Microbial mediation of biogeochemical cycles revealed by simulation of global changes with soil transplant and cropping

Despite microbes'' key roles in driving biogeochemical cycles, the mechanism of microbe-mediated feedbacks to global changes remains elusive. Recently, soil transplant has been successfully established as a proxy to simulate climate changes, as the current trend of global warming coherently causes range shifts toward higher latitudes. Four years after southward soil transplant over large transects in China, we found that microbial functional diversity was increased, in addition to concurrent changes in microbial biomass, soil nutrient content and functional processes involved in the nitrogen cycle. However, soil transplant effects could be overridden by maize cropping, which was attributed to a negative interaction. Strikingly, abundances of nitrogen and carbon cycle genes were increased by these field experiments simulating global change, coinciding with higher soil nitrification potential and carbon dioxide (CO₂) efflux. Further investigation revealed strong correlations between carbon cycle genes and CO₂ efflux in bare soil but not cropped soil, and between nitrogen cycle genes and nitrification. These findings suggest that changes of soil carbon and nitrogen cycles by soil transplant and cropping were predictable by measuring microbial functional potentials, contributing to a better mechanistic understanding of these soil functional processes and suggesting a potential to incorporate microbial communities in greenhouse gas emission modeling.     

气候变化对陆地生态系统土壤有机碳储量变化的影响

通过研究气候变化对土壤有机碳储藏的影响,对预测未来气候变化下土壤有机碳动态变化与深入理解陆地生态系统变化和气候变化之间的相互作用有着极其重要的意义。本文归纳了土壤类型法、模型模拟法等途径对土壤有机碳储量估算的结果并分析它们各自的不确定性,综述了气候变化对土壤碳贮藏影响机理的研究与相应过程模拟的模型研究进展,并综合分析了当前研究中还存在的问题与不足。     

大气CO2浓度升高对土壤碳库稳定性的影响

工业革命以来,大气CO₂浓度持续上升,升高的CO₂浓度会改变植物光合产物积累、土壤碳库的碳输入和碳输出过程,进而通过影响有机碳组成和周转特征来调控土壤碳库动态变化。土壤碳库是陆地生态系统碳库的重要组成部分,其碳储量的微小变化都会对大气CO₂浓度和气候变化产生巨大影响。但目前关于CO₂浓度升高对土壤碳库动态和稳定性的影响还不清楚,很大程度上限制了预测陆地生态系统碳循环对气候变化的反馈。系统综述国内外大气CO₂浓度升高对植被生产力、植被碳输入和土壤碳库影响的研究进展,旨在揭示土壤碳库物理、化学组成以及周转特征对CO₂浓度升高的响应过程和机理,探讨CO₂升高情境下土壤微生物特征对土壤碳库稳定性的影响和驱动机制,为深入理解全球变化下的土壤碳循环特征提供理论支撑。     

外源碳输入对苔原土壤有机碳矿化和组分的影响

苔原生态系统土壤碳储量巨大,其微小的变化都可能显著影响大气CO₂的浓度,对调节全球碳平衡有着重要的意义。长白山岳桦(Betula ermanii)林下的草本植物入侵苔原,导致苔原植被发生显著变化,为揭示不同外源碳输入对土壤有机碳矿化及组分的影响,开展了120 d的室内培养实验。选取苔原带原生灌木优势种牛皮杜鹃(Rhododendron aureum)和入侵草本优势种小叶章(Deyeuxia angustifolia)的凋落物,采集牛皮杜鹃样方内表层土样(0-15 cm),设置6个凋落物处理模拟不同外源碳输入。研究结果表明:(1)与灌木凋落物输入相比,随着草本外源碳输入比例的提高,增加了土壤有机碳矿化速率、土壤有机碳累积矿化量和正激发效应,特别是培养初期的土壤有机碳矿化速率和正激发效应增加更为显著;并且较高品质的混合凋落物输入使各项测量指标高于品质更高的单一草本植物凋落物输入。(2)与灌木凋落物输入相比,随着草本外源碳输入的增加,减少了土壤有机碳库中总有机碳的数量和重组有机碳的比例,增加了微生物量碳、可溶性有机碳、易氧化有机碳和轻组有机碳的比例,而且也增加了土壤中速效养分的含量。(3)通过相关分析、一级动力学单指数模型和一级动力学双指数衰减模型拟合表明,高品质的外源碳输入促进土壤有机碳的矿化,而低品质的外源碳输入有利于土壤有机碳的稳定。综上,随着草本植物入侵程度的加重,苔原土壤有机碳库变得越来越不稳定,而当未来草本植物完全代替灌木植物时,苔原土壤有机碳库又会变得相对稳定一些。     

