Responses of microbial communities to atmospheric nitrogen deposition within different soil horizons in High Arctic tundra

Arctic environments are subject to acute nitrogen deposition events, in which 40% or more of annual atmospheric N input can be deposited as acidic rainfall in less than one week. The overall aim of this research was to investigate the impact of acute N deposition events upon soil microbial communities in High Arctic tundra. A plot scale field experiment, established on the High Arctic tundra (Ny-Ålesund, Svalbard), and a microcosm experiment, were used to simulate acute N deposition over the summer by the application of NH4NO3 solution at ~pH 4, at rates of 0.4, 4 and 12 kg N ha-1 yr-1. Changes in soil characteristics were measured on soil samples from the organic and mineral horizons. Variation in the structure and abundance of bacterial, archaeal, and fungal communities and in the presence and abundance of N-cycling functional guilds were investigated using molecular (DNA)-based approaches such as Terminal Restriction Fragment Polymorphism (T-RFLP) and quantitative-PCR.
T-RFLP analysis revealed significant (P < 0.001) differences in the structure of bacterial and archaeal communities between the organic and mineral horizons within field plots, whilst the fungal community structure showed high variability between soil horizons. Both bacterial and fungal abundance were higher within the mineral horizon in 2009 at the end of the summer, but similar between soil horizons in 2010 earlier in the summer. In contrast, archaeal abundance was always higher in the mineral than in the organic horizon, despite sharp decreases in C, N and water content from organic to mineral horizons. Significant changes (P < 0.05) in both bacterial and fungal community structure in the 12 kg N ha-1 yr-1 plots were found in the mineral horizon only after one and seven days post treatment, respectively, with no subsequent changes thereafter. Bacterial community structure within the mineral horizon also varied with lower N addition rate (4 kg N ha-1 yr-1) one day post N application. The microcosm experiment showed that addition of water had a stronger effect on bacterial and archaeal community structure and bacterial abundance within both soil horizons than N addition. Overall, this study showed that microbial community in High Arctic tundra can be rapidly affected by acute N deposition event.