Contingent Effects of Liming on N2O-Emissions Driven by Autotrophic Nitrification

Liming acidic soils is often found to reduce their N 2 O emission due to lowered N 2 O/(N 2 O + N 2 ) product ratio of denitrification. Some field experiments have shown the opposite effect, however, and the reason for this could be that liming stimulates nitrification-driven N 2 O production by enhancing nitrification rates, and by favoring ammonia oxidizing bacteria (AOB) over ammonia oxidizing archaea (AOA). AOB produce more N 2 O than AOA, and high nitrification rates induce transient/local hypoxia, thereby stimulating heterotrophic denitrification. To study these phenomena, we investigated nitrification and denitrification kinetics and the abundance of AOB and AOA in soils sampled from a field experiment 2–3 years after liming. The field trial compared traditional liming (carbonates) with powdered siliceous rocks. As expected, the N 2 O/(N 2 O + N 2 ) product ratio of heterotrophic denitrification declined with increasing pH, and the potential nitrification rate and its N 2 O yield ( Y N2O : N 2 O-N/NO 3 – -N), as measured in fully oxic soil slurries, increased with pH, and both correlated strongly with the AOB/AOA gene abundance ratio. Soil microcosm experiments were monitored for nitrification, its O 2 -consumption and N 2 O emissions, as induced by ammonium fertilization. Here we observed a conspicuous dependency on water filled pore space (WFPS): at 60 and 70% WFPS, Y N2O was 0.03-0.06% and 0.06–0.15%, respectively, increasing with increasing pH, as in the aerobic soil slurries. At 85% WFPS, however, Y N2O was more than two orders of magnitude higher, and decreased with increasing pH. A plausible interpretation is that O 2 consumption by fertilizer-induced nitrification cause hypoxia in wet soils, hence induce heterotrophic nitrification, whose Y N2O decline with increasing pH. We conclude that while low emissions from nitrification in well-drained soils may be enhanced by liming, the spikes of high N 2 O emission induced by ammonium fertilization at high soil moisture may be reduced by liming, because the heterotrophic N 2 O reduction is enhanced by high pH.