Denitrification in dry soils: Unexpected N emissions under environmental extremes

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 thermodynamically unfavorable 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 δ¹⁵N^SP-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 thermodynamically 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.