Winter legume cover crop benefits to corn: Rotation vs. fixed-nitrogen effects

The use of winter legume cover crops for erosion control and to provide additional N to the soil is well established. Other potential benefits to legume cover crops besides N additions have been recognized, but have not been quantified. The objective of this study was to separate the fixed-N effects from the rotation effects in a winter legume cover cropping system. A field study was initiated in 1989 on a Norfolk loamy sand (fine, loamy, siliceous, thermic Typic Kandiudult) in east-central Alabama. Corn (Zea mays L.) was grown following (i) 'Tibbee' crimson clover (Trifolium incarnatum L.), (ii) a partially ineffective-nodulating crimson clover, CH-1, (iii) rye (Secale cereale L), and (iv) winter fallow. The plots were split into four rates of fertilizer N (0, 56, 112, and 168 kg N ha^-1) in a split-plot experimental plan. An evaluation of different methods of distinguishing fixed-N vs. rotation effects of the winter annual legume cover crop to a subsequent corn crop was made. Regression analysis of the effect of N application rates on N₂ fixation by crimson clover (fertilized with 45 kg N ha^-1) indicated that CH-1 clover biomass contained approximately 40 and 101 kg N ha^-1 and Tibbee clover contained approximately 51 and 119 kg N ha^-1 in 1990 and 1991, respectively. In both years of the study, crimson clover substantially increased corn yield compared with winter fallow, with a yield increase at the highest fertilizer N application level of 7 and 22% for 1990 and 1991, respectively. Estimates of yield increases due to rotation ranged from negative to 40%. The data indicated that winter cover crops improve corn yield and that besides soil N availability, there was very little difference between the beneficial effects of clover and the rye cover crops to corn.