Nitrogen fertilization effects on soil carbon and nitrogen in a dryland cropping system

No-till (NT) increases the potential to crop more frequently in the Great P lains than with the conventional-till (CT) crop-fallow farming sl stem. Mor e frequent cropping requires N input to maintain economical yields. We eval uated the effects of N Fertilization on crop residue production and its sub sequent effects on soil organic C (SOC) and total soil N (TSN) in a dryland NT annual cropping system. Six N rates (0, 22, 45, 67, 90, and 134 kg N ha (-1)) were applied to the same plots from 1984 through 1994, except 1988 wh en rates sere reduced 50%, on a Weld silt loam (fine, smectitic, mesic Arid ic Argiustoll). Spring hal leg (Hordeum vulgare L.), corn (Zea mays L.),win ter wheat (Triticum aestivum L.), and oat (Avena sativa L.)-pea (Lathyrus t ingitanus L.) hay were grown in rotation. Crop residue production varied wi th crop and gear. Estimated average annual aboveground residue returned to the soil (excluding hay years) was 2925, 3845, 4354, 4365, 4371, and 4615 k g ha(-1), while estimated annual contributions to belowground (root) residu e C were 1060, 1397, 1729, 1992, 1952, and 2031 kg C ha(-1) for the above N rates, respectively. The increased amount of crop residue returned to the soil with increasing N rate resulted in increased SOC and TSN levels in the 0- to 7.5-cm soil depth after 11 crops. The fraction of applied N fertiliz er in the crop residue decreased with increasing N rate. Soil bulk density (D-b) in the 0- to 7.5-cm soil depth decreased as SOC increased, The increa se in SOC with N fertilization contributes to improved soil quality and pro ductivity, and increased efficiency of C sequestration into the soil. Carbo n sequestration can be enhanced by increasing crop residue production throu gh adequate N fertility.