Characteristics of nitrous oxide emissions in grain and vegetable rotation system in South China Region under organic fertilizer substitution

This study aims at exploring the nitrous oxide (N₂O) emissions and key impact factors under alternative organic fertilizer substitution managements from South China region, which has hot and rainy climate, acidic soils, and high multiple cropping index cultivation. We conducted one year in situ observations (from December 3rd, 2022, to December 1st, 2023) by using the static opaque chamber method. Chinese flowering cabbage season (P1), lettuce season (P2), sweet corn season (P3), and Chinese flowering cabbage season (P4) were cultivated in turn in this study. Five treatments were implemented: no nitrogen fertilizer application (CK), chemical fertilizer application (R), substitution 15% and 30% of chemical fertilizer nitrogen with commercial organic fertilizer (M15% and M30%), and substitution 15% of chemical fertilizer nitrogen with composted vegetable residue (S15%). Results showed that, there was no significant difference in crop yields between organic fertilizer replacement treatments and R treatment. During the whole observation period, the N₂O emission fluxes of each treatment ranged from 6.52 to 2 441.42 μg(N₂O)∙m^−2∙h⁻¹, and the N₂O peaks of each nitrogen application treatment generally occurred within 2−5 d following the fertilization and irrigation events. The annual N₂O emission of R treatment was 2.66 kg(N)∙hm⁻², there was no significant difference between M15% treatment and R treatment, but the annual N₂O emission of M30% treatment and S15% treatment increased by 52.25% and 38.72% (P<0.05). Under R treatment, the seasonal N₂O emissions of P1, P2, P3, P4, and leisure period accounted for 8.30%, 11.32%, 46.79%, 13.21% and 20.38% of the annual emissions, respectively. The impacts of substituting organic manure for compound treatments on N₂O emission varied among different crop seasons. The structural equation model revealed that soil temperature had great contribute to N₂O emissions (P<0.001), while water filled pore space (WFPS), soil NO₃⁻-N content and NH₄⁺-N content barely effected N₂O emissions. According to one-year observations, we recommend organic fertilizer substitution managements applied in vegetable growing seasons under patterns of grain and vegetable rotation in South China region. Considering both crop yields and annual N₂O emission, we suggested substitution 15% of chemical fertilizer nitrogen with commercial organic fertilizer management.