Winter rye cover crop following soybean under conservation tillage: residual soil nitrate

Use of a winter rye (Secale cereale L.) cover crop following soybean [Glycine max (L.) Merr.] has been shown to reduce the soil erosion potential in a corn (Zea mays L.)-soybean rotation system, but little is known about the effect of rye on residual soil NO3(-)N (RSN). An irrigated field study was conducted for 4 yr on a Sharpsburg silty clay loam (fine, smectitic, mesic Typic Argiudoll) to compare crop rotation and winter rye cover crop following soybean effects on RSN under several tillage practices and N fertilization rates. Treatments each year were (i) tillage: no-till or disk; (ii) rotation: corn following soybean/rye (Cbr) or soybean/rye following corn (BRc), corn following soybean (Cb) or soybean following corn (Bc), and corn following corn (Cc); and (iii) N rate: 0, 100, and 300 kg N (ha(-1) (applied to corn). Rye in the Cbr/BRc rotation was planted in the fall following soybean harvest and chemically killed in the spring of the following year prior to corn planting. Each spring, before tillage and N application, RSN was determined to a depth of 1.5 m, at 30-cm intervals. The net spring-to-spring change in RSN between subsequent spring seasons was computed for each plot, and annual aboveground N uptake for rye, corn, and soybean were determined. Rye, rotation, N rate, and tillage significantly influenced RSN in the top 1.5 m of soil. The presence of rye (BRc) reduced total spring RSN between 18 and 33% prior to corn planting in 2 of the 3 yr, compared with the no-rye system (Bc), as rye immobilized from 42 to 48 kg N ha(-1) in aboveground dry matter. Recycling of N in high-yielding rye cover crop residues led to an increase in RSN accumulation after corn in the succeeding spring. Up to 277 kg RSN ha(-1) accumulated at high rates of N following corn in the Cbr rotation, compared with 67 kg RSN ha(-1) in the no-rye system (Cb) in 1992. Regardless of the presence of rye, significant accumulation of RSN occurred following corn in the rotation sequence, while RSN declined following soybean. Less RSN was found in the top 1.5 m of soil under continuous than rotation corn, and disking tended to increase NO3(-) accumulation in rotation systems at high rates of N application. Although RSN declines following a rye cover crop, the ready release of this immobilized N suggests that some N credit should be given, reducing N recommendation for corn following winter rye cover, to minimize potential NO3(-) leaching under corn-soybean/rye rotations.