Maize–soybean relay cropping increases land-equivalent ratio, but shading often limits soybean productivity. Optimizing strip relay configurations improves the light environment for soybean, enhancing its photosynthetic capacity and yield. In a four-year trial, we tested maize–soybean relay strip cropping at interspecific distances of 30, 45, 60 and 75 cm, and monocropping soybean. We measured photosynthetic characteristics, photosynthate allocation, root traits, nitrogen (N) uptake and yield to elucidate the canopy-root synergy driving spacing-induced yield gains and identify the optimal interspecific distance. Increasing interspecific distance significantly improved canopy transmittance and photosynthetically active radiation (PAR). The 60 cm treatment (MS60) increased transmittance and upregulated leaf antioxidant enzyme activity, thereby enhancing leaf area index, SPAD and net photosynthetic rate. Compared with other relay cropping treatments, MS60 increased 13C content and sucrose accumulation by 17.8%–69.7% and 7.1%–34.9%, respectively, and increased N uptake by 20.1% on average. The dual boost in carbon and nitrogen accumulation led to an 11.6%–29.3% yield increase under MS60, with a soybean yield of 1.9 t ha−1 that was close to the monocropped soybean yield of 2.1 t ha−1. This yield advantage was attributed to increased canopy radiation and carbon (C) accumulation that increased root development and N uptake. MS60 optimizes the balance between interspecific compensation and intraspecific competition in the relay strip cropping system, increasing maize yield while maintaining soybean yield at monoculture levels.
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The exogenous plant growth regulator, diethyl aminoethyl hexanoate (DA-6), in combination with suitable varieties and planting densities, is important to increase yield in the maize–soybean strip intercropping system. To identify the role of DA-6 in mitigating high-density stress and increasing yield, we conducted a two-year field experiment examining changes in branching architecture and other yield traits of soybeans in maize–soybean strip intercropping systems. In the planting system, two soybean cultivars (ND: Nandou 25 and QH: Qihuang 34) were grown under three planting densities (D1: 102,000 plants ha−1, D2: 130,000 plants ha−1, D3: 158,000 plants ha−1) with DA-6 treatments (DA0: water control; DA60: 60 mg L−1; DA100: 100 mg L−1). Applying DA-6 at 60 mg L−1 at the fourth trifoliolate leaf stage increased soybean yield, with QH yield rising by 22.4% and 29.5% at D3 density, and ND yield by 29.5% and 30.0% at D2 density in 2022 and 2023, respectively, compared with D1 under DA0. DA-6 improved photosynthesis in both varieties under D2 density, with DA60 increasing ND canopy photosynthetic rate by 15.1%–16.4% and QG by 9.1%–20.6% over two years. In ND, DA-6 enhanced branching, raising the leaf area index by 37%, branch number from 3.6 to 4.7 per plant, and total pod number by 19.7%. In QH, yield grains were mainly due to a 17% increase in the number of stem pods and a 6.5% improvement in hundred-grain weight. In the maize–soybean strip intercropping system, QH achieved a high yield by forming a high-density (D2 to D3) main stem pod, and ND by combining moderate density (D1 to D2) with DA-6-induced branching.
The aim of this study was to explore the characteristics of leaf green retention, photosynthesis and system yield of maize and soybean under different planting modes and nitrogen (N) application levels.
The effects of planting methods (maize monoculture (MM), soybean monoculture (SS), maize intercropping (IM), soybean intercropping (IS)) and N application levels (0 N application (NN), reduced N application (RN: 180 kg N·hm-2) and constant N application (CN: 240 kg N·hm-2)) on leaf stay-green, photosynthetic characteristics, dry matter accumulation and system yield of maize and soybean leaves were studied by field positioning experiment.
