A novel coupled cycling of abiotic denitrification and N2O-reduction is frequent across tropical peatlands

Abstract:

Fluxes of N₂O are large in tropical regions although high variation and regional uncertainties persist due to limited available measurements and a lack of mechanistic understanding of N₂O origin and fate particularly in intact ecosystems. New extensive regions of peatlands have been identified in the tropics. Denitrification is a widespread process achieved by soil microbial communities, however chemo (abiotic) denitrification has been shown in anoxic peat soils but not broadly assessed. Here, we report that, in anoxic peat soils of Central and South America (Costa Rica, Brazil and Peru), N₂O production can be primarily driven by abiotic reactions (originating 98%-40% of N₂O rates), and that these reactions are highly competitive to microbial rates. Moreover, we found that microbial N₂O reduction accompanies abiotic production, forming a previously unknown coupling of abiotic-biotic cycling of N₂O. To elucidate the fate of N₂O, we used doubly labeled (¹⁵N)₂O to measure N₂O reduction rates under in-situ conditions. We detected anaerobic N₂O consumption at all diverse soils indicating an active N₂O-reducing microbial community even at soils with pH 3.7. Based on quantitative PCR of nosZ gene amplicons, proportions of the novel clade II N₂O-reducing microbes increased with N₂O consumption rates in high-N₂O soils. Our findings identify abiotic reactions as significant source of N₂O in tropical peat soils in a close interaction with microbial consumption, forming a coupled cycling not previously considered and a novel possible mechanism for N₂O flux variation in the tropics.