Effect of drought on ammonia and nitrite oxidisers

Nitrification, the oxidation of ammonia via nitrite to nitrate, contributes to nitrogen (N) losses in agricultural soils. When nitrification is a two-step process, it depends on the successful metabolic interaction between ammonia oxidising archaea (AOA) and bacteria (AOB), and nitrite oxidising bacteria primarily within Nitrobacter (NIB) and Nitrospira (NIS). Implications for the communities of these microbial guilds, their association, and the fate of N in soils due to an increasing frequency and severity of dry spells caused by climate change remain unclear. Here we subject four distinct soils to either one long or two shorter drying-rewetting cycles in a microcosm experiment to evaluate the hypothesis that drying-rewetting stress triggers distinct responses in the functional guilds due to differences in environmental preferences and adaptation strategies. Resulting shifts in community compositions and co-associations could ultimately change nitrification activity. AOA were most sensitive to drying among the four guilds and decreased in relative abundance in line with reduced ammonia oxidation rates, while AOB were highly resistant. We observed almost full recovery of AOA one week after rewetting. NIS but not NIB, however, were strongly affected by rewetting with no recovery during the experiment, showing shifts in community composition and relative abundance. Network analysis revealed that drying-rewetting affected co-occurrences between ammonia and nitrite oxidisers in a soil-dependant manner, possibly indicating a destabilisation of their metabolic interaction. Overall, this study emphasises the importance to consider weather extremes like drought on soil nitrifier community dynamics and the fate of N in soils.