Biochar amendment benefits 15N fertilizer retention and rhizosphere N enrichment in a maize-soil system

• Biochar amendment benefited maize N uptake and fertilizer N retention in soil. • Biochar amendment increased N reserved as soil MBN, PON and MTN. • Biochar application benefited fertilizer N enrichment in rhizosphere. Biochar amendments regulate fertilizer nitrogen (N) transformation in soil and crop N uptake. To elucidate the effect of biochar amendment on the translocation of fertilizer N in a crop-soil system during crop growth, a maize pot experiment was conducted using an isotope tracer approach. The treatments were: without N application; 15 N-labelled urea; and 15 N-labeled urea plus biochar. During maize growth, biochar amendment increased root biomass, total crop N uptake, and urea N retention in soil. In bulk and rhizospheric soils, biochar amendment significantly (P < 0.05) increased urea N retention in soil total N (TN), microbial biomass N (MBN), particulate organic N (PON) and mineral associated total N (MTN) from the eighth leaf stage (V8) to physiological maturity (R6) and decreased fertilizer N retention in inorganic N and dissolved organic N from stages V8 to silking (R1). Relative to bulk soil, rhizospheric soil promoted urea N retention in TN, inorganic N, MBN, and MTN under both fertilizer treatments, and biochar application enlarged this difference for MBN; rhizospheric soil enhanced PON mineralization resulting in a significantly (P < 0.05) lower PON from stages R1 to R6 when compared with bulk soil, and biochar application retarded this mineralization. These findings unveiled that biochar application increased fertilizer N retention by increasing MBN, PON, and MTN and that biochar application benefited the enrichment of fertilizer N in rhizospheric soil.