Objectives Under low-temperature conditions, the degradation of corn straw is limited by the complex structure and recalcitrance of lignocellulose. This study aimed to develop a Fe-Mg salt-urea composite amendment to enhance corn straw degradation under cold environmental conditions.

Methods A laboratory incubation experiment was conducted at three temperatures (10℃, −5℃, and −20℃). Nine FeCl₃–MgCl₂–urea formulations were designed using an orthogonal experimental design, with deionized water treatment (T0) and untreated straw (CK) as controls. Straw samples were collected on days 7, 14, 21, and 28 to determine cumulative degradation rates and degradation rates, and the optimal formulation was screened under different temperature conditions. Changes in cellulose, hemicellulose, and lignin contents were determined for the optimal treatment and CK. Illumina-based high-throughput sequencing of bacterial 16S rRNA genes and fungal ITS regions was performed to characterize microbial community composition and diversity, and the relationships between microbial characteristics and lignocellulose degradation were analyzed to elucidate the microbial regulatory effects of the composite amendment.

Results Temperature, amendment formulation, and their interaction significantly affected the cumulative degradation rate of maize straw, with values ranging from 18.00% to 25.86%. After 28 days of incubation, treatment T4 (2 mg FeCl₃-5 mg MgCl₂-200 mg urea/g straw) showed the highest degradation performance at 10℃, achieving a cumulative degradation rate of 25.86%, which was significantly higher than those of other treatments, with an average degradation rate of 0.0924 g/d. Compared with CK, T4 significantly increased the relative contents of cellulose and hemicellulose by 3.57 and 9.21 percentage points, respectively, while decreasing the relative lignin content by 7.66 percentage points (P<0.05). At −5℃, treatment T7 (2 mg FeCl₃-8 mg MgCl₂-100 mg urea/g straw) exhibited the highest cumulative degradation rate (24.17%), with an average degradation rate of 0.0863 g/d. At −20℃, treatments T2 (5 mg FeCl₃-2 mg MgCl₂-200 mg urea/g straw) and T9 (8 mg FeCl₃–8 mg MgCl₂–200 mg urea /g straw) showed significantly lower cumulative degradation rates than the other seven treatments. Among the remaining treatments, no significant differences were observed, and T1 showed the highest cumulative degradation rate, with an average degradation rate of 0.0732 g/d. Microbial community analysis revealed that T4 significantly enriched potential functional microorganisms associated with lignocellulose and aromatic compound degradation at 10℃, including bacterial genera Sporosarcina and Brevundimonas and fungal genera Aspergillus, Wallemia, and Meyerozyma.

Conclusions The Fe-Mg salt–urea composite amendment altered microbial community composition and structure in the straw degradation system and promoted the enrichment of potential functional microorganisms involved in lignocellulose and aromatic compound degradation, thereby accelerating straw decomposition. The optimal Fe-Mg-urea formulation depended on temperature conditions. The optimal combination for a gram of dry straw is 2 mg FeCl₃-5 mg MgCl₂-200 mg urea at 10℃, and 2 mg FeCl₃-8 mg MgCl₂-100 mg urea at −5～−20℃.