Soil Nitrogen Accumulation in Fertilized Pastures of the Southern Plains

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Abstract
Marginal farmlands seeded to grasses in the Southern Great Plains usually have been depleted in N by cropping, cultivation, and erosion. This study measured soil N accumulation over 20 to 22 years in N fertilized weeping lovegrass (Eragrostis curvula) or Old World bluestem (Bothriochloa caucasica) pastures seeded into old fields as compared to adjacent unfertilized old field pastures dominated by sand dropseed (Sporobolus cryptandrus). Significantly more (P < 0.05) total N was found in the surface 5 cm of soil from the fertilized pastures. Total N was not significantly different between the old field and N fertilized pastures at greater depths. Two different samplings resulted in an estimated 8 and 5 kg N ha-1yr-1 (standard error of the mean difference 2.4, n = 4 and 2.0, n = 10, respectively) greater N accumulation in the N fertilized pastures as compared to the old field pastures. Nitrogen input into the N fertilized pastures as fertilizer and protein supplement was 45 kg N ha-1yr-1 greater than into the old field pastures. Thus, a relatively small proportion of the N input into the N fertilized pasture was accounted for as increased soil N. The N accumulation rate in the N fertilized pastures appears to be considerably slower than the N depletion rate under past farming practices.