Objectives This study aimed to investigate the effects of different straw interlayer thicknesses on nutrient dynamics, extracellular enzyme activities, microbial resource limitations, and organic carbon fractions in the soil profile of salinized soil, to determine the optimal straw interlayer application rate, and to reveal the synergistic driving mechanism of soil water-salt, nutrients, and microorganisms on the dry matter accumulation of sunflower.

Methods A root box simulation experiment was conducted using salinized soil as the research object. Four treatments were established: no straw interlayer (CK) and straw interlayers with thicknesses of 3 cm (S3, 6000 kg/hm²), 5 cm (S5, 12000 kg/hm²), and 7 cm (S7, 18000 kg/hm²). Soil physicochemical properties, carbon-, nitrogen-, and phosphorus-acquiring enzyme (C/N/P-acq) activities as well as oxidoreductase activities, enzymatic stoichiometric characteristics, organic carbon fractions, and nutrient and dry matter accumulation of sunflower were systematically measured. The interrelationships among these indicators were analyzed using redundancy analysis (RDA) and partial least squares path modeling (PLS-PM).

Results Compared with CK, straw interlayer significantly decreased soil water and salt contents in the 0–40 cm soil layer, increased soil organic matter (SOM) contents across the soil profile and available nutrient contents in the 40–60 cm soil layer, but had no significant effect on soil pH. Straw interlayer also enhanced C/N-acquiring enzyme activities throughout the soil profile while reducing oxidoreductase activities. Soil nutrient and ecoenzymatic stoichiometric characteristics indicated that straw interlayers alleviated phosphorus limitation in the 0–20 cm soil layer and microbial nitrogen and phosphorus limitations throughout the soil profile, with the S5 treatment showing the greatest alleviation effect. Relative to CK, straw interlayer increased soil organic carbon (SOC) stocks, microbial biomass carbon (MBC), and dissolved organic carbon (DOC) by 0.7%−12.2%, 80.7%−215.1%, and 7.7%−47.7%, respectively. Redundancy analysis revealed that cellobiohydrolase (Cello) activity was closely associated with SOC stocks and MBC and contributed significantly to carbon fraction accumulation in the 40−60 cm soil layer. Straw interlayer increased per-plant dry matter accumulation by 18.5%−34.4%, with significant increases observed in the S3 and S5 treatments. Furthermore, straw interlayer increased the proportion of dry matter allocated to roots and promoted plant nitrogen and phosphorus accumulation, with S5 showing the greatest overall improvement. Mantel tests showed that grain nutrient accumulation and dry matter production were closely associated with soil water-salt status, SOM, and C/N-acquiring enzyme activities. Partial least-squares path modeling indicated that soil water-salt conditions and extracellular enzyme activities positively regulated soil nutrient availability and stoichiometric characteristics (path coefficients = 0.68 and 0.43, respectively), thereby indirectly promoting grain nutrient accumulation (path coefficient = 0.90) and enhancing grain dry matter production.

Conclusions The straw interlayer regulated soil water movement in the soil profile to reduce salt content in the plow layer, and increased the soil organic carbon to total phosphorus ratio (C/P), the activities of C/N-acq, as well as their ratios to P-acq activity. These changes synergistically promoted the release of available nutrients, alleviated microbial resource limitations, and enhanced SOC fractions, ultimately leading to a significant increase in nutrient and dry matter accumulation of sunflower. An application rate of 12000 kg/hm² (5 cm thickness) is recommended. This study provides technical support and a theoretical basis for the improvement of salinized soil and the sustainable development of sunflower production.