Carbon (13C) uptake and allocation in pasture plants following field pulse-labelling

The allocation of carbon (C) to plant roots and conversion to soil organic matter is a major determinant of the size of the terrestrial C pool in pastoral ecosystems. The aim was to quantify C allocation to roots in contrasting pastoral ecosystems. Pastures on long-term research sites in Canterbury, New Zealand were pulse-labelled using (CO2)-C-13 within portable gas-tight enclosures. Sites included Winchmore (with or without superphosphate fertiliser, and with or without irrigation) and Tara Hills (low, medium or high grazing intensity with continuous or alternating grazing). Separate micro-plots were labelled in late spring, summer and autumn at Winchmore and in spring at Tara Hills. Herbage label C-13 recoveries were greatest one hour after pulse labelling and declined by 21 days, whereas in roots they were initially lower but generally continued to increase until 21 days. The greatest recoveries of C-13 in roots, one hour and 21 days after labelling, were in summer and autumn respectively. The proportion of label C-13 allocated to roots by 21 days was 0.50 in the absence of superphosphate and 0.41 in the superphosphate treatment, and was 0.39, 0.43 and 0.51 respectively in spring, summer and autumn. Irrigation had no significant effect on root allocation. The low stocking rate at Tara Hills, which had the greatest herbage biomass, also had greater total C-13, tussock herbage C-13 and root C-13 recoveries than the higher stocking rate treatments. Inter-tussock root recovery and allocation of C-13 to roots increased with increasing stocking rate, whereas tussock root allocation was greatest in the high and least in the medium stocking rate treatment. By 21 days there was a greater inter-tussock and tussock root recovery and lower inter-tussock herbage recovery in the continuous than in the alternating grazing management treatment. The root allocation was generally greater in the continuous than in alternating grazed treatments, except for tussocks one hour after labelling where the reverse was the case. In conclusion the C-13 pulse labelling showed pasture plants allocate more C to roots with low soil fertility, high grazing intensity, continuous grazing, and in autumn.