Quantification and partitioning the contributions of abiotic and biotic processes to soil N-oxides emissions in the Dead Sea valley, Israel.

Wetting of dry soil after prolonged drought triggers emissions of N-oxides (nitric oxide; NO and nitrous oxide; N₂O) and this post-wetting burst may contribute disproportionately to annual soil N-oxides emissions in drylands. During the wetting, nitrification and denitrification were shown to be the major sources of soil N-oxides emissions. Several abiotic reactions involving the nitrification intermediates- e.g. nitrite (NO₂^-), however, may also contribute to the production of N-oxides in soils. The contribution of these abiotic reaction to N-oxides emissions, despite potential importance is not well quantified. To quantify and partition the contribution of abiotic and biotic processes to post-wetting N-oxides emissions in drylands, we measured soil NO and N₂O production in a laboratory incubation with live and gamma-irradiated soils. Samples were collected under canopies of dominant local (Acacia tortilis) and invasive (Prosopis juliflora) trees, as well as from bare soils outside the canopy cover.

We found that that while the overall dynamics of soil NO and N₂O emissions were similar in gamma irradiated and live soils under both P. juliflora and A. tortilis trees, as well as in bare soils, the magnitudes and rates of emissions exhibited significant disparities. In particular, gamma irradiated soils under A. tortilis canopies after eight hours’ incubation, emitted ~10 times less NO (~5 ng N g^-1) and ~4 times less N₂O (~10 µg N g^-1) compared to the live soils. While gamma irradiated soils under P. juliflora canopies emitted ~2 times less NO (~7 ng N g^-1) and similar N₂O (~7 µg N g^-1) compared to the live soils, and in the bare soils, ~9 times less NO (~5 ng N g^-1) and similar N₂O (~10 µg N g^-1). Our findings suggest that both biotic and abiotic pathways contribute to N-oxides production following dry soil wetting, however, the relative contribution is dependent on the landscape position and affected by plant presence and species. Specifically, abiotic processes contributed 10% to soil NO and 25% to N₂O production in soils beneath A. tortilis canopies and between 10% and 75% in the bare soils. In soil under P. juliflora canopies abiotic processes contributed five times more to the NO production (50%) while N₂O production was solely from abiotic activity.