Isaac Yagle

Pulse emissions of soil trace gases (carbon dioxide: CO₂, nitrous oxide: N₂O, and nitric oxide: NO) following the wetting of dry soils may contribute disproportionately to total annual trace gas emissions in drylands. These pulses are driven by coupled biotic–abiotic processes that are challenging to partition. Here we measured soil trace gases emissions after the wetting of γ-irradiated and live intact cores of soils with different soil C and N pools, exchangeable cations, and microbial activity. For all soils, the emissions pulse of CO₂ and N-oxides was measured within 5 min of wetting. Post-wetting CO₂ emission pulses from the live soils were approximately twice as large as those from the γ-irradiated soils and were correlated with soil C content. However, the γ-irradiated soil emitted up to 5 times more N₂O and up to 13 times more NO compared with the live soil and emissions were correlated with soil N content. After normalization against the soil inorganic N content, the immediate post-wetting N₂O and NO emissions were the same from the live and γ-irradiated soils. The dependence of the post-wetting N₂O and NO burst on substrate pools, regardless of the presence of live microbes, suggests an abiotic origin of immediate post-wetting fluxes.

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.

Vegetation patchiness is hypothesized to affect the spatial heterogeneity of resources and soil nutrient distribution in drylands. Nutrient accumulation under perennial vegetative patches leads to faster nitrogen (N) cycling in times of water availability. Compared to perennials, annual plant patches have a shorter life cycle, and labile nutrient buildup can occur more quickly in these patches due to faster nutrient turnover rates of litterfall and root death. The buildup of these labile nutrient pools, in surface soils under annual plant patches over time, may indirectly facilitate succession by other plants, thus aiding in the establishment of fertility islands. To understand how the establishment of annual plant patches affects soil nutrient dynamics, we planted replicated patches of a widespread local summer annual plant, saltwort (Salsola inermis Forssk.), and assessed how these patches influence the soil N cycle and soil N oxides (N2O and NO) emissions. We also assessed rates of surface litter decomposition of the saltwort plant. We found that rates of soil N transformations and soil N oxides emissions were highest under the plant patch, while they decreased across the patch-to-bare-soil gradient. Water extractable organic carbon (WEOC) accumulation increased in the surface soil beneath the plants and was associated with a large burst in soil N oxides emissions within the patch, following dry soil wetting by the first winter rains. Soil N2O emission pulse increased by 5.2 folds, whiles NO increased by 95.8 folds. Each N-oxide gas, however, had a different post-wetting pattern, with N2O peaking a few hours after wetting and NO after one day. We measured a second pulse in soil N oxide emissions after the third rain event. This pulse occurred only with the plant patch and not outside the patch and was reduced by 54% and 31% for N2O and NO respectively. However, the temporal (peaking) pattern of the second N-oxides pulse was similar to that of the first pulse. Suggesting a reduction in substrate availability as a cause of the reduced pulse. We also found 43% mass loss from the plant litter after 12 months of decomposition. Together, these results suggest that the establishment of saltwort plants affects soil nutrient dynamics and accumulation, thus creating nutrient-rich microsites for potential succession by other annuals and perennials, leading to fertility island establishment in the Negev Desert ecosystem.