Optimizing organic substitution: Balancing carbon sequestration and priming effects of a six-year field experiment for sustainable vegetable production

Organic substitution has been recognized to markedly influence soil organic carbon (SOC) dynamics and carbon sequestration. However, the priming effect can alter SOC mineralization, contingent upon the varying proportions and intensities of mineralization introduced by fresh organic substrates. A comprehensive assessment to pinpoint the organic substitution level in intensive vegetable cultivation remains elusive. Thus, we carried out a 6-year field experiment, following four fertilization treatments at consistent nitrogen levels: solely mineral fertilizer; and organic fertilizers replacing 20 %, 50 %, and 100 % of mineral nitrogen fertilizer. We assessed carbon dioxide emissions derived from various sources, soil properties, nutrient availability, enzyme activity, 13C-phospholipid fatty acid, SOC stability, and nutrient stoichiometries via a 63-day incubation with 13C glucose. Our results showed that different levels of organic substitution significantly affected soil mineralization and priming effect. The microbial community was the primary determinant of priming effect, accounting for 59.4 % of the variation, with gram-positive bacteria (i.e., a15: 0) being instrumental. Moreover, organic fertilizers provided a large amount of labile substrate, diminishing the microbial requirement for enzyme production in SOC decomposition. Notably, 50 % organic substitution consistently outperformed others, consistently yielding 1.17 to 1.29 times more than alternative treatments. Furthermore, this approach exhibited the lowest cumulative priming effect, which declined by 26.9 % to 44.8 %, attributed to enhanced SOC stability. Therefore, a 50 % organic substitution effectively balances carbon sequestration and the priming effect, contributing to sustainable vegetable production. Collectively, our results highlight the importance of appropriate organic substitution rates in promoting carbon sequestration and reducing greenhouse gas emissions for producing higher vegetable yield. Further work should emphasize long-term field evaluations to validate our priming effect determinants and explore the comprehensive impacts on soil health and sustainable agriculture.