Interspecific competition and nitrogen application alter soil ecoenzymatic stoichiometry, microbial nutrient status, and improve grain yield in broomcorn millet/mung bean intercropping systems

Abstract

• Broomcorn millet consistently dominated the intercropped systems. • Nitrogen fertilizer application intensified the differences in growth and competition between the species. • Intercropping and N fertilization affected soil nutrient availability and soil microbial activity. • High N fertilizer application rates decreased the intercropping advantages. Positive interactions in cereal/legume intercropping may drive high system productivity and ensure the sustainable development of modern agricultural practices. In this study, the effect of nitrogen (N) fertilizer application rates on soil ecoenzymatic stoichiometry, microbial nutrient status and interspecific competition during the cogrowth stages in broomcorn millet ( Panicum miliaceum L.)/mung bean ( Vigna radiata L.) intercropping were investigated. Two-year field experiments (2018 and 2019) involved the use of the following N treatments: N0, 0 kg N ha −1 (control); N1, 45 and 60 kg N ha −1 ; N2, 90 and 120 kg N ha −1 ; and N3, 135 and 180 kg N ha −1 for mung bean and broomcorn millet, respectively. Intercropped broomcorn millet was the dominant species determined by positive competitive ratios and aggressivity values at the early (1.70 and 1.81), middle (2.06 and 1.98), and the later cogrowth stages (3.02 and 2.48), respectively. Nitrogen application increased the competitive ratio and aggressivity by 29.4 % and 24.5 %, respectively, compared with the control. Leaf photosynthesis and aboveground biomass of the two crops were altered by the N treatment, with high N fertilizer inputs significantly reducing the intercropping advantages and land use efficiency. Intercropping and N fertilizer improved interspecific competition and boosted grain yield in 2018 and 2019, and values were 6331.8 kg ha −1 and 6531.3 kg ha −1 for broomcorn millet and 905.0 kg ha −1 and 979.8 kg ha −1 for mung bean, respectively. Moreover, intercropping and N fertilization markedly affected the soil nutrient content, microbial biomass, and enzyme activity involved in carbon (C), nitrogen (N), and phosphorus (P) acquisition in both species. The soil ecoenzymatic C:N:P acquisition ratios were strongly associated with nutrient acquisition by microbes in the rhizosphere, highlighting the importance of microbial metabolic changes driven by nutrient stoichiometry to meet the soil nutrient demands. This study provides an explanation for considering more sustainable agricultural practices of cereal/legume intercropping based on reduced fertilizer application compared with monocropping.