Partitioning and translocation of photosynthetically fixed C-14 in grazed hill pastures

Information on carbon (C) flows and transformations in the rhizosphere is vital for understanding soil organic matter dynamics and modelling its turnover. We followed the translocation of photosynthetically fixed C in three hill pastures that varied in their phosphorus (P) fertility, using a C-14-CO2 pulse-labelling chamber technique. Pasture shoot, root and soil samples were taken after 4 h, 7 days and 35 days chase periods to examine the fluxes of C-14 in the pasture plant-root-soil system. Shoot growth over 35 days amounted to 114, 179 and 182 gm(-2) at the low (LF), medium (MF) and high (HF) fertility pasture sites, respectively. The standing root biomass extracted from the soil did not differ significantly between sampling periods at any one level of fertility, but was significantly different across the three levels of fertility (1367, 1763 and 2406 g m(-2) at the LE MF and HF pastures, respectively). The above- and below-ground partitioning of C-14 was found to vary with the length of the chase period and fertility. Although most C-14 (74%, 65% and 57% in the LE MF and HF pastures, respectively) was in the shoot biomass after 4 h, significant translocation to roots (23-39%) was also detected. By day 35, about 10% more C-14 was partitioned below-ground in the LF pasture compared with the HF pasture. This is consistent with the hypothesis that, at limiting fertility, pasture plants allocate proportionally more resource below-ground for the acquisition of nutrients. In the LF site, with an annual assimilated C of 7064 kg ha(-1), 2600 kg was respired, 1861 kg remained above-ground in the shoot and 2451 kg was translocated to roots. In the HF pasture, of the 17313 kg ha(-1) C assimilated, 7168 kg was respired, 5298 remained in the shoot and 4432 kg was translocated to the roots. This study provides, for the first time, data on the fluxes and quantities of C partitioned in a grazed pasture. Such data are critical for modelling C turnover and for constructing C budgets for grazed pasture ecosystems.