Nitrogen addition increases wetland soil organic carbon in China: A meta-analysis

Wetland ecosystems, with substantial organic carbon (C) storage, have the potential to regulate soil C dynamics under increased nitrogen (N) deposition resulting from climate change. Thus, altering N inputs in wetland ecosystems has implications for global climate change and soil health. Quantifying SOC responses to N is essential for nitrogen stewardship in wetland conservation, coastal restoration, and paddy management. China, as a key contributor to global wetland ecosystems, requires a clear understanding of how SOC responds to N addition in these habitats—a crucial step for addressing global climate change. However, the direction and magnitude of this response remain uncertain. We conducted a global meta-analysis using a database of 129 sites to examine the impact of N addition on SOC in freshwater wetlands, coastal wetlands, and paddies. Across wetlands, N addition increased SOC by 4.7% (P < 0.05). Effects differed among wetland types, with coastal wetlands ( + 9.38%, P < 0.05), paddy fields ( + 5.93%, P< 0.05), and freshwater wetlands ( + 1.72%, P > 0.05). SOC responses were strongest at 50–150/kg N /(ha·yr) ( + 7.9%, P < 0.05) and when duration exceeded five years ( + 8.8%, P < 0.05); responses were independent of N form. Patterns were consistent after accounting for soil layer and other moderators. Meta-regression showed that, across ecosystems, mean annual temperature (MAT) and the microbial biomass C: N ratio (MBC: MBN) significantly moderated SOC responses to N addition. Within wetlands, bulk density (BD) and electrical conductivity (EC) were the strongest soil-physical moderators. Our findings indicated that the effects of N deposition depended on an appropriate N addition rate and duration. Additionally, our study highlighted the importance of considering soil EC and BD when estimating soil C cycling under future N deposition scenarios in wetlands.