Biochar amendment has been widely recognized for its potential to promote soil carbon sequestration and improve crop productivity; however, the microbial mechanisms underlying carbon sequestration at varying biochar application rates remain insufficiently understood. In this study, a field experiment was conducted in a typical fluvo-aquic soil region of the North China Plain under a maize–wheat rotation, with one-time biochar application at four levels: CK (0 t ha⁻¹), B5 (5 t ha⁻¹), B10 (10 t ha⁻¹), and B20 (20 t ha⁻¹). The effects of these treatments on soil physicochemical properties, organic carbon fractions, microbial community structure, and enzyme activities were systematically examined. The results showed that soil total nitrogen (TN) and pH increased consistently with higher biochar application rates, reaching maximum values under B20 treatment, where TN and pH rose by 35.56% and 7.00% relative to CK, respectively. In contrast, the contents of NH₄⁺-N, available phosphorus (AP), and available potassium were mostly enhanced under B5 during the maize season, while in the wheat season, NH₄⁺-N peaked under B10 and AP peaked under B5. Biochar addition significantly increased soil organic carbon fractions and the carbon pool management index (CMI). In the maize season, soil organic carbon (SOC), microbial biomass carbon (MBC), particulate organic carbon (POC), and CMI under B20 rose by 55.99%, 39.67%, 79.69% and 180.54% over CK, respectively, whereas dissolved organic carbon (DOC) peaked under B5. Throughout the wheat season, SOC, MBC, and POC contents under B20 were 53.70%, 64.31% and 147.81% higher than CK, while DOC peaked under B5 (+56.98%). Soil enzyme activities, including catalase, urease, invertase and alkaline phosphatase, were strongly stimulated by biochar, with B20 increasing their activities by 4.49–18.18%, 3.19–19.77%, 6.14–26.14% and 12.25–33.19%, respectively. Biochar also reshaped microbial community structure: the during maize season, it reduced the relative abundance of Glomeromycetes (65.31%) and Oligohymenophorea (51.64%) while enhancing Deltaproteobacteria (46.15%) and Gammaproteobacteria (29.03%); during wheat season; it enhanced Eurotiomycetes (85.77%) and Dothideomycetes (16.28%) but suppressed Deinococci (74.08%) and Alphaproteobacteria (4.39%). Pathway analysis further indicated that biochar amendments indirectly increased SOC fractions and CMI by simultaneously altering nutrient availability, regulating microbial community structure, and stimulating soil enzyme activities. Collectively, these findings highlight that the effects of biochar are dosage-specific: moderate rates (e.g., 5 t ha ⁻¹) are more suitable for the short-term improvement of soil fertility, while higher rates (e.g., 20 t ha⁻¹) are more effective for long-term carbon sequestration; depending on the objective, biochar application can thus substantially modify soil physicochemical and biological processes to promote agroecosystem sustainability in the North China Plain.

Late-season foliar nitrogen (N) application is widely employed to improve wheat grain yield and protein concentration, particularly during later growth stages when root activity declines and N uptake becomes less efficient. However, the reported effects of foliar N application on grain yield and the quality of wheat remain inconsistent. This meta-analysis, based on 51 field trials encompassing 1498 observations, quantitatively evaluates the impact of late-season foliar N application on wheat yield and protein concentration. The results demonstrate that late-season foliar N application significantly enhances both grain yield (+4.1%) and protein concentration (+5.9%) compared to control treatments. Notably, split foliar N application primarily increased protein concentration (+6.3%), whereas late-season N supplementation enhanced both yield (+3.4%) and protein concentration (+6.0%). Subgroup analyses reveal that the effectiveness of foliar N application is influenced by N management practices. Split foliar N application significantly increased both yield and protein concentration at N rates of 101–200 kg N/ha, whereas late-season N supplementation was beneficial only at higher N rates (≥200 kg N/ha). Moreover, the timing of foliar N application played a crucial role: application at anthesis resulted in the greatest increases in both yield (+5.3%) and protein concentration (+5.8%), while applications at booting or post-anthesis stages primarily increased protein concentration with minimal yield effects. Additionally, late-season foliar N application mitigated the conventional negative correlation between wheat yield and protein concentration, particularly when applied as a split N strategy, allowing for improved grain quality without reducing yield potential. This study highlights the importance of optimizing foliar N timing, the method, and N rate to maximize both wheat yield and quality while improving N use efficiency.

Soil necromass carbon (C) has significant potential for C sequestration in combination with minerals. Global warming and nitrogen (N) deposition affect necromass C, but these effects vary greatly across different climate conditions, land uses, and soil properties, and the role of regional specificity remains poorly understood. We synthesized 197 paired observations from 50 climate change studies to investigate these effects in China. Our results indicate that warming and N addition significantly increase necromass C accumulation by 17% and 9%, respectively. Warming strongly enhanced necromass C incroplands (+44%), cool (+16%) and semi-arid regions (+15%), and soils rich in soil organic carbon (SOC) (+17%) or loam (+22%), whereas N addition further promoted sequestration in croplands (+12%), forests (+10%) SOC-rich soils (+16%), and clay (+7%) or clay–loam (+12%) soils. In the context of climate change, soil C management requires attention to land use, climate, and soil properties. However, heterogeneous responses of microbial necromass C to global warming and N addition are still poorly understood. In the future, global warming is projected to enhance necromass C in croplands, cool or semiarid regions, SOC-rich and loam soils, whereas N addition is expected to further promote its sequestration in croplands, forests, SOC-rich, and clay-based soils. These findings demonstrate the targeted management of necromass C, particularly through optimized nitrogen application in clay-rich croplands and conservation tillage in cool and semiarid regions, offering a nature-based solution to complement global climate mitigation.