Wang Q., Wen Y.X., Zhao B., Hong H.L., Liao R., Li J.W., Liu J.C., Lu H.L. and Yan C.L.. 2021. Catena, 207:105709.

Mangroves are among the most carbon-rich forests in the tropics, but the spatial variation in soil organic carbon (SOC) storage and the mechanisms controlling their stability remain contentious. Here, we proposed a new framework based on distinct coastal soil textures (muddy, sandy, and mud-sand mixed soil) and found that coastal soil texture exhibits regional-scale variations in mangrove soil C:N:P stoichiometry and SOC storage. Using this soil texture classification framework, we identified the important role of mud-sand mixed soil, which functions as a blue carbon hotspot in coastal wetlands. We showed that mangrove SOC storage was underestimated by approximately 37% in mud-sand mixed soil and overestimated by approximately 54% in muddy soil. We further revealed that the chemical composition of SOC and clay mineralogy are responsible for SOC stability by combining physical and chemical characterizations. As a microbial-derived C, glomalin-related soil protein (GRSP) contained higher alkyl C (~58%) and lower O-alkyl C (~17%) than mangrove soils, indicating that GRSP contributes to the stable SOC pool by its recalcitrant structure. We further found that the C:N:P stoichiometric signature of GRSP can capture variations in mangrove SOC storage compatible with distinct coastal soil textures. These results highlight the role of GRSP in regulating SOC stabilization mechanisms, potentially attenuating coastal blue carbon-climate feedback. Regional-scale variation in SOC linked to coastal soil textures can provide more robust estimates of the contribution of mangrove SOC to global C dynamics.

Figure 1. The coastal soil texture framework offers an alternative standpoint to the latitude- and species-based framework proposed to explain the spatial variation in SOC storage. Red and blue dashed lines indicate the SOC density of the total average and distinct soil textures, respectively. The sizes of the ellipses are proportional to the mangrove area for each soil texture, indicating the mangrove blue carbon hotspots. The SOC stability in distinct coastal soil textures, as regulated by the chemical composition of SOC, C:N:P stoichiometry, clay minerals and stubborn structure of GRSP, determines the mangrove SOC storage.