Effects of land use changes on the spectroscopic characterization of hot-water extractable organic matter along a chronosequence: Correlations with soil enzyme activity

•Soil organic C and dissolved organic C decreased after land use changes.•Land use conversion affected the structural properties of dissolved organic matter.•The stabilities of soil organic matter of different aged croplands are different.•Soil enzymes were affected by the structural properties of dissolved organic matter.

Given the importance in soil C cycling, changes in the composition of hot-water extractable organic matter (HWEOM) due to the land use conversion are less known. The purposes of this study were to elucidate the structural characteristics of HWEOM from soils collected from a native wetland and five croplands which originated from the wetlands by conversions for 3, 8, 16, 22, and 24 years, and to relate the structural characteristics to soil enzyme properties. We examined the structural characteristics of HWEOM by UV absorption and fluorescence. The values of specific absorption at 254 nm, humification index and fluorescence efficiency index of HWEOM showed a tendency to increase initially compared to the values of wetland (the highest value was found in the cropland with cultivation-time of 16 years), and then the values tended to decline with time. Soil β-glucosidase and N-acetyl-β-glucosaminidase activities were negatively affected by the structural characteristics of soil HWEOM. Our results indicate that the HWEOM from the croplands contained more highly condensed aromatic compounds, and were less mobile than that from the native wetland. Furthermore, the stabilities of HWEOM in the croplands with short/long cultivated-time were different.