Nitrogen (N) fixation is essential in the development of sustainable agriculture, but nodulation of legumes is usually inhibited by N fertilization. In this study, we evaluated the increased density of intercropped maize (Zea mays L.) as a means to alleviate the inhibitory effect of soil mineral N on intercropped pea (Pisum sativum L.) and improve system performance. A field experiment was conducted in the Hexi Corridor region of northwestern China from 2012 to 2014. The experiment consisted of monoculture pea, monoculture maize, and a pea/maize strip-intercropping system. Two levels of N fertilization were evaluated in both cropping systems during the co-growth period of intercropping, i.e., 0 kg N ha⁻¹ (N0) and 135 kg N ha⁻¹ (N1), and three maize densities were evaluated with both levels of N fertilization in the intercropping system, i.e., 45,000 plants ha⁻¹ (D1), 52,500 plants ha⁻¹ (D2), and 60,000 plants ha⁻¹ (D3). The application of N reduced the number of nodules of intercropped pea by 135% at D1 and by 9% at D2 compared to no application of N, in all the years examined. The alleviation of the inhibitory effect of soil mineral N on the nodulation of intercropped pea (C_is) was calculated as the percentage increase in nodulation with intercropping relative to monoculture for a given level of N fertilization. With the application of N, C_is was improved by increased intercropped maize density (D3 > D2 > D1) at all stages. The internal efficiency of nitrogen (IEN) of pea was improved with intercropping and, on average, was 19% and 12% greater at D3 than at D1 and D2, respectively. These results demonstrate that increased maize density can alleviate the inhibitory effect of soil N on the nodulation of pea and sustain the productivity of maize/pea intercropping while reducing N fertilizer requirements in arid regions.

No-till and cereal–legume intercropping have been recognized as favorable cropping practices to increase crop yields while maintaining soil quality in arid and semiarid environments, but the biological mechanisms are poorly understood. The present study aimed to determine the response of yields, soil properties, enzyme activities, and microbial community diversity and composition in mono- and inter-cropping under conventional and no-tillage conditions. We initiated a field experiment in Wuwei, a typical arid area of China, in 2014. Soil was sampled in August 2022 and, yields, soil properties, enzyme activities, and the microbial community diversity and composition were determined in the maize and pea strips in inter- and mono-cropping systems. Results revealed that the maize and pea strips in the no-till intercropping significantly increased yields, total and organic carbon stocks, decreased NO₃⁻-N, and obtained the highest total and organic P in the soil. No-tillage significantly enhanced the Shannon index and Pielou evenness of the bacterial community and total microbial community over conventional tillage, with the α-diversity of the bacterial community and total microbial community distinctly higher in the NTIM treatment than in the CTIM treatment. The α-diversity of the total microbial community was significantly related to yield, soil IC and OC, and the α-diversity of the archaea community was significantly related to soil TC, TC/TP, TN/TP, and BX. Meanwhile, the α-diversity of the eukaryote community was significantly related to soil yield, soil TC/TP. Both no-tillage and intercropped maize significantly increased the abundance of archaea phylum Thaumarchaeota and bacterial phylum Nitrospirae, and were significantly positively associated with soil OC and NH₄⁺-N, benefiting nitrogen fixation of intercropped pea from the atmosphere under the no-tillage cereal/legume intercropping. No-till intercropping was conducive to the accumulation of organic carbon, while decreasing the abundance of Proteobacteria, Acidobacteria, and Verrucomicrobia. Limited soil enzyme activities (ACP, ALP, DP, NAG, BG, AG, CB) led to decreases in organic carbon turnover and utilization. Intercropping altered soil microbial community diversity and composition due to changes in soil properties and enzyme activities. These findings suggest that no-tilled cereal–legume intercropping is a sustainable cropping practice for improving soil properties and enhancing microbial (archaea, bacterial, eukaryota) diversity, but the persistence is not conducive to rapid turnover of soil nutrients due to limited enzyme activities.

Cereal/legume intercropping enhances legume nodulation and improves nitrogen use efficiency (NUE) in cereal crops. This facilitation of symbiotic nitrogen fixation (SNF) in intercropped legumes involves a complex eco-physiological mechanism driven by multiple factors. Among them, interspecific root interactions (IRIs) are a key factor influencing SNF in intercropped legumes. Currently, it remains unclear whether and how IRIs modulate SNF to affect NUE and yield formation in legume species. In this study, maize/pea intercropping with different types of root separation [no barrier (NB) and plastic barrier (PB)] and pea monocropping (IP) were simulated in a nitrogen (N)-free nutrient matrix in pots, and the SNF, N metabolism, and N partitioning were investigated. We demonstrated that IRIs optimize SNF performance. N assimilation is positively regulated following increases in enzyme activity and gene expression in intercropped roots and nodules. Furthermore, IRIs facilitate amino acid (AA) export from nodules to roots and shoots, which is followed by an increase in AA levels in leaves (source) and leaf exudates (sink). Overall, intensive SNF drives N metabolism and alters source-to-sink N partitioning, thereby increasing NUE (by 23%) and yield (by 15%) in intercropped pea. This study reveals the positive roles of IRIs to the NUE and yield and provides useful reference material for increasing N contents derived from SNF to maximize NUE and crop yields in intercropped legumes.

