Effects of coastal wetland reclamation on soil organic carbon, total nitrogen, and total phosphorus in China: A meta-analysis

The reclamation of coastal wetlands for agriculture has expanded considerably over the last 40 years in China. Wetland reclamation could substantially alter soil properties, but so far, a systematic analysis of the changes in soil chemical elements after reclamation is missing on a national scale. In this study, we synthesized 418 pairs of data from 41 experimental studies and investigated the responses of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) contents to coastal wetlands reclamation in China. The results showed that SOC significantly decreased by 24.77% after reclamation, while TN and TP were significantly increased by 23.57% and 13.55%, respectively. The changes in SOC, TN, and TP after reclamation of coastal wetlands mainly occurred in the 0-40 cm soil layer but not in the deep soil layer. For different land use types, SOC was significantly reduced by 49.48%, 44.57%, and 32.91% after reclaiming wetlands to woodlands, paddy fields, and drylands, respectively. TN and TP were significantly increased by 32.22% and 13.54% after the reclamation of wetlands to drylands, respectively. The reclamation duration also had significant effects on those elements. Specifically, SOC gradually decreased with increasing reclamation duration, while TN and TP increased with the increased reclamation duration. Anthropogenic factors, such as land use types and reclamation duration, rather than climatic factors, dominated changes in SOC, TN, and TP contents after coastal reclamation. Further, our study showed a significant SOC reduction following reclamation, and given the importance of coastal blue carbon, it is essential to reduce wetland reclamation to protect coastal blue carbon.