Nitrogen Fertilization Effects on Soil Carbon and Nitrogen in a Dryland Cropping System

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Abstract
No‐till (NT) increases the potential to crop more frequently in the Great Plains than with the conventional‐till (CT) crop–fallow farming system. More frequent cropping requires N input to maintain economical yields. We evaluated the effects of N fertilization on crop residue production and its subsequent 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 when rates were reduced 50%, on a Weld silt loam (fine, smectitic, mesic Aridic Argiustoll). Spring barley (Hordeum vulgare L.), corn (Zea mays L.), winter wheat (Triticum aestivum L.), and oat (Avena sativa L.)–pea (Lathyrus tingitanus L.) hay were grown in rotation. Crop residue production varied with crop and year. Estimated average annual aboveground residue returned to the soil (excluding hay years) was 2925, 3845, 4354, 4365, 4371, and 4615 kg ha−1, while estimated annual contributions to belowground (root) residue 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 fertilizer in the crop residue decreased with increasing N rate. Soil bulk density (Db) in the 0‐ to 7.5‐cm soil depth decreased as SOC increased. The increase in SOC with N fertilization contributes to improved soil quality and productivity, and increased efficiency of C sequestration into the soil. Carbon sequestration can be enhanced by increasing crop residue production through adequate N fertility.