Nitrogen is a key factor in maize (Zea mays L.) grain and biomass production. Inappropriate application with sub-optimum plant density and irrigation can lead to low productivity and inefficient use. A two-year field experiment was conducted to determine which nitrogen rate, plant density, and irrigation level optimize grain, biomass yield, and water use efficiency. Three nitrogen rates of urea (46–0–0 of N–P₂O₅–K₂O) (N0 = 0 kg N ha⁻¹, N1 = 270 kg N ha⁻¹, and N2 = 360 kg N ha⁻¹), with three maize densities (D1 = 75,000 plants ha⁻¹, D2 = 97,500 plants ha⁻¹, and D3 = 120,000 plants ha⁻¹), and two irrigation levels (W1 = 5250 m³/hm² and W2 = 4740 m³/hm²) were investigated. The results show that both grain and biomass yields were affected by the main factors. The interaction between nitrogen rate and irrigation level significantly (p < 0.001) affected grain yield but not biomass. It was observed that the grain yield increased correspondingly with nitrogen rate and plant density, while it decreased as the irrigation level increased. Water use efficiency was significantly (p < 0.001) affected by the main factors and their interactions. Nevertheless, water use efficiency was highest at (5250 m³/hm²) × 270 kg N ha⁻¹; × 360 kg N ha⁻¹ × 120,000 plants ha⁻¹ and increased from 62% to 68%. In addition, the highest biomass yield was recorded at 5250 m³/hm² × 270 kg N ha⁻¹; × 360 kg N ha⁻¹ × 120,000 plants ha⁻¹ while the interaction of either irrigation level with 0 and 270 kg ha⁻¹ or 97,500 and 120,000 plants ha⁻¹ yielded the lowest water use efficiency. Thus, optimized nitrogen rates, plant density, and alternate irrigation levels can support optimum grain and biomass yields. It can also improve nitrogen and water use efficiency in maize production.

Conservation tillage is widely recognized as an important way to improve soil quality, ensure food security and mitigate climate change. However, relatively little attention has been paid to the subject in terms of sustainable evaluation of environmental and economic benefits of the combination of no tillage and straw returning for maize production in arid irrigated areas. In this study, grain yield (GY) and water use efficiency based on grain yield (WUE_GY), soil carbon emission characteristics and economic benefits were investigated, and a sustainability evaluation index based on the above indicators was assessed in maize production under a wheat–maize rotation system from 2009 to 2012. Four wheat straw returning approaches were designed: no tillage with 25 to 30 cm tall wheat straw mulching (NTSMP), no tillage with 25 to 30 cm tall wheat straw standing (NTSSP), conventional tillage with 25 to 30 cm tall wheat straw incorporation (CTSP), and conventional tillage without wheat straw returning (CTP). The results showed that NTSMP treatment could effectively regulate water consumption characteristics of maize fields and meet the water conditions for high grain yield formation, thus gaining higher GY and WUE_GY. NTSMP increased GY and WUE_GY of maize by 13.7–17.5% and 15.4–16.7% over the CTP treatment, and by 5.6–9.0% and 2.3–11.2% over the CTSP treatment, respectively. Meanwhile, compared with CTP, the NTSMP treatment could effectively reduce carbon emissions from maize fields, where average soil carbon emission fluxes (AC_f), carbon emission (CE) and water use efficiency based on carbon emission (WUE_CE) were reduced by 17.7–18.9%, 11.1–11.2% and 8.8–12.8% and carbon emission efficiency (CEE) was increased by 10.2–14.7%. In addition, the NTSMP and NTSSP treatments could effectively increase total output and reduce human labor and farm machinery input, resulting in higher economic benefit. Among them, the NTSMP treatment was the most effective, net income (NI) and benefit per cubic meter of water (BPW) were increased by 16.1–34.2% and 19.1–31.8% over the CTP treatment, and by 13.2–13.3% and 9.8–15.6% over the CTSP treatment, respectively. The sustainability analysis showed that the NTSMP treatment had a high sustainability evaluation index and was a promising field-management strategy. Therefore, no tillage with 25 to 30 cm tall wheat straw mulching is a sustainable maize-management practice for increasing economic benefits and improving environmental impacts in arid irrigated areas.

Leaf photophysiological characteristics are the main indexes that determine crop yield formation. However, it remains unclear whether photosynthesis is systematically regulated via the cropping pattern and nitrogen supply when maize crops are planted with a high density. So, a field experiment that had a three-factor split-plot arrangement of treatments was conducted from 2020 to 2021. The main plot was two cropping patterns that included the sole cropping of maize and wheat–maize intercropping. The split plot had two nitrogen application rates: a traditional nitrogen application rate (N2, 360 kg ha⁻¹) and one reduced by 25% (N1, 270 kg ha⁻¹) for maize. The split–split plot had three planting densities: a traditional density (M1, 78,000 plant ha⁻¹), a medium density (M2, 10,400 plant ha⁻¹), and a high density (M3, 129,000 plant ha⁻¹) for sole maize; the corresponding densities of intercropped maize were 45,000, 60,000, and 75,000 plant ha⁻¹, respectively. The grain yield, the photosynthetic traits, and chlorophyll a fluorescence of the maize were assessed. The results showed that a 25% nitrogen reduction and dense planting had a negative impact on the individual maize’s photosynthesis. However, intercropping could alleviate these drawbacks. When the maize was grown in the intercropping system at a lower nitrogen level and a medium planting density (IN1M2), the photosynthetic traits were better or similar to those of the traditional treatment (SN2M1) at the reproductive growth stage. Moreover, IN1M2 improved the light energy distribution among photochemistry, photo-protective and heat dissipation process of maize compared with SN2M1. A grey relation analysis demonstrated that the Pn and Tr of the individual maize played the most significant role in the group’s productivity. Thus, the IN1M2 treatment achieved the highest grain yield and can be recommended as a feasible agronomic practice in oasis-irrigated regions.