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 13C tracer technique of applying C4 dung to a C3 grass pasture allowed us to succesfully quantify the fate of cattle dung in the soil environment. Dung was collected from beef steers fed on either maize (C4) or perennial ryegrass (C3). The C4 dung (δ13C−15.4‰) or C3 dung (δ13C=−25.7‰) was applied in circular patches to a temperate (C3) grassland, with a bulk soil δ13C 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 μm-filtered) were analysed for δ13C 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.