Land use changes and edaphic properties control contents and isotopic compositions of soil organic carbon and nitrogen in wetlands

Land use change in wetlands leads to significant losses of soil organic matter (SOM). Stable carbon (C) and nitrogen (N) isotopes offer insights into changes in C3/C4 vegetation, SOM sources, and decomposition processes. Yet, predicting the spatial–temporal dynamics of SOM contents and isotopes under land use changes remains challenging. This study delves into the effects of land use changes on soil organic carbon (SOC), total nitrogen (TN), δ¹³C and δ¹⁵N values, and soil physico-chemical properties and lignin phenols. Our results highlight the significance of soil water content (SWC) in determining the outcomes of land use changes. The conversion of wetland to cropland, forestland and construction land, led to notable reductions in SOC contents (8.71–56.33 %), and TN contents (7.87–37.12 %). Wetland conversion resulted in an enrichment of ¹³C and ¹⁵N abundance, with wetlands exhibiting the lowest δ¹³C (−25.57 to –22.89 ‰) and δ¹⁵N (2.66 to 6.67 ‰) values. A significant correlation occurred between δ¹³C and δ¹⁵N values in wetlands, but underwent considerable changes after wetland conversion. Key parameters, including bulk density (BD), C:N, the acid-to-aldehyde of vanillyl ((Ad/Al)v), lignin content (Λ₈), and total phosphorus (TP), were identified as influencing factors for both SOC and TN contents. When evaluating δ¹³C values, the most influential factors included silt, C:N, SOC, sand, and BD. These indicate the importance of soil chemical group (from 41 % to 21 %) in elucidating δ¹³C values declined, while lignin group’s (from 9 % to 28 %) importance increased from topsoil to subsoil. The acid-to-aldehyde of syringyl ((Ad/Al)s), Λ₈, C:N, BD and the cinnamyl-to-vanillyl ratio (C/V) were identified as the primary factors influencing δ¹⁵N values, with chemical group accounting for 36 % and lignin group for 48 % in topsoil, while physical group dominated 42 % in subsoil. Our findings underscore the shifts in SOM sources and distinct mechanisms of degradation/preservation of SOM following land use changes.