<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, 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 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.干旱气候会对生物区系（biota）造成严重影响。氮（N）循环是理解生物区系对干旱气候响应的关键科学信息。然而，目前仍不清楚在古干旱气候背景下，关键氮循环过程是否以及如何响应干旱程度的变化。本研究通过分析古干旱气候条件下黑色页岩中残留的微量硝酸盐（NO₃⁻）的含量及其氮、氧（O）同位素组成，构建了耦合双氮氧同位素的稳态模型，分别量化了低干旱度和高干旱度气候下微生物硝酸盐生产（硝化作用）与消耗（反硝化作用、同化作用）的通量。研究发现，低干旱度条件下硝化作用速率是大气氮输入量（0.29 kg N ha⁻¹ yr⁻¹）的11倍，但在高干旱度条件下其速率下降至原水平的1/16。类似地，低干旱度条件下反硝化作用速率是微生物同化作用速率（0.39 kg N ha⁻¹ yr⁻¹）的6倍，但在高干旱度条件下其速率下降至原水平的1/37，此时反硝化作用速率与同化作用速率（0.33 kg N ha⁻¹ yr⁻¹）已相当。相关性分析证实，在干旱气候背景下，硝化作用（斜率=0.692，决定系数R²=0.928）与反硝化作用（斜率=0.706，决定系数R²=0.645）对干旱程度均表现出敏感响应，但微生物硝酸盐同化作用（斜率=0.101，决定系数R²=0.037）则无明显响应。本研究为量化旱地古生态系统的氮循环通量提供了新的同位素记录与地球化学方法，有助于理解生物区系对历史干旱事件的响应机制，同时可为现代旱地生态系统的氮循环模拟提供参考。