Interactions between biochar and nitrogen impact soil carbon mineralization and the microbial community

Adding biochar (BC) to soils is proposed to enhance carbon sequestration in agricultural soils. However, there is limited knowledge regarding the effects of the interactions between BC and nitrogen (N) on soil organic carbon (SOC) mineralization at an interannual scale. We conducted a three-year field experiment to systematically reveal the impacts of BC combined with N fertilizer on a silty day soil, including its physicochemical properties, soil respiration characteristics and microbial community. BC applied once at 0, 20 and 40 t ha(-1) (B0, B1 and B2, respectively) was combined with three N fertilization levels (0, 120 and 240 kg N ha(-1); N0, N1 and N2, respectively). Only winter wheat (Triticum aestivwn L.) was cultivated, and it was cultivated in a winter wheat-summer fallow crop system. BC application persistently increased the SOC content by 36.3-91.6% over three years. Several soil parameters were also improved by BC combined with N after one year, such as bulk density, NO3- and available P content. Compared with its values with BO treatments with N1 or N2, the total soil CO2 emitted decreased by 6.7-8.9% in response to BC combined with N1 but increased by 5.9-7.5% in response to BC combined with N2. B1N1 not only significantly increased microbial biomass but also decreased qCO(2). In addition, B1N1 in particular increased the relative abundances of members of the microbial population associated with increased microbial C use efficiency, such as Rhizopus and Helotiaceae. However, B2N1 and B2N2 strongly disturbed the soil microbial ecosystem; moreover, the lowest alpha diversity of the bacterial community was observed in B2N2 soils. In summary, adding BC at a high rate to N-fertilized soil strongly disturbs the soil microbial ecosystem and reduces the C-sequestering potential of soil, while B1N1 treatment is recommended to enhance soil C sequestration and improve soil fertility under dryland farming.