[Effects of grazing exclusion on the abundance of functional genes involved in soil nitrogen cycling and nitrogen storage in semiarid grassland].

We investigated the effects of grazing exclusion on the abundance of functional genes (nifH, amoA-AOA, amoA-AOB, narG, nirK, nirS, and nosZ) involved in soil nitrogen cycling in soil profiles (0-10, 10-20, 20-40 and 40-60 cm) from a chronosequence of grazing exclusion (0, 7, 18, 27 and 35 years) in the semiarid grasslands of the Loess Plateau. The relationship between abundance of functional genes and soil nitrogen storage was evaluated. The results showed that 35 years exclusion increased the abundance of nifH and amoA-AOB genes by 67.8% and 17.6% compared with the grazed grassland, respectively, and decreased that of nirK genes. The abundance of nifH, narG, and nirS genes in surface soil (0-10 cm) were significantly higher than that in deep soil (20-40 and 40-60 cm), indicating that those genes had surface accumulation effects. Grazing exclusion increased soil nitrogen storage. Soil nitrogen storage in 0-60 cm layer was the highest at 27 years (20.96 mg·hm-2), indicating that 27 years might be the optimum for grazing exclusion. The abundance of nifH, amoA-AOA and amoA-AOB had a significant linear relationship with nitrogen storage, suggesting that microbes harboring these genes played an important role in soil nitrogen accumulation. Total nitrogen, bulk density, and available phosphorus content were the dominant factors affecting the abundance of functional genes involved in soil nitrogen cycling. Our results provided a scientific reference for understanding soil nitrogen cycling and restoration of degraded grassland.以黄土高原云雾山不同禁牧(0、7、18、27和35年)草地为对象,研究禁牧对不同土层(0～10、10～20、20～40和40～60 cm)土壤氮循环微生物功能基因(nifH、amoA-AOA、amoA-AOB、narG、nirK、nirS和nosZ)丰度的影响,并分析了土壤氮循环微生物功能基因与土壤氮储量之间的关系.结果表明: 与放牧相比,禁牧35年提高了nifH和amoA-AOB的基因丰度,分别增加了67.8%和17.6%;禁牧降低了nirK基因丰度.表层土壤(0～10 cm)nifH、narG和nirS基因丰度显著高于深层土壤(20～40和40～60 cm),表明nifH、narG和nirS基因具有表聚效应.禁牧提高了土壤氮储量,在27年时0～60 cm土层土壤氮储量最高(20.96 mg·hm-2),说明27年可能为最适禁牧年限.nifH、amoA-AOA和amoA-AOB基因丰度与氮储量具有显著线性关系,表明含有这些基因的微生物对于提高土壤氮储量具有重要作用.土壤全氮、容重以及速效磷含量是影响土壤氮循环基因的主要因素.本研究结果为深入理解土壤氮循环过程及退化草地的恢复提供了科学参考.