干湿交替下草地生态系统土壤呼吸变化的微生物响应机制研究进展

草地是陆地生态系统中最重要、分布最广的生态系统类型之一,对全球碳循环和气候调节有着重要的作用和效应.我国拥有极为丰富的草地资源,是巨大的陆地碳储存库,也是全球碳循环重要组成部分.干湿交替是土壤中普遍发生的自然现象,这种现象的发生可能会加速土壤的碳矿化过程、激增土壤呼吸以及影响微生物的活性和群落结构等.在全球变化日趋显著的背景下,降雨量、降雨强度以及降雨频率的变化将会加速土壤干湿交替进程,进而带来微生物活性、群落结构以及土壤呼吸的变化,并对全球碳循环过程产生重要影响.本文综述了近十年来国内外的相关文献,对干湿交替条件下,土壤释放CO₂消耗碳源、土壤呼吸随时间的动态变化趋势以及土壤呼吸与微生物量、微生物活性和微生物群落结构之间的关系进行了分析和总结,以期为更好地理解干湿交替过程中草地生态系统土壤呼吸的微生物学响应机制,更准确地预测和评估未来的全球陆地生态系统的碳收支与气候变化提供一定的理论基础.     

消落区植物群落对土壤团聚体稳定性的影响研究进展

水位脉动、干湿交替是影响消落区植物群落分布和土壤稳定性的关键环境驱动力,消落区植物群落是影响土壤团聚体稳定的重要因素,研究消落区植物群落结构和功能性状对土壤团聚体稳定性的影响有助于预测消落区植物演替过程和揭示其对岸带稳定的影响机制。本文总结了国内外相关研究,综述了植物群落对干湿交替环境的响应,同时基于植物功能性状对干湿交替的响应重点阐述了其对土壤团聚体粒径稳定性的影响。未来研究应该重点关注根系构型性状和根际微生物性状对土壤团聚体的影响,并加强消落区不同胁迫强度和演替阶段植物群落组成、地上-地下性状对土壤团聚体稳定性的研究,进而探究水位脉动条件下植物群落对土壤团聚体稳定的影响机制,为消落区生态调节、生态恢复提供理论支撑。     

干湿交替条件下土壤氨基糖含量的动态变化

通过室内模拟培养试验,研究了恒湿和干湿交替条件下土壤中3种微生物来源氨基糖含量的动态变化,并且利用氨基葡萄糖和胞壁酸的比值分析了干湿交替条件下土壤真菌和细菌对土壤有机质转化的相对贡献.结果表明:恒湿条件下,细菌来源的胞壁酸在土壤中的分解速率大于真菌来源的氨基葡萄糖,氨基半乳糖在土壤中的分解速率较慢;干湿交替改变了土壤中3种氨基糖的分解特征,与恒湿处理相比,干湿交替培养前期以胞壁酸为代表的细菌残余物的分解速率高于以氨基葡萄糖为代表的真菌残余物,随着干湿交替频率的增大,以氨基葡萄糖为代表的真菌残余物分解速率高于以胞壁酸为代表的细菌残余物.可见,干湿交替条件改变了以氨基糖为代表的土壤氮素的微生物转化过程.     

冻融对土壤氮素损失及有效性的影响

土壤冻融交替是寒冷生态系统土壤氮素循环的重要驱动力。已有研究表明冻融交替作用能够促进氮素周转,从而缓解因土壤有效氮素缺乏而引起的植物生长限制。即便如此,冻融环境下土壤有效氮素供应量远高于其利用量,过剩的氮素会通过气态(N2O-N)排放、淋溶和径流等途径损失。论述了季节冻融环境和模拟冻融条件下土壤氮素损失状况;同时分析了影响冻融土壤N2O生产的相关因素、产生途径及冻融期N2O大量排放的机制;针对冻融交替过程中土壤氮素有效性问题,探讨了氮矿化、可溶性有机氮(DON)和微生物量氮(MBN)与氮素损失的关系。评述了土壤冻融研究中存在的不足,认为模型研究、土壤微生物功能、氮素转化中间产物、土壤-植物界面过程是未来值得关注和深入探讨的研究方向。     

