Acid-treatment of the solid separate of pig slurries for reducing greenhouse gas emissions and improving nutrient balance

Composted animal slurries can be valuable sources of soil organic matter and nutrients, however their narrow N:P ratio can cause a nutrient imbalance in the soil, leading to a progressive accumulation of P and increased potential for P losses and adverse environmental impacts. Moreover, losses of ammonia and greenhouse gases can occur both during the composting phase and after soil application of the amendment. In this experiment, we tested the effects of pig slurry acidification and of acidification with nutrient correction on ammonia volatilization, gas emissions and soil nutrient status, with respect to an untreated slurry. The solid separate was acidified with elemental sulfur prior to stabilization by composting, and subsequently pressed into pellets to favour precise mechanized distribution. In addition, a nutrient corrected pellet was produced by adding calcium and potassium nitrate to the acidified solid separate, thus obtaining a balanced nutrient source for crops. The impacts on soil nutrient balance and greenhouse gas emission were evaluated under controlled conditions. The different kinds of pellets were incubated with two soils having different initial P saturation. The gaseous emissions were measured during a time span of 92 days, and the rate of organic matter mineralization and changes in soil chemical properties were measured during 63 days of incubation. Finally, potential P losses in solution and in particulate form were evaluated. Acidification resulted in a slight improvement of the N:P ratio of the pellets, lower pH, increased nitrate concentration in soil and reduced emission of ammonia and carbon dioxide, although it did not affect organic matter dynamics. The increase in available P content and potential P environmental mobility mostly depended on the dose of pellet application rather than on the acidification treatment. The samples receiving greater amounts of compost dispersed less fine particles, due to the aggregating effect of organic matter, however, the overall potential P loss increased because of the enhanced P enrichment of the fine particles.