Sixty years of seasonal irrigation affects carbon storage in soils beneath pasture grazed by sheep

For sixty years at Winchmore, South Island, New Zealand (43 degrees 48'S, 171 degrees 48'E, 160 masl), stoney soils under continuous pasture grazing by sheep have received rainfall (nil irrigation) or rainfall and irrigation as required during summer. This consistently managed, replicated field trial presents a unique opportunity to examine long-term treatment effects on pastoral soil. Samples were recently excavated at intervals to a depth of 1 m and the total carbon (C) storage measured. In the irrigated plots, soil C storage (9.1 +/- 0.3 kg C m(-2), mean +/- standard error, n = 3) was significantly less (p < 0.05) than in plots receiving rainfall alone (13.4 +/- 0.8 kg C m(-2)). We estimated irrigation induced a 36% increase of C inputs to the soil on an annual basis, mostly as litter fall. Using a respiration model based on soil temperature and water content inputs, irrigation was also estimated to have induced a 97% increase in rate of annual C loss to the atmosphere. On this basis, the estimated irrigation effects had reduced C storage by 61% (97-36%), reasonably accounting for the 47% treatment effect determined by soil sampling. (c) 2011 Elsevier B.V. All rights reserved.