马尾松人工林土壤和团聚体有机碳矿化对穿透雨减少的响应

为研究气候变化背景下降水格局对森林土壤碳排放机制的影响,在南亚热带马尾松人工林中模拟穿透雨减少50%,采用室内恒温培养法研究减水处理对土壤和不同粒级团聚体有机碳矿化的影响.结果表明: 1～2 mm团聚体有机碳累积矿化量高于其他粒级团聚体.干湿季减雨样地表层土壤含水量分别是对照样地的82.1%和82.7%,而其0.106～0.25 mm土壤团聚体质量分数分别比对照增加1.8%和4.2%.与对照相比,穿透雨减少显著降低了干季土壤和微团聚体易矿化碳库的矿化速率(k₁),增加了土壤和<1 mm团聚体难分解碳库的分解速率(k₂),但对土壤有机碳累积矿化量无显著影响.相关分析表明,土壤和微团聚体k₁呈显著正相关,土壤和0.25～1 mm团聚体k₂呈显著正相关.受团聚体结构、水分条件和土壤有机碳含量的影响,穿透雨减少对干季土壤易矿化和难分解有机碳的矿化分别起抑制和促进作用.     

水分条件变化对土壤微生物的影响及其响应机制研究进展

土壤微生物在维持陆地生态系统服务中扮演着重要的角色.土壤水分条件是影响微生物活性与生态系统功能的重要因素之一,全球气候变化所引起的极端干旱与降雨必将加速土壤水分的剧烈变化.由于不同土壤微生物对干旱胁迫的耐受性不同及其对水分变化的响应差异,使得土壤水分条件变化直接改变了土壤微生物活性与群落结构,进而对微生物介导的关键过程与土壤生态系统功能造成深刻的影响.因此,全面深入地理解水分条件变化下土壤微生物群落的结构变化特征与响应机制具有重要意义.本文在总结土壤水分条件变化对土壤微生物活性(土壤呼吸与酶活性)和微生物群落结构的影响的基础上,进一步阐述了土壤微生物对干旱胁迫与水分条件变化的响应机制和生态学策略,包括: 1)积累胞内溶质、产生胞外聚合物、进入休眠状态等应对干旱胁迫的细胞生理策略;2)微生物之间、微生物与植物之间相关抗逆性基因的转移及土壤微生物群落的功能冗余等应对水分变化的微生物机制.研究水分条件变化下土壤微生物群落结构及生态系统功能之间的内在联系,不仅有助于进一步剖析微生物介导的土壤生态过程,而且能够为今后陆地生态系统对气候变化的响应研究和模型预测提供理论依据.     

冻融作用下土壤物理和微生物性状变化与氧化亚氮排放的关系

在中、高纬度及高海拔地区,土壤冻融交替现象普遍存在.冻融作用影响土壤的物理和微生物性状,影响土壤的氮素转化过程和强度,导致土壤温室气体氧化亚氮排放增多,使冻融区土壤成为氧化亚氮的重要排放源.冻融作用改变了土壤水分特征和土壤团聚体的稳定性;冻融作用使土壤微生物量、微生物的组成和结构发生改变,导致微生物标识物氨基糖种类和数量改变.本文概述了上述变化与氧化亚氮排放的关系,简要提出了冻融作用下土壤氧化亚氮产生、排放的理论问题及其研究去向.     

