Nitrogen Immobilization In a Rye (Secale cereale L.) Roll-Killed System.

Abstract: The use of cover crops has been well documented in organic cropping systems for their ability to suppress weeds, enhance soil organic matter and prevent nutrient leaching. Rye (Secale cereale L.), along with leguminous cover crops, have been deployed in organic cropping systems to achieve many of these beneficial effects. One system that has been shown to both suppress weeds and aid in soil fertility is the roll-killed system. This study investigated the nitrogen immobilization dynamics of soybeans under the rye roll-killed system. The ability of high C:N ratio residues to cause N immobilization is well known, but the impact of surface mulches on immobilization is not well characterized.  A two site-year study was conducted in North Carolina during the 2009 growing season at Kinston and Goldsboro. Two treatments were evaluated, one treatment with rye that was roll-killed (R) and the other without rye (i.e. bare-ground) (WR). Both treatments received 50 kg ha^-1-N in mid February 2009, and the no rye treatment (WR) was sprayed with glyphosate and left fallow until the planting of soybeans (Glycine max L. ‘Hutcheson’) in May 2009. The rye biomass varied between the sites with approximately 8000 kg ha^-1 at Kinston and 4200 kg ha^-1 at Goldsboro.  During the season, the flow of soil inorganic nitrogen was monitored via ion exchange probes along with soil extractions from two depths (0-10.2 cm and 10.2-25.4 cm). Tissue data was collected on both soybeans and redroot pigweed (Amaranthus retroflexus) to determine the percent nitrogen. For both sites, peak nitrogen immobilization occurred between 4 and 6 weeks. At Kinston there was a soil inorganic total nitrogen (NO₃ and NH₄) difference of 1 ppm-N between the rye treatment (R) and the bare-ground no rye treatment. During the same sampling period at Goldsboro a difference of 2.25 ppm-N was detected. The ion-exchange probes correlated with the findings at both sites. Surface mulches appear able to cause a significant N immobilization effect in the soil profile.  Future work is needed to determine whether the effect is primarily from transport of soil N to the surface residue or from dissolved C leaching into the soil.