Compared effects of long-term pig slurry applications and mineral fertilization on soil denitrification and its end products (N2O, N2)

The long-term (9 years) effect of pig slurry applications vs mineral fertilization on denitrifying activity, N2O production and soil organic carbon (C) (extractable C, microbial biomass C and total organic C) was compared at three soil depths of adjacent plots. The denitrifying activities were measured on undisturbed soil cores and on sieved soil samples with acetylene method to estimate denitrification rates under field or potential conditions. Pig slurry applications had a moderate impact on the C pools. Total organic C was increased by +6.5% and microbial biomass C by >= 25%. The potential denitrifying activity on soil suspension was stimulated (x1.8, P < 0.05) 12 days after the last slurry application. This stimulation was still apparent, but not significant, 10 months later and, according to both methods of denitrifying activity measurement (r(2)=0.916, P < 0.01 on sieved soil; r(2)=0.845, P < 0.001 on soil cores), was associated with an increase in microbial biomass C above a threshold of about 105 mg kg(-1). The effect of pig slurry on denitrification and N2O reduction rates was detected on the surface layer (0-20 cm) only. However, no pig slurry effect could be detected on soil cores at field conditions or after NO3- enrichments at 20 degrees C. Although the potential denitrifying activity in sieved soil samples was stimulated, the N2O production was lower (P < 0.03) in the plot fertilized with pig slurry, indicating a lower N2O/(N2O + N-2) ratio of the released gases. The pig-slurry-fertilized plot also showed a higher N2O reduction activity, which is coherent with the lower N2O production in anaerobiosis.