Microbial biomass and respiration responses to nitrogen fertilization in a polar desert

How microbial communities respond to increases in available nitrogen (N) will influence carbon (C) and nutrient cycles. Most studies addressing N fertilization focus on mid-latitude ecosystems, where complex aboveground-belowground interactions can obscure the response of the soil microbial community, and little is known about how soil microbial communities of polar systems, particularly polar deserts, will respond. The low C content and comparatively simpler (low biomass and biodiversity) soil communities of the McMurdo Dry Valleys of Antarctica may allow easier identification of the mechanisms by which N fertilization influences microbial communities. Therefore, we conducted a microcosm incubation using three levels of N fertilization, added in solution to simulate a pulse of increased soil moisture, and measured microbial biomass and respiration over the course of 4.5 months. Soil characteristics, including soil pH, conductivity, cation content, chlorophyll a, and organic C content were measured. Soils from two sites that differed in stoichiometry were used to examine how in situ C:N:P influenced the N-addition response. We hypothesized that negative influences of N enrichment would result from increased salinity and ion content, while positive influences would result from enhanced C availability and turnover. We observed that microbes were moderately influenced by N addition, including stimulation and inhibition with increasing levels of N. Mechanisms identified include direct inhibition due to N toxicity and stimulation due to release from N, rather than C, limitation. Our results suggest that, by influencing microbial biomass and activity, N fertilization will influence C cycling in soils with very low C content.