Microbial nitrogen immobilization reduces competitive advantage of nitrophilous plants in legume systems

One way to alter the competitive advantage of plants is through changing resource availability. Plant species that are more competitive in high nitrogen soils may have greater reductions in growth in low resource soils, and plants with greater investment in belowground growth may then become more competitive. A useful system to test this hypothesis is in row-crop production, where high resource inputs have selected for problematic weed species that are highly responsive to nitrogen. We tested the ability of soil nitrogen immobilization to reduce the competitive advantage of nitrophilous weeds over nitrogen-fixing soybeans (Glycine max). This experiment was conducted in a greenhouse with two soybean varieties, a nodulating and a non-nodulating variety with the same lineage, and three common nitrophilous weeds: Palmer amaranth (Amaranthus palmeri S. Watson), Powell amaranth (Amaranthus powellii S. Watson), and common lambsquarters (Chenopodium album L.). The plants were grown in monoculture and in polyculture in carbon amended and unamended soils. We measured soil microbial growth and community composition, soil nitrogen availability, and plant growth after 7.5 weeks. Weed species biomass and competitive advantage was lower in carbon amended soils than in unamended soils, but soybean growth was unaffected. In carbon amended soils we observed nitrogen immobilization processes occurring with increased microbial biomass nitrogen and reduced plant-available nitrogen. These trends were mirrored in the microbial community, which had a decreased abundance of nitrifying bacteria, an increased abundance of bacteria with low nutrient requirements, and a high potential ability to degrade compounds. There were also differences in belowground bio-geo-chemical cycling based on plant polyculture compared to plant monoculture competition, which may have impacted plant competition.