Differences in carbon and nitrogen retention and bacterial diversity in sandy soil in response to application methods of charred organic materials

The use of charred organic materials (biochar) as soil amendment can alter soil nutrient, microbial abundance, and their diversities. These alterations can be influenced by the biochar source, application method and amount, but the details are still unknown. Thus, this study aimed to compare two methods of biochar application (surface or mixed) on carbon (C) and nitrogen (N) losses with their related effect on microbial community under C-depleted sandy soil. Chicken manure (CM), rice husk (RH) and rice straw (RS), pyrolyzed at 350 °C to produce biochar, were tested in a pot trial. The biochar was applied singly, and in combined forms (CM + RH and CM + RS) under dent corn, as mixed (incorporated) or surface application, at different rates of 0, 15, and 30 g kg −1 soil (equivalent to 0, 7.5 and 15.0 Mg ha −1 ). The soil respiration ( S R ), leaching loss of organic-C and inorganic-N (as nitrate-N), microbial biomass C at top 10 cm soils, bacterial relative abundance and community structure were measured. The application methods' effect on S R was unclear but the surface application reduced the C leaching loss and increased nitrate-N loss compared to the mixed application, in general. The microbial biomass C was found to be significantly higher ( P < 0.001) under the surface applied (459 ± 252 mg C kg −1 soil) compared to the mixed applied (76.1 ± 18.5 mg C kg −1 soil). Surface application of biochar also increased microbial diversity on soil surface. The increase in diversity was characterized by an increase in OTU numbers within the phylum Actinobacteria and Proteobacteria . We concluded that surface application of biochar increased the microbial diversities in the soil and thus, can retain C in low C soils through higher immobilization of C, but might be unsuitable when biochar is applied with the aim being to reduce nitrate-N loss in soils. • Surface application of biochar materials increased microbial diversity and evenness. • Microbial biomass was increased in surface application even at lower biochar rate. • Mixed biochar material application reduced nitrate release from the soil. • Overall, there was higher carbon mineralization in mixed applied biochar.