Species interactions enhance root plasticity and microbial diversity in a wheat-soybean intercropping system

Belowground facilitative interactions in legume-cereal intercrops may improve resource availability and lead to rhizosphere acclimation to soil nutrient constraints such as phosphorus (P) deficiency. To advance our knowledge of belowground mechanisms involved in P tolerance (root plasticity and microbial diversity), soybean (Glycine max, SB) and wheat (Triticum aestivum) were grown in two cropping treatments, mono- and intercropped, under P-deficiency and P-sufficiency in soil-filled plexiglass rhizoboxes. We hypothesized that intercropping induces root plasticity that promotes P acquisition and stimulates higher microbial diversity in the rhizosphere. Results showed that total root dry weight (RDW), length, and surface area significantly (p<0.05) increased in P-deficient intercropped wheat and SB as compared to P-deficient monocropped treatment. Greater allocation of roots (RDW, length, and surface area) to a deeper soil layer was evident for P-deficient intercropped wheat as compared to all remaining wheat treatments. Similarly, P-deficient SB had induced root allocation to deeper soil layers, especially for RDW (p<0.001) compared to root length and surface (p<0.05) under intercropping. Analysis of root acid phosphatases activity (APase) showed a significant (p<0.001) stimulation in shallow roots of P-deficient intercropped wheat. Conversely, shallow roots of SB monoculture exhibited higher APase irrespective of P treatment. Cropping treatment had a significant effect on both bacterial (p<0.01) and fungal (p<0.05) community composition, and the interaction “cropping x P” had additional significant effects on root bacterial communities (p<0.05). Fungal diversity was significantly higher in intercropped wheat roots, whereas bacterial diversity was higher in both intercropped SB and wheat compared to monocropped treatments. Large differences in bacterial and fungal community composition were found between mono- and intercropped-wheat. This enhanced microbial diversity along with root plasticity provides evidence of belowground rhizosphere heterogeneity that was strongly influenced by cropping design rather than P status.