Alteration of pasture root carbon turnover in response to superphosphate and irrigation at Winchmore New Zealand

Grassland net primary productivity, carbon (C) allocation to roots, root production and subsequent turnover time in grazed pastures have significant implications for modelling landuse effects on global carbon dynamics. The objective of this work, using the decline of assimilated 13 C, was to quantify C allocation to roots, root production and turnover time in response to long‐term fertiliser and irrigation treatments. The treatments on the pastures on a long‐term research site at Winchmore, Canterbury, New Zealand were 0 or 375 kg ha −1 y −1 superphosphate under irrigation and, on a nearby irrigation experiment, unirrigated or irrigated (with fertilisation of 250 kg ha −1 y −1 superphosphate) when the soil water content fell to 20% w/w (50% available moisture). The pasture treatments were pulse‐labelled using 13 CO 2 within portable gas‐tight enclosures. Separate micro‐plots were 13 CO 2 pulse‐labelled in late spring, summer and autumn. Below ground net primary production (206 g C m −2 y −1 ) was similar in unfertilised, unirrigated, fertilised and irrigated pastures, despite marked differences in above ground production. Unfertilised/irrigated and unirrigated/fertilised treatments had greater root biomass, root C allocation and longer root turnover time (1.9 y and 2.0 y, respectively) than fertilised and irrigated treatments (1.3 y). These root turnover times appeared to be consistent with the improved substrate quality attributed to the species found in the irrigated and fertilised treatments.