The effects of different exogenous carbon types on the chemical structural characteristics and stability of soil organic carbon in dryland farmland remain unclear. Based on a four-year fixed-site experiment in a typical dryland farmland on China’s Loess Plateau, this study systematically analyzed the impacts of different carbon sources on soil enzyme activities, organic carbon content, chemical structural characteristics, and their interrelationships under five treatments: (i) no fertilization (T₀); (ii) 100% chemical nitrogen, phosphorus, and potassium fertilizers (CK); (iii) 50% CK + fermented cattle manure (T₁); (iv) 50% CK + corn straw (T₂); (v) 50% CK + mixed fermented cattle manure/corn straw (T₃). The results showed that the activities of β-glucosidase and N-acetylglucosidase ranked in the order T₁ > T₂ > T₃ and T₃ > T₂ > T₁, respectively. Specifically, β-glucosidase activity under T₁ increased by 35.26% compared to CK, while N-acetylglucosidase activity under T₃ increased by 30.78% relative to CK. Compared to CK, the T₁, T₂, and T₃ treatments increased soil organic carbon by 26.84%, 11.27%, and 18.63%, and alkyl carbon content by 7.67%, 2.91%, and 5.57%, respectively. Additionally, T₁ and T₃ treatments elevated aromatic carbon content by 20.59% and 176.47% relative to CK. The organic carbon activity index under T₁ was the lowest, decreasing by 10.04% compared to CK. Structural equation modeling (SEM) path analysis revealed that the addition of different exogenous carbon sources in dryland farming primarily influenced the structure and stability of soil organic carbon by directly or indirectly enhancing the activities of glucosidase, β-acetylglucosidase, and alkaline phosphatase, with T₁ demonstrating the most significant improvement.

The mechanisms through which different types of exogenous carbon enhance the soil organic carbon sequestration rate (C_seq), nitrogen use efficiency (NUE), and corn yield (CY) in rainfed farmland on the Loess Plateau remain inadequately elucidated. This study established a four-year fixed-site experiment in the context of organic materials to increase soil organic carbon storage and enhance corn yield in the dry-farmed areas of the mountainous southern Ningxia region. The research investigates the effects of adding different types of exogenous carbon materials on C_seq, NUE, and CY. The soil type at the experimental base is loessial soil (Huangmian soil), with a soil pH of 8.28 and a baseline organic carbon content of 8.20 g kg⁻¹. The main crop cultivated in this area is corn. The experimental treatments were as follows: (i) N, no fertilization; (ii) CK, 100% nitrogen, phosphorus, and potassium fertilizers; (iii) C, 50%CK + corn straw (pulverized); (iv) M, 50%CK + fermented cow manure; (v) C/M, 50%CK + fermented cow manure + corn straw (1:1). The results show that compared with the CK treatment, the C_seq of C, M, and C/M treatments increased by 488.89%, 355.56%, and 527.78%, respectively. Compared with the CK treatment, the NUE of C, M, and C/M treatments increased by 15.04%, 7.70%, and 12.20%, respectively. Compared with the CK treatment, the CY under the C, M, and C/M treatments were increased by 7.91%, 19.10%, and 11.59%, respectively. The linear regression results show that the C_seq had a significant positive effect on CY (R² = 0.37) and NUE, R² = 0.39) (p < 0.0001). The TOPSIS (technique for order preference by similarity to ideal solution) evaluation results indicate that the C/M treatment was the optimal measure for achieving increased corn yield while enhancing Cseq and NUE. Therefore, incorporating a 1:1 mixture of corn straw and cattle manure in rainfed farmland in the mountainous area of southern Ningxia may be the best strategy to improve C_seq and NUE.

Soil degradation and low productivity in dryland areas require sustainable measures to improve the resilience of agroecosystems. The experimental design was based on the principle of organic matter substitution. To evaluate the effects of the combined application of chemical fertilizer and organic amendments on soil quality and microbial communities in rainfed loessial farmland in southern Ningxia, a field experiment was conducted in 2021 with maize as the test crop. Five treatments were established: control with no fertilizer (CK); chemical fertilizer (N—240 kg/ha, P₂O₅—150 kg/ha, and K₂O—120 kg/ha) (F); 50% F (N—120 kg/ha, P₂O₅—75 kg/ha, and K₂O—60 kg/ha) combined with decomposed cow manure (18,000 kg/ha) (MF1); 50% F (N—120 kg/ha, P₂O₅—75 kg/ha, and K₂O—60 kg/ha) combined with decomposed cow manure (30,000 kg/ha) (MF2); and 50% F (N—120 kg/ha, P₂O₅—75 kg/ha, and K₂O—60 kg/ha) combined with straw (14,527 kg/ha), equal in carbon content to the decomposed cow manure applied with MF1) (SF). Regular methods and high-throughput sequencing techniques were used to determine the physical and chemical properties, soil water stability aggregates, and the diversity and community structure of the soil bacteria and fungi. The results showed that the MF2 treatment resulted in the highest soil organic matter (SOM), available phosphorus (AP), available potassium (AK), and total nitrogen (TN) contents. Compared with CK, MF2 significantly increased SOM, AP, AK, and TN by 5.84%, 112.16%, 52.13%, and 16.88%, respectively. The MF1 and MF2 treatments increased the soil fungal diversity and richness. The BG and ALP activities with the MF2 treatment increased by 95.32% and 22.38%, respectively, compared with the F treatment, while the NAG and CBH activities with the SF treatment increased by 44.10% and 17.18%, respectively, compared with the F treatment. The MF2 treatment increased the relative abundances of the soil bacteria Chloroflexi and Acidobacteria and the soil fungi Mortierellomycota and Basidiomycota. In soils amended with mature cow manure, the quantity of water-stable aggregates with a diameter greater than 0.25 mm (R0.25), soil organic matter (SOM), β-glucosidase (BG) activity, and alkaline phosphatase (ALP) activity were the primary factors influencing the soil bacterial Chao1 index and Shannon index, while R0.25 and ALP activity were the main factors affecting the fungal Observed species index. Overall, 50% chemical fertilizer combined with decomposed cow manure (MF2) is an effective strategy to improve the physicochemical properties and microbial function in soil of farmland. These findings provide a theoretical basis for rational fertilization and soil quality improvement in rainfed farmland in southern Ningxia.