Nitrogen mineralization as affected by cropping system

The quantity of crop residues returned to the soil is controlled by the cropping system and in turn these residues affect the amount of mineral nitrogen (N) released by decomposition of organic matter. This study was conducted to evaluate the effect of four 20-year-old cropping systems which returned varying amounts of crop residue on N mineralization potential (N-0) of a Misteguay silty clay soil. N-0 was estimated from a long-term incubation study using exponential and hyperbolic models. The N-0 ranges from 70 to 109 mg.kg(-1) for the exponential model and 86 to 144 mg.kg(-1) for the hyperbolic model. Values of N-0 are closely related to the estimated amount of crop residue returned. It requires 0.33 Mg.ha(-1) of crop residue returned to increase N-0 one mg.kg(-1). Estimated N-0 values are consistently greater for the hyperbolic than for the exponential model. However, there is a close relationship between the instantaneous rates of reaction of the two models suggesting both models can be used in laboratory incubation studies to estimate N-0 using a nonlinear least-squares fitting technique. This study shows that the exponential and hyperbolic models are equally effective in distinguishing both the qualitative and quantitative changes in soil organic N due to the cropping system in place.