The effect of soil moisture on nitrous oxide flux and production pathway in different soil types

Non-Peer ReviewedUnderstanding the production pathways of potent greenhouse gases, such as nitrous oxide (N2O), is essential for accurate flux prediction and for developing effective adaptation and mitigation strategies in response to climate change. Yet, there remain surprising gaps in our understanding and precise quantification of the underlying production pathways. Soil-derived N2O is a product of nitrification and denitrification microbially driven processes that depend on the aeration status of the soil. As a result, the relative contributions of nitrification and denitrification are often determined based their relationship to soil water-filled-pore space (WFPS), which acts as a proxy for aeration status. A powerful (if underutilized) approach for quantifying the relative contribution of nitrification and denitrification to N2O production involves determining the intramolecular 15N distribution of N2O. Recent developments in laser spectroscopy have made it easier to quantify the concentrations and relative abundances of the isotopomers of N2O (i.e., 14N15NO and 15N14NO). Using one such technique (cavity ring-down spectroscopy [CRD]), we conducted short-term (24-h) incubations of three soils (differing in texture and organic matter content) at water contents equivalent to 20-105% WFPS. The volumetric water content of the soils was adjusted to a predetermined value and the soils were packed to a constant bulk density (the value of which was soil dependent) in a small (5.9-cm i.d.) petri dish. The petri dishes were then placed in 1-L jars and sealed; headspace samples were collected after 24-h and the samples analyzed for total N2O concentrations and isotopomers. Relatively low N2O fluxes and high 15N2O site-preference values resulted from dry soil conditions, whereas at higher soil moisture, peak N2O emissions coincided with a sharp decline in 15N2O site-preference. This pattern supports the classic N2O production curves from nitrification and denitrification published by Davidson et al. 1991. However, the WFPS transition-zone between nitrification and denitrification widely differed, depending on the soil type