Reclamation leads to loss of soil organic carbon and molecular complexity: Evidence from natural to reclaimed wetlands

The molecular diversity of soil organic matter (SOM) is recognised as a key factor influencing soil organic carbon (SOC) accumulation, and the molecular diversity of SOM may change as SOC content changes during land use change. However, the relationship between SOM molecular diversity and SOC before and after natural wetland reclamation remains unclear. Here, we selected seven groups of natural wetland–reclaimed wetlands for spatially paired sampling. SOM molecular diversity was assessed using pyrolysis–gas chromatography–mass spectrometry (py-GC/MS), and factors driving changes in SOM molecular diversity (including microbial community characteristics, enzyme activities, carbon mineralisation rate and soil environmental factors) were investigated. The results showed that molecular diversity (Shannon diversity, Richness) tended to increase with increasing organic carbon content in both wetland and paddy soils. And the soil mineralisation rate decreased with the increase of molecular diversity. This suggests that the relationship between molecular diversity and organic carbon content is not decoupled, even in anaerobic or cyclic anaerobic environments. Therefore, the molecular diversity of soil organic matter can be used as an indicator of the sustainability of soil carbon pools. Microbial biomass and enzyme activity characteristics were important factors influencing soil carbon dynamics and molecular diversity. Molecular diversity decreases with a loss of soil organic carbon after wetland reclamation. Compared to those in natural wetlands, the relative proportions of both aliphatic and alkyl compounds decreased, and the relative proportions of nitrogenous compounds increased in paddy field soils. In addition, the rate of soil carbon mineralisation increases despite the presence of a greater proportion of recalcitrant carbon (phenols and aromatics) in paddy soils. Our results also suggest a positive role for molecular diversity in suppressing soil mineralization rates. Our study provides a molecular diversity-based perspective for understanding wetland soil organic carbon dynamics under the influence of reclamation.