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

The aim of this study was to explore nitrous oxide (N₂O) emissions and key impact factors under alternative organic fertilizer substitution management in South China in the context of the regional conditions (hot and rainy climate, acidic soils) and high multiple cropping index cultivation. We conducted one-year in situ observations (from December 3 in 2022 to December 1 in 2023) 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% chemical fertilizer nitrogen with commercial organic fertilizer (M15% and M30%), and substitution 15% of chemical fertilizer nitrogen with composted vegetable residue (S15%). Compared to R treatment, there was no significant difference in crop yield under the organic fertilizer replacement treatments. 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 under R treatment was 2.66 kg(N)∙hm⁻², there was no significant difference between M15% and R treatment. However, compared to R treatment, the annual N₂O emission under M30% and S15% treatment increased by 52.25% and 38.72% (P<0.05), respectively. Under the R treatment, the N₂O emissions of P1, P2, P3, P4, and the leisure period accounted for 8.30%, 11.32%, 46.79%, 13.21%, and 20.38% of the annual emissions, respectively. The effects of substituting chemical fertilizer with organic fertilizer on N₂O emissions varied among the different crop seasons. The structural equation model revealed that soil temperature showed the greatest contribution to N₂O emissions (P<0.001), while the effects of water filled pore space (WFPS), soil NO₃⁻-N content and NH₄⁺-N content on N₂O emissions were not significant. Based on one-year observations, we recommend organic fertilizer substitution management applied during vegetable growing seasons under crop and vegetable rotation pattern in South China. Considering both crop yield and annual N₂O emissions, we suggest substituting 15% of chemical fertilizer nitrogen with commercial organic fertilizer.