Ammonia volatilization and selected soil characteristics following application of anaerobically digested pig slurry

Ammonia volatilization occurs shortly following land application of pig slurry. Several slurry and soil characteristics modulate the intensity of this process, and their net effect on volatilization is still hard to predict. Our aim was to compare volatilization following application of anaerobically stored (ASPS) and anaerobically digested (ADPS) pig slurry to a bare loamy soil (loamy, mixed, frigid, Aeric Haplaquept). Ammonia volatilization was measured using wind tunnels. Soil pH and water, NH4+, NO3-, and volatile fatty acid (VFA) contents were monitored in the 0- to 0.5-, 0.5- to 2-, 2- to 5-, and 5- to 10-cm soil layers to explain volatilization rates. Following slurry application, pH increased by 1 to 3 units in the top 2 cm of soil, resulting in high volatilization rates in the first 6 h of experiment. Thereafter, pH decreased more slowly in ASPS than ADPS plots, possibly due to the degradation of VFAs present in ASPS. After 2 d, 35% of slurry-added NH4+-N was lost as NH3-N for both slurries, corresponding well to the net decrease found in soil NH4+-N content. After 9 d, net soil NH4+-N disappearance accounted for about 60% of slurry-added NH4+-N for both slurries, whereas NH3-N losses represented only 40%. Therefore, for the first 2 d of the experiment NH3 volatilization explained most of the decline in soil NH4+. Afterwards, biological processes, such as immobilization and nitrification, were assumed to play a significant role in slurry NH4+ disappearance. Despite marked changes in slurry properties, anaerobic digestion did not significantly modify the proportion of slurry N that was lost as NH3. Ammonia volatilization was related mostly to soil pH and NH4+ content in the top 2 cm of soil. Below 5-cm depth, slurry application had little effect on soil pH, water, VFA, or mineral N content. This finding stresses the importance of stratified soil sampling when studying the short-term effects of pig slurry on NH3 volatilization and associated soil properties.