Linking feedstock and application rate of biochars to N₂O emission in a sandy loam soil: Potential mechanisms

Biochar amendment has been proposed as a strategy to improve soil quality and mitigate climate change. Yet the extent and regulating mechanisms of soil nitrous oxide (N₂O) emission in response to amendment of biochars derived from different feedstock have not been clearly demonstrated. A 12-month field trial was conducted to study the impact of two distinct biochar feedstocks (pine chip and peanut shell, at 10 and 30 t ha⁻¹) with (+N) and without nitrogen (N) (−N) fertilization on soil N dynamics, N₂O emissions, plant performance and microbial gene abundance. The cumulative N₂O emissions varied greatly with biochar feedstock and application rate, and N fertilization. Pine biochar (at 30 t ha⁻¹) significantly increased N₂O emissions (by 48%) compared to control under −N, while peanut shell biochar (at 30 t ha⁻¹) significantly increased N₂O emissions (by 131%) compared to control under +N. Overall, biochar amendments had no impact on soil mineral N (NH₄ ⁺ and NO₃ ⁻) under both −N and +N. Under −N, biochar amendments showed no effect on plant yield, whereas under +N yield was significantly improved by 25–27% from pine biochar treatments. Under +N, only pine biochar (at 10 t ha⁻¹) significantly increased plant N uptake (by 47%), and only peanut shell biochar (at 30 t ha⁻¹) significantly enhanced soil AOB, narG, nirS and nosZ gene abundance. Increased soil N₂O emission under −N from pine biochar amendment is likely to be regulated by improved water retention. However, increases in N₂O emission from peanut shell biochar under +N could be driven by its large N inputs, labile organic C concentration and the increased soil pH which might largely facilitate biotic processes as evidenced by higher narG, nirS and AOB gene abundance. Positive feedback to N₂O emission by biochar application revealed from this study is inconsistent with many current biochar studies, showing that caution should be exercised when considering use of biochar to mitigate N₂O emission.