干湿交替对生物滞留系统中氮素功能微生物群落的影响

【目的】为探究生物滞留系统干湿交替下环境因子对氮素功能微生物群落的影响。【方法】应用高通量测序技术(Illumina MiSeq PE300),并以amoA和nirS功能基因为分子标记,对无植物型和植物型生物滞留系统在干湿交替下不同土壤空间位置(种植层、淹没层)的硝化和反硝化细菌的多样性和群落结构进行研究,并对微生物群落与环境因子的相互关系进行相关性分析。【结果】微生物种群的功能基因存在显著的空间差异,相比淹没层,种植层的功能细菌更丰富。种植层的OTUs高于淹没层,而进水再湿润促使两种功能基因在种植层和淹没层的OTUs占比差异性增大。群落组成分析表明,amoA型硝化细菌和nirS型反硝化细菌的优势细菌门均为变形菌门(Proteobacteria)。虽然植物根系对氮素功能微生物的多样性指数影响不显著,但在属水平上,植物系统种植层的反硝化菌群种类高于淹没层,而无植物系统则刚好相反。CCA/RDA分析表明,土壤空间位置是影响硝化和反硝化菌群结构的最重要环境因子。【结论】本研究证实干湿交替运行下生物滞留系统中的氮素功能微生物群落受土壤空间位置、水分含量和植物根系的共同调控,其机制有待进一步研究。     

冻融交替对土壤氮素循环关键过程的影响与机制研究进展

冻融交替是由于季节或昼夜热量的变化,在表土及以下一定深度形成的反复冻结和解冻的过程,是普遍存在于中、高纬度及高海拔地区的一种自然现象。在全球变暖的背景下,部分地区的土壤环境将经受更广泛和频繁的冻融交替作用,这将对土壤氮素循环关键过程产生深远的影响。冻融交替主要通过改变土壤的理化性质使土壤微生物量、微生物群落的组成和结构发生改变,进而影响氮素在土壤中的迁移与转化,是陆地生态系统氮循环的重要影响因素。目前,关于冻融交替对土壤氮素循环关键过程影响的研究结果还不尽一致,其影响机制尚不明晰,研究方法也还有待进一步创新。重点论述了冻融交替对土壤氮素循环各个关键过程的影响效应,归纳总结了冻融交替对土壤氮素循环的影响机制,简要指出了目前研究过程中存在的一些不足,并对未来研究中值得重点关注和深入研究的科学问题进行了探讨与展望。     

荒漠-绿洲土壤微生物群落组成与其活性对比

结合野外观测与实验室研究方法,对比研究了准葛尔盆地南缘盐生荒漠与绿洲农田土壤微生物活性与其群落组成的变化特征,并分析了土壤温度与湿度对荒漠-绿洲土壤微生物活性的影响。结果表明:荒漠开垦为绿洲后,土壤细菌明显增加,真菌无明显变化,放线菌显著减少。细菌在绿洲农田土壤矿化作用中占主导,真菌则在荒漠中占优势,绿洲农田土壤微生物活性(包括真菌与细菌活性)明显高于荒漠。温度对荒漠-绿洲土壤微生物活性的影响只在一定土壤湿度范围内作用显著,绿洲农田受其影响较大;荒漠有机质含量明显高于绿洲农田,但水分与盐分因素抑制了微生物对其的分解和矿化。不同土地利用方式导致了荒漠绿洲间土壤湿度及盐份的较大差异,加之与土壤温度极显著的交互作用,使得开垦后土壤有机碳的易得性增强,微生物群落结构发生显著改变,进而有机碳的矿化速率加快,土壤碳库随之消减。     

Reappraisal of drying and wetting effects on C and N mineralization and fluxes in soils

