西南喀斯特植被正向演替过程中土壤氮素转化特征

The knowledge of soil N biogeochemical cycling along vegetation succession is crucial for understanding plant adaptation, optimization of plant communities and plant community regulation. So far, little is known in terms of the dynamics of soil N biogeochemical cycling processes along vegetation succession in the Karst region, southwest China. The soil N cycling processes in the karst region may be different from those in other regions due to their unique soil physicochemical conditions, including hydrological cycling, high Ca levels. In order to improve our understanding of soil N cycling in karst region, we propose to investigate soil cycling dynamics along a vegetation succession gradient. What we propose to study include: 1) gross N transformation rates measured by 15N isotope dilution technique; 2) Nitrate leaching and N2O emissions; 3) net N transformation rates which are the balance of gross N production and consumption; 4) soil microbial indices and physicochemical properties which will be used to determine the regulating factors on N cycling dynamics. The major objectives are to 1) quantify nitrate leaching and N2O emissions along vegetation succession, 2) determine the rates of N cycling processes and relate them to nitrate leaching and N2O emissions, 3) find the main regulating factors on soil N cycling dynamics along vegetation succession. The results are helpful to reveal the mechanisms of soil N retention and accumulation along the vegetation succession. In the meanwhile, this project is helpful to develop strategies to reduce N loss and to improve soil fertility in this region. This project can also provide some supporting information for the ongoing ecoengineering practices (e.g., grain for green and sloppy cropland regulation) in the karst region.

植被演替过程中物质的生物地球化学循环规律是研究植物适生性、物种优化配置和适应性生态系统调控机理的关键基础。至今对喀斯特植被演替进程中土壤氮素生物地球化学循环过程的认识非常有限。喀斯特区独特的土壤条件决定了其氮循环过程可能有别于其它地区。本项目针对当前研究的不足，对植被演替不同阶段土壤氮总转化过程进行研究，并将氮转化速率与土壤硝酸盐淋失与N2O排放相联系，比较氮总转化速率和氮净转化速率与土壤氮动态的关系，同时通过对土壤微生物与理化指标的测定揭示影响氮转化的因素。总目标是阐明喀斯特区植被演替过程中土壤氮素生物地球化学循环过程变化特征，在一定程度上揭示土壤氮素累积与流失的机理。结果有助于增强我们对区域氮循环的调控与管理能力，进而达到固碳、减少氮流失、保持土壤肥力的目的；同时也能为“退耕还林还草”和“坡耕地整治”等重大生态工程的深入开展提供科技支撑。

氮是植物生长发育的必需营养元素之一，也是陆地生态系统最普遍且最重要的一种限制性养分元素，其循环在很大程度上影响生态系统的结构、功能、演替进程以及生态系统对气候环境变化的响应。目前对喀斯特植被演替进程中土壤氮素转化特征的认识非常有限。项目采用空间代替时间的研究方法，研究了不同植被演替阶段土壤理化性质变化、微生物群落结构、土壤初级氮转化速率、土壤氮流失特征等，阐明植被演替过程中土壤氮转化的主要控制因子与机制。结果表明：西南喀斯特区随着植被的正向演替，土壤有机碳和全氮快速累积，0-15cm土层总氮的累积速率为12.4 g N m-2 yr-1，在退耕后约67年达到原生林水平，同时发现土壤交换性钙是喀斯特土壤有机质稳定的最主要因素，对土壤氮快速累积具有重要意义；土壤微生物生物量显著增加，细菌、真菌、放线菌群落生物量相应增加，但真菌:细菌比值并未发生显著变化；土壤无机氮生成速率（即氮总矿化速率和总硝化速率）增加，但固持速率（硝态氮异化还原为铵及微生物氮固持速率）不变或下降，同时发现土壤蛋白酶活性及脲酶活性均与氮总矿化速率或总硝化速率显著相关，表明土壤蛋白质与尿素水解过程是喀斯特土壤氮转化的限制性环节；不同氮素有效性指标包括硝氮含量、净矿化速率、净硝化速率、硝铵比、DIN：DON等比值都表明随着喀斯特植被演替，土壤氮有效性逐步提高，氮循环愈加开放；土壤硝化作用强度与土壤硝态氮含量及硝态氮淋失都有显著的正相关关系，表明土壤内部氮转化过程深刻影响生态系统氮流失。