Restoration from grazing on the Tibetan plateau: Pathway-specific soil MAOC sequestration in meadow and peat wetlands

Mineral-associated organic matter (MAOM) constitutes a significant portion of carbon (C) pools in wetlands, determining their role as C sinks or sources. Nevertheless, detailed knowledge of the response of MAOM formation pathways to wetland restoration is lacking. Here, we collected 0-20 cm soil samples from two typical wetlands (meadow and peat wetlands) subjected to both grazing and restoration treatments (through micro dam construction) in the Zoige area. Lignin phenols and amino sugars within the MAOM fraction were analysed to clarify the MAOM formation pathways and their potential contributions to MAOM formation. The results demonstrated that both meadow and peat wetland restoration enhanced MAOM sequestration. Compared to grazed sites, restored meadow wetlands significantly increased the mineral-associated organic carbon (MAOC) content in soils but not the C content of the MAOM fraction; restored peat wetlands increased the C content of the MAOM fraction by 58% but not the MAOC content in soils. Notably, plant-derived C and microbial necromass C strongly and dominantly influenced the C content of the MAOM fraction in the meadow (path coefficient = 0.73) and peat wetlands (path coefficient = 0.75), respectively. Compared with those in the grazed peat wetlands, the MAOM fraction in restored peat wetlands contained 1.93, 1.61, and 2.09 times higher total, bacterial, and fungal necromass C contents, respectively. Moreover, soil properties influence lignin phenols and necromass C in conjunction with the C content of the MAOM fraction rather than directly. It seems reasonable to assume that meadow wetland restoration from grazing enhanced MAOM formation in soils via the accumulation of lignin phenols (through the "direct sorption" pathway). Peat wetland restoration from grazing enhanced MAOM formation in soils via the accumulation of lignin phenols (through the "direct sorption" pathway) and the "microbial turnover" pathway.