A new look at an old concept: using ¹⁵N ₂O isotopomers to understand the relationship between soil moisture and N ₂O production pathways

Understanding the production pathways of potent greenhouse gases, such as nitrous oxide (inline-formulaN₂O), 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 – such as the relationship between soil moisture and inline-formulaN₂O production pathways. A powerful, but arguably underutilized, approach for quantifying the relative contribution of nitrification and denitrification to inline-formulaN₂O production involves determining inline-formula¹⁵N₂O isotopomers and inline-formula¹⁵N site preference (SP) via spectroscopic techniques. Using one such technique, we conducted a short-term incubation where inline-formulaN₂O production and inline-formula¹⁵N₂O isotopomers were measured 24 h after soil moisture treatments of 40 % to 105 % water-filled pore space (WFPS) were established for each of three soils that differed in nutrient levels, organic matter, and texture. Relatively low inline-formulaN₂O fluxes and high SP values indicted nitrification during dry soil conditions, whereas at higher soil moisture, peak inline-formulaN₂O emissions coincided with a sharp decline in SP, indicating denitrification. This pattern supports the classic inline-formulaN₂O production curves from nitrification and denitrification as inferred by earlier research; however, our isotopomer data enabled the quantification of source partitioning for either pathway. At soil moisture levels < 53 % WFPS, the fraction of inline-formulaN₂O attributed to nitrification (inline-formulaF_N) predominated but thereafter decreased rapidly with increasing soil moisture (inline-formulax), according to inline-formula $M16inlinescrollmathml{F}_{\mathrm{normal\; N}}=\mathrm{normal\; 3.19}-\mathrm{normal\; 0.041}x$ 93pt12ptsvg-formulamathimg179172eaab607bfbc3736a7783f7ccc8 soil-5-265-2019-ie00001.svg93pt12ptsoil-5-265-2019-ie00001.png , until a WFPS of 78 % was reached. Simultaneously, from WFPS of 53 % to 78 %, the fraction of inline-formulaN₂O that was attributed to denitrification (inline-formulaF_D) was modelled as inline-formula $M19inlinescrollmathml{F}_{\mathrm{normal\; D}}=-\mathrm{normal\; 2.19}+\mathrm{normal\; 0.041}x$ 101pt12ptsvg-formulamathimge0450b7e1d7864138c45f9c33114d214 soil-5-265-2019-ie00002.svg101pt12ptsoil-5-265-2019-ie00002.png ; at moisture levels of > 78 %, denitrification completely dominated. Clearly, the soil moisture level during transition is a key regulator of inline-formulaN₂O production pathways. The presented equations may be helpful for other researchers in estimating inline-formulaN₂O source partitioning when soil moisture falls within the transition from nitrification to denitrification.