Optimal biochar amendment rate reduced the yield-scaled NO emissions from Ultisols in an intensive vegetable field in South China.

Nitrous oxide (NO) emissions, vegetable yields, and soil microbial properties were studied in response to different rates of rice-straw biochar applied to an intensive vegetable soil (Ultisol) in South China. The study was conducted over a one-year period as a block-designed field experiment (n = 3) with two successive crops and five harvests in total. Biochar was applied at rates of 0, 10, 20, 30 and 40 Mg ha and splits of nitrogen (N) fertilizer were added in the form of urea (1010 kg N in total). References without biochar and N fertilization were included. Biochar significantly decreased the cumulative annual NO emissions by 34-67%, which concurred with decreased denitrification enzyme activity and increased nosZ gene abundance in the vegetable soil. The absolute NO mitigation increased with increasing flux rates, which were positively correlated to soil temperature and water-filled pore space. Conversely, weak increases of NO emissions were recurrently induced by biochar when the soil temperature was lower than 20 °C and the absolute fluxes were low. A significant 17-29% increase in vegetable yield was induced by biochar, which also ameliorated soil fertility by increasing the soil carbon content and the cation exchange capacity. Overall, biochar significantly decreased the yield-scaled NO emissions by 44-71% with the lowest yield-scaled NO emissions for the intermediate biochar application rate of 20 Mg ha. Higher biochar application rates failed to further decrease the yield-scaled NO emissions, but rather caused weak increases. Based on the present results, a biochar application rate of 20 Mg ha combined with N fertilization seemed to be recommendable to achieve highest vegetable yield with lowest NO emissions in intensive vegetable production in South China.