In the next decades, many soils will be subjected to increased drying/wetting cycles or modified water availability considering predicted global changes in precipitation and evapotranspiration. These changes may affect the turnover of C and N in soils, but the direction of changes is still unclear. The aim of the review is the evaluation of involved mechanisms, the intensity, duration and frequency of drying and wetting for the mineralization and fluxes of C and N in terrestrial soils. Controversial study results require a reappraisal of the present understanding that wetting of dry soils induces significant losses of soil C and N. The generally observed pulse in net C and N mineralization following wetting of dry soil (hereafter wetting pulse) is short‐lived and often exceeds the mineralization rate of a respective moist control. Accumulated microbial and plant necromass, lysis of live microbial cells, release of compatible solutes and exposure of previously protected organic matter may explain the additional mineralization during wetting of soils. Frequent drying and wetting diminishes the wetting pulse due to limitation of the accessible organic matter pool. Despite wetting pulses, cumulative C and N mineralization (defined here as total net mineralization during drying and wetting) are mostly smaller compared with soil with optimum moisture, indicating that wetting pulses cannot compensate for small mineralization rates during drought periods. Cumulative mineralization is linked to the intensity and duration of drying, the amount and distribution of precipitation, temperature, hydrophobicity and the accessible pool of organic substrates. Wetting pulses may have a significant impact on C and N mineralization or flux rates in arid and semiarid regions but have less impact in humid and subhumid regions on annual time scales. Organic matter stocks are progressively preserved with increasing duration and intensity of drought periods; however, fires enhance the risk of organic matter losses under dry conditions. Hydrophobicity of organic surfaces is an important mechanism that reduces C and N mineralization in topsoils after precipitation. Hence, mineralization in forest soils with hydrophobic organic horizons is presumably stronger limited than in grassland or farmland soils. Even in humid regions, suboptimal water potentials often restrict microbial activity in topsoils during growing seasons. Increasing summer droughts will likely reduce the mineralization and fluxes of C and N whereas increasing summer precipitation could enhance the losses of C and N from soils.     

Alternate partial root-zone irrigation induced dry/wet cycles of soils stimulate N mineralization and improve N nutrition in tomatoes

Given the same amount of irrigation volume, applying alternate partial root-zone irrigation (PRI) has improved crop N nutrition as compared to deficit irrigation (DI), yet the mechanisms underlying this effect remain unknown. Therefore, the objective of this study was to investigate whether PRI induced soil dry/wet cycles facilitate soil organic N mineralization hereby contributing to the improvement of N nutrition in tomatoes. The plants were grown in split-root pots in a climate-controlled glasshouse and were subjected to PRI and DI treatments during early fruiting stage. ¹⁵N-labeled maize residues were incorporated into the soils. Results showed that PRI resulted in 25% higher net ¹⁵N mineralization than did DI, indicating that the enhanced mineralization of soil organic N alone could account for the 16% increase of N accumulation in the PRI than in the DI plants. The higher net N mineralization under PRI was coincided with an intensified soil microbial activity. In addition, even though soil chloroform fumigation labile carbon (CFL-C, as an index of microbial biomass) was similar for the two irrigation treatments, a significant increase of chloroform fumigation labile nitrogen (CFL-N) was found in the PRI wetting soil. Consequently, the C:N ratio of the chloroform fumigation labile pool was remarkably modified by the PRI treatment, which might indicate physiological changes of soil microbes or changes in labiality of soil organic C and N due to the dry/wet cycles of soils, altering conditions for net N mineralization. Moreover, in both soil compartments PRI caused significantly less extractable organic carbon (EOC) as compared with DI; whilst in the PRI wetting soil significantly higher extractable organic nitrogen (EON) was observed. A low EOC:EON ratio in the PRI wetting soil may indicate an increasing net mineralization of the organic N as a result of microbial metabolism. Conclusively, PRI induced greater microbial activity and higher microbial substrates availability are seemingly responsible for the enhanced net N mineralization and improved N nutrition in tomato plants.     

Effect of mild drying on the mineralization of soil nitrogen

Summary Drying soil to –100 kPa increased the subsequent mineralization of nitrogen under optimal moisture conditions. The effect was greater when the soils were dried to –1500 Pa. Mineralization was greater after four cycles of wetting and drying than after one. Depending on the drying conditions, the amount of nitrogen mineralized after drying to –1500 Pa was between 6.8 and 18.2% of that mineralized after chloroform fumigation. After drying the soils the average ratio of CO₂-C respired to min N was 21.1–22.3 depending on the drying conditions, whereas after chloroform treatment and autoclaving the ratio was 6.0 and 9.9 respectively. The effect of drying on nitrogen mineralization is attributed to two causes: the death and subsequent lysis of a small proportion of the soil organisms, and to the desorption of organic substances with a wide C/N ratio.Because of the stimulation of even mild drying conditions, marked differences in mineralization rates of soil nitrogen between cropping seasons must be expected.     

