Microplastics (MPs) have emerged as a significant pollutant, threatening agricultural ecosystem sustainability and global food security. However, relatively few studies have investigated biochar remedial effects on plant growth and biochemical properties in soils contaminated with MPs. In polyethylene microplastic (PE-MP)-contaminated soil, we investigated corn stover biochar effects on soybean growth, soil nutrient content, enzyme activity, and microbial biomass and assessed its impact on soil microbial resource limitations. The addition of MPs inhibited soybean growth in various forms across four stages. Conversely, the addition of biochar to MPs improved soybean growth to some extent, where above-ground biomass increased by 5.82% after adding biochar to soils containing microplastics. In soil treated with MPs and biochar, nitrate nitrogen (N), available phosphorus (P), and available potassium (K) increased by 20.1, 27.4, and 57.2%, respectively, while available nitrogen significantly decreased to 128.3 mg kg⁻¹ compared to the MP-only treatment. PE-MPs alone significantly reduced soil carbon (C), N, and P enzyme activities, as well as microbial biomass, with β-glucosidase, leucine aminopeptidase, and acid phosphatase activities decreasing by 29.9, 27.8, and 25.5%, respectively. Interestingly, biochar addition to MPs significantly alleviated these detrimental effects. Microbial biomass C, N, and P increased by up to 56.0, 22.5, and 96.6%, respectively, following biochar addition to soils containing MPs. Analysis of vector lengths, vector angles, and scatter plots indicated that the presence of MPs reduced soil N and P availability. Overall, while MPs inhibited soybean development, biochar addition alleviated this effect to some extent. Furthermore, partial least-squares path modeling revealed that MPs negatively affected soil chemical properties, microbial biomass, and enzyme activities, whereas biochar positively influenced soil enzyme activities.

Soil organic matter (SOM) is an important carbon pool in terrestrial ecosystems and plays a key role in soil functions. Nevertheless, the effects of fertilization practices on the physical, chemical, biological, and comprehensive stability of SOM are still unclear. We carried out a long-term field experiment in the northeast black soil region in China with four different fertilization practices: no fertilizer (CK), single chemical fertilizer (NPK), chemical fertilizer + straw (NPKJ), and chemical fertilizer + organic manure (NPKM). The content of particulate organic matter (POM) and mineral-associated organic matter (MAOM), compound composition of SOM, carbon mineralization characteristics, active soil organic matter (ASOM), and inert soil organic matter (ISOM) were tested. The results showed that the application of fertilizers significantly increased the contents of POM and MAOM to 2.59–4.65 g kg⁻¹ and 32.69–34.65 g kg⁻¹ (p < 0.05), but decreased the MAOM/POM values by 37.8–42.4%, indicating reduced the physical stability of SOM. Fertilization practices increased the contents of aromatic, nitrogen-containing compounds and decreased the oxygen compounds of SOM, representing enhancement of the chemical stability. The contents of ASOM and ISOM increased in fertilization practices, while the biological stability index (BSI) under the NPKJ and NPKM treatments was lower than the CK treatment, suggesting that the biological stability decreased under the manure and straw application. In addition, the comprehensive stability of SOM increased by 26–116% through a reduction in the physical and biological stability, coupled with an increase in the chemical stability. Collectively, our study demonstrated that the application of manure and straw enhanced both the comprehensive stability and content of SOM and reduced the physical and biological stabilities while increasing the chemical stability, which made the largest contribution to the comprehensive stability.

