Effects of Rock Powder Additions to Cattle Slurry on Ammonia and Greenhouse Gas Emissions

For several decades, farmers have been mixing rock powders with livestock slurry to reduce its NH3 emissions and increase its nutrient content. However, mixing rock powders with slurry is controversial, and there is currently no scientific evidence for its effects on NH3 and greenhouse gas (GHG) emissions or on changes in its nutrient content due to element release from rock powders. The major aim of this study was therefore to analyse the effects of mixing two commercially established rock powders with cattle slurry on NH3, CO2, N2O and CH4 emissions, and on nutrient release over a course of 46 days. We found that rock powders did not significantly affect CO2 emission rates. NH3 and N2O emission rates did not differ significantly up until the end of the trial, when the emission rates of the rock powder treatments significantly increased for NH3 and significantly decreased for N2O, respectively, which coincided with a reduction of the slurry crust. Cumulative NH3 emissions did not, however, differ significantly between treatments. Unexpected and significant increases in CH4 emission rates occurred for the rock powder treatments. Rock powders increased the macro- and micronutrient content of the slurry. The conflicting results are discussed and future research directions are proposed