Soil carbon stock beneath an established irrigated pasture grazed by dairy cattle

Soils were sampled at intervals to 0.8 m depth on an irrigated dairy farm in the Canterbury region of New Zealand and a nearby (control) area with unimproved grasses which was not known to have been irrigated, fertilised or grazed by farmed animals. Eleven years earlier, the dairy farm had been converted from an unirrigated sheep farm. For each depth interval, we measured the bulk density and organic carbon (C) concentration. By volumetric calculations to depth 0.3 m, the irrigated sites mean soil C stock was 28% greater than the control (10.0 ± 0.3 kg C m−2 [±standard error] vs 7.8 ± 0.6 kg C m−2), a statistically significant difference (P < 0.05). In contrast, by volumetric calculations to depth 0.8 m and on an equivalent soil mass basis, the corresponding differences were not statistically significant (13.0 ± 0.4 vs 12.9 ± 0.2 kg C m−2 and 11.6 ± 0.4 vs 12.9 ± 0.2 kg C m−2, respectively). Although C stock change was inferred by sampling soils on irrigated and control sites, these results suggested changes can depend on the sampling depth and calculation basis.