The reduction of plant-available nitrogen by cover crop mulches and subsequent effects on soybean performance and weed interference

A 3 site-year study was conducted to investigate the impact of roller-crimped rye (RC) (Secale cereale L.) mulches on soil N immobilization and subsequent effects on weed suppression and soybean [Glycine max (L.) Merr.] yield. Treatments consisted of: (i) RC, (ii) conventional tillage with neither rye cover crop nor weed control measures (WC), (iii) conventional tillage plus herbicide weed control (CT+HB), and, (iv) roller-crimped rye plus herbicide (RC+HB). The rye biomass varied between the sites with 4400, 8300, and 7084 kg ha^-1 dry matter (DM) for Goldsboro 2009, Kinston 2009, and Kinston 2010, respectively. During the season, the flow of soil inorganic N was monitored via ion-exchange probes and by direct extractions at two depths (0-10 and 10-25 cm) every 2 wk. Tissue data was collected every 2 wk on soybean and redroot pigweed (Amaranthus retroflexus L.) to determine the C/N ratio. For all sites, peak N immobilization occurred between 4 and 6 weeks after planting (WAP), indicated by a reduction in soil inorganic N. Results from the ion-exchange probes showed similar trends of the extractable soil inorganic N at all sites. Pigweed C/N ratios revealed a growing divergence between the two systems, with a severe N deficiency in the RC. Even with varying rye biomass production across environments the RC system created an extremely low N environment, suggesting that when a cereal cover crop is paired with a legume cash crop, reduced weed crop interference may result, with little reduction in soybean yield.