Carbon mineralization, nitrogen immobilization and pH change in soil after adding volatile fatty acids

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Abstract
Summary: Volatile fatty acids (VFAs) accumulate in animal manure when it is stored anaerobically, and they quickly decompose when the manure is applied to soil. In this study the influence of VFAs on the immobilization of N and mineralization of C in soil was investigated by incubating mixtures of acetate, propionate and butyrate in soils containing varying amounts of clay. The oxidation of VFAs (300 μg C g−1 soil) caused a significant increase in pH (0.6–2.2 pH units), with the largest increase in the most coarse‐textured soil. The maximum net immobilization of N resulting from decomposition of the VFAs was 33–77 mg N g−1C and was maximal after 1–5 weeks of decomposition. After this time immobilized N was remineralized, and after 12 weeks the VFAs caused no net immobilization of N in the two most sandy soils. Despite this, the concentration of N in the microbial biomass was still greater in the soil amended with VFAs than in the control. After 12 weeks, the mineralization of C from the decomposition of the VFAs was equivalent to 60–113% of the applied C. It seems that mineralization of native soil C and N was stimulated by adding VFAs, except in the most clayey soil. This stimulation was presumably caused by the increase in the soil's pH as the VFAs oxidized.