Nitrogen Transformations in Aerated Swine Manure Slurries

Abstract

A laboratory study was conducted to evaluate the effect of aeration rate, temperature, and manure loading rate on nitrogen transformations in swine manure slurries. Periodic samples were collected and analyzed for residual organic N, NH₄‐N, NO₂‐N, and NO₃‐N, as well as NH₃ volatilized from the aerated slurries. It was found that organic N decreased rapidly leaving a rather constant fraction after the first several weeks. At the end of the 83‐day study at 25C, one‐fourth of the N remained in solution in organic forms. This represents N that was not mineralized and consequently would not be lost during the denitrification process. From 16 to 26% of the added N was volatilized as NH₃‐N and from 48 to 87% of the added C was evolved as CO₂‐C during the study. Increasing temperature, increasing loading rate, and increasing aeration tended to increase volatilization of NH₃ and CO₂. An N balance at the conclusion of the study showed that 22 to 48% of the N was unaccounted for. While the mechanism responsible for the N losses was not determined, NH₃ volatilization apparently predominated. The quantities of lost N increased with an increase in temperature. There appeared to be no relationship between the losses of N and changes in concentration of oxidized forms of N in solution. While nitrification occurred in extensively aerated swine‐manure slurries, about 1/4 of the original total N was not mineralized. As this represents a rather stable form and because only 3% of added N occurred in a mineral form at any given time, the extent of denitrification would be limited. Consequently, nitrification‐denitrification schemes probably have limited application to N removal from waste substances.