Combined application of biochar and nitrogen fertilizer benefits nitrogen retention in the rhizosphere of soybean by increasing microbial biomass but not altering microbial community structure.

The application of nitrogen fertilizer and biochar (N-biochar) is one of promising management practices recommended to improve soil fertility. A field trial based on seven treatments (control, urea, urea + biochar, KNO3, KNO3 + biochar, (NH4)2SO4, (NH4)2SO4 + biochar) were carried out in the field. Both rhizosphere and bulk soils were collected after harvest to investigate the response of microorganisms to N-biochar and the influence on N retention. The results showed that N-biochar increased total microbial biomass but did not change the relative proportions of microbial groups compared with the control. Principal component analysis suggested that microbial community structure in the rhizosphere differed from that in bulk soils, with lower ratios of G+/G- bacterial PLFAs, Saturated/Monounsaturated PLFAs and Cyclopropyl/Precursor PLFAs in the rhizosphere. More abundant biomass and better developed community structure in the rhizosphere could be ascribed to labile C from root exudates. The increased N content in soil-plant system as well as the relative high concentration of total N in the rhizosphere indicated that N-biochar benefited N retention towards near-root areas. Redundancy analysis and correlation analysis suggested that both fungi and bacteria were involved in promoting N retention, especially G- bacteria. Biochar-induced high soil pH favored G- bacteria development while the importance of labile C availability from biochar to G- bacterial activity.