Relationship between N immobilization and volatile fatty acids in soil after application of pig and cattle slurry

Summary A laboratory study was performed to determine decomposition of fatty acids and mineralization of C and N from slurries in soil. Fatty acids present in slurries decomposed within 1–2 days at 25°C in soil. Parallel to the fatty acid decomposition, immobilization of N was measured in soil. The correlation between the initial fatty acid concentrations in the slurries and the amounts of N immobilized were found to be highly significant (R ²=0.97). It was concluded that fatty acids act as an easily decomposable C source for microorganisms and cause immobilization of N. Immobilization of N was followed by a curvilinear mineralization of N in all slurrytreated soils. Despite mineralization, only fresh pig slurry and anaerobically digested pig slurry showed a net release of N over 70 days whereas cattle slurry and anaerobically fermented pig slurry did not. The percentage of slurry C evolved during 70 days was fresh pig slurry, 65%; anaerobically fermented pig slurry, 48%; anaerobically digested pig slurry, 45%; and anaerobically fermented cattle slurry, 42%.