Effect of biochar additions to soil on nitrogen leaching, microbial biomass and bacterial community structure

Previous studies already demonstrated that biochar addition reduces nitrogen (N) leaching in soil, but little information is available about its effects on N leaching and bacterial community structure under the application of organic N. This study investigated the effects of corn-straw biochar under the application of urea (250 kg N ha−1) in layered soil columns. The PCR-amplified partial 16S rRNA genes in soil were sequenced before and after biochar treatment in order to assess the change of bacterial diversity and community structure utilizing the Illumina technology. With the application of 2% (B2), 4% (B4) and 8% (B8) biochar (mass ratio), the cumulative amount of total leached nitrogen was reduced by 18.8%, 19.5% and 20.2%, respectively (P < 0.05). More than 90% of the total nitrogen leaching was in the form of nitrate, and increasing amount of biochar resulted in reduced amount of N leaching. The water holding capacity, microbial biomass, pH, electrical conductivity, net N mineralization and respiration rate of the soil were all increased under biochar treatments, except that the B8 treatment decreased soil respiration rate and net N mineralization in comparison with B4. Bacterial diversity increased in biochar-amended soil and was positively correlated with the addition ratio of biochar. Dominant phyla across all samples were Proteobacteria, Acidobacteria, Chloroflexi, Bacteroidetes, Actinobacteria, Nitrospirae and Gemmatimonadetes. The relative abundance of Acidobacteria, Chloroflexi and Gemmatimonadetes decreased under biochar treatments, while that of Proteobacteria, Bacteroidetes and Actinobacteria increased. Overall, biochar increased water holding capacity, enhanced microbial biomass and changed bacterial community structure of the soil which may all have contributed to the reduction of nitrogen leaching.