The maize yield increased with the increase of N application, and the soybean yield increased first and then decreased with the increase of N application; Under RN, the seed dry matter accumulation of IM was the largest, the total yield of maize-soybean intercropping system was the highest, and the system productivity index (SPI) was the largest too. Under intercropping, the leaf green period of each crop was longer, the photosynthetic characteristics were more stable than that of monoculture, and better than that of monoculture at seed formation stage; Under all N application levels, the percentage of green leaves under intercropping treatment was significantly higher than that under monoculture. The maximum green leaf attenuation rate of IM appeared 7 d, 5 d and 1d later than that of MM, respectively, while IS was 7 d, 0 d and 11 d later than SS, respectively. Compared with monoculture, the intercropping could significantly reduce the average attenuation rate of maize leaves, prolong the days of maximum attenuation rate and reduce the attenuation degree of green leaves. The photosynthetic rate of each crop was higher under intercropping than monoculture, and the reduced N application was higher than the constant N application. At R2 stage, the photochemical quenching coefficient (QP) under IM was 12.78% higher than that under MM, and the non-photochemical quenching coefficient (NPQ) was 21.30% lower; NPQ decreased with the increase of N application level, while the ratio of RN to NN decreased by 17.11%. The fluctuation range of SPAD value of intercropping was weaker than that of monoculture, and showed a stable upward trend. In maize R2 stage, IM was 34.52% higher than MM; In soybean R2 and R6 stage, IS was 10.39% and 29.48% higher than SS, respectively, and the SPAD value of RN was the highest. At R2 stage, IMRN was 17.46% higher than IMNN, and MMRN was 35.02% higher than MMNN; in soybean R6 stage, ISRN was 7.71% and 6.67% higher than that of ISNN and ISCN, and SSRN was 10.03% higher than that of SSCN.
Under reduced N application condition, the maize-soybean intercropping significantly prolonged the green holding period of leaves; After flowering, the photosynthetic rate of leaves, the function of PS Ⅱ photosynthetic mechanism and chlorophyll remained at a high level were more stable than that of monoculture, and the accumulation of seed dry matter was enhanced, which gave full play to the production potential of maize and increased the yield of soybean, so that the total yield of intercropping system was significantly increased.
Soybean and maize strip intercropping is an important cultivation pattern for soybean productivity improvement project in China. In this experiment, the effects of straw returning and irrigation methods on the emergence and seedling growth quality in soybean and maize strip intercropping were investigated, in order to provide the theoretical support for the strip intercropping to solve the problem of seedling emergence.
The experiment was carried out in Yucheng City, Dezhou City, Shandong Province and Anju District, Suining City, Sichuan Province in 2022 and 2023, respectively. Two-factor split plot zone design was adopted, three straw returning methods (non-returning straw (S1), returning straw with no stubble (S2), and returning straw with stubble (S3)) and three irrigation methods (non-irrigation (W1), check irrigation before sowing (W2), and spray irrigation after sowing (W3)) were set up, and their effects on seedling emergence and growth of strip intercropping crops were researched.
Irrigation could significantly increase soil water content and alleviate soil compactness, thus effectively improving the emergence rate of soybean and maize, and shortening the emergence time. The seedling emergence rates under W3, W2 and W1 with S2 were 71.00%, 45.70% and 38.50% in Yucheng, respectively. The seedling emergence rates under W3, W2 and W1 with S2 were 90.17%, 88.50% and 61.67% in Anju, respectively. The emergence time under W2 and W3 was 3.29 d and 2.92 d shorter than that under W1, respectively. Under different irrigation methods, there was no significant difference in the seedling emergence rate of maize, which could reach more than 90%, but the seedling emergence time was significantly different. The emergence time of maize under W2 and W3 was 1.9 d and 3.1 d shorter than that under W1 in Yucheng, respectively. The emergence time of maize under W2 and W3 was 0.96 d and 0.6 d shorter than that under W1 in Anju, respectively. Straw returning significantly increased soil water content and decreased soil compactness, and the effects in both places were S2>S3>S1. Straw returning to the field under W1 could significantly improve the soybean seedling emergence rate in Yucheng, in which S2 was 45.08% higher than S1. Straw returning had no significant effect on the emergence rate of maize in the two places. Straw returning and irrigation significantly increased the activity of soybean lipase (LPS) and maize α-amylase (α-AL). The LPS of W2 and W3 were 26.86% and 37.77% higher than that of W1, respectively. The LPS under S2 was 14.14% and 18.05% higher than that under S3 and S1, respectively. The α-AL under W2 was 189.47% higher than that under W1, and the α-AL under S2 was 61.52% and 127.33% higher than that under S3 and S1, respectively. The two irrigation treatments could promote the growth and development of soybean and maize, and improve the seedling growth rate and uniformity. The soybean plant height under W3 and W2 was 21.74% and 15.70% higher than that under W1, respectively, wihle the stem diameter was 12.52% and 28.15% higher, respectively, and the leaf area was 11.84% and 38.78% higher, respectively. The maize plant height under W3 and W2 was 21.80% and 20.62% higher than that under W1, respectively, while the stem diameter was 37.69% and 26.39% higher than that under W1, respectively, and the leaf area under W3 was 36.56% and 73.33% higher than that under W2 and W1, respectively. Straw returning significantly affected the growth of maize seedlings in Yucheng, showing S3>S1>S2. The plant height under S3 was 19.92% and 27.31% higher than that under S1 and S2, respectively, while the stem diameter was 27.59% and 59.80% higher, respectively, and the leaf area was 42.76% and 68.54% higher, respectively.
