Winter Rye Cover Crop Biomass Production, Degradation, and N Recycling.

Cover crops are used to reduce nitrate transport to water systems in row crops. A winter rye (Secale cereale L.) cover crop can utilize residual soil nitrate; however, does the rye N uptake affect N recycling to soil and subsequent N fertilization rate for corn (Zea mays L.). The rye C:N ratio could also influence N recycling. The objectives of this study were to evaluate rye biomass degradation, and C/N recycling after spring rye termination in a no-till corn-soybean [Glycine max. (L.) Merr.] rotation.  A two year experiment (2010-2011) was conducted at four Iowa sites. Treatments included N rate applied to prior-year corn (0, 135, and 225 kg N ha^-1) and time for rye degradation after rye termination. Only time was considered for rye following soybean. Rye was collected from representative plots, put into nylon mesh bags, and placed on the surface of corresponding plots. Bags were collected after 1, 3, 9, and 15 weeks to measure remaining dry biomass dry matter, and C and N concentration. The average rye biomass production and rye N uptake were low but variable across sites (< 2300 kg ha^-1 and < 45 kg N ha^-1, respectively). Rye following corn had a higher C:N ratio compared to rye following soybean (20 vs. 14). Rye biomass, C, and N consistently decreased across the 15 weeks. An average 64% of the rye following corn and 60% of the rye following soybean was degraded after 15 weeks. Nitrogen recycling was 60% with rye following corn and 77% with rye following soybean during the same period. Nitrogen uptake by the rye cover crop and release over time was low, and thus there would be little influence expected on the economic optimum N fertilization rate for corn.