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°48′S, 171°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.