Assistant Professor University of California, Riverside Riverside, California, United States
Abstract: Moisture is a key factor governing soil nitrogen (N) biogeochemistry; it controls microbial activity and, therefore, the cycling of N. Ongoing climate change is altering precipitation regimes, increasing the frequency and intensity of droughts with implications for ecosystem N retention and loss. In particular, wetting dry soils can produce large emission pulses of nitrous oxide (N2O), a potent greenhouse gas, but the mechanisms governing the N losses remain elusive. Especially because denitrification, arguably the most important process producing N2O, is not favored in dry soils. Disentangling how drought can alter the balance between ecosystem N retention and loss is further challenged by the multiple biotic and abiotic processes that interact to control N availability and emissions, requiring multiple analytical approaches.
To advance understanding of N cycling in dry soils, we studied drylands in southern California that can experience >6 months without rain and whose contrasting soils developing under shrub canopies (soils known as “islands of fertility”), or in the bare interspaces between shrubs, allow us to interrogate biotic–abiotic interactions. Using isotopologues of N2O coupled with chloroform fumigations to slow microbial activity, we found that N retention and loss trade off as dry conditions intensify. In particular, N2O emissions were undetectable from soils in the interspaces between plants, but exceeded 1000 ng N-N2O m-1 s-1 in islands of fertility, rivaling emissions observed in global hotspots like tropical forests and temperate agroecosystems. Despite the hot and dry conditions, isotope tracers and natural abundance isotopologues of N2O indicated NO3- was reduced to N2O within 15 minutes of wetting dry desert soils, and that both denitrification and N2O reduction to N2 contributed to the observed patterns, with δ15NSP-N2O values averaging 12.8 ± 3.9 ‰. Consistent with isotope values, fumigating soils in the lab with chloroform decreased NO3--derived N2O emissions by 59%, suggesting denitrifiers were able to reduce NO3- immediately after wetting these summer-dry desert soils. In contrast to NO3-, the 15N-NH4+ tracer was not found in N2O, suggesting nitrification is not an important pathway governing N2O emissions in these systems. Despite the hot and dry conditions known to make denitrification unfavorable in many drylands, denitrifiers can endure through hot and dry summers and are key to producing the surprisingly large N2O emissions when dry desert soils wet up.