Quantification of priming and CO2 respiration sources following slurry-C incorporation into two grassland soils with different C content.

The fate of incorporated slurry-C was examined in a laboratory experiment using two UK grassland soils, i.e. a Pelostagnogley (5.1 %C) and a Brown Earth (2.3 %C). C3 and C4 slurries were incorporated into these two wet-sieved (C3) soils (from 4-10 cm depth). Gas samples were collected 0.2, 1, 2, 3, 4, 6, 9, 20, 30 and 40 days after slurry application and analyzed for CO2 concentration and delta13C content. Slurry incorporation into the soil strongly increased soil CO2 respiration compared with the unamended soil. Total (40 day) cumulative CO2 flux was higher for the Pelostagnogley than the Brown Earth. The 13C natural abundance tracer technique enabled quantification of the sources of respired CO2 and priming effects (days 0-9). Proportionally more slurry-derived C was respired from the Pelostagnogley (46%) than the Brown Earth (36%). The incorporated slurry-C was lost twice as fast as the native soil C in both soils. Slurry incorporation induced a priming effect, i.e. additional release of soil-derived C, most pronounced in the Pelostagnogley (highest C content). The majority of respired soil-derived C (>70%) was primed C. The study indicated that potential reductions in ammonia volatilisation following slurry injection to grasslands might be negated by enhanced loss of primed soil C (i.e. pollution swapping).