Inorganic N Dynamics from Soils Amended with Low-P Manure from Genetically Modified Pigs (Enviropig^TM)

Abstract

Genetically modified pigs have been developed to enable them to use phosphorus (P) in the feed more efficiently and thereby reduce the amount of P excreted in the manure. However, there is no information available about how improved P digestibility affects nitrogen (N) and carbon contents in the pig manure and the subsequent dynamics that occur when the manure is applied to soil. An 8-week incubation study was conducted to determine the changes in inorganic N contents of two soils (a clay loam and a sandy loam) treated with pig manure (115 mg N kg⁻¹ soil) from these transgenic pigs as well as from conventional pigs. In addition, both the transgenic and conventional pigs were fed either a low-P diet or a conventional P diet and the effects of diet type on N contents in the manure and the subsequent soil dynamics were examined. There was no difference in total manure N content between transgenic and conventional pigs. However, the ammonium content was lower and the soluble carbon level was greater in manure produced by transgenic pigs than by conventional pigs. The manure excreted by pigs fed with low-P diet contained more total N and ammonium N than the manure from pigs fed with conventional-P diet. Ammonium-N was nitrified completely in the first week of incubation in manure-amended clay loam, while the nitrification process took 4 weeks to complete in manure-amended sandy loam soil. Manure from transgenic pigs not only contained less inorganic N but this N was more rapidly immobilized and/or denitrified compared to the inorganic N produced by conventional pigs.