Interactions of plant species mediated plant competition for inorganic nitrogen with soil microorganisms in an alpine meadow

Sources of competition for limited soil resources, such as nitrogen, include competitive interactions among different plant species and between plants and soil microbes. We hypothesized that plant interactions intensified plant competition for inorganic nitrogen with soil microorganisms. To test these competitive interactions, one dominant species (Kobresia humilis Serg) and one less abundant gramineous herb (Elymus nutans Griseb) in an alpine ecosystem were selected as target species to grow under interactions with their neighboring plants and without interaction treatments in field plots. N-15-labeled ammonium and nitrate were used to quantify their partition between plants and soil microorganisms for 48 h after tracer additions. Responses of K. humilis to interactions from their surrounding plants were negative, while those of E. nutans were positive. Species identity, inorganic nitrogen forms, and plant interactions significantly affected the total amount of nitrogen utilization by soil microorganisms and plants. Although plant interactions have negative effects on nitrogen uptake of K. humilis, there is an increase in microbial immobilization of nitrogen under presence of its neighbors. For E. nutans, facilitation from surrounding plants is in favor of their nitrogen uptake. Compared with K. humilis, competition for N-15 - NO3- and 15N - NH4+ was less intensive between E. nutans and microorganisms. N-15 - NH4+ recovery by soil microorganisms and plants were not more than or much lower than their utilization of N-15 - NO3- under different interaction treatments. These results suggested that the partitioning of inorganic nitrogen between plants and soil microorganisms is mediated by plant - plant interactions and interactions between plants and soil microorganisms.