降雨格局变化对内蒙古草地生态系统土壤微生物与氮转化的影响

The frequency, magnitude and intensity of precipitation events are known to influence dry-land ecosystems. Increased intensity in precipitation events and longer periods of water deficit are predicted as a general trend under future climate scenarios with potentially large effects on terrestrial ecosystem function. Nitrogen regulates the Earth's climate system by constraining the terrestrial primary production. Alterations in global and regional precipitation patterns are expected to affect soil microbial activity and nitrogen ecosystem productivity especially in water-limited ecosystems. Plant productivity in deserts, may be more directly responsive to soil water availability than to precipitation. However, measurement of soil moisture alone may not be enough to elucidate plant responses to precipitation pulses, as edaphic factors may influence productivity when soil moisture is adequate. . There is an increasing demand for the sustainable management of arid and semi-arid grassland in northern China, which have developed on degraded grassland during past 20 years, to regain forage yield, biodiversity, and soil fertility. The first objective of the study was to determine the responses of the aboveground annual net primary productivity (ANPP) in a semi arid grassland to variation in increase and decrease in seasonal precipitation (supplemental and decrease watering in spring and summer). Secondly, nitrogen transformations response to other key environmental and soil parameters were explored during dry, average, and wet years. Thirdly, microbial predictors for precipitation intensity and frequency were dynamic. High ANPP was positively associated with higher soil NH4-N and frequent larger precipitation events, while that was positively correlated with frequent small summer precipitation events with short inter-pulse periods.

全球气候变化大背景下的降雨格局改变已经成为不争的事实，对陆地生态系统的结构和功能正产生或已经产生巨大的影响。IPCC第五次报告预测降雨的不确定性在本世纪将继续增大，极端降雨事件尤其是极端干旱将增多。目前我们对生态系统过程响应气候变化的理解还很有限，缺乏降雨格局改变对草地生态系统生产力重要影响因子——土壤微生物氮转化过程的研究。本研究以内蒙古草地生态系统为研究对象，通过降雨量控制实验(9个梯度)、降雨梯度样带原位调查及室内培养实验，研究内蒙古半干旱草地土壤微生物参与的氮循环关键过程对降雨格局改变的响应特征，揭示降雨格局变化调控土壤微生物氮转化与草地生产力的规律，进一步探讨土壤微生物和氮转化过程对降雨梯度的响应以及影响草地生产力的机制。本研究可为半干旱草原生态系统初级生产力预测和氮循环模型提供参数，进一步加强对全球变化下半干旱草地生态系统初级生产力和氮循环的认识，为草地管理提供必要的数据支持。

全球气候变化背景下的降水格局变化已经成为不争的事实，而氮元素是草地生态系统的主要限制元素之一，因此，在全球气候变化背景下，研究微生物氮循环过程对降水变化的响应及其机制变得十分必要且十分紧迫。本研究选取内蒙古西乌珠穆沁旗典型草原降水控制实验平台，及降水梯度的样带原位调查，采用15N库稀释法测定了不同降水条件下的土壤总氮转化速率，并采用定量PCR的方法测定了氮转化相关微生物的功能基因的丰度。同时，我们采用传统的氯仿熏蒸法测定土壤微生物生物量，并运用高通量测序、磷酸脂肪酸（phospholipid fatty acid，PLFA）的方法测定土壤微生物多样性和群落结构。通过比较不同降水条件下微生物群落、氮转化速率及氮转化相关基因丰度，进一步发掘了微生物氮转化过程对降水变化的响应机制。（1）降水控制实验的主要研究结果如下：a). 土壤总氮矿化速率在4-9月份波动较小，月份之间差异不显著，而10月份的总氮转化速率较其它月份显著降低。降水变化对土壤总氮矿化速率影响显著，随着降水量的增加而增加；b). 降水变化对土壤微生物生物量碳氮及碳氮比的影响不显著；c). 降水增加60%和降水减少60%均增加了土壤微生物多样性，其中Chao指数的变化最为明显，说明降水变化对稀有种影响较大。但降水变化并未显著影响微生物群落结构；d). 降水变化对半干旱草地土壤微生物组分在2015年至2017年有显著影响，同时这种影响并非线性，表现为降水增加60%和降低60%均增加各组分的量。（2）降水梯度的野外样带的主要研究结果如下：a). 总氮转化速率与降水的从东到西的降低趋势不一致，表现为典型草原高于草甸草原和荒漠草原；b). 典型草原的amoA AOA丰度约为草甸草原和荒漠草原的4倍和2.7倍，而amoA AOB丰度在内蒙古三种不同草地类型中无显著差异。