Vegetation degradation along water gradient leads to soil active organic carbon loss in Gahai wetland

Soil active organic carbon responds quickly to soil disturbances and is a sensitive indicator of early changes in soil organic carbon (SOC). In order to identify the differences in the distribution of dissolved organic carbon (DOC), light fraction organic carbon (LFOC), SOC and their changes as affected by vegetation degradation degree along water gradient in wetland, we analyzed DOC, LFOC, and SOC in the 0-100 cm soil layer under four vegetation degradation degrees: non-degradation (ND), lightly degradation (LD), moderately degradation (MD) and heavily degradation (HD). The results showed that soil DOC, LFOC and SOC in the 0-100 cm layer of ND wetland was significantly higher than the other three degradation levels. DOC, LFOC and SOC contents decreased with increasing soil depth under the four degradation degrees and the contents of soil DOC, LFOC and SOC were mainly concentrated in the soil surface (0-20 cm). The DOC, LFOC, and SOC contents in the 0-20 cm layer under all four degradation levels showed obvious seasonal changes, while the DOC, LFOC, and SOC contents in the 20-100 cm layer showed little fluctuation over the plant growing season. There was a significant positive correlation between soil DOC and SOC, and between LFOC and SOC, with correlation coefficients of 0.948 and 0.911, respectively. There was also a very significant correlation between DOC and LFOC(R-2 = 0.904). Soil DOC and LFOC in the 0-100 cm layer under the four degree of degradation were linearly correlated with SOC. While there was a linear correlation between DOC and LFOC in the non-degradation wetland soils, DOC and LFOC in the three degradation soils correlated exponentially correlated with SOC.