The influence of stochastic soil moisture dynamics on gaseous emissions of NO, N₂O, and N₂

Even though the fundamental role played by nitrogen oxides in several important atmospheric processes is generally acknowledged, the understanding and quantitative evaluation of some mechanisms involved in the biogenic emission of N gases is still incomplete. Because of the number and variety of chemical, biological and physical processes involved, their mutual interaction and the strong nonlinearities, the dynamics of biogenic nitrogen oxide emission are complex and difficult to study. In this context, this study is centred on the link between soil water content and gaseous emissions of N₂ and oxidized nitrogen gas, NO and N₂O, from the ground. In particular, the study investigates how the probability distribution of soil moisture affects the distribution of the emission rate of nitrogen and nitrogen oxides when other limiting factors are absent. Some analytical tools are provided for the estimation of the steady-state probability density function of the emission rate. These tools are then used to investigate the influence of characteristics of rainfall regimes, plant transpiration, and soil texture on the emission process. It results that: (a) both frequency and depth of the storms strongly influence the shape and the moments of the probability density function of the emission rates; (b) higher values of plant transpiration favour nitric oxide emissions and give rise to higher variances in the emission rates; and (c) N₂O emissions are greater in fine-textured soils than in coarser soils, while NO emissions are higher in coarser soils and can be very low in clayey soils where, instead, N₂ emissions become significant.