Belowground nitrogen (N) transfer from legumes to non-legumes provides an important N source for crop yield and N utilization. However, whether root contact facilitates N transfer and the extent to which N transfer contributes to crop productivity and N utilization have not been clarified. In our study, two-year rain shelter experiments were conducted to quantify the effect of root contact on N transfer in a maize/alfalfa intercropping system. N transfer occurred mainly one direction from alfalfa to maize during the growth period. Following the N0 treatment, the amount of N transfer from alfalfa to maize was 204.56 mg pot⁻¹ with no root barrier and 165.13 mg pot⁻¹ with a nylon net barrier, accounting for 4.72% and 4.48% of the total N accumulated in maize, respectively. Following the N1 treatment, the amount of N transfer from alfalfa to maize was 197.70 mg pot⁻¹ with no root barrier and 139.04 mg pot⁻¹ with a nylon net barrier, accounting for 3.64% and 2.36% of the total N accumulated in the maize, respectively. Furthermore, the amount of N transfer without no root barrier was 1.24–1.42 times higher than that with a nylon net barrier regardless of the level of N addition. Our results highlight the importance and the relevance of root contact for the enhancement of N transfer in a maize/alfalfa intercropping system.

Biochar (BC) and cattle manure (CM) are carbon-nutrient-rich organic substances and have long been used to improve crop yield and soil fertility. Nevertheless, their combined effect with potassium (K) fertilizer remains unknown. Against the previous context, a 2-year (2021–2022) field experiment was conducted to assess the effect of K fertilization coupled with BC and CM on the growth and yield of maize and soil physio-chemical characteristics. The K application combined with BC and CM increased (p ≤ 0.05) the majority of the growth indices of maize crop compared with CK. Compared with CK, the combined application of K (60 kg K ha⁻¹) with BC and CM resulted in an increased number of seeds cob⁻¹ by up to 451 and 465, and up to 383 and 396, the 1000-seed weight up to 22 and 23 g, and up to 27 and 34 g, and the grain yield up to 1979 and 2900 and up to 3240 and 3341 kg ha⁻¹, respectively, in 2021 and 2022. The integrated application of these inputs increased the chlorophyll of maize crops by 29 and 36% and by 30 and 44%, respectively, in 2021 and 2022. Such application also increased the photosynthetic activities of maize such as transpiration rate (Tr), stomatal conductance (Gs), and photosynthetic rate (Pn) by 21 and 23%, 143 and 110%, and by 64 and 66% in 2021 and by 19 and 30%, 163 and 118%, and by 63 and 72% in 2022. Similarly, the combined application of K, BC, and CM increased the K uptake of maize due to an increase in the soil extractable K. Equally, soil total N and organic matter improved under the combined application of K, BC, and CM. However, it did not affect the soil extractable P in 2021 but increased it in 2022. Conversely, these applications reduced (p < 0.05) the soil electrical conductivity, sodium adsorption ratio, and bulk density. This suggests that K fertilization combined with BC and CM enhances the growth and yield of maize by improving the soil nutrients availability, increasing soil organic matter, and enhancing soil structure and moisture retention.

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.

Maize–soybean intercropping is widely practised to improve land use efficiency, but shading from maize often limits soybean growth and productivity. Melatonin, a plant signaling molecule with antioxidant and growth-regulating properties, has shown potential in mitigating various abiotic stresses, including low light. This study investigated the efficacy of applying foliar melatonin (MT) to enhance shade tolerance and yield performance of soybean under intercropping. Four melatonin concentrations (0, 50, 100, and 150 µM) were applied to soybean grown under mono- and intercropping systems. The results showed that intercropping significantly reduced growth, photosynthetic activity, and yield-related traits. However, the MT application, particularly at 100 µM (MT₁₀₀), effectively mitigated these declines. MT₁₀₀ improved plant height (by up to 32%), leaf area (8%), internode length (up to 41%), grain yield (32%), and biomass dry matter (30%) compared to untreated intercropped plants. It also enhanced SPAD chlorophyll values, photosynthetic rate, stomatal conductance, chlorophyll fluorescence parameters such as Photosystem II efficiency (ɸPSII), maximum PSII quantum yield (Fv/Fm), photochemical quenching (qp), electron transport rate (ETR), Rubisco activity, and soluble protein content. These findings suggest that foliar application of melatonin, especially at 100 µM, can improve shade resilience in soybean by enhancing physiological and biochemical performance, offering a practical strategy for optimizing productivity in intercropping systems.