Effect of long-term land use change on soil organic carbon fractions and functional groups

Soil organic matter (SOM) fractions and functional groups can be used to study the short-term as well as the long-term effects of land use change on soil organic carbon (SOC) dynamics. In this study, soil samples of grassland (native vegetation), planted forest and cropland (both converted from grassland) were collected from three soil pedogenetic horizons of A-horizon, AB-horizon and Bk-horizon in a semiarid area of Northeast China. The amounts of SOC, particulate organic carbon (POC), and mineral-associated organic carbon (MOC) were evaluated. After grassland conversion into planted forest and cropland, the amount of SOC decreased by 5.71 and 13.59%, POC decreased by 17.39 and 34.72%, and MOC decreased by 2.84 and 8.36%. Based on the analysis of MOC composition, the average content of humic extract-C decreased by 2.19% and humin-C decreased by 3.08% for planted forest, meanwhile humic extract-C decreased by 14.14% and humin-C decreased by 6.24% for cropland, compared to grassland. This result suggested that labile SOC fractions were more affected by land use change. Solid-state 13C nuclear magnetic resonance (NMR) spectra of bulk soil showed that native grassland SOC had a higher proportion of alkoxy C but a lower proportion of aromatic C, and the aromaticity was less than that of planted forest and cropland. Our results suggested that although the chemical structure of SOM tend to be stable, SOM fraction levels were not enough to recover. Reasonable grassland management measures should be formulated to prevent the continued conversion of native grassland.