Cumulative effects of experimental nitrogen deposition on soil chemistry in a desert steppe: A 12-year field study

Although the effects of human-enhanced atmospheric nitrogen (N) deposition are well documented, the response of dryland soils to N deposition remains unclear owing to the divergence in hydrological outputs and soil heterogeneity. We selected a typical desert steppe in western China to simulate the effects of long-term N deposition by applying 0 (CK), 3.5, 7, and 14 g N m^-2 yr^-1 for 12 consecutive years. We found that, compared with the CK plots, the total N content of the upper (0-10 cm) and lower (10-20 cm) soil layers in fertilized plots increased by 8.3-14.6 % and 2.4-8.2 %, respectively. Correspondingly, the available, NH₄⁺-, and NO₃^--N contents in the upper soil significantly increased by 25.5-68.3 %. However, in the lower soil, available and NO₃^--N contents were significantly lower than those in the CK plots, and their variation trend was opposite to that of NH₄⁺-N, implying N turnover and leaching. As a result, the upper and lower soil pH in fertilized plots significantly decreased by 0.36-0.53 and 0.31-0.37 units; however, their CaCO₃ content significantly increased by 9.8-22.8 % and 7.2-30.3 %, respectively. The total phosphorus (P) content in the upper and lower soil layers in fertilized plots significantly increased and decreased by 3.6-51.3 % and 16.7-62.5 %, respectively, however, both significantly decreased along the N fertilization gradient. Furthermore, the upper and lower soil organic carbon (SOC) content in the fertilized plots significantly increased by 57.7-78.1 % and 19.2-27.4 %, respectively. Pearson's correlation analysis revealed that available soil P was significantly negatively correlated with plant shoot Mn content (a proxy for rhizosphere carboxylates), whereas dissolved OC, SOC, and CaCO₃ were significantly positively correlated, suggesting that Ca cycling is involved in P cycling and SOC sequestration. Our study suggests that long-term N input exacerbates P limitation in desert steppes, however, enhances SOC sequestration.