Distinct microbial communities associated with buried soils in the Siberian tundra

Cryoturbation, the burial of topsoil material into deeper soil horizons by repeated freeze–thaw events, is an important storage mechanism for soil organic matter (SOM) in permafrost-affected soils. Besides abiotic conditions, microbial community structure and the accessibility of SOM to the decomposer community are hypothesized to control SOM decomposition and thus have a crucial role in SOM accumulation in buried soils. We surveyed the microbial community structure in cryoturbated soils from nine soil profiles in the northeastern Siberian tundra using high-throughput sequencing and quantification of bacterial, archaeal and fungal marker genes. We found that bacterial abundances in buried topsoils were as high as in unburied topsoils. In contrast, fungal abundances decreased with depth and were significantly lower in buried than in unburied topsoils resulting in remarkably low fungal to bacterial ratios in buried topsoils. Fungal community profiling revealed an associated decrease in presumably ectomycorrhizal (ECM) fungi. The abiotic conditions (low to subzero temperatures, anoxia) and the reduced abundance of fungi likely provide a niche for bacterial, facultative anaerobic decomposers of SOM such as members of the Actinobacteria, which were found in significantly higher relative abundances in buried than in unburied topsoils. Our study expands the knowledge on the microbial community structure in soils of Northern latitude permafrost regions, and attributes the delayed decomposition of SOM in buried soils to specific microbial taxa, and particularly to a decrease in abundance and activity of ECM fungi, and to the extent to which bacterial decomposers are able to act as their functional substitutes.     

Climate change cannot be entirely responsible for soil carbon loss observed in England and Wales, 1978–2003

We present results from modelling studies, which suggest that, at most, only about 10–20% of recently observed soil carbon losses in England and Wales could possibly be attributable to climate warming. Further, we present reasons why the actual losses of SOC from organic soils in England and Wales might be lower than those reported.     

农田生态系统土壤有机碳库及其影响因子

土壤有机碳(SOC)的数量和质量在很大程度上与维持和提高土壤肥力密切相关。农田生态系统土壤碳库研究一直是农业、生态和环境领域的一个主要方向。土地利用、耕作、作物类型、种植密度、灌溉、施肥以及其他人为活动等,对农田生态系统土壤有机碳库的变化均能产生影响。本文综合评述了农田生态系统土壤有机碳库及其影响因子,土壤碳截获潜力,维持和提高土壤有机碳库的措施,以及农田土壤碳截获在温室气体减排及气候变化中的潜在作用等,最后提出了农田生态系统土壤有机碳库研究的主要方向。     

Soil carbon stocks and bulk density: spatial or cumulative mass coordinates as a basis of expression?

Accounting for CO₂ fluxes by determining changes in stocks of soil carbon (C) as a result of land use change is an option for complying nations under the Kyoto Protocol. The 1996 IPCC guidelines for C accounting recommend that soil C stocks to a depth of 30 cm be used in such accounting. However, the soil bulk density often changes with land use and the soil C per unit ground area to a fixed depth will also change even without any change in the mass fraction of C in dry soil. This problem will generally arise when soil C accounting is taken to a fixed depth (i.e. uses ‘spatial coordinates’). For accuracy in determining the land use change effects on soil C, soil sampling should be referred to a fixed dry soil mass per unit ground area (i.e. use ‘cumulative mass coordinates’). There has been intermittent literature‐discussion about this issue over several decades. Methods to accomplish C accounting on a mass coordinate basis, none of them accurate or efficient, have been suggested. Here, we propose a simple, accurate methodology for determining soil C stocks using cumulative mass coordinates, which does not involve repeat sampling trips, nominal specification of the location of boundaries between soil horizons, or independent sampling for determining soil bulk densities. Each core is taken a little (say 10 cm) below the nominal mass/depth required and the retrieved core is sliced into two at a point a little above the nominal mass/depth (say 10 cm above). An accurate determination of the depth of the core or slice is not needed, but an accurate determination of the dry mass of soil above and below the slice‐point is required. Linear interpolation between these two measurements is then used to estimate the cumulative soil C per unit ground area to the target dry soil mass per unit ground area. Even though this method eliminates the need for reporting soil bulk densities for C accounting, it is urged that the bulk densities and density changes still be routinely reported. This is because such information is of fundamental importance for understanding and predicting the movement of fluids and substances carried in them within the soil and between the soil and the environment. Hence, these data are likely to be of fundamental importance in developing our future understanding and predictive capacity of soil C changes with land use change.