Nitrogen and Oxygen Isotopes of Shale Nitrate Recorded Soil Nitrate Biogeochemistry Under Arid Paleoclimates

Abstract

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 the contents and N and oxygen (O) isotopes of trace nitrate (NO₃⁻) remaining 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, R² = 0.928) and denitrification (slope = 0.706, R² = 0.645) were sensitive to the aridity but microbial NO₃⁻ assimilation (slope = 0.101, R² = 0.037) was not sensitive to arid climates. This study provides new isotopic records and geochemical methods for quantifying N-cycle fluxes of dryland paleoecosystems, which are useful for understanding the mechanisms of biological responses to historical arid events and modeling N cycles in modern dryland ecosystems.