Mangrove soil carbon stocks varied significantly across community compositions and environmental gradients in the largest mangrove wetland reserve, China

Mangroves play a crucial role in offsetting the increasing levels of carbon dioxide as their soils could store a significant amount of carbon. Despite that patterns of mangrove soil carbon and related driving factors at the global scale have been widely investigated, data at the regional scale (particularly in Southern China) are still missing, which is critical for climate change adaption and mitigation. Therefore, we conducted a field investigation in Zhanjiang, the largest mangrove wetland reserve of China, across a variety of mangrove communities, and sampled 87 soil cores from 29 sites, aiming to determine the spatial patterns of mangrove soil carbon stocks in this region and their potential driving factors. Our results revealed that the soil carbon stocks in Zhanjiang ranged from 16.89 to 300.58 Mg C/ha, with an average of 126.74 ± 70.93 Mg C/ha. The soil carbon stocks of mangrove forests differed significantly across community compositions. Single mangrove communities had higher soil carbon stocks than mixed communities, probably due to the fact that most mixed mangrove forests in this study were planted in recent years, with the soil having a limited ability to store carbon compared with mature mangrove forests. Introduction of the non-native Sonneratia apetala did not enhance carbon burial. Soil total nitrogen and total phosphorus were the primary driving factors influencing soil carbon stocks of mangroves. This may be due to pollution from anthropogenic development, which has resulted in the discharge of large quantities of nutrient-rich wastewater into local mangroves. Our findings emphasize the pressing need to diminish the introduction of nitrogen and phosphorus at the local scale, and our study highlights the importance of employing native mangrove communities with high carbon stocks to augment the capacity for carbon burial in blue carbon ecosystems.