Increased retention of soil nitrogen over winter by biochar application: Implications of biochar pyrolysis temperature for plant nitrogen availability

While soil freeze-thaw cycles can decrease soil nutrient retention over winter by increasing leaching losses and greenhouse gas emissions, biochar as a soil amendment could mitigate these effects. Nevertheless, there is often variation in the effectiveness of different biochar formulations with respect to soil nutrient retention. We added N-15 tracer to soil mesocosms to examine the effects of biochar produced under a series of pyrolysis temperatures (250-600 degrees C) on soil nitrogen retention in response to variation in soil freeze-thaw cycle intensity (-10 degrees C vs. 0 degrees C following spring melt). We also examined the subsequent effects on plant nitrogen uptake, soil nitrous oxide ((NO)-O-2) emissions and nitrogen leaching losses. As we predicted, increased soil freezing increased inorganic nitrogen losses through leaching and decreased the biomass of the test crop Arugula (Eruca sativa) the following growing season. Biochar amendment increased both soil N-15 retention over winter and the subsequent plant N-15 uptake, with the biochar generated at the highest temperature exhibiting the strongest effects on plant N-15 uptake. Biochar addition also significantly mitigated the negative soil freezing effect on subsequent plant biomass. Nevertheless, biochar addition combined with freezing increased N2O emissions. Overall, our results confirm that biochar application can mitigate soil nitrogen losses over winter, although it may also interact with soil freezing to increase emissions of the greenhouse gas N2O. (C) 2016 Elsevier B.V. All rights reserved.