Wet Spots as Hotspots: Moisture Responses of Nitric and Nitrous Oxide Emissions From Poorly Drained Agricultural Soils

摘要

A classic framework for soil nitrogen (N) cycling, the hole in the pipe (HIP) model, posits a trade-off in emissions of nitric oxide (NO) and nitrous oxide (N_2O) as a function of soil moisture. This has been incorporated into ecosystem models but not tested experimentally and remains an important uncertainty for understanding potential hotspots of reactive N emissions: poorly drained agricultural soils that experience episodically high moisture following intensive fertilization. We incubated soils at moisture ranging from 44% to 100% water-filled pore space (WFPS). Counter to HIP, we did not observe a consistent trade-off in NO and N_2O emissions at intermediate moisture levels following fertilization, and prefertilization emissions were low. Emissions of N as N_2O exceeded NO by 2-200-fold at all moisture levels and peaked at 73-82% WFPS. Emissions of NO declined with moisture but remained significant even under saturated conditions. Increases in nitrite and reduced iron at high moisture indicated possible NO production from chemodenitrification. Potential nitrification rates were 100-1,000-fold greater than potential denitrification. Emission factors for fertilizer N ranged from 0.05% to 0.58% (mean = 0.2%) for NO and from 0.4% to 16.9% (mean = 5.3%) forN_2O.Our results caution the use ofWFPS to predictNO:N_2Oemission ratios as oftenemployed in ecosystem models. Subsurface N cycling may suppress emissions of NO relative to N_2O, and N_2O emissions can persist under saturated conditions. Elevated N_2O emissions from in-field wet spots comprising a small landscape extent could potentially address disparities between top-down and bottom-up N_2O budgets.