Effect of chemical fertilizer reduction with return of Chinese milk vetch (Astragalus sinicus L.) on soil labile organic carbon and carbon conversion enzyme activities

Objectives To evaluate the effects of reduction of chemical fertilizer with return of Chinese milk vetch (Astragalus sinicus L. hereafter vetch) on paddy soil labile organic carbon content and C conversion enzyme.

Methods Field experiments were carried on the paddy soil derived from quaternary red clay in Jinhua City, Zhejiang Province. Vetch cultivar Ningbodaqiao was used in this experiment. The vetch was incorporated into soil at flowering stage in April every year. The field experiments continued for six years and had seven treatments, i.e. CK (no fertilizer), CF (only application of chemical fertilizers), MV (vetch return without chemical fertilizers), MV + 20%CF (vetch return plus 20% chemical fertilizers), MV + 40%CF (vetch return plus 40% chemical fertilizers), MV + 60%CF (vetch return plus 60% chemical fertilizers), and MV + 80%CF (vetch return plus 80% chemical fertilizers). The amount of fresh vetch returned was 45 t/hm². The amount of chemical fertilizer was 210 (N), 56.3 (P₂O₅) and 112.5 (K₂O) kg/hm² in the CF treatment. After the rice harvest, soil samples were collected in October, 2016. Contents of soil labile organic carbon, including dissolved organic carbon (DOC), hot-water extractable organic carbon (HEOC), microbial biomass carbon (MBC), particle organic carbon (POC), readily oxidizable organic carbon (ROC), and soil total organic carbon (TOC), and C-conversion enzyme (cellulase, invertase, β-glucosidase, phenol oxidase and peroxidase) activities were investigated.

Results In comparison with CF treatment, MV treatment significantly increased the contents of DOC, HEOC and ROC, but had no effect on the contents of MBC and TOC. The contents of DOC, HEOC, MBC, ROC and TOC in CF and MV treatments were significantly lower than those in MV + 20%CF, MV + 40%CF, MV + 60%CF和MV + 80%CF treatments. With the increase of chemical fertilizers application rates, the contents of DOC, HEOC, MBC and ROC increased first and then decreased, with the highest contents in MV + 60%CF treatment, while TOC content showed little change. The content of POC in MV treatment was higher than that in CF treatment, but lower than that in MV + 60%CF treatment. MV treatment significantly increased the values of DOC/TOC and HEOC/TOC compared with CF treatment. The values of DOC/TOC and ROC/TOC in CF treatment were significantly lower than those in MV + 20%CF, MV + 40%CF, MV + 60%CF and MV + 80%CF treatments. The increase in chemical fertilizer application amount had no significant influence on the ratio of soil labile organic carbon to TOC. The carbon management index in CF treatment was significantly lower than that in MV, MV + 20%CF, MV + 40%CF, MV + 60%CF and MV + 80%CF treatments. The carbon management index tended to increase with the increase of chemical fertilizer application amount after vetch incorporation. Compared with CF treatment, MV treatment significantly increased the activities of cellulase, invertase and β-glucosidase but had little influences on the activities of phenol oxidase and peroxidase. The cellulase, invertase, β-glucosidase, phenol oxidase and peroxidase activities in both CF and MV treatment were significantly lower than those in MV + 20%CF, MV + 40%CF, MV + 60%CF and MV+80%CF treatments. With increase of chemical fertilizer application rates, the five enzyme activities increased first and then decreased, with the highest contents in MV+60%CF treatment. Geometric mean enzyme activity and total enzyme activity were significantly influenced by different fertilizer treatment in the order of CK < CF < MV < MV + 20%CF < MV + 40%CF and MV + 80%CF < MV + 60%CF (P < 0.05). MV treatment showed significantly higher relative activities of cellulase and invertase (the ratio of enzyme activities to microbial biomass carbon) compared with CF treatment. CF treatment had significantly lower relative activities of invertase and β-glucosidase but higher relative activity of peroxidase compared with MV + 20%CF, MV + 40%CF, MV + 60%CF and MV + 80%CF treatments. Path analysis showed that cellulase activity had the highest positive direct effect on HEOC content, invertase activity had the highest positive direct effect on DOC and MBC contents, and β-glucosidase activity had the highest positive direct effect on POC and ROC contents. Furthermore, phenol oxidase activity had the highest positive indirect effect on DOC content, β-glucosidase activity had the highest positive indirect effect on HEOC, and peroxidase activity had the highest positive indirect effect on MBC, POC and ROC.

Conclusions Return of vetch along with chemical fertilizers could significantly increase soil labile organic carbon contents and C-cycling involved enzyme activities, with the best effects in the application of 60% chemical fertilizers treatment. Cellulase, invertase and β-glucosidase activities had direct positive effects on soil labile organic carbon contents. Phenol oxidase and peroxidase activities showed indirect effects on soil labile organic carbon contents.