霍林河流域湿地土壤碳氮空间分布特征及生态效应

对霍林河流域湿地土壤有机碳及全氮空问分布特征及其生态效应的研究表明,有机碳和全氮的水平分异和垂直分异都十分显著,干湿交替周期是引起分异的关键因子;表层土壤有机碳与全氮含量显著相关(r=0．977),土壤碳氮比基本沿湿度梯度变化;土壤pH值对土壤表层碳氮含量及碳氮比值影响显著;流域湿地土壤与流域草原土壤碳氮比与土壤碳氮含量的相关性差异显著;其生态效应主要表现在生产效应和净化效应两方面．     

冻融对温带土壤可溶性氮库、氮转化过程及细菌群落多样性的影响

土壤冻融会影响土壤氮的有效性。氮的转化与土壤微生物密不可分,而土壤冻融对温带土壤细菌群落的影响并不十分清楚。假设: 冻融影响细菌群落结构多样性及其组成,从而改变土壤可溶氮含量和氮转化过程。为了验证这一假设,本研究设计了不同冻融循环次数(分别为6次和15次循环)的培养试验,并以2 ℃恒温培养作为对照。结果表明: 随着冻融循环次数的增加,可溶性全氮、可溶性无机氮、微生物生物量氮和净氮矿化率均显著降低。冻融循环次数对细菌α多样性(包括Chao1和Shannon指数)无显著影响,恒温培养的培养周期数与细菌的α多样性呈显著正相关。冻融处理显著影响细菌群落功能和组成,但冻融循环次数对细菌群落结构的影响较小。偏冗余分析表明,冻融处理下细菌群落结构和功能多样性与土壤可溶氮含量和氮转化过程密切相关。     

Recent research progress on soil microbial responses to drying–rewetting cycles

Climatic change, such as increases in extreme drought and rainfall events and changes in rainfall intensity and pattern, has been strongly influencing soil moisture. The climatic change impact is particularly common in arid, semi-arid and Mediterranean regions, which is causing dramatic changes in the intensity and frequency of soil drying–rewetting cycles. The soil drying–rewetting cycle is a natural phenomenon that the soil experiences drying, then wetting, and then drying and rewetting again and again. When a dry soil is being rewetted, the amount of soil microbial biomass and its activity can be sharply increasing in a short time period, and then a large amount of gaseous carbon (C) and nitrogen (N) erupts from the soil. The sudden release of gaseous C and N is caused by the stimulation of the soil microbes. Such a phenomenon is called “Birch effect”. The drying–rewetting cycles have direct and indirect effects on soil microbes, and soil microbial responses to the drying and rewetting events play an important role in the feedbacks of terrestrial ecosystems. From aspects of soil microbial biomass, microbial activities and microbial structure, we review recent advances on studies regarding microbial responses to soil drying–rewetting cycles. We interpret the microbial responses using five different types of mechanisms: (1) Microbial stress mechanism: when a soil becomes dry, microorganisms must accumulate compatible solutes such as carbohydrates and aminoacids so that the soil microbes can equilibrate with their environment in order to avoid dehydrating and being killed. When the soil is rewetted, soil microbes must dispose of those osmolytes rapidly by transforming them into carbon dioxide (CO₂), dissolved organic carbon (DOC) and nutrients in order to prevent water from being flowing into the cells. (2) Substrate supply mechanism: low soil moisture may result in the physical disruption of soil aggregates which leads to the exposure of new soil surfaces and of previously protected organic matter. When the soil is rewetted, its physical structure is further disrupted by swelling. The increased new soil surfaces and previously protected organic matter will improve the microorganism’s nutrient availability. (3) Soil hydrophobicity mechanism: soil hydrophobicity can cause the reduction of soil moisture and nutrient availability and inhibition of microbial decomposition of soil organic matter. Therefore, soil hydrophobicity is an important factor of explaining the activity of microorganism in drying and rewetting events. (4) Diffusive limitations mechanism: transportation of the soil microbe is limited in a dry soil. When soil moisture is increasing, soil microbial activity is enhanced along with the increased availability of substrate nutrients. (5) Predation mechanism: a moist soil is usually conducive to the increase of bacteria and fungi populations. In response, protozoa and nematodes also increase, leading to the fluctuation of the soil microbial community structure. On the basis of the literature review, we propose five important aspects to be considered in the future: (1) assessing soil microbes’ concrete adapting ways to the drying–rewetting cycles, (2) evaluating the microbial responses to the drying–rewetting cycles based on suitable indicators, (3) interpreting microbial responses to the drying–rewetting cycles by combining field investigation and laboratory controlling experiment, (4) investigating the microbial responses to the drying–rewetting cycles at different temporal and spatial scales.     

