Soil C:N impacts on soil biological health and consequences on weed control in soybean and corn systems

Nitrogen availability has an important influence on agricultural weed growth since many weeds in annual cropping systems are more competitive in high-nitrogen soils. A potential method to control nitrogen availability is through soil carbon amendments, which stimulate soil microbial growth and immobilize nitrogen. Additionally, carbon amendments may alter soil microbial community composition, increase soil biological functioning, and improve soil health. In a two-year field experiment in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.], we implemented five amendment treatments to test their ability to alter weed and crop growth through soil nitrogen availability and soil biological functioning. The treatments included: an untreated control, an unamended weed-free control, rye hay adding 3,600 kg C·ha-1 and 3,400 kg C·ha-1 each year, sawdust adding 5,000 kg C·ha-1 and 4,400 kg C·ha-1 each year, and a rye hay and sawdust combined treatment adding 8,600 kg C·ha-1 and 7,700 kg C·ha-1 each year. Each treatment was replicated 5 times in corn and 6 times in soybean. Each season, we explored correlations between crop and weed biomass and weed community composition and nitrogen immobilization measured through soil respiration and nitrogen availability. We also explored changes to the soil microbial community composition and soil health as a secondary result of the carbon amendment treatments. Nitrogen availability was lowest in plots treated with the highest C:N amendment. Increasing carbon improved soil health metrics, but the microbial community composition was most affected by the rye hay treatment. Amendments with high C:N reduced weed growth in both soybean and corn plots but only selected for specific weed communities in soybeans, leading to improved soybean competitiveness against weeds. In corn, crop growth and weed community composition remained consistent across amendment treatments. Targeted nitrogen immobilization may improve leguminous crop competition in some weed communities as part of an integrated weed management program.