Estimating seasonal and annual carbon inputs, and root decomposition ratesin a temperate pasture following field C-14 pulse-labelling

Using a C-14 pulse-labelling technique, we studied the seasonal changes in assimilation and partitioning of photoassimilated C in the plant-root-soil components of a temperate pasture. Pasture and soil samples were taken afte r 4-h, and 35-day chase periods, to examine these seasonal C-14 fluxes. Tot al C and C-14 were determined in the shoot, root and soil system. The amoun ts of C translocated annually to roots and soil were also estimated from th e seasonal C-14 distribution and pasture growth. The in situ field decompos ition of newly formed roots during different seasons, also using C-14-label ling, was studied for one year in undisturbed rhizosphere soil. The C-14-la belled roots were sampled five times and decomposition rates were calculate d assuming first-order decomposition. Annual pasture production at the site was 16 020 kg DM ha(-1), and pasture growth varied with season being highest (75-79 kg ha(-1) d(-1)) in spring a nd lowest (18-20 kg ha(-1) d(-1)) in winter. The above- and below-ground pa rtitioning of C-14 also varied with the season. The respiratory C-14-CO2 lo sses, calculated as the difference between the total amounts of C-14 recove red in the soil-plant system at 4 h and 35 days, were high (66-70%) during the summer, autumn and winter season, and low (37-39%) during the spring an d late-spring season. Pasture plants partitioned more C below-ground during spring compared with summer, autumn and winter seasons. Overall, at this h igh fertility dairy pasture site, 18 220 kg C/ha was respired, 6490 kg rema ined above-ground in the shoot, and 6820 kg was translocated to roots and 1 320 kg to soil. Root decomposition rate constant (k) differed widely with t he season and were the highest for the autumn roots. The half-life was high est (111 days) for autumn roots and lowest (64 days) for spring roots. Abou t one-third of the root label measured in the spring season disappeared in the first 5 weeks after the initial 35 Day of allocation period. The late s pring, summer, late summer and winter roots had intermediate half-lives (88 -94 days). These results indicate that seasonal changes in root growth and decomposition should be accounted for to give a better quantification of ro ot turnover.