Increased N availability in grassland soils modifies their microbial communities and decreases the abundance of arbuscular mycorrhizal fungi

Two complementary studies were performed to examine (1) the effect of 18 years of nitrogen (N) fertilization, and (2) the effects of N fertilization during one growing season on soil microbial community composition and soil resource availability in a grassland ecosystem. N was added at three different rates: 0, 5.44, and 27.2 g N m^-2 y^-1. In both studies, Schizachyrium scoparium was the dominant plant species before N treatments were applied. Soil microbial communities from each experiment were characterized using fatty acid methyl ester (FAME) analysis. Discriminant analysis of the FAMEs separated the three N fertilizer treatments in both experiments, indicating shifts in the composition of the microbial communities. In general, plots that received N fertilizer at low or high application rates for 18 years showed increased proportions of bacterial FAMEs and decreased fungal FAMEs. In particular, control plots contained a significantly higher proportion of fungal FAMEs C18:1(cis9) and C18:2(cis9,12) and of the arbuscular mycorrhizal fungal (AMF) FAME, C16:1(cis11), than both of the N addition treatment plots. A significant negative effect of N fertilization on the AMF FAME, C16:1(cis11), was measured in the short-term experiment. Our results indicate that high rates of anthropogenic N deposition can lead to significant changes in the composition of soil microbial communities over short periods and can even disrupt the relationship between AMF and plants.