Effects of indigenous arbuscular mycorrhizal fungi and nitrogen forms on plant nitrogen utilization and the influencing factors in a pepper-common bean intercropping system

In recent years, continuous cropping of peppers and unreasonable application of high-residue nitrogen (N) fertilizer have been the main factors preventing high yields and high-quality cultivation of peppers. A study of indigenous arbuscular mycorrhizal fungi (AMF) and intercropping to enhance the utilization of different forms of N by vegetables, combined with the feedback effect of soil hypha density, available N nutrients, and enzymes may provide a basis for efficient utilization of N in protected culturing soil and reduced soil N residues. A pot experiment with different planting options (pepper-common bean intercropping, pepper monocropping, and common bean monocropping), different AMF treatmentsno AMF (NM), and indigenous AMF inoculation and different forms of N treatmentsno N, inorganic N (ammonium bicarbonate, 120 mg·kg^-1, ION), and organic N (glutamine, 120 mg·kg^-1, ON) was conducted to reveal the effects of indigenous AMF, N form, and pepper-common bean intercropping on mycorrhizal colonization, soil enzyme activity, and N utilization by plants under greenhouse conditions. The results showed that, compared with NM treatment, inoculation of indigenous AMF significantly increased plant biomass and N uptake of peppers and common beans, except in the case of common bean monocropping-ON treatment, and decreased the contents of NH₄⁺-N and NO₃^--N in rhizosphere soil. Whether inoculated with AMF or not, N application increased plant biomass and N uptake of peppers and common beans, with the order of ON > ION, except common bean monocropping-AMF treatment. In comparison with monocropping-ON-AMF treatment, intercropping-ON-AMF treatment increased the N uptake of peppers and common beans significantly by 39.9% and 93.0%, respectively. Intercropping and inoculating with indigenous AMF increased protease, urease, and nitrate reductase activities and organic matter content in rhizosphere soils to different extents. Correlation analysis showed that N uptake in peppers and common beans was significantly positively correlated with mycorrhizal colonization percentage, while soil NH₄⁺-N and NO₃^--N contents were significantly negatively correlated with mycorrhizal colonization percentage. In addition, the activities of soil protease, urease, and nitrate reductase were positively correlated with N uptake of peppers and common beans. Our results indicated that pepper-common bean intercropping combined with inoculation by indigenous AMF and application of an appropriate quantity of organic N significantly promoted pepper and common bean growth and N utilization in protected cultures.