Biochar can influence soil microbial biomass. It is not clear how biochar amendment affects soil microbial biomass carbon and nitrogen (MBC and MBN) and dissolved organic carbon and nitrogen (DOC and DON) in double-cropping rice soils. To address this problem, two subtropical double-cropping rice soils (S1 and S2) were selected for an incubation experiment. S1 is developed from granite-weathered red soil and S2 is developed from Quaternary red clay. The following three wheat straw-derived biochar application rates were used, without N fertilizer, in each paddy soil:0%, 1%, and 2% of soil weight, represented by CK, LB, and HB, respectively. After a 70 d incubation, soil mean MBC was 877.03 mg·kg-1, 832.11 mg·kg-1, and 849.30 mg·kg-1 in S1 for the three application rates, and 902.94 mg·kg-1, 874.19 mg·kg-1, and 883.22 mg·kg-1, respectively, in S2. S1+LB, S1+HB, and S2+LB treatments reduced soil mean MBC compared to the CK treatment (P<0.05). This may be attributed to biochar inhibiting microbial growth by adsorbing soil organic carbon and other low-molecular-weight organic matter. Low biochar application rates decreased mean soil MBN by 9.45% compared to the CK treatment in S1 (P<0.05). No significant differences in mean MBC/MBN were observed among the S1 treatments, but LB reduced MBC/MBN in S2 (P<0.05). Due to the soluble organic carbon content and strong alkalinity of biochar, biochar amendment increased mean soil DOC by 4.42%-22.20% and 10.57%-35.47% in S1 and S2, respectively (P<0.05). However, biochar amendment (except for the S2+HB treatment) decreased mean soil DON in both paddy soils. This may have resulted from the adsorption of soil organic nitrogen by biochar and N consumption during the decomposition of the organic carbon within biochar. Biochar amendment increased mean soil DOC/DON in both paddy soils (P<0.05) and mean DOC/DON increased with an increase in the biochar application rate. Based on these results, biochar amendment increased soil dissolved organic carbon, decreased soil microbial biomass, and enhanced the nitrogen deficit in double-cropping paddy soils. Therefore, biochar should be combined with the application with fertilizer in double-cropping rice systems in subtropical central China.
Keywords: biochar; dissolved organic carbon, nitrogen; double rice cropping system; microbial biomass carbon, nitrogen; paddy soils.