Optimal biochar amendment rate reduced the yield-scaled N 2 O emissions from Ultisols in an intensive vegetable field in South China.

Nitrous oxide (N ₂ O) 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 ^-1 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 N ₂ O emissions by 34-67%, which concurred with decreased denitrification enzyme activity and increased nosZ gene abundance in the vegetable soil. The absolute N ₂ O mitigation increased with increasing flux rates, which were positively correlated to soil temperature and water-filled pore space. Conversely, weak increases of N ₂ O 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 N ₂ O emissions by 44-71% with the lowest yield-scaled N ₂ O emissions for the intermediate biochar application rate of 20 Mg ha ^-1 . Higher biochar application rates failed to further decrease the yield-scaled N ₂ O emissions, but rather caused weak increases. Based on the present results, a biochar application rate of 20 Mg ha ^-1 combined with N fertilization seemed to be recommendable to achieve highest vegetable yield with lowest N ₂ O emissions in intensive vegetable production in South China.
Competing Interests: Declaration of competing interest The authors declare no competing financial interests.
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