Short-term Nitrogen Mineralization Potential in Soils of Biofuel Cropping Systems

Plant-available N in soil originates both from fertilizer N amendments and from mineralization of soil organic matter and crop residues. This study compared the N mineralization potential of soil in five biofuel cropping systems in Iowa (continuous corn [Zea mays], continuous corn with a winter rye [Secale cereale] cover crop, perennial prairie, N-fertilized perennial prairie, and corn-soybean [Glycine max] rotation). Net N mineralization potential was measured by a 4-week incubation and leaching method. To evaluate the impact of plant residues on mineralization, net N mineralization potential was measured with and without macroscopic in situ residues. To explore the effect of freezing and thawing on net N mineralization, some soil samples were frozen and thawed before incubation. Cropping systems had a significant effect (P < 0.05) on net N mineralization rate, in the following order: N-fertilized perennial prairie approximate to continuous corn with winter rye >= corn-soybean rotation >= continuous corn >= perennial prairie. The freezing and thawing treatment increased the 30-day net N mineralization rate approximately two-fold. The presence of plant residues alone did not affect the N mineralization rate, but N mineralization was significantly and positively correlated with the amount of plant residue N in the soil.