Short-term C and N dynamics in a soil amended with pig slurry and barley straw: a field experiment

Interactions between animal slurries and crop residues can impact on soil N availability during decomposition. Our objective was to study the short-term decomposition of pig slurry and barley straw incorporated alone or in combination. A field experiment was conducted on a sandy loam unamended (control) or amended with 60 m3 ha–1 pig slurry (PS) or 4 Mg ha–1 barley straw (BS), or both (PSBS). Surface CO2 and N2O fluxes, soil water content and temperature, microbial biomass C, and NO3− and NH4+ contents were monitored during 28 d in the 0- to 20-cm soil layer. Large CO2 fluxes occurred during the first 4 h of the experiment in slurry-amended plots that were attributed to carbonate dissociation when slurry was mixed to the soil. Specific respiration activity (ratio of CO2-C fluxes-to-microbial biomass C) was increased in slurry-amended soils for the first 7 d, likely due to the rapid oxidation of volatile fatty acids present in slurry. After 28 d, 26% more C had been evolved in PSBS than the sum of C released from PS and BS, indicating a synergistic interaction during decomposition of combined amendments. Adding straw caused a net but transient immobilisation of soil N, especially in PSBS plots where 36% of slurry-added NH4+ was immobilised after 3 d. Slurry-NH4+ was rapidly nitrified (within 10 d), but N2O production was not a significant source of N loss during this study, representing less than 0.3% of slurry-added NH4+. Nevertheless, about twice the amount of N2O was produced in PS than in PSBS after 28 d, reflecting lower soil N availability in the presence of straw. Our study clearly illustrates the strong interaction existing between soil C and N cycles under field conditions as slurry mineral N appeared to stimulate straw-C mineralisation, whereas straw addition caused a net immobilisation of slurry N.