Conversion of cropland to forage land and grassland increases soil labile carbon and enzyme activities in northeastern China

Soil labile carbon (C) and enzyme activities are valuable indicators of changes in soil quality and health. Understanding the changes in soil labile C and enzyme activities under different land uses is important to maintain soil quality and health and for sustainable land use. The primary objective of this study was to investigate the short-term influences of different land uses on SOC, soil labile C and enzyme activities in semiarid alkaline grassland of northeastern China. The experiment was organized as a block design with four replications of each land use treatment. Land use treatments were corn cropland (Corn), alfalfa forage land (Alfalfa), Lyemus chinensis grassland (AG), Lyemus chinensis grassland for mowing (AG+Mow) and restored grassland (RG), which were applied for five years. Total soil organic carbon (SOC), three labile C pools (oxidizable labile C; water-extractable organic C; microbial biomass C) and the activities of four soil enzymes (catalase; urease; alkaline phosphatase; invertase) were determined at the 0–20cm depth in the five land use treatments. Results showed that soil labile C and enzyme activities were sensitive indicators of land use change. Conversion of cropland to forage land and grassland increased SOC (40.42%), soil labile C measures (25.50%) and enzyme activities (55.60%). However, the responses of different forms of soil labile C and enzyme activities to different land uses were not similar. Under Corn, AG+Mow, AG and RG land uses, the geometric means of labile C (27.01%, 10.95%, 17.52% and 5.11%, respectively) and enzyme activities (40.92%, 13.54%, 11.38% and 7.38%, respectively) were lower than those under Alfalfa, demonstrating that soil labile C and enzyme activities improved more under Alfalfa than under other land uses in northeastern China. Significant correlations were also obtained between SOC, soil labile C measures and enzyme activities. To conclude, soil labile C and enzyme activities can be expected to gradually increase with the conversion of cropland to grasslands and forage land, and planting to alfalfa offers a profitable and sustainable solution to our requirement for pairing forage production with rapid restoration of soil quality in the areas in which soils are not suitable for growing crops in the Songnen Grassland.