Returning crop residues to the field after harvesting is a proven effective strategy for improving soil fertility, carbon sequestration, and crop productivity. However, the relationships between crop residue return modes, SOC and nutrient contents, and crop yields are still unclear. In this study, a field trial was conducted to investigate the effects of different maize straw return modes, i.e., straw mulching (SMU), straw deep ploughing (SDP), and control without straw return (CK), on soil organic carbon (SOC) and nutrient contents in soil layers of 0–40 cm in a Mollisol. The relationships between straw return modes, SOC and nutrient contents, and maize yield were evaluated. Compared with CK, SMU and SDP significantly increased SOC, total nitrogen (N), available N, total phosphorus (P), and available P contents in all soil layers. Relative to SMU, SOC, total N, available N, total P, and available P contents were significantly lower in soil layers of 0–10 cm, but they were significantly higher in soil layers of 20–40 cm in SDP. Redundancy analysis indicated that total N, available N, and SOC were major factors controlling maize yield. Structural equation modeling further showed that straw return modes indirectly affected maize yield by directly and preferentially affecting total N and available N contents. The results indicated that SMU and SDP were beneficial for increasing SOC and nutrient contents at the surface and subsurface soils, respectively. Optimizing a nitrogen management strategy is important to achieve high maize yield with straw return.

The aim of this study is to accurately evaluate the quality characteristics of whole tillage cropland and deepen the knowledge of sub-tillage soil quality evaluation in Jilin Province, China. In this study, top-tillage and sub-tillage soil samples were collected from 185 maize continuous cropping areas in Jilin Province, and 12 physicochemical indexes (pH, cation exchange capacity (CEC), soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), total potassium (TK), available nitrogen (AN), available phosphorus (AP), available potassium (AK), sand, silt, and clay) were used to evaluate the whole tillage layer soil quality index (SQI). The results showed that the whole tillage soil physicochemical indexes in Jilin Province were generally above the moderate level, and nutrient contents increased from West to East among the regions. The minimum data set SQI (SQI-MDS) of the top-tillage and sub-tillage layers were 0.22–0.98 (0.46) and 0.23–0.93 (0.55), respectively. The suitable ranges of MDS parameters for reasonable tillage layers were as follows: top-tillage layer SOM ≥ 34.5 g kg⁻¹, 31.5% ≤ sand ≤ 53.5%, AP ≥ 32.1 mg kg⁻¹, and TK ≥ 15.18 g kg⁻¹; sub-tillage layer 31.3% ≤ sand ≤ 51.2%, TN ≥ 1.48 g kg⁻¹, 6.4 ≤ pH ≤ 7.1, and AK ≥ 157.6 mg kg⁻¹. In summary, the SQI and evaluation indexes of the top-tillage and sub-tillage layers in different ecological zones are varied. It is necessary to adjust the evaluation index thresholds in combination with the actual conditions to establish a more accurate evaluation index system of the whole tillage soil quality.

Long-term intensive cultivation has degraded soil structure and reduced soil organic carbon (SOC) retention in the black soils of Northeast China. However, the processes underlying SOC accumulation under integrated soil–crop system management (ISSM) remain poorly understood. An 11-year field experiment compared farmers’ practice (FP), high-yield and high-efficiency practice (IP), super-high-yield practice (HY), and ISSM. We assessed SOC stocks, aggregate stability, organic carbon chemistry, extracellular enzyme activities, microbial carbon-use traits, microbial community composition, and microbial necromass carbon (MNC), and used partial least squares path modeling as an exploratory analysis of potential linkages. Compared with FP, ISSM increased SOC content and stock by 87.0 and 77.0%, respectively, and achieved an SOC sequestration rate of 2.8 Mg C ha⁻¹ yr⁻¹. ISSM also increased mean weight diameter and geometric mean diameter by 45.0 and 61.0%, increased microbial biomass carbon and carbon use efficiency by 205.2 and 100.0%, and decreased qCO₂ by 71.1%. Bacterial and fungal necromass carbon increased by 100.0 and 70.0%, respectively. The relative abundance of taxa operationally classified as growth-oriented also increased under ISSM. However, MNC/SOC did not differ significantly among treatments, while SOC aromaticity and hydrophobicity declined. Path analysis suggested that microbial carbon transformation was positively associated with necromass formation, whereas aggregate stability was associated with MNC and SOC retention. These results indicate that ISSM synchronously enlarged SOC and microbial necromass pools through sustained carbon inputs, efficient microbial transformation, and aggregate-mediated physical protection rather than through increased chemical recalcitrance.

Livestock manure amendment improves soil fertility and promotes carbon sequestration, but long-term application leads to heavy metal (HM) accumulation with unknown ecological consequences. Based on a 13-year field experiment in a continuous maize cropping system, we compared chemical fertilizer (NPK) with four organic amendments (cattle, pig, chicken manure, and compost) applied on an isocarbon basis. Organic amendments significantly increased total organic carbon (TOC) by 15.8–24.3% and available phosphorus (AP) by 1.9- to 6-fold relative to NPK. Compost achieved the highest maize yield. However, pig and chicken manure led to substantial accumulation of Cu and Zn due to high background levels. Despite this, grain HM concentrations remained below safety thresholds, indicating no immediate food chain risk. Metagenomic analysis revealed that HM stress acted as a deterministic filter on the soil microbiome. Cattle manure fostered the most complex co-occurrence network (average degree: 2.70), while pig manure reduced network complexity and increased modularity (>0.92), reflecting a shift toward fragmented, survival-oriented interactions. This structural reorganization was coupled with functional shifts, including enrichment of stress-tolerant taxa (Chitinophagales, Nitrosotalea) and detoxification pathways. We recommend prioritizing cattle manure or compost over raw pig and poultry manure to balance fertility, productivity, and ecological safety in black soil regions.