Soil Water Content and the Ratio of Nitrous Oxide to Nitric Oxide Emitted from Soil

Abstract

Nitrification and denitrification are oxidative and reductive microbial processes, respectively, that occur simultaneously at different microsites within the soil, and each process produces both nitric oxide (NO) and nitrous oxide (N₂O). Although much is known about regulation of these processes in the laboratory, the salient factors appropriate for use in regional and global models of N trace gas emissions have not been identified. However, trends exist in the growing global database of N₂O and NO emissions that are consistent with soil water content functioning as an important controller of the ratio of N₂O to NO emitted from soils. Below about 60% water-filled pore space (WFPS), soil microbial processes are often limited by diffusion of organic-C and inorganic-N substrates in soil water films, whereas diffusion of O₂ in the soil atmosphere limits aerobic processes when WFPS > 60% (60% WFPS is roughly equivalent to field capacity in many soils). In laboratory studies with nitrification inhibitors, the aerobic process, chemoautotrophic nitrification, was the dominant source of both N₂O and NO when soil water content was < 60% WFPS, and NO emis­sions exceeded N₂O emissions. When WFPS > 60%, the anaerobic process, denitrification, was the dominant source, and N₂O emissions were 10 or more times greater than NO emissions. Emission ratios of N₂O to NO were < 1 when WFPS < 60% and were > > 1 when WFPS > 60%. Few field studies report WFPS, but even gravimetric soil water content, which does not account for textural differences among soils, crudely predicted emission ratios of N₂O to NO. Prediction of ratios of emissions of N₂O and NO based on WFPS (or a related parameter of soil water content) would be most useful when combined with ecosystem models of N cycling dynamics that predict total N gas emissions from indices of N availability or site fertility.