基于15N示踪研究西南喀斯特主要恢复生态系统氮素转化与存留过程

Nitrogen deficiency retards positive succession of the karst restoration ecosystem, and induces more succession to stay on the grass and shrub, which seriously weakens the resistance and stability of the karst restoration zone, and the community may reverse succession and aggravate rocky desertification. However, the current study mainly focused on measure the nitrogen availability of different karst restoration ecosystems, and little is known in terms of the dynamics of nitrogen transformation, turnover and retention in the karst ecosystem, southwest China. In the proposed project, a field 15N tracer experiment will be performed to measure the ecosystem nitrogen turnover and retention by tracing the 15N allocation, including plant uptake, microbial immobilization and soil adsorption which are beneficial to nitrogen retention, and leach and gas release which accelerate nitrogen loss, in the rainy and dry season respectively in the grass and shrub ecosystems in the karst region. Additionally, laboratory 15N isotope dilution method combined with nitrogen cycling process model are used to measure the gross nitrogen transformation rates, including nitrogen mineralization, autotrophic and heterotrophic nitrification, microbial ammonium and nitrate immobilization and dissimilatory nitrate to ammonium. The results are expected to reveal the nitrogen limited key processes of the karst restoration ecosystem, and can provide scientific basis for the karst ecosystem recovery and improve the stability of karst restoration zone.

氮素限制减缓喀斯特恢复生态系统正向演替进程，致使更多演替停留在草、灌丛阶段，严重影响恢复区抗干扰能力和系统稳定性，群落极易发生逆向演替，加剧石漠化。然而现有研究主要集中在恢复生态系统氮素含量的表象对比研究，对造成氮素限制的内在过程的研究非常缺乏。为此，本项目以我国西南喀斯特主要恢复生态系统草丛和灌丛为研究对象，分别在旱季和雨季运用野外15N（15NH4+和15NO3-）示踪技术，阐明不同氮素15N去向、周转和存留（土壤、微生物、植物、淋溶液、气体）；结合室内15N同位素稀释法和氮循环模型估算土壤氮素转化速率（氮矿化、自养和异养硝化、微生物铵态氮和硝态氮固持、硝酸盐异化还原成铵）；综合分析旱雨季野外和室内研究结果，明确喀斯特恢复生态系统氮素限制的关键过程。研究结果将有助于我们通过增强土壤利用与管理来调控氮素的生物地球化学循环，为喀斯特生态系统恢复和提高恢复区系统稳定性提供理论依据和科技支撑。

喀斯特石漠化过程中61%-85%的土壤氮素发生流失，导致西南喀斯特恢复生态系统普遍处于“氮限制”状态。因此，明晰喀斯特演替初期生态系统氮素输入、存留和去向等过程，有助于我们从氮素循环的角度有针对性的进行石漠化生态恢复及其维持。研究结果表明（1）降雨氮沉降是喀斯特石漠化演替初期阶段一项重要的氮素补给，生态系统净氮获得达到7.73 kg N/ha/y，87.6%为无机氮。（2）喀斯特演替初期阶段氮素循环闭合，微生物、土壤固持和植物吸收能够存留83%的氮素，气体释放和淋溶损失<0.5%，地下漏失15%，且主要发生在雨季。（3）喀斯特演替初期阶段生态系统处于氮素限制状态，特别是雨季植物生长高峰期氮素限制更强烈，其关键限制过程是土壤供氮潜力低，以及供氮形式不匹配。总体而言，氮素缺乏和形式不匹配在一定程度上导致喀斯特石漠化次生演替缓慢，但闭合的氮素循环以及降雨氮素的输入有助于演替的进行。