<b>Nitrogen and oxygen isotopes of shale nitrate recorded soil nitrate biogeochemistry under arid paleoclimates</b>

Arid climates severely influence the biota. Nitrogen (N) cycling is crucial information for understanding responses of biota to arid climates. However, it remains unclear whether and how key N-cycling processes respond to aridity under arid paleoclimates. By investigating contents and N and oxygen (O) isotopes of trace nitrate (NO₃^-) remained in black shale under arid paleoclimates, here we established a steady-state model combining dual N and O isotopes to quantify fluxes of microbial NO₃^- production (nitrification) and consumption (denitrification, assimilation) under low- and high-aridity climates, respectively. We found that nitrification was 11 times higher than atmospheric inputs (0.29 kg N ha^-1 yr^-1) under low aridity but declined 16-fold under high aridity. Similarly, denitrification was six times higher than microbial assimilation (0.39 kg N ha^-1 yr^-1) under low aridity but decreased 37-fold, becoming comparable to assimilation (0.33 kg N ha^-1 yr^-1) under high aridity. Correlation analyses confirmed that nitrification (slope = 0.692, <i>R</i>² = 0.928) and denitrification (slope = 0.706, <i>R</i>² = 0.645) were sensitive to the aridity but microbial NO₃^- assimilation (slope = 0.101, <i>R</i>² = 0.037) was not under arid climates. This study provides new isotopic records and geochemical methods for quantifying N-cycle fluxes of dryland paleoecosystems, which are useful for understanding mechanisms of biological responses to historical arid events and modeling N cycles in modern dryland ecosystems.