Tracing dung-derived carbon in temperate grassland using 13C natural abundance measurements

To understand the role of dung-derived carbon in the carbon cycle of grazed temperate grasslands, we need a procedure to trace dung-derived C. The natural C-13 tracer technique of applying C-4 dung to a C-3 grass pasture allowed us to successfully quantify the fate of cattle dung in the soil environment. Dung was collected from beef steers fed on either maize (C-4) or perennial ryegrass (C-3). The C-4 dung (delta(13)C - 15.4 parts per thousand) or C-3 dung (delta(13)C = -25.7 parts per thousand) was applied in circular patches to a temperate (C-3) grassland, with a bulk soil delta(13)C value of -27.9%. Triplicate samples were taken from 1-5, 5-10 and 10-20 cm depth in the soil, and from zero tension lysimeters (installed at 30 cm depth) at time intervals of 150 days following dung application. The soil and lysimeter solution samples (< 0.7 mu m-filtered) were analysed for delta(13)C and total C. Dung C was readily detectable in the upper 5 cm of the soil profile, but not below that depth. After 150 days, only 16.6% of the applied dung-C was accounted for, with 12.6% of dung C being recovered in the soil (1-5 cm depth) and 4.0% in leachate waters collected in lysimeters (installed at 30 cm depth). Apparently, only a minor proportion of dung C is retained in the grassland ecosystem. (C) 2000 Elsevier Science Ltd. All rights reserved.