开垦对温性草甸草原土壤氮转化与氮平衡的影响及微生物学机制

As the most profound land use, reclamation can obviously change soil nitrogen cycling and nitrogen budget in grassland ecosystems, which will affect ecosystem productivity and stability. However, the responses of soil nitrogen transformation and retention to various reclamation time in the temperate meadow steppes as well as microbial mechanisms involved in it are not well understood. This project will be conducted at Hulunber grassland ecosystem research and observation station (HGEROS). Natural grassland, artificial grassland (reclamation for 3 years), and cropland (reclamation for 30 years) are selected to build up a reclamation gradient experiment. Soil nitrogen pools, soil nitrogen loss fluxes, and soil gross nitrogen transformation rates will be measured using field observation and 15N tracing experiment and tracing model, respectively. Also, the abundances and community composition of functional microbes mediating soil nitrification and denitrification (amoA, narG, nirK, nirS, and nosZ) will be determined using molecular biological techniques (qPCR, cloning and sequencing), which will favor to clarify the effects of reclamation on soil nitrogen transformation and nitrogen budget as well as microbial mechanisms responsible for it. Our researches will be helpful to evaluate the contribution of long-term reclamation to soil nitrogen budget in HulunBuir grassland, to improve the nitrogen cycling model in grassland ecosystems, and to provide theoretical basis for the restoration and management of grassland ecosystems.

开垦是影响草地生态系统最大的扰动方式之一，显著改变土壤的氮循环和氮平衡，进而影响生态系统生产力与稳定性。然而，目前有关温性草甸草原土壤氮转化和固持过程对开垦的响应及其微生物学机理尚不清楚。本项目拟以呼伦贝尔羊草草地、新开垦3年的栽培草地、开垦30年的农田为研究对象，构建不同开垦年限的梯度实验。利用原位监测、15N示踪实验结合15N示踪模型研究开垦对土壤氮储量、氮通量和转化过程的影响；利用分子生物学技术（qPCR、克隆测序）研究开垦对硝化-反硝化微生物功能基因（amoA、narG、nirK、nirS,nosZ）和群落组成的影响，揭示开垦对土壤氮转化和氮平衡影响及其微生物学机制。研究结果有助于合理评估长期开垦对呼伦贝尔温性草甸草原土壤氮平衡的贡献，完善草地生态系统氮循环模型，并可为退化草地生态系统的恢复与管理提供理论依据。

开垦是影响草地生态系统最大的扰动方式之一，显著改变土壤的氮循环和氮平衡，进而影响生态系统生产力与稳定性。然而，目前有关温性草甸草原土壤氮转化和固持过程对开垦的响应及其微生物学机理尚不清楚。因此，本项目目的在于揭示开垦对土壤氮转化和氮平衡影响及其微生物学机制。研究结果表明：（1）人工草地开垦后，施氮显著提高了土壤有效氮素（NO3--N、NH4+-N）含量，并且牧草种植、水氮添加之间具有明显的协同作用，种植无芒雀麦土壤NO3--N、NH4+-N累积最明显；（2）施氮会促进土壤活性碳组分（例如POC）积累、降低惰性有机碳组分（例如MAOC），从而可能降低土壤碳库的稳定性。水氮添加显著影响土壤酶活性，并且土壤微生物对碳、氮（或磷）养分需求的差异是调控活性有机碳组分周转的重要驱动力。苜蓿-雀麦混播在旱季补水条件下有助于缓解施氮对SOC稳定性的负面效应，对维持呼伦贝尔开垦后的人工草地土壤碳固持及稳定性有重要意义；（3）施氮显著促进开垦后的人工草地土壤N2O排放，而旱季补水会显著促进土壤CO2和N2O排放，并促进CH4吸收。土壤N2O通量的响应比与土壤NO3-N的响应比正相关性，说明N2O排放增加主要是土壤硝化过程所致。土壤N2O通量与土壤氮转化相关功能基因丰度有显著相关性，说明氮转化相关功能微生物是调控N2O通量的关键因素。其中土壤NO3--N含量和N2O通量的变化均与AOB amoA变化正相关，说明温性人工草地开垦后，土壤N2O排放主要受AOB amoA介导。另外，土壤氧化酶的活性增加与土壤N2O通量的增加有显著正相关性，说明真菌介导的异养硝化过程也可能在开垦的人工草地土壤氮循环中起着重要的作用。