Long-term release of carbon from grassland soil amended with different slurry particle size fractions: a laboratory...

Application of animal manure to agricultural soils enhances both native soil carbon (C) and overall (native soil C and added C) respiration. CO(2) effluxes were measured in a laboratory incubation study for 1465 days after the application of different slurry fractions (>2000, 425-2000, 250-425, 150-250, 45-150 and <45 µm) to a grassland soil. The slurry-derived C present in the soil was traced using the natural abundance δ(13)C method. We used two kinetic (single and two pool) models to fit the experimental data and to test the model validity with respect to long-term data sets. Mean residence times (MRTs) of the particle size based slurry-C fractions were estimated using these models and a linear (13)C natural abundance based approach. The results showed that slurry-C degradation in soil over time varied between the different particle size based slurry treatments. The two kinetic soil-C models were successful to predict medium- to long-term carbon release from soil amended with animal slurry. The estimated MRTs did vary between the linear (3.8-5.6 years) and non-linear based (0.8-3.8 years) (model) approaches. Slurry-derived C could still be (isotopically) detected in the soil 4 years after slurry application using the natural abundance δ(13)C method. This suggests that it may take a decadal timescale or longer before the entire amount of C introduced through whole slurry amendments to grassland soils is fully dissipated.