The influence of dung amendments on dissolved organic matter in grassland soil leachates - Preliminary results from a lysimeter study

Understanding the carbon (C) cycle in grassland pasture systems requires more information about the fate of decomposing dung material within the soil. In this soil lysimeter study we successfully applied the natural C-13 abundance labelling technique to trace dung-C within a temperate grassland soil. Dung was collected from beef steers fed on either maize (a C-4 plant) or perennial ryegrass (a C-3 plant) silages, and applied to a freely draining (C-3) grassland soil. Leachates were collected from soil lysimeters (0 - 2.5) and (0 - 10 cm soil depth) to determine the organic carbon and C-13 content of < 0.7 mu m filtered solution. Leachates were taken from (i) control, no dung added, (ii) C-3 dung and (iii) C-4 dung amended soil. Results showed that, (i) the addition of dung resulted in a tenfold increase in C lost from the lysimeters in drainage waters, (ii) up to 50 % of the C present in the leachates was 'native' soil C and (iii) the application of dung produced a 'priming' effect. Further work is required to verify; (i) whether increased leaching of native C following dung application is a 'true priming' phenomenon, or merely the result of 'displacement' or 'pool substitution' of soil C, and (ii) the precise conditions and mechanisms under which organic amendments induce a true 'priming' effect in grassland and other agricultural soils.