Intact and washed biochar caused different patterns of nitrogen transformation and distribution in a flooded paddy soil

Abstract

Abstract Biochar soil amendment has been proposed as a novel way of mitigating greenhouse gas emissions and improving crop productivity. However, the effects of biochar on N2O emissions and rice growth are inconsistent. Here, we investigated how biochar and its residue after washing affected nitrogen (N) transformation and rice growth and how far these effects remain from biochar using novel three-compartment pots. Intact biochar, washed biochar, washed biochar plus extract were placed in the central compartment of the pot, while rice plants were grown in the near-char zone ( 5\xa0cm). The addition of biochar and its extract generally increased N concentration in the roots and grains for the plants grown in the near-char zone but decreased the N concentrations for those in the far-char zone. The intact biochar increased the aboveground biomass, especially for the plant grown in the near-char zone, whereas the washed biochar had little influence on plant growth at the maturity and on dissolved organic C, total organic N, NH4+-N and available P. However, washed biochar, as well as intact biochar, still lowered N2O emissions from the near- and far-char zones. The treatments, sampling times and rice growth all played important roles in regulating N-related functional genes, while sampling zones only affected the abundances of amoA and nifH genes. The 16S rRNA, bacterial amoA and narG gene abundances were highest with the intact biochar. The abundance of nirS and nirK genes increased with the washed biochar plus its extract but not with washed biochar alone. The N-functional microorganisms responded to the intact biochar, its soluble component and washed biochar differently and were closely correlated to microbial biomass C, NH4+-N, available P and electrical conductivity. Future research should be devoted to studying the effects of soluble component from biochar and the residual effects of biochar loading on plant growth, soil nutrients and N2O emissions.