Forms and Functions of Meso and Micro-niches of Carbon within Soil Aggregates

Soil aggregates include sand/silt/clay, water, ion and organic matter contents combined with natural dry/wet (D/W) cycling alters both the formation and function of intra-aggregate pore continuity, connectivity, dead-end storage volumes, and tortuosity. Surface aggregates in the 0-5 cm depths of most soils experience from 34 to 57 D/W cycles that exceed differences in water contents >10%. Both the rates of drying or wetting, (intensity) and the D/W range of soil water contents (severity) alter the transport of water, C and N through micro and mesofaunal habitats among multiple size domains. This report identifies micro-niche locations of accumulating soil C within soil aggregate regions that may affect nematode residence sites and migration pathways. Recent advances in X-ray microtomography enable the examination of intact pore networks within soil aggregates at resolutions as small as 4 microns. Geostatistical and multi-fractal methods provide concise characteristics of pore spatial distributions within the aggregates and are useful for comparing these alterations among soils. Aggregates subjected to multiple D/W cycles developed greater spatial correlations that parallel increases in the (13)C sorption within aggregate interiors were compared with locations of soil microbial communities. Past research indicates microbial activities within the soil aggregate matrix are spatially heterogeneous due to complex pore geometries within aggregates. Illumination of the "blackbox" interiors of soil aggregates includes a discussion of natural and anthropogenic alterations of solution flow and carbon sequestration by soil aggregates containing biophysical gradients.     

Further reading in geomicrobiology

Microbial volatilization of selenium (Se) may be an effective bioremediation technique to remove Se from dewatered sediments. In this laboratory study, soil management parameters (wetting and drying cycles, aeration, mixing, aggregate size, and water quality) were assessed for their influence upon Se volatilization. Selenium volatilization rates were higher under continuously moist conditions (—33 kPa) compared with wetting and drying cycles. After 6 months of incubation, a continuously moist seleniferous soil had lost approximately 21% of the Se inventory, whereas the same soil incubated under wetting and drying cycles had dissipated 7% of the total Se. Incubation under anoxia (N₂ atmosphere) increased evolution of dimethyl selenide (DMSe) 1.4‐fold compared with aerated conditions. When soil samples were incubated under static versus continuously mixed conditions, the latter treatment enhanced volatilization 1.8‐fold. This was attributed to increased availability of the Se to the methylating soil microbiota. The optimum aggregate size to promote volatilization of Se was 0.53 mm when compared to 0.15, 1, and 2 mm. The application of saline well water (7.5 dS m^‐1) over 6 months, compared with deionized water, had little effect on volatilization rates of Se from a highly saline (22 dS mr^‐1) seleniferous dewatered sediment. Each of these parameters should be considered in promoting volatilization of Se as a bioremediation approach in the cleanup of seleniferous sediments.