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.
KeywordsNitrogen fertilization Water pulses Fungal biomass Bacterial biomass Desert ecosystems Soil respiration
We thank Paul Zietz for assistance with sample processing and analysis. We appreciate methodological assistance provided by Jill Mikucki, James Scott, Brad Taylor, and Kathy Welch. Additional laboratory support was provided by Clara Chew. We thank three anonymous reviewers for their thoughtful insight which led to significant improvements to this manuscript. This research was supported by National Science Foundation Office of Polar Program grants to the McMurdo Long-Term Ecological Research Program ANT-0423595.
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