The Fate of Nitrogen from Injected Liquid Manure in a Silt Loam Soil

A field experiment was conducted on a Piano silt loam (Typic Argiudoll) in southcentral Wisconsin to monitor N transformations following liquid manure injections. Liquid manure with and without a nitrification inhibitor [nitrapyrin, 2-chloro-6-(trichloromethyl)pyridine, 13 mg a.i. L⁻¹ liquid manure] was injected into an uncropped area in June 1985. Soil samples were taken using a grid placed over the vertical cross-section of the injected bands permitting spatial sampling in the vertical plane. Sampling was done 5, 12, 26, 39, 67, 99, and 350 d after manure application by progressive exposure of the band. Soil water content, pH, NH₄-N, (NO₂ + NO₃)-N, Cl, and water-soluble C were determined. The synagraphic computer program, SYMAP, was used to evaluate spatial changes in inorganic N concentrations in the manure injection zone. Nitrapyrin was not effective in inhibiting nitrification at the rate used. Nitrification occurred at the outer fringes of the band within 5 d; maximum concentrations of 300 to 800 mg NO₃-N kg⁻¹ occurred throughout the injection zone at 26 d. High respiratory activity in the injection zone was indicated by an almost threefold decrease in water-soluble C between day 5 (750 mg C kg⁻¹) and day 12 (270 mg C kg⁻¹). Water content remained higher than the surrounding soil in the injection zone for 99 d. Once NO₃ leaching occurred, NO₃-N/Cl ratios decreased with depth under the injection zone. The abundant supply of NO₃ and readily oxidizable C, combined with the high moisture environment, led to the conclusion that liquid manure injections created an environment conducive for denitrification. Contribution of the College of Agricultural and Life Sciences, Univ. of Wisconsin-Madison. This research was partially funded by a grant from the Univ. of Wisconsin-Madison Graduate School and Dow Chemical, USA.