The accumulation of microbial residues decreases with time after wetland-to-cropland conversion

Soil organic carbon (SOC) serves as a pivotal carbon reservoir within terrestrial ecosystems, exerting profound influences on soil fertility, agricultural productivity, and global carbon cycling. Wetland reclamation alters the micro-environment of soils and influences soil biogeochemical processes, disrupts the natural balance of SOC dynamics. However, the microbial involvement in SOC dynamics in croplands converted from wetlands across different years remains unclear. To address this issue, we investigated changes in SOC and microbial residues in the tillage layer (0–25 cm) and plow pan (25–35 cm) of wetlands converted to farmlands over six decades (0, 20, 30, 40, 50, 60 years) in northeast China. The results revealed that the SOC content decreased from 3.97 to 0.95 % over the 60-year cultivation period. The content of microbial biomass and residues was higher in the tillage layer than in the plow pan. In the tillage layer, microbial biomass generally decreased, whereas in the plow pan, it initially increased and then decreased with increasing duration of farming. The content of the fungal and bacterial residues initially increased and subsequently decreased at all depths with increasing duration of farming. The accumulation efficiency of microbial residues in soil showed the same trend. Structural equation modeling revealed that both the duration of farming and soil layer influenced soil physicochemical properties and the free iron content. These factors in turn directly affected microbial community composition and residues, while indirectly affecting SOC. Our findings provide a theoretical foundation for assessing carbon storage capacity and persistence following wetland cultivation.