Abstract: The variation of land use pattern significantly changes the nitrogen cycling in grassland,and further affects the function of grassland ecosystem. Soil nitrification is an important part of soil nitrogen cycling. Ammonia oxidation process driven by ammonia oxidizing microorganisms is a speed-limited step of nitrification,and plays a key role in the soil nitrogen availability. At present,the response of soil ammonia-oxidizing microorganisms to the exclosure was rarely reported in leymus chinensis grassland in Inner Mongolia. In this study,we investigated the abundance of ammonia oxidizing microorganisms with different land use patterns (graze and exclosure) using real-time PCR approaches. Furthermore,we analyzed the relationship between the abundance of ammonia oxidizing microorganisms and potential nitrification rate in leymus chinensis grassland in Inner Mongolia. The results showed that the abundance of Ammonia-Oxidizing Archaea(AOA)was significantly higher than Ammonia-Oxidizing Bacteria(AOB)in both graze and exclosure treatments. AOA-amoA and AOB-amoA gene abundances were around (1.82~3.40)×10⁸ copies per gram dry soil and (1.05~1.17)×10⁶ copies per gram dry soil,respectively. Compared with graze treatment,the abundance of AOA decreased significantly,up to 46.5%,but the abundance of AOB did not change significantly after long-term exclosure. After long-term exclosure,the soil potential nitrification rate decreased significantly in leymus chinensis steppe. The potential nitrification rate was positively correlated with the abundance of AOA,however,it was not significantly correlated with the abundance of AOB. The abundance of AOA was more sensitive to land use pattern changes,which might be the dominant population affecting soil potential nitrification rate. This study provided a scientific basis for understanding the microbial mechanism of nitrification process in the leymus chinensis steppe in Inner Mongolia.

Key words: Leymus chinensis steppe, Graze, Exclosure, Ammonia oxidizing microorganisms, Abundance, Potential nitrification rate

摘要： 草地利用方式变化会显著改变草地的氮素循环，进而影响草原生态系统功能。土壤硝化作用是土壤氮素循环的重要环节，由氨氧化微生物驱动的氨氧化过程是硝化作用的限速步骤，在土壤氮素可利用性等方面起着关键作用。目前，土壤氨氧化微生物对于羊草（Leymus chinensis）草原利用方式变化响应的研究仍然欠缺。本研究应用定量PCR的方法，对内蒙古羊草草原不同利用方式（放牧和围封）下氨氧化微生物丰度及其与硝化潜势的关系进行研究。结果表明：放牧和围封样地的氨氧化古菌（Ammonia-Oxidizing Archaea，AOA）丰度均明显高于氨氧化细菌（Ammonia-Oxidizing Bacteria，AOB）丰度，土壤AOA-amoA和AOB-amoA基因丰度范围分别为每克干土（1.82~3.40）×10⁸个拷贝数和（1.05~1.17）×10⁶个拷贝数；长期围封后，AOA丰度显著下降，下降了46.5%，AOB丰度则无显著变化；长期围封后，羊草草原的硝化潜势显著降低，硝化潜势与AOA丰度呈显著正相关关系，与AOB丰度没有显著性相关。与AOB相比，AOA丰度对于草地利用方式的改变更敏感，可能是影响土壤硝化潜势的优势种群。该研究为理解内蒙古羊草草原硝化过程的微生物机制提供科学依据。

关键词: 羊草草原, 放牧, 围封, 氨氧化微生物, 丰度, 硝化潜势