Effects of long-term fertilizer management on soil labile organic carbon fractions and hydrolytic enzyme activity under a double-cropping rice system of southern China.

Fertilization is an effective way to improve soil quality, increase soil fertility and soil microbial diversity in paddy soil. To explore the changes of soil labile organic carbon (C) fractions and hydrolytic enzyme activity after 34 years fertilization treatments in a field experiment in double-cropping rice system of southern China. There were four treatments, including chemical fertilizer alone (MF), rice residue and chemical fertilizer (RF), 30% organic matter and 70% chemical fertilizer (OM), and the control without fertilizer input (CK). We measured soil organic carbon (SOC) content, soil labile organic C fractions, SOC related hydrolytic enzyme activity, correlation coefficients of soil enzyme activity with SOC content and its labile organic C fractions. The results showed that MF, RF and OM increased SOC content by 4.5%, 22.4% and 53.5%, respectively. Compared with MF and CK, RF and OM increased soil labile organic C fractions [cumulative C mineralization (Cmin), permanganate oxidizable C (KMnO-C), particulate organic C (POC), dissolved organic C (DOC), light fraction organic C (LFOC), microbial biomass C (MBC)] and the proportion of each labile organic C fractions to total organic C. The contents of Cmin, KMnO-C, POC, DOC, LFOC and MBC under OM treatment were 3.5, 3.1, 3.7, 1.9, 1.2 and 1.9 times higher than CK treatment, respectively. The proportion of labile organic C fractions to total organic C of RF and OM treatments was significantly higher than that in CK. The order of soil hydrolytic enzyme activity [α-glucosidase (αG), β-glucosidase (βG), β-xylosidase (βX), cellobiohydrolase (GBH), and N-acetyl-β-glucosaminidase (NAG)] was OM>RF>MF>CK. The soil hydrolytic enzyme activity under OM treatment increased by 111.8%, 14.1%, 127.3%, 285.6% and 91.4% compared with CK, respectively. Furthermore, RF and OM treatments were beneficial to soil peroxidase (POD) activity. MF treatment was beneficial to soil polyphenol oxidase (PPO) activity. There was a significant positive correlation between soil hydrolytic enzyme activity and SOC content and its labile organic C fractions. In conclusion, the combined application of organic manure, rice straw returning and chemical fertilizer is an effective method to improve soil labile organic C fractions and hydrolytic enzyme activity in a double-cropping rice paddy field of southern China.