Urea hydrolysis and calcium carbonate precipitation in gypsum-amended broiler litter.

Authors: BURT, CHRISTOPHER - University Of Georgia; CABRERA, MIGUEL - University Of Georgia; Rothrock, Michael; KISSEL, DAVID - University Of Georgia

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2017
Publication Date: 1/12/2018
Citation: Burt, C., Cabrera, M., Rothrock Jr, M.J., Kissel, D.E. 2018. Urea hydrolysis and calcium carbonate precipitation in gypsum-amended broiler litter. Journal of Environmental Quality. 47(1):162.169. 10.2134/jeq2017.08.0337.
DOI: https://doi.org/10.2134/jeq2017.08.0337

Interpretive Summary: Broiler litter (BL) contains significant amounts of organic nitrogen (N) in the form of urea which is subject to ammonia (NH3) volatilization. Previous work has shown that the addition of gypsum to BL can increase nitrogen (N) mineralization, and decrease NH3 losses due to a decrease in pH but the mechanisms responsible for these differences are not well understood. Therefore, the goal of this study was to compare the rates of urea hydrolysis in gypsum-amended litter and unamended litter, and to identify the mechansim responsible for decreases in litter pH. The addition of gypsum to BL increased ammonium (NH4+-N) concentrations (p < 0.0033) and decreased litter pH by 0.43 to 0.49 pH units after 5 days (p <0.0001); however, the rate of urea hydrolysis in treated litter only increased on day 0 for broiler litter with Low (0.31 g H2O g -1) and High (0.78 g H2O g -1) water contents, and on day 3 for litter with Medium (0.44 g H2O g -1) water content (p < 0.05). Amending BL with gypsum also caused an immediate decrease in litter pH (0.22 pH units) due to the precipitation of calcium carbonate (CaCO3) from gypsum-derived calcium and litter bicarbonate. Furthermore, as urea was hydrolyzed in treated litter, more urea derived carbon precipitated as CaCO3 than in untreated litter (p < 0.001). Findings of this study indicate that amending BL with gypsum favors the precipitation of CaCO3 which buffers against increases in litter pH that are known to facilitate NH3 volatilization.

Technical Abstract: Broiler litter (BL) contains significant amounts of organic nitrogen (N) in the form of urea which is subject to ammonia (NH3) volatilization. Previous work has shown that the addition of gypsum to BL can increase nitrogen (N) mineralization, and decrease NH3 losses due to a decrease in pH but the mechanisms responsible for these differences are not well understood. Therefore, the goal of this study was to compare the rates of urea hydrolysis in gypsum-amended litter and unamended litter, and to identify the mechansim responsible for decreases in litter pH. The addition of gypsum to BL increased ammonium (NH4+-N) concentrations (p < 0.0033) and decreased litter pH by 0.43 to 0.49 pH units after 5 days (p <0.0001); however, the rate of urea hydrolysis in treated litter only increased on day 0 for broiler litter with Low (0.31 g H2O g -1) and High (0.78 g H2O g -1) water contents, and on day 3 for litter with Medium (0.44 g H2O g -1) water content (p < 0.05). Amending BL with gypsum also caused an immediate decrease in litter pH (0.22 pH units) due to the precipitation of calcium carbonate (CaCO3) from gypsum-derived calcium and litter bicarbonate. Furthermore, as urea was hydrolyzed in treated litter, more urea derived carbon precipitated as CaCO3 than in untreated litter (p < 0.001). Findings of this study indicate that amending BL with gypsum favors the precipitation of CaCO3 which buffers against increases in litter pH that are known to facilitate NH3 volatilization.