Nitrogen is an essential element for plant growth and development and is the element most closely related to yield. In a gramineous and leguminous intercropping system, nitrogen content was clearly promoted nitrogen content and yield in the gramineous crop, and improved land productivity [21]. In the current study, maize had marginal advantage under IMP. The nitrogen content in the IM roots, stems and leaves was significantly higher than SM and MIM, respectively (Fig. 1), which was consistent with the findings of another [4]. Due to the adjustment of root length density and root distribution, the nitrogen uptake per unit root length was increased, compared with that of monoculture [22]. Combined with other research reports, maize competes strongly for nitrogen and absorbs more nitrogen than peanut, so significantly increasing the nitrogen content in IM and promoting the yield of maize [23]. In the middle and late growth period, the shading of IM was intensified, and photosynthesis was weakened on the IP, which further affected the absorption of nutrients by the peanut (Fig. 1). In the intercropping of maize and soybean, light transmittance was increased after defoliation of the top two leaves of the maize, and the nitrogen absorption of soybean was increased by 5% (grain), 10%(stem) and 14% (root), respectively [24]. The shading of maize inhibited the growth of peanut and the yield of intercropped peanut decreased (Table S1). Ear length, number of grains per spike and fruit weight per hundred were the main yield components (Table S1), which were the same as the findings of previous studies [4].
In this study, it was found that interspecific root interactions of IMP promoted soil nutrient uptake and utilization, compared with monoculture (Table 1). The results were consistent with the decrease of soil TN in intercropping Chinease milk vetch and rape[25]. It is reported, the root density distribution was different under different soil depths, which affected the absorption and utilization of soil nutrients [26], and IMP maintained the basic stability of soil chemical properties and the diversity effect of soil nitrogen accumulation (Table S2-3) [14, 27].
It is needed to explore the interactions of rhizosphere microbial community in intercropping, and the inner mechanism of IMP was elucidated through analyzing the physicochemical properties of and rhizosphere soil microbial community [26]. Soil microorganisms are one of the main sources of soil enzymes, and there is a correlation between soil enzyme activity and microorganisms [11, 28], it’s consistent with this study show that RB41, Candidatus-udaeobacter and Chaetomium were significantly positively correlated with soil POD, and negatively correlated with Pro and DHO (Fig. 8). Besides, Fusarium, Stropharia and Penicillium also were negatively correlated with Pro and DHO (Fig. 8-B). Candidatus-udaeobacter within the Verrucomicrobia phylum is pervasive in soils around the world, sacrificing metabolic versatility for efficiency to become dominant in the soil environment. The Chaetomium, which is a beneficial fungi with biocontrol effect and antagonistic to soil pathogenic bacteria [29].In our study, the soil POD was negatively correlated with soil TN content across all soil (Figure S1), due to it is involved in the degradation of hydrocarbons and their intermediates [30]. Soil Pro and DHO activities were positively correlated with TN (Figure S1), which was consistent with previous studies [31]. Soil Pro are involved in the conversion of amino acids and other nitrogen-containing organic compounds present in soil, and their hydrolysates are one of the nitrogen sources for higher plants [32]. Soil DHO participates in soil carbon cycle and promotes dehydrogenation of carbohydrates and organic acids [31].
