Exogenous organic materials applied to paddy field improving soil microbial biomass C, N and dissolved organic C, N

Abstract: Soil microbial biomass C, N and dissolved organic C, N are the most active organic C components, and they can serve as important and also sensitive indexes for soil in response to disturbance. The change in characteristics of soil microbial biomass C (MBC), N (MBN) and dissolved organic C (DOC), N (DON) and their interrelations were studied by setting the field plot experiment with an addition of exogenous organic materials at different organic carbon rate. The results showed that soil microbial biomass C, N and dissolved organic C, N under the combined application of mineral fertilizers and biochar, maize straw, cow dung and pine needle were significantly (P<0.05) greater than those under no fertilizer (CK) and mineral fertilizers application treatments, respectively, with an average of 23.52% and 12.66% (MBC), 42.68% and 24.02% (MBN), 14.70% and 9.99% (DOC), 22.32% and 21.79% (DON) higher than those under CK and chemical fertilizers, respectively. For the treatments of combination of mineral fertilizers and different organic materials, soil microbial biomass C, N and dissolved organic C, N in the treatment of mineral fertilizers plus cow dung were the highest, they were 26.20% (MBC), 49.54% (MBN), 19.29% (DOC) and 32.81% (DON) respectively more than those of CK treatment, and next came the treatments of mineral fertilizers plus biochar or maize straw. Soil microbial biomass C, N and dissolved organic C, N were the lowest in the treatment of mineral fertilizers with pine needle incorporated. These differences were caused by the carbon, nitrogen contents and their component differences in quality of organic materials, different microbial communities and their ability to use carbon, nitrogen in organic materials, etc. The content of soil dissolved organic C (308.87 mg/kg soil) was less than microbial biomass C content (474.71 mg/kg soil), and the content of soil dissolved organic N (53.07 mg/kg soil) was less than microbial biomass N content (34.79 mg/kg soil). The combined application of mineral fertilizers and organic material significantly (P<0.05) reduced the MBC/MBN and DOC/DON compared to no fertilization treatment, and the average rates of decrease were 24.57% and 7.71%. With the increase of soil organic carbon (SOC), total nitrogen (TN), the MBC and DOC, MBN and DON were the significant (P<0.05) linearly increase. There were the significant positive correlations (P<0.01) between MBC, MBN, DOC, DON, DOC+MBC and DON+MBN. The degree of correlation between different forms of carbon and nitrogen, DOC+MBC and DON+MBN better illustrated the changes of soil active organic carbon and nitrogen pools than DOC, MBC, DON, MBN alone, and they were the more effective indicators for evaluation of soil fertility and quality. The study provided valuable information for improving soil nitrogen supplying capacity, enhancing soil nitrogen fixation, reducing soil nitrogen loss from farmland in Erhai Lake Valley and protecting water quality and safety of Erhai Lake.