Various cropping patterns are extensively used on the North China Plain, which greatly alter various soil properties. Although these soil properties respond differently to the different cropping patterns, little is known about the possible effects of cropping patterns on desalinized soils. To assess the effects of the different cropping patterns on soil properties and enzyme activities, a long-term field experiment was conducted at the Nanpi Agro-Ecological Station in Hebei Province, China. The cropping patterns under study were the single-cropping patterns of winter wheat (Triticum aestivum L.), spring maize (Zea mays), and spring cotton (Gossypium hirsutum), and the double-cropping pattern of winter wheat–summer maize (WW–SM), which is the predominant cropping pattern on the North China Plain. Soil samples were collected at a depth of 0–100 cm, depending on the soil properties. Our results indicated that soil available phosphorous (P) and available potassium (K) concentrations at 0–20 cm were greater than those of the subsurfaces. Soil organic matter declined with depth, and WW–SM had the highest amount in the deep levels. The WW–SM cropping pattern also generally showed greater levels of enzyme activities than did the single-cropping patterns, proving that diverse crops can enhance enzyme activities. Soil pH generally increased with depth despite there being no significant differences between the cropping patterns. No significant effects were observed on soil electrical conductivity (EC). However, EC generally increased with depth in spring cotton and winter wheat and declined in spring maize and WW–SM. Winter wheat had a significant aggregate size (<0.053 mm) at 20–40 cm depth and generally showed the lowest amount of nutrients. This study suggests that a fallow period allows the soil to regenerate its structure, whereas WW–SM enhances high soil organic matter in the deep soil layers and promotes increased soil enzyme activities. This indicates that crop rotation could sustainably exploit soil resources without reducing fertility over a long period. There is a need to incorporate different soil management practices for single-cropping patterns to increase their productivity, especially in spring maize, whose organic matter declined the most. Our results also show a need to diversify to different crop rotations to utilize their benefits and enhance soil productivity while increasing crop output.

Maintaining healthy soil is fundamental to sustainable agriculture, and soil organic carbon (SOC) is a key indicator of soil health. Although the application of straw and biochar is known to enhance SOC, their specific effects on the soil health index (SHI) and the underlying microbial drivers across different soil layers—particularly in the 20–40 cm subsoil layer—remain insufficiently quantified. This study therefore aimed to evaluate the effects of straw and biochar application on the SHI and associated microbial indicators. The experiment included five treatments: conventional planting with straw removed (CK), homogeneous incorporation of mechanically crushed straws into the 0–20 cm topsoil layer (SR), biochar burial at 40 cm (SD), combined application of returning mechanically crushed straws in the topsoil and biochar burial at 40 cm (SB), and biochar incorporation into the topsoil combined with burial of entire maize plants at 40 cm (BS). Results showed that SR, SD, SB and BS all enhanced SHI values compared to CK in the topsoil, with BS exhibiting the largest increase (23.07%). In the subsoil, SB exhibited the highest SHI, representing increases of 64.52% and 18.60% compared with CK and BS, respectively. This was attributable to the greater enhancement of key subsoil parameters under SB than under BS, including microbial biomass carbon (17.52%), microbial biomass nitrogen (24.20%), leucine aminopeptidase (91.20%), and SOC (181.38%). Furthermore, surface straw application reduces fungal diversity, whereas biochar application in the subsoil significantly enhanced bacterial diversity. Biochar improved SHI both directly through SOC increase and indirectly via the increase in microbial activity and enzyme content mediated by SOC.

Biochar amendment holds promise for improving saline soils, yet its efficacy is often constrained by the uncertainty of application rates. In this study, a large field trial and associated statistical modeling were conducted to explore the mechanisms by which biochar affects wheat yield in coastal saline soils of northern China. Results showed that biochar application significantly increased soil organic carbon (SOC) content (R² = 0.615, p < 0.001) but induced marked spatial heterogeneity across the field, with the coefficient of variation (CV) reaching 30.2%. Given the difficulty of uniformly applying biochar in the field, subplot-level SOC was used as a proxy for effective biochar distribution. Stepwise regression identified soil electrical conductivity (EC) as the dominant yield constraint (standardized coefficient = −0.69), rather than water and nutrients, and a quadratic relationship was observed between SOC and EC. Structural equation modeling (SEM) further suggested a trade-off: SOC was associated with higher yield through reduced bulk density (BD) (path coefficient = −0.603), whereas high SOC levels were also associated with increased EC under this coastal saline field setting (path coefficient = 0.243), thereby indirectly constraining growth. Consequently, the agronomic response showed a threshold-like transition: the peak wheat yield occurred at an SOC threshold of 13.87 g kg⁻¹ (equivalent to 44.41 t ha⁻¹), which exceeded the point of minimum salinity (11.71 g kg⁻¹, equivalent to ~29.90 t ha⁻¹ biochar). These results suggest that the agronomic benefit of biochar in saline soils depends on maintaining application within an estimated beneficial buffering zone.

Returning crop residues to soil is a central strategy for increasing soil organic carbon (SOC), yet whether biochar-associated persistent free radicals (PFRs) actively regulate the stabilization of residue-derived carbon remains unresolved. Here, graded PFR activities were generated by selective radical quenching while maintaining similar biochar physicochemical properties, and ¹³C-labeled straw was used to trace residue-derived carbon dynamics in two contrasting agricultural soils. Biochar PFRs enhanced hydroxyl radical (·OH) production in a dose-dependent manner, with the highest-PFR treatment generating 6.1- and 18.1-fold higher ·OH levels than the straw-only control in black soil and fluvo-aquic soil, respectively. This abiotic oxidation accelerated early-stage (0–14 d) straw carbon mineralization without increasing net carbon loss. Instead, it redirected straw-derived carbon toward aromatic intermediates that preferentially accumulated as mineral-associated organic matter (MAOM), increasing the proportion of MAOM carbon derived from straw from 59.4% to 64.2% in black soil and from 53.8% to 60.0% in fluvo-aquic soil. Structural equation modeling further demonstrated that PFRs promoted MAOM formation primarily through ·OH-driven chemical transformation, whereas microbial communities acted mainly as secondary responders. These findings demonstrate that biochar-derived PFRs redirect, rather than simply accelerate, straw-derived carbon turnover by coupling abiotic oxidation with microbial processing. This study establishes a mechanistic link between biochar surface redox chemistry and soil carbon persistence, providing a mechanistic basis for designing biochars with enhanced carbon sequestration potential. Graphical Abstract