Carbon mineralization in the soils under different cover crops and residue management in an intensive protected vegetable cultivation

Continuous cropping under plastic greenhouses, a common practice in intensive Chinese vegetable production systems, has led to the decline of soil productivity and crop yields. A 4-year greenhouse experiment on cucumber double-cropping systems was conducted in Changping country, Beijing, China, to investigate the effects of summer cover crops and residue management on soil microbial biomass carbon (MBC), C mineralization and cucumber yield. The treatments included sweet corn with residue removal after harvest (SR), sweet corn with residue incorporation after harvest (SI), common bean with residue removal after harvest (CR), common bean with residue incorporation after harvest (CI), Garland chrysanthemum and edible amaranth as summer cover crops (GR), and bare fallow during the summer period (Control). The experiment was a randomized complete block design with three replicates. C mineralization was analyzed by trapping the respired CO₂ using air-tight Mason jars containing NaOH vials. After 4 years of the trial, the treatments SR, SI and GR showed significantly (P<0.05) higher levels of potentially mineralized readily available carbon, Cₐ (μgCO₂-Cg⁻¹), than control. The Cₐ values for residue retention treatments were significantly related to the biomass inputs for both sweet corn- (r=0.981; P<0.01) and common bean-related (r=0.994; P<0.01) treatments. Generally, the SI and GR treatments showed greater cumulative C mineralization (CCM) than the other treatments. The relatively low CCM:MBC observed for the SI and GR treatments were significantly (P<0.05) related to increased cucumber fruit yields from 2005 to 2008. On the contrary, the CR and CI treatments resulted in significant declines in cucumber fruit yields over the same time periods. The responses of cucumber yields to cover crop treatments were related to the dynamics of soil C. These results suggested that the treatments SI and GR could increase C sequestration and improve soil productivity. Further research is required to test how much loss of C (SOC) occurs from the soil due to the protected cultivation using the in situ testing method.