CO2浓度升高对我国北方冬小麦田土壤微生物资源限制和代谢活动的影响机制

Shifts in microbial resource limitation related to elevated CO2 (eCO2) can exert strong influence on microbial metabolism, which consequently lead to uncertainties in predicting soil carbon sink under the scenario of global change. Recently, stoichiometry of extracellular enzymes involved in microbial carbon (C), nitrogen (N), and phosphorus (P) acquisition has been suggested as a useful indicator of microbial resource limitation in the changing environment. Although researchers have paid close attention to the feedbacks of several extracellular enzyme activities under global change, most studies generally failed to reveal their stoichiometric response to eCO2. Considering that farmland ecosystem has a great potential in soil C sequestration, it is crucial to understand how enzymatic stoichiometry determines resource limitation and metabolism of microorganisms in response to eCO2. This research focuses on the winter wheat field of Northern China. Based on the free air CO2 enrichment (FACE) site located in Changping, Beijing, comprehensive experiments will be conducted to investigate the spatial-temporal patterns and driving forces in regarding to microbial energy (e.g. C) and nutrient (e.g. N and P) limitations from the novel insight into enzymatic stoichiometry. Furthermore, microbial C use efficiency and soil organic C decomposition will be simulated by enzyme-catalyzed processes and microbial allocation model, then used to predict whether soils of winter wheat field will serve as C sink under eCO2. The object of this research is to provide scientific evaluation of increasing soil C sink as an important measure to cope with global change.

CO2浓度上升会改变微生物的资源限制，强烈影响其代谢活动，导致全球变化对土壤的碳汇作用具有不确定性。最新研究指出，参与微生物碳、氮、磷获取的胞外酶的化学计量关系可以有效指示环境变化对微生物资源限制的影响；但目前关注CO2升高的研究往往只探讨了其中几种胞外酶，没有系统考察对酶化学计量的影响。鉴于农田土壤碳库增汇的潜力十分可观，理解CO2升高作用下酶化学计量如何决定微生物资源限制及其代谢活动至关重要。本项目聚焦我国北方冬麦区，通过位于北京昌平的FACE系统模拟CO2升高，以酶化学计量的新颖视角揭示不同时（生育期）空（土层）格局下土壤微生物能量（碳）和养分（氮、磷）限制的变化规律，并探索内在的驱动机制；利用酶分解模型模拟微生物碳利用效率和土壤有机碳分解，并结合对地上碳同化的观测，评估CO2升高对麦田土壤碳汇形成的潜在影响。项目旨在为我国制定麦田土壤碳库增汇措施及应对全球变化提供科学依据。

本项目结合全球变化、土壤碳氮循环和农田生态系统领域的前沿问题，选择北方冬麦区为研究区域，依托自由式二氧化碳富集（FACE）平台，在田间原位条件下模拟未来CO2浓度升高情景，以微生物碳氮磷获取酶为关键研究对象，开展CO2浓度升高对华北冬小麦田土壤微生物资源限制和代谢活动的影响机制研究。研究结果表明，CO2浓度升高提高了收获时小麦叶片中固持的碳氮量，明显增加了作物回收物料中的养分含量，减少了叶片向籽粒的氮素转运，表明除生长稀释效应外，CO2浓度升高对叶片转运能力的影响也是造成籽粒氮素降低的可能原因。由于土壤中的活性碳库明显增加，可溶性养分化学计量比在CO2浓度升高处理下显著上升，导致土壤养分供应更趋向于氮限制。微生物采用降低自身碳利用效率的策略来缓解这种氮限制，进而引起碳限制，最终表现为CO2浓度升高将微生物面临的资源限制从氮限制为主转变为碳氮共限制。长期CO2浓度升高还引起冬小麦田有机质分解的正激发效应，同时参与C获取的胞外酶活性也相应增强，为CO2浓度升高引起的正激发效应提供了解释机制，表明地上光合作用固定的碳可能通过微生物代谢活动被返还，形成对CO2浓度升高的正反馈。对胞内酶活性的进一步量化分析表明，这种正激发效应不仅可以来自微生物直接增加酶生产，还可以通过调节酶在细胞内外的分配实现。驱动胞外和胞内酶活性的关键环境因素在不同CO2浓度水平下存在差异，其中大气CO2浓度水平下，胞外酶活性随土壤pH升高而降低，胞内酶活性随土壤含水量升高而升高，而在高CO2浓度水平下，仅观察到胞内氮获取酶受土壤全氮影响。CO2浓度升高削弱了环境因子对微生物酶活性的影响，可能增强未来气候变化条件下对微生物功能预测的不确定性。