Nutrient uptake and rhizosphere microbial community as related to yield advantage in broomcorn millet‒alfalfa intercropping under different row configurations

Abstract

To investigate the effects of row ratio configurations on intercropping advantages and related rhizosphere microbial communities, a field experiment involving five treatments of different rows of broomcorn millet, i.e., P1M1 (1 row of broomcorn millet intercropped with 1 row of alfalfa), P2M3, P1M2, P1M3 and broomcorn millet alone (SP), was conducted on the Loess Plateau of China. We analyzed the yield, nutritional content of broomcorn millet, the soil nutrient availability and the diversity and community composition of AMF (arbuscular mycorrhizal fungi) and diazotrophs in the rhizosphere of broomcorn millet. The results showed that compared with monocultures, alfalfa-millet intercropping system under different row ratio configurations significantly increased the yield of broomcorn millet and the absorption of PTP and PTK (total phosphorus and potassium of broomcorn millet). In addition, the broomcorn millet-alfalfa intercropping system also improved soil nutrition, with the decrease of the row ratio of broomcorn millet, the changes of TN, NH₄⁺-N and microbial biomass in the rhizosphere of broomcorn millet were consistent, which was opposite to NO₃^--N. Moreover, co-occurrence network and PLS-PM (partial least squares path modelling) analysis showed alfalfa-broomcorn millet intercropping system changed the community diversity and composition of soil microorganisms, increased the improvement of soil nutrition (TN, NH₄⁺-N and microbial biomass), and promoted the absorption of different nutrients by plants (N, P and K) mainly through the negative regulation of AMF and the synergistic effect of AMF on diazotrophs, and finally increased crop yield. This shows that broomcorn millet-alfalfa intercropping can increase plant nutrient content by adjusting soil nutrients and soil microbial activities, thereby increasing yield. Furthermore, we found that 1P2M was the best ratio of alfalfa-millet intercropping system, which may provide reliable suggestions and selection basis for future agricultural production practices.

Keywords: Intercropping system; Microorganism-plant interactions; Rhizosphere microbial community; Row ratio configuration.

Fig. 1

Schematic diagrams of experimental design. SP: Monoculture of broomcorn millet, P1M1: one row of alfalfa intercropped with one row of broomcorn millet, P1M2: two rows of alfalfa intercropped with one row of broomcorn millet, P1M3: three rows of alfalfa intercropped with one row of broomcorn millet, P2M3: three rows of alfalfa intercropped with two rows of broomcorn millet

Fig. 2

Nutrition of the broomcorn millet and its rhizosphere soil samples (a), relationship between ¹³C‰, ¹⁵N‰, C%, N% and C/N (b). Note: ¹³C‰: soil stable carbon isotope; ¹⁵N‰: soil stable nitrogen isotope; C%: percentage of total carbon; N%: percentage of total nitrogen; C/N: soil carbon to nitrogen ratio. The results are means ± (standard error). Different letters in chart indicate significant difference (P < 0.05) in different row ratio configurations by the ANOVA and LSD multiple test

Fig. 3

The alpha diversity (a), community composition and Veen (b) and random forests analysis (c) of microbial communities of AMF and nifH in the rhizosphere of broomcorn millet under different row ratio configurations. All the data of alpha diversity is presented based on the box line plots of Chao1, Goods_coverage, Shannon, and Observed_species of ASVs/OTUs. The number under the diversity index label is the p-value of the Kruskal-Wallis test. The relative abundance of taxa and Veen show levels of genus in AMF and nifH. Random Forests Analysis at genus show in AMF and nifH

Fig. 4

The non-metric multidimensional scaling (NMDS) (a) and analysis of inter-group differences (b) between microbial communities in the rhizosphere of broomcorn millet under different row ratio configurations

Fig. 5

The co-occurrence networks patterns (a) and features (b) of AMF and diazotrophic bacteria communities in different row ratio configurations. Connections indicate strong (Spearman’s |r|>0.8) and significant (P < 0.01) correlations. The green node indicates AMF and the blue node denotes nifH. The node size represents the degree of ASV/OTUs. The red line indicates a positive interaction between two separate nodes and the green line denotes a negative interaction. Numbers represent the nodes proportion

Fig. 6

Network heat map (a), PLS-PM (b) and standardized effects from PLS-PM (c) between broomcorn millet and soil nutrition and diversity of AMF and diazotrophic bacteria under different row ratio configurations. The significant and nonsignificant relationships between different variables are represented by continuous and dashed arrows. Red arrows showed positive impacts while blue arrows showed negative impacts. The goodness-of-fit (GOF) statistics were examined and indicated that the model adequately fitted the data