Wheat straw and its biochar had contrasting effects on soil C and N cycling two growing seasons after addition to a Black Chernozemic soil planted to barley

Application of crop residues and its biochar produced through slow pyrolysis can potentially increase carbon (C) sequestration in agricultural production systems. The impact of crop residue and its biochar addition on greenhouse gas emission rates and the associated changes of soil gross N transformation rates in agricultural soils are poorly understood. We evaluated the effect of wheat straw and its biochar applied to a Black Chernozemic soil planted to barley, two growing seasons or 15 months (at the full-bloom stage of barley in the second growing season) after their field application, on CO2 and N2O emission rates, soil inorganic N and soil gross N transformation rates in a laboratory incubation experiment. Gross N transformation rates were studied using the N-15 isotope pool dilution method. The field experiment included four treatments: control, addition of wheat straw (30 t ha(-1)), addition of biochar pyrolyzed from wheat straw (20 t ha(-1)), and addition of wheat straw plus its biochar (30 t ha(-1) wheat straw + 20 t ha(-1) biochar). Fifteen months after their application, wheat straw and its biochar addition increased soil total organic C concentrations (p = 0.039 and < 0.001, respectively) but did not affect soil dissolved organic C, total N and NH4 (+)-N concentrations, and soil pH. Biochar addition increased soil NO3 (-)-N concentrations (p = 0.004). Soil CO2 and N2O emission rates were increased by 40 (p < 0.001) and 17 % (p = 0.03), respectively, after wheat straw addition, but were not affected by biochar application. Straw and its biochar addition did not affect gross and net N mineralization rates or net nitrification rates. However, biochar addition doubled gross nitrification rates relative to the control (p < 0.001). Our results suggest that land application of biochar, as opposed to the application of the raw wheat straw, could suppress CO2 and N2O emissions and enhance soil C sequestration. However, the implications of the increased soil gross nitrification rate and NO3 (-)-N in the biochar addition treatment for long-term NO3 (-)-N dynamics and N2O emissions need to be further studied.