Response of soil microbial communities and nitrogen thresholds of Bothriochloa ischaemum to short-term nitrogen addition on the Loess Plateau

China is becoming the world's third largest area of nitrogen (N) deposition, which is attracting increasing attention. Understanding how N affects soil microbial communities and determining the thresholds for the effect of N on microorganisms and ecosystems are critical. We investigated the changes to the characteristics of microbial communities after two years of adding N to soil with the perennial grass Bothriochloa ischaemwn, simulating N deposition on the Loess Plateau, at four rates of N addition (0 (CK), 2.5 (N1), 5.0 (N2), and 10.0 (N3) g N m(-2)y(-1)) and a control BL (bare land without vegetation or N addition). Soil microbial biomass carbon (C) and N contents and microbial activity increased in N1. The lowest rate of N addition (Ni) increased soil total, bacterial, fungal, and actinomycetic phospholipid fatty acid (PLFA) contents, but excessive N addition decreased bacterial and actinomycetic PLFA contents. N addition did not alter microbial-community structure. The effect of N addition on soil microbial properties was influenced by soil C content (SOC and DOC), increased the diversity and evenness of the microbial community and decreased the diversity of the bacterial community. Soil microbial biomass and activity increased in N1, ivhich was beneficial to the stability of the soil ecosystem on the plateau and defined the threshold of N addition for microorganisms and the ecosystem. More attention should thus be paid to depositional level represented by N2 (5 g N m(-2) y(-1)), which might limit microbial communities. The microbial community was inhibited, the diversity decreased, and the ecological system was affected by the level represented over N2.