Regulation of Precipitation on Soil Dissolved Organic Matter in Perturbed Mangrove Ecosystems

Carbon sinks in mangrove soils play a critical role in climate change mitigation globally. Soil dissolved organic matter (DOM) is a major form of labile organic matter and influences carbon cycling in wetland ecosystems. However, the factors regulating DOM pools in mangrove soils on a regional scale are not well understood. Here, we used a novel approach to assess soil DOM dynamics and its environmental drivers in mangrove wetlands on a national scale in China. Soil samples were collected from 43 sampling sites and distributed in mangroves across the coastline of China. DOM extracted from mangrove soils was characterized by fluorescence spectroscopy. We estimated that dissolved organic carbon (DOC) content ranged from 0.20 +/- 0.02 g/kg to 3.85 +/- 0.09 g/kg and nationally averaged 0.73 +/- 0.07 g/kg in mangrove soils. Soil DOM was composed of humic-like substances, including humic acid-like (53.46 +/- 8.74%) and fulvic acid-like (46.54 +/- 8.74%). DOM pools in mangrove soils were identified to be perturbed by terrestrial inputs. Besides, the coupling interactions of environmental controls on the soil DOM pool were validated by establishing the structural equation modeling (SEM). We found that precipitation is the most important driver, which controlled directly the inputs and outputs of DOM pools. It also indirectly influenced DOM pools by regulating soil parameters through the cascade reactions. Cu, salinity, and clay are key mediators among soil parameters for precipitation affecting DOM pools. Precipitation influences soil DOC negatively and CDOM and FDOM positively markedly. The results provide novel insights into the labile carbon pool in mangrove soils and are beneficial for improving the assessment frameworks in the blue carbon ecosystems.