冻融条件下NH4+强吸附态量从-4.25 mg/kg呈线性升高到25.12 mg/kg。由于冻融作用降低了强吸附态NH4+吸附解吸达到平衡时土壤溶液中NH4+的浓度,因而有利于降低土壤中铵根离子的淋失。冻融作用主要影响的是以离子交换形式吸附于土壤的NH4+。[结论]该研究为控制土壤氮素过量输入水体,防治水体富营养化奠定了理论基础。     

冻融交替对土壤CO2及N2O释放效应的研究进展

在秋冬交替和冬春交替时期高纬度地区和高海拔生态系统表层土壤常有冻融交替频繁发生。由于冻融交替作用通过改变土壤水热性质而对土壤物理、化学、生物学特性产生效应。冻结通常导致土壤团聚体破裂、微生物细胞及细根死亡,释放出活性较高的有机物,增强随后融解的土壤的反硝化和呼吸活性,从而影响土壤生物、生物化学过程以及生物地化循环。已有对苔原、泰加林等北极和亚北极生态系统的研究表明,土壤冻融交替次数、冻融极端温度、土壤水分、土壤团聚体结构变化等对CO2和N2O的释放通量影响较为显著,一般在冻融的最初几个循环温室气体排放会增加,随后会降至一个较为稳定的水平。目前,冻融循环变化背景下的温室气体排放研究主要是针对北方高纬度地区,而且对冻融交替影响土壤温室气体排放的机理研究也不够。我国面积广大的青藏高原高海拔地带在全球增温背景下,轻微增温会导致季节性冻土表层冻融交替次数增加,甚至冻土季节消失,加强全球增温背景下我国高山亚高山季节性冻土生态系统效应和过程研究,特别是土壤暖化导致的温室气体排放变化通量和变化机理的研究,对揭示全球变化的区域效应以及高海拔生态系统的管理都具有重要作用。     

Hydrological and sediment connectivity under freeze–thaw meltwater compound erosion conditions on a loessal slope

Freeze-thaw processes can influence hydrology, soil erosion, and morphological development by altering the connectivity between active pathways of water and sediment transport. Concentrated flow experiments were conducted involving frozen, shallow thawed, and unfrozen soil slopes under 1, 2, and 4 L/min runoff rates at a temperature of approximately 5 °C. In this study, hydrological connectivity was analysed via the simplified hydrological curve and relative surface connection function. Sediment connectivity was analysed via the sediment structure connectivity and sediment functional connectivity. Results indicated that hydrological connectivity was greatest on frozen slopes (FS), followed by shallow thawed slopes (STS), and unfrozen slopes (UFS) given a constant flow rate. Hydrological connectivity increased with increasing runoff rate for each freeze-thaw condition. Freezing condition and runoff rate exhibited a positive response to the hydrological connectivity. Sediment structure connectivity increased with increasing runoff rate for each slope condition. The ordering of sediment structure connectivity across freeze-thaw condition was that FS was greater than STS while STS was greater than UFS independent of flow rate. Sediment functional connectivity included longitudinal, lateral, and vertical connectivity components. Sediment longitudinal and vertical connectivity indicated a trend of first increasing and then decreasing under the different runoff rates and freeze–thaw conditions. For a given runoff rate, the ordering of sediment longitudinal and vertical connectivity across freeze-thaw condition was that FS was greater than STS while STS was greater than UFS. Sediment lateral connectivity exhibited a trend of first decreasing and then stabilizing. The ordering of sediment lateral connectivity across freeze-thaw condition was that UFS was greater than STS while STS was greater than FS. FS could more easily reach longitudinal and vertical penetration. Sediment longitudinal and vertical connectivity rates demonstrated increasing trends with increasing runoff rate after runoff generation stabilization and gradually approached unity. This research further improves our understanding of the hydrological and erosional mechanisms of meltwater and the generation of flooding in frozen soil conditions.