Microbial nitrogen immobilization reduces competitive advantage of nitrophilous plants with soybean

Due to differential plant resource allocation, changing soil resource availability can alter the competitive advantage of plants. A useful system to test this hypothesis is in row-crop production, where high resource inputs have selected for weed species that are highly responsive to nitrogen (N). We tested whether soil N immobilization can reduce the competitive advantage of nitrophilous weeds over N-fixing soybeans (Glycine max).

This experiment was conducted in a greenhouse with two soybean varieties 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 polyculture in carbon (C) amended and unamended soils. We measured soil microbial growth and community composition, soil N availability, and plant growth after 7.5 weeks.

Weed species biomass was lower in C amended soils than in unamended soils, but soybean growth was unaffected. In C amended soils we observed increased microbial biomass N and reduced plant-available N, which indicate N immobilization processes. 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 plants grown in polyculture compared with plants grown in monoculture.

Altering soil resource availability has cascading effects in bio-geo-chemical cycling that result in changes in plant competitive outcomes.