Factors controlling nitrification and denitrification: A laboratory study with gel-stabilized liquid cattle manure

Nitrification and denitrification were studied in a millimeterscale microenvironment using a two-phase system with a liquid manure-saturated layer. Samples consisted of liquid cattle manure and air-dried soil stabilized with silica gel, placed between two aerobic soil phases with a water content near field capacity. A high potential for NH₄ ⁺ oxidation developed within 0–2 mm distance from the interface, and NH₄ ⁺ diffused only 10–20 mm into the soil. Some NH₄ ⁺ was probably immobilized by microorganisms in the soil between 0 and 4 days, after which nitrification was the only sink for NH₄ ⁺. A potential for denitrification developed within the manure-saturated zone. Maximum rates of both potential and actual denitrification were recorded by Day 4, but denitrification continued for at least 2–3 weeks. The potential for nitrification peaked after 14 days. When the pH of the manure was adjusted to 5.5, nitrification was reduced close to the interface, and NH₄ ⁺ penetrated further into the soil before it was oxidized. The pH adjustment had an inhibitory effect on denitrification: Both potential and actual rates of denitrification were almost eliminated for several days. The size of the manure-saturated layer strongly affected denitrification losses. With layers of 8 and 16 mm thickness, losses equivalent to 33 and 40% of the original NH₄ ⁺ pool, respectively, were estimated. When manure corresponding to a 12 mm layer was homogeneously mixed with the soil, only 0.3% was lost.Offprint requests to: S. O. Petersen.