Use of 13C abundance to study short-term pig slurry decomposition in the field

Applying pig slurry (PS) on agricultural soils is a common practice. However, its impact on soil organic C dynamics is not clear. This experiment investigated the use of natural 13C abundance to study the short-term C mineralization of anaerobically stored PS under field conditions. Measurements of δ13C–CO2 were made on soil air samples obtained from a bare sandy loam during 22 d following incorporation of either PS alone, PS+barley straw, or barley straw alone; an unamended treatment was used as a control. Slurry C was enriched in 13C (−20.0‰) because of the high corn (Zea mays L.) content of the animal diet. This value contrasted with δ13C of −28.4‰ for the soil organic matter and of −29.0‰ for the barley straw. A peak of high δ13CO2 values (average of −9.2‰) was observed on the day of PS application and was attributed to the dissociation of PS carbonates when mixed with the relatively acidic soil. After this initial burst, 36% of the evolved CO2 originated from the decomposing PS. After 22 d of incubation, approx. 20% of the PS-C had been lost as CO2. This short-term field study did not show any priming effect of PS on the mineralization of straw or native soil C. Due to its heterogeneity, the use of the isotopic composition of the evolved CO2 for estimating PS decomposition requires precaution either through the use of a specific experimental design involving comparable C3 and C4 treatments, or calculations to account for the presence of 13C-enriched inorganic C in the PS.