Successful emergence and construction of a good seedling population were the basis for achieving high yield. Spray irrigation after sowing improved the physical structure of the plough layer, thus promoted the emergence of seedlings and shortened the emergence time in strip intercropping, and provided favorable conditions for subsequent crop growth. Returning straw with stubble could improve the growth quality of strip intercropping seedlings in Yucheng. Returning straw with no stubble had the characteristics of water storage and alleviating soil compaction, it is beneficial to soybean emergence without irrigation, and has a promoting effect on the formation of strong seedlings of strip intercropping crops in Anju.
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Flavonoids produced by legume roots act as signaling molecules that induce the expression of nod genes in symbiotic rhizobia. However, the role of flavonoids in root exudates under intercropping systems in promoting soybean nodulation remains unclear. Two consecutive years of field experiments were conducted using maize–soybean strip intercropping with interspecific row spacings of 30 cm (MS30), 45 cm (MS45), and 60 cm (MS60), along with sole cropping of soybean (SS) and maize (MM). Root interactions were manipulated using either no root barrier (NB) or a polyethylene plastic barrier (PB) to assess the relationship between flavonoids in root exudates and soybean nodulation. We found that root–root interaction between soybean and maize increased nodule number and fresh weight in intercropped soybean, with enhancement gradually increasing as interspecific distance widened. The proportion of nodules with diameters exceeding 0.4 cm was higher in intercropped soybean under NB compared to PB. Additionally, the expression of nodule-related genes - GmENOD40, GmNIN2b, and GmEXPB2 - was up-regulated. Furthermore, compared to monocropping, isoflavone secretion by soybean roots decreased, whereas flavonoid and flavonol secretion by both maize and soybean roots increased under intercropping. The abundance of differentially secreted flavonoid metabolites in the rhizosphere of both species declined when root contact was prevented by the barrier. In soybean roots, the expression of GmCHS8 and GmIFS1 was up-regulated, while GmICHG was down-regulated under root interaction. Most flavonoid and flavonol compounds showed positive correlations with nodule diameter. Nodule number, fresh weight, and the proportion of nodules larger than 0.2 cm increased in diverse soybean genotypes treated with maize root exudates, which contributed to enhanced nitrogen fixation capacity. Therefore, maize– soybean strip intercropping, combined with optimal row spacing, enhances the positive effects of underground root interactions and improves nodulation and nitrogen fixation in intercropped soybean.
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Optimized nitrogen (N) management can increase N-use efficiency in intercropping systems. Legume-nonlegume intercropping systems can reduce N input by exploiting biological N fixation by legumes. Measurement of N utilization can help in dissecting the mechanisms underlying N uptake and utilization in legume-nonlegume intercropping systems. An experiment was performed with three planting patterns: monoculture maize (MM), monoculture soybean (SS), and maize-soybean relay intercropping (IMS), and three N application levels: zero N (NN), reduced N (RN), and conventional N (CN) to investigate crop N uptake and utilization characteristics. N recovery efficiency and 15N recovery rate of crops were higher under RN than under CN, and those under RN were higher under intercropping than under the corresponding monocultures. Compared with MM, IMS showed a lower soil N-dependent rate (SNDR) in 2012. However, the SNDR of MM rapidly declined from 86.8% in 2012 to 49.4% in 2014, whereas that of IMS declined slowly from 75.4% in 2012 to 69.4% in 2014. The interspecific N competition rate (NCRms) was higher under RN than under CN, and increased yearly. Soybean nodule dry weight and nitrogenase activities were respectively 34.2% and 12.5% higher under intercropping than in monoculture at the beginning seed stage. The amount (Ndfa) and ratio (%Ndfa) of soybean N2 fixation were significantly greater under IS than under SS. In conclusion, N fertilizer was more efficiently used under RN than under CN; in particular, the relay intercropping system promoted N fertilizer utilization in comparison with the corresponding monocultures. An intercropping system helps to maintain soil fertility because interspecific N competition promotes biological N fixation by soybean by reducing N input. Thus, a maize-soybean relay intercropping system with reduced N application is sustainable and environmentally friendly.
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