Impact of Cropping Systems on Net Nitrogen Mineralization Rate.

N fertilizer is an expensive input for managing crop land. However, the added nitrogen is not all taken up by plants. Under some management systems, a large amount of mineralized N may be leached from the soil, negatively impacting surface-water systems. But under alternative management systems, the nitrogen could be slowly mineralized to a plant-available form and less likely to be leached. Better predictions of N mineralization rates are essential to improve management decisions for cropping systems. The research questions for this experiment were: (1) Does the cropping system have an impact on the net N mineralization rate? (2) Does the presence of decomposing plant residues influence the rate of mineralization? Net nitrogen mineralization rates were compared among five biofuel cropping systems: (a) corn in corn-soybean rotation, (b) continuous corn, (c) continuous corn with a winter rye cover crop, (d) unfertilized perennial prairie, and (e) fertilized perennial prairie. Soil samples were composited from four replicate blocks per cropping system in a complete randomized-block field experiment near Ames, Iowa. To look at the impact of plant residues on mineralization, N mineralization was measured in soil samples with intact residues as well as in soil samples with the plant residues removed. We used the laboratory incubation-leaching method: 30 g of field-moist soil were placed in a column and incubated under aerobic conditions at 35°C for 30 days. During the incubation period, the soil column was leached every five days with 5 mM CaCl₂ solution, followed by equilibration with a nutrient solution that contained no N. The net mineralization rate was calculated from the cumulative, leached mineralized N (measured as nitrate-N). When the two residue treatments were lumped together, the least significant difference statistical analysis indicated that the net rate of N mineralization in the fertilized perennial prairie system (1.12 mg N/kg soil/day) was significantly larger than that of the other cropping systems, except for the continuous corn with rye cover crop (1.02 mg N/kg soil/day). The mineralization rates of the unfertilized prairie and continuous corn treatments were significantly smaller than those of the other cropping systems (e.g., 0.86 mg N/kg soil/day for the continuous corn cropping system) (α = 0.05). Across all five cropping systems, the N mineralization rate was larger in the presence of plant residues than in their absence (p = 0.05). In addition, the amount of plant residues in the soil samples had a positive effect on the N mineralization rates (p = 0.01).