Interactive water-nitrogen effects on tissue-specific maize straw degradation in mollisols

Crop residue decomposition plays a critical role in soil organic carbon (SOC) sequestration. However, the interactive effects of tissue-specific quality, soil moisture, and nitrogen (N) fertilization on this process remain insufficiently understood. To address this gap, we conducted a 120-day incubation experiment using 13 C-labeled maize straw tissues (stem, leaf, and sheath) to monitor residue chemical evolution and microbial community dynamics under three soil moisture levels (45%, 65%, and 90% water-holding capacity [WHC]) and three N addition rates (0, 24, and 36 mg N per 100 g soil). The results demonstrated that residue quality regulated decomposition patterns. Leaf residues exhibited the greatest degradation extent and SOC contribution, exceeding those of stems and sheath by 23.88–82.11% and 8.91–99.38%, respectively, attributable to their lower lignin content and higher nutrient concentrations. Soil moisture exerted tissue-dependent effects. Leaf degradation increased linearly with rising moisture, driven by enhanced leaching of labile compounds and improved substrate–microbe contact. In contrast, sheath degradation responded non-linearly, with both drought and excessive moisture promoting greater decomposition than normal moisture conditions, albeit via distinct mechanisms— K -strategist dominance under drought and physical fragmentation under waterlogging. Nitrogen effects were dose-dependent across all tissues. Low N addition (24 mg N) stimulated decomposition by alleviating microbial N limitation and increasing the fungal-to-bacterial (F/B) ratio, whereas high N addition (36 mg N) suppressed decomposition due to microbial N saturation, pH-mediated community inhibition, and lignin–N complex stabilization. Under drought conditions, low N promoted stem degradation by increasing the F/B ratio by 93.00%, thereby selecting for K -strategist decomposers, yet suppressed leaf degradation by limiting labile carbon leaching. Conversely, high N reversed this pattern, suppressing stem degradation while enhancing leaf decomposition. Under excessive moisture, low N inhibited degradation of all tissues by reducing the F/B ratio, whereas high N enhanced decomposition through increased microbial biomass and improved substrate accessibility. A mixed-effects model revealed tissue-specific driver hierarchies: moisture explained the largest proportion of variance in stem (39.97%) and leaf (45.34%) decomposition, whereas N dominated sheath decomposition (69.31%). Random forest analysis further identified G - bacteria as the primary predictor of stem carbon transformation and actinomycetes as the key predictors of leaf decomposition. These findings underscore the pivotal role of water–N–microbe interactions in regulating tissue-specific residue decomposition, with microbial community composition acting as a central driver. The results provide a mechanistic basis for developing diversified residue return strategies tailored to tissue type and local hydro-nutrient conditions, thereby enhancing soil carbon storage and promoting the sustainability of agricultural systems under changing climatic conditions. • Water–nitrogen interactions drive tissue-specific decomposition of maize residues via microbial and chemical pathways. • Under drought, low N promotes fungal decomposition of stems, while under sufficient moisture, high N enhances bacterial degradation of leaves. • Leaves exhibit the highest contribution to soil organic carbon, with carbon sequestration efficiency being regulated by the combined effects of moisture and nitrogen on microbial activity and residue chemistry.