Mineralization of nitrogen in Danish soils,as affected by short-, medium- and long-term annual inputs of animal slurries

At four sites across Denmark with varying medium- and long-term histories of annual slurry applications, N turnover rates and crop N use efficiencies were measured in 2000. No significant effect of medium-term (in this study, 3 years) annual slurry applications on gross N turnover was observed. However, a significant effect of long-term (in this study, >25 years) annual slurry applications was observed. At one site in Denmark with short-term (4 days before measurement) slurry application, N turnover was measured in 2001. Gross N turnover was 4–5 times higher in the slurry-amended soil compared to the unamended soil. In both years, net N turnover was unaffected by the slurry application. Generally, the crops had higher use efficiency of slurry NH₄ ⁺-N than of mineral fertilizer-N, indicating that the crops were able to extract slurry organic-N, independently of the net mineralization. The measured net N mineralization rate was generally higher than the difference between gross rates. The application of ¹⁵NH₄ ⁺ to soil (a prerequisite for the determination of N mineralization and N immobilization turnover), probably stimulated the gross N immobilization rate in soil with little native NH₄ ⁺, since NH₄ ⁺ is the substrate for immobilization. The results suggest that gross immobilization estimates should be interpreted with caution.