Decomposition of sorghum [Sorghum bicolor (L.) Moench] and wheat (Triticum aestivum L.) plant residues can immobilize enough surface-applied N to cause a deficiency in successive winter wheat crops. This experiment examined the effects of conventional and no-till grain sorghum and wheat residues on N requirements of dryland winter wheat. Field experiments conducted from 1987-1991 on an Austin silty clay (fine-silty, carbonatic, thermic Udorthentic Haplustoll) soil included sorghum and wheat residue treatments with conventional till (CT), no-till (NT), and residue removal (RR). All residue plots received four preplant N rates (0, 45, 90, and 135 kg N ha-1), with subplots planted to three winter wheat cultivars in 1988 and 1989, and two cultivars in 1990 and 1991. Grain and stover yields were significantly lower when wheat followed sorghum than under continuous wheat. Wheat grain yields at N application rates < 90 kg ha-1 were 39% lower in NT plots vs. CT plots, 5% lower in CT plots compared with yields in RR plots, and 39% lower in sorghum-wheat rotation compared with continuous wheat. Wheat N uptake at N application rates < 90 kg ha-1 was 41% lower in NT plots vs. N uptake in CT plots, 10% lower in CT plots vs. N uptake in RR plots, and 36% lower in sorghum-wheat rotation vs. continuous wheat. Grain yield and N uptake of wheat at the 135 kg N ha-1 rate were not significantly different in NT and CT plots. Preplant soil NO3-N analysis indicated a need for the application of N fertilizer at planting in CT and NT sorghum residues, and basal stem NO3-N analysis showed wheat growing in NT residues and sorghum-wheat rotations had higher N fertilizer requirements than continuous CT wheat. Microbial immobilization of surface-applied N was responsible for N deficiencies observed in NT winter wheat, while a decrease in fallow time between sorghum harvest and wheat planting dates was the primary cause for reduced mineralization rates of residue-derived and indigenous soil N in sorghum-wheat rotations. A fallow period following grain sorghum and/or band application of fertilizer N could alleviate N deficiencies observed in this study.
