While soil freeze-thaw cycles can increase soil nutrient losses and greenhouse gas emissions, biochar as a soil amendment could mitigate these effects, potentially boosting crop yields. Nevertheless, there is often variation in the effectiveness of different biochar formulations with respect to soil nutrient retention. Therefore, we asked 1) to what extent pyrolysis temperature affects the ability of biochar to increase N retention over winter and spring melt, and 2) to what extent the latter affects soil N availability and plant productivity over the subsequent growing season. We used 15N tracer in soil mesocosms to examine the effects of biochar produced under a series of pyrolysis temperatures (250-600 oC) on soil nitrogen retention in response to variation in soil freeze-thaw cycle intensity (-10 oC vs. 0 oC following spring melt), and we examined the subsequent effects on plant nitrogen uptake.
Results/Conclusions
Nitrate leaching increased with freezing, but it decreased with increasing biochar pyrolysis temperature. N2O emissions peaked on the 4th day after melt, and there was a significant interaction between biochar addition and freezing, with the highest emissions occurring at the highest pyrolysis temperatures (> 450 oC) in response to freezing. For plant biomass, there was a significant interaction between freezing and biomass addition, whereby freezing alone decreased plant biomass, but the freezing effect was mitigated by biochar addition. Biochar application also increased over-winter soil 15N retention. For the two highest pyrolysis temperatures, biochar addition increased both plant 15N concentrations and the total amount of 15N uptake. Overall, our results revealed that biochar as a soil amendment can enhance soil nitrogen retention over winter, with benefits for plant N uptake and biomass production in the following growth season. Thus, increased benefits of biochar amendment may be realized in the context of increasing soil freezing in northern temperate regions that are experiencing reduced snow cover. Nevertheless, the effects of biochar on potentially increasing soil emissions of N2O, a potent greenhouse gas, must also be considered in evaluating its use as a soil amendment.