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

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 (N2O) 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 N2O emissions at intermediate moisture levels following fertilization, and prefertilization emissions were low. Emissions of N as N2O 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%) for N2O. Our results caution the use of WFPS to predict NO:N2O emission ratios as often employed in ecosystem models. Subsurface N cycling may suppress emissions of NO relative to N2O, and N2O emissions can persist under saturated conditions. Elevated N2O emissions from in-field wet spots comprising a small landscape extent could potentially address disparities between top-down and bottom-up N2O budgets. Plain Language Summary Nitric oxide and nitrous oxide are environmentally harmful gases: they contribute to climate change, the loss of good ozone high in the atmosphere that protects us from damaging sunlight, and the production of bad ozone that pollutes our air. The activities of microorganisms in soils are important sources of these gases to the atmosphere, and fertilized agricultural soils have particularly high emissions. Conceptual and mathematical models are often used to predict how nitrous and nitric oxide emissions respond to environmental changes and to guide policy and management. Here we tested a key assumption of a classic conceptual model that is widely used to estimate soil nitrous and nitric oxide emissions, using a laboratory experiment with two poorly drained agricultural soils. We found that there was not a clear trade-off in the emissions of nitric versus nitrous oxide as soil moisture varied from low to high levels, in contrast to predictions. Rather, emissions of nitrous oxide were greater than expected, and production of nitrous oxide and nitric oxide persisted under very wet conditions. Poorly drained patches of intensively fertilized agricultural soils could represent important sources of nitrous oxide to the atmosphere.