The combined application of chemical fertilizers with organic materials contributes to higher contents of bioavailable phosphorus. However, the underlying mechanism remains poorly understood. A field experiment including four treatments, chemical fertilizer (CF), chemical fertilizer with biochar (CB), chemical fertilizer with organic fertilizer (CO), and chemical fertilizer with biochar and organic fertilizer (CBO), was conducted to explore how the combination of fertilizer applications enhanced soil phosphorus bioavailability using metagenomic sequencing technology. The results showed that chemical fertilizers combined with organic materials (CB, CO, and CBO) significantly increased citrate-extractable phosphorus by 34.61–138.92% and hydrochloric acid-extractable phosphorus contents by 72.85–131.07% compared to CF. In addition, the combined applications altered the microbial community structure and increased the abundance of phoR, spoT, and ppnK genes, but decreased those of gcd, phoD, and ppk1 genes. A partial least squares path model indicated that the combined applications regulated the microbial community composition and gene abundance of phosphorus-cycling microorganisms by influencing soil physicochemical properties, thereby enhancing soil phosphorus cycling. Correlation analysis indicated that pH, total phosphorus, and available phosphorus were the key factors influencing microbial communities, while available nitrogen and total nitrogen primarily regulated phosphorus cycling gene abundance. In addition, the CO and CBO treatments significantly increased maize yield by 14.60% and 21.04%, respectively. Overall, CBO most effectively enhanced bioavailable phosphorus content and maize yield. This study provides a foundation for developing rational fertilization strategies and improving soil phosphorus use efficiency.

Soil aggregates, which form the basic framework of soil structure, exert significant control over soil quality and crop yield. However, the influence of organic amendments on the relationships between aggregate formation and crop yield are still unclear. To investigate this issue, a long-term field experiment was established including four fertilizer treatments: control without fertilization (CK), chemical fertilizer (NPK), NPK combined with straw (NPKS), and NPK combined with organic manure (NPKM). Soil aggregates were fractionated into >2 mm (LMA), 2–0.25 mm (MMA), 0.25–0.053 mm (SMA), and <0.053 mm (MIC) fractions. NPKS and NPKM treatments increased the proportion of large macroaggregates (LMAs) by 8–12% and significantly elevated soil organic carbon (SOC) and nutrient levels relative to CK. NPKS and NPKM significantly increased the soil quality index (SQI) of LMA and MIC by 45.5–116.7% and 21.1–32.1%, compared with CK and NPK. Random forest (RF) analysis revealed that among the nutrient variables across the four aggregate fractions, the SOC content in LMA and the total phosphorus (TP) content in MIC contributed the highest to soybean yield. Partial least squares modeling further confirmed that the SQI of LMA was the dominant factor influencing soybean yield. Therefore, long-term organic amendments improve soybean yield mainly by enhancing soil quality at the aggregate scale, providing a practical pathway for sustaining soil quality and crop productivity.

Soil organic carbon (SOC) and total nitrogen (TN) are fundamental indicators of soil fertility and long-term agricultural sustainability. However, intensive cultivation, residue removal, and imbalanced fertilization have resulted in substantial declines in SOC and TN across many agroecosystems, particularly in Northeast China. This study investigated SOC and TN dynamics within the 0–35 cm profile of four representative soils in Northeast China under a continuous maize cropping system. Five treatments were assessed: conventional tillage (CT), deep tillage (DT), deep tillage with straw (SDT), deep tillage with organic fertilizer (MDT), and deep tillage combined with straw and organic fertilizer (SMDT). Compared with DT, organic amendment treatments increased SOC and TN contents in the 0–20 cm layer by 9.41–57.57% and 5.29–60.76%, respectively. The SMDT treatment achieved the highest SOC and TN stocks (65.03 Mg ha⁻¹ and 7.91 Mg ha⁻¹) and enhanced nutrient accumulation in the 20–35 cm layer. In the subsoil, the ratio of soil C and N (C/N) under SMDT increased by 3.11%, 11.08%, 2.10%, and −7.01% across the four soils, indicating improved C–N balance and reduced nutrient stratification. SOC and TN stocks were linearly correlated with cumulative C input, confirming that organic amendments were among the main drivers of C and N sequestration. Mantel and path analyses further revealed that clay content and mean annual precipitation enhanced SOC and TN storage by improving soil structure and C–N balance through increased C input and reduced bulk density. Overall, deep tillage combined with amendments strengthened C–N coupling, improved soil fertility, and provided a mechanistic basis for reconstructing fertile tillage layers and sustaining productivity in Northeast China.