Nutrient supply enhances positive priming of soil organic C under straw amendment and accelerates the incorporation of straw-derived C into organic C pool in paddy soils

Straw return accelerates the decomposition of soil organic C (SOC), a phenomenon referred to as the priming effect. However, the interactive influence of nutrient supply levels on priming effect intensity and SOC sequestration in paddy soils still needs to be better understood. In this study, we investigated the dynamics of the priming effect and associated changes in phospholipid fatty acids, enzyme activity, and microbial necromass following the addition of ¹³C-labelled rice straw (98 % atom) to soils under three nutrient supply levels during a 300-d incubation period. Our results showed that the addition of straw (5 g C kg⁻¹ soil) with no-nutrient (S + Nu₀), low nutrient (S + Nu_low, 42 mg N kg⁻¹, 10 mg P kg⁻¹), and high nutrient (S + Nu_high, 126 mg N kg⁻¹, 30 mg P kg⁻¹) supply increased total CO₂ production by 42.9 %, 59.0 %, and 97.3 %, respectively, compared to the control soil. After 300 d, the cumulative priming effect was nearly doubled in the S + Nu_low and tripled in the S + Nu_high compared to the S + Nu₀. Moreover, the intensity of priming varied with the incubation stage under nutrient treatments. Similar patterns of priming effect were observed across all straw amendments during the early incubation stages; however, the priming effect increased with the nutrient supply levels in the later stages. These patterns are linked to microbial metabolic limitation and resource acquisition strategies, as evidenced by a lower C-to-N stoichiometry of extracellular enzymes and necromass in the S + Nu_low S + Nu_high. A greater proportion of straw-derived C incorporation into SOC (indicated by higher levels ¹³C-SOC) in nutrient-enriched was found, which largely offset the native SOC losses, resulting in high SOC content by the end of incubation. Our findings highlight the critical role of nutrient supply in regulating the priming effect and the balance of SOC after straw return in paddy soils.