Soil microorganisms play a key role in soil nutrient cycling and crop nutrient uptake [13]. Through the analysis of the dominant bacteria in the soil components, it was found that the intercropping varieties had increased the relative abundance on the dominant bacteria species (Fig. 5-A). The Ramlibacter and MND1 were significantly enriched in IP and II, respectively (Fig. 6-A). The functions of MND1 need to be investigated more deeply. The Ramlibacter within the Proteobacteria, comprising an enormous range of metabolic diversity [26]. In addition, Sphingomonas also increased in IP than SP (Fig. 5-B), which is the characteristics of promoting nitrogen fixation and dehydrogenation [33] and the uptake of nutrients in the rhizosphere, improving the soil environment in the rhizosphere of IP, and maintaining the soil nitrogen balance. This result could be related to the reduced abundance of denitrifying bacteria Haliangium, which reduces nitrates in the soil to nitrogen and releases them into the air [34]. So that, intercropping improved the bacterial community structure and increased the abundance of beneficial bacteria. IMP changed the abundance of bacteria in soil, affected soil enzyme activities and soil nutrients, and ultimately affected crop nutrient uptake (Fig. 9). Other bacterial genera had no significant correlation with soil enzyme activities (Fig. 8). The reason is that there are many kinds of bacteria in the soil, and the correlation between soil enzyme activity and bacteria is not specific and unique. A single bacterium can affect a variety of enzyme activities, so it is necessary to further explore or study the functional properties of each bacterium and the mechanism of soil enzyme activities themselves [35]. It was found that the transport and metabolism of amino acids, carbohydrate transport and metabolism, and metabolism of other amino acids in the secondary functional areas were relatively high in abundance, which was consistent with previous findings [32, 36]. Furthermore, it was speculated that the relative abundance of RB41 and Candidatus-udaeobacter increased. Soil microorganisms to transport amino acid metabolism, carbohydrate metabolism to produce a series of material, when they were plants as signal perception, can cause related enzyme activity in plant or related gene expression changes, ensure the survival of the bacteria, and then adjust the plant physiological metabolism and nutrient accumulation levels, which promote plant growth and development [37, 38].
We found that, compared with SP, the change in OTUs and richness of IP was higher than those of SP (Figure S2 and 4). These results were consistent with the promotion of fungal community growth in the IMP, intercropping promotes fungal community growth [39]. On the one hand, many biotic and abiotic factors can alter the fungal community, such as soil chemical properties, plant functional diversities and management practices. On the other hand, the variety and quantity of root exudates affect the abundance of the rhizosphere fungal community owing to the presence of different crop species [40, 41].
Soil fungi decompose organic matter in crop residues and fertilizers, which is beneficial to soil nutrient supply and crop growth, but also affects soil enzyme activity [35]. Mortierella decomposes organic matter and promotes mineral uptake by plant roots. It also has the potential to secrete antimicrobials that inhibit pathogenic bacteria such as Fusarium [42–44]. However, many fungi are plant pathogens and cause fungal diseases [35]. The relative abundance of soil pathogens such as Fusarium increased (Fig. 5-B). IMP can reduce the damage of pathogenic fungi in soil to plants. The unique of Saccharomyces promoted crop yield increases under IMP. One possible reason is the secretion of hormones such as gibberellin (GA), cytokinin (CTK) and auxin (IAA), which promote metabolic activity and stimulate crop growth and development. Another reason may be that the large concentrations of bacteria around the rhizosphere are advantageous to Saccharomyces, effectively reducing pathogenic bacterial infection and improving crop resistance. The reason for the increased abundance of Saccharomyces in II is, however, unclear. Overall, the fungal structure in IMP, SM and SP was relatively stable but still had some differences in distribution. This is because different crops release chemical substances to the surrounding environment through allelopathy produced by secondary substances in intercropping. Another reason is that the physicochemical properties of soil are related. Saprotrophic fungi are concentrated in rhizosphere soil as a dominant functional group and can obtain nutrients by degrading dead host cells (Fig. 7-C). They are closely involved in the cycle of decomposition of organic matter and nutrients and can also produce a series of hydrolases and oxidases, which contribute to the decomposition of carbohydrates and increase the nutrients in soil organic matter [45]. Compared with that of sole cropping, the soil IMP micro ecological environment is complex and microorganisms and plants are interdependent. This characteristic provides a theoretical basis for further understanding the mechanism of plant nutrient absorption. Our results indicated that the staggered superposition of roots and secretion of secondary metabolites among root systems in IMP promoted the reproduction of rhizosphere fungi and improved microbial diversity [12]. In this regard, managing rhizosphere microbes and maintaining the balance of the soil microbial community assist plant growth and nutrient uptake.