Soil organic carbon availability determines the role of crop straws in regulating N2O emissions from maize agroecosystems

Straw return enhances soil quality and cropland sustainability but increases risks of inducing larger nitrous oxide (N 2 O) emissions due to the simultaneous input of carbon and nitrogen (N). However, the response of N 2 O emissions to straw remains controversial, depending on the management-driven dynamics of soil organic carbon (SOC), especially its availability. Here, two fields with the same SOC quantity but distinct availability (HCA and LCA, i.e., high vs. low lability) were established with a factorial design of fertilization and straw, to track the straw decomposition, N 2 O fluxes, available substrates, and N-cycling microbes. Furthermore, a meta-analysis was integrated with the field experiments to decipher how SOC availability regulates N 2 O emissions responding to straw returning. Meta-analysis revealed that SOC availability mediates the effect of straw return on N 2 O emissions from maize agroecosystems, with dissolved organic carbon (DOC) identified as the primary regulator of response direction and magnitude, and exhibits a negative correlation with N 2 O emission response. Consistently, our field experiments showed that straw incorporation combined with fertilization significantly reduced N 2 O emissions by 22 % in HCA soils but increased emissions by 55 % in LCA soils. Straw incorporation into HCA facilitated bacterial growth while reducing AOB abundance and ammonium supply, suggesting restricted nitrification due to promotion of N immobilization, which ultimately suppressed N 2 O emissions. Conversely, straw in LCA soils exhibited a faster decomposition rate and stimulated denitrifiers ( nirS and nirK ) metabolism by increasing DOC concentration and biodegradability, leading to increased N 2 O emissions. Consequently, the fertilizer-induced N 2 O emission factor decreased from 1.53 % to 1.11 % by straw for HCA, while increased from 0.67 % to 1.19 % for LCA. Overall, these findings highlight that SOC availability determines the direction and magnitude of straw’s impact on N 2 O emissions, which should be fully considered into sustainable straw management strategies to balance SOC improvement against climate change. • Field and meta-analysis showed SOC availability drives N₂O response to straw return. • Straw induced opposite N 2 O responses from soils with contrasting SOC availability. • Straw reduced N 2 O emissions in high C-availability soil via boosting N immobilization. • Straw increased N 2 O emissions in low C-availability soil via promoting denitrification. • Labile C supply more facilitated the nosZI - than nosZII -harbouring bacteria.