氮沉降增加情景下植物-土壤-微生物交互对自然生态系统土壤有机碳的调控研究进展

大气氮沉降增加倾向于促进受氮限制陆地生态系统地上生物量,但是对地下碳过程和土壤碳截存的影响结果迥异,导致陆地生态系统“氮促碳汇”的评估存在很大的不确定性。大气氮沉降输入直接影响微生物活性或间接影响底物质量,改变凋落物和土壤有机质(SOM)的分解速率和分解程度,进而影响土壤有机碳(SOC)的积累与损耗过程。过去相关研究主要集中在土壤碳转化过程和碳储量动态方面,缺乏植物-微生物-SOM交互作用的理解,对土壤碳截存调控的生物化学和微生物学机理尚不清楚。本文以地下碳循环过程为主线,分别综述了氮沉降增加对植物地下碳分配、SOC激发效应、微生物群落碳代谢过程的影响,深入分析SOM化学稳定性与微生物群落动态的关系。该领域研究的薄弱环节体现在:(1)增氮倾向于降低根系的生长和周转,对根际沉积碳分配(数量和格局)的影响及驱动因素不明确;(2)虽然认识到氮素有效性影响土壤激发效应的方向和强度,但是氧化态NO-3和还原态NH+4输入对有机质激发效应的差异性影响及潜在机理知之甚少;(3)微生物碳利用效率(CUE)是微生物群落碳代谢的关键表征,能够很好地解释土壤碳的积累与损耗过程;由于缺乏适宜的测定方法,难以准确量化土壤微生物的CUE及微生物生物量的周转时间;(4)增氮会抑制土壤真菌群落及其胞外酶活性,对细菌群落组成的影响尚未定论,有关SOM化学质量与土壤微生物群落活性、组成之间的耦合关系尚不清楚。未来研究应基于长期的氮添加控制实验平台,结合碳氧稳定性同位素示踪、有机质化学、分子生物学和宏基因组学等方法,深入分析植物同化碳的地下分配规律、微生物碳代谢和周转、有机质化学结构与功能微生物群落的耦合关系等关键环节。上述研究将有助于揭示植物-土壤-微生物交互作用对SOC动态的调控机制,完善陆地生态系统碳-氮耦合循环模型,有效降低区域陆地碳汇评估的不确定性,并可为陆地生态系统应对全球变化提供科学依据。

Regulation of plant-soil-microbe interactions to soil organic carbon in natural ecosystems under elevated nitrogen deposition: A review

Increased atmospheric nitrogen (N) deposition generally promotes aboveground biomass in N-limiting terrestrial ecosystems, but the effects on underground carbon (C) processes and soil C sequestration remain controversial. This leads to considerable uncertainties in the evaluation of the C sequestration capacity caused by N deposition in terrestrial ecosystems. Atmospheric N deposition affects soil organic C (SOC) accumulation and depletion by directly changing microbial activity and/or indirectly changing substrate quality, and thereby changing the soil organic matter (SOM) decomposition. Previous research primarily focuses on soil C transformation processes and storage dynamics; however, limited information is available on the interaction among plants, microorganisms, and SOM, especially the biophysical and biochemical mechanisms involved in regulating plant-microorganism-SOM interactions with soil C sequestration. In this review, we summarize the effects of elevated N deposition on plant belowground C distribution, SOC priming effect, and microbial C metabolism, and analyzed the relationship between SOM chemical stability and microbial community dynamics. We identified a number of research topics which are in urgent needs of mechanistic investigation in the following decades:first, increased N input tends to reduce root growth and turnover, but the effects on C allocation in rhizosphere and associated mechanisms are unclear; second, although N availability can affect the direction and magnitude of the SOM priming effect, the contrasting effects of oxidized NO₃^- and reduced NH₄⁺ and the potential mechanisms on SOM priming effect are far from certain; third, microbial C use efficiency (CUE) is a crucial characterization of C metabolism of microbial communities, the bottleneck process for soil carbon emission. It is challenging to accurately quantify the microbial CUE and microbial turnover time owing to a lack of appropriate measurement methods; fourth, increased N input inhibits the activities of soil fungal communities and their extracellular enzymes, but the effects on the activity and composition of the soil bacterial community are inconsistent; moreover, the association between SOM chemical quality and soil microbial activity and composition is elusive. Therefore, we call for a long-term N control experiment platform to fully investigate the above-mentioned topics in a systems perspective. The most advanced techniques, such as stable C and oxygen isotopic tracer, organic matter chemistry, molecular biology, and macro genomics, will be used to analyze the belowground allocation of the plant-assimilated C, microbial C metabolism and turnover, and coupling between the SOM chemical structure and microbial functional groups. This long-term experiment could help understand the mechanism of plant-soil-microbial interaction and its contribution to SOC dynamics, improve the soil carbon models, and reduce the uncertainty of regional C sink assessment, and further lay a cornerstone for scientific managing terrestrial ecosystem in a changing world.