Impacts of irrigation on soil C and N stocks in grazed grasslands depends on aridity and irrigation duration

•Soil C and N stocks were measured at 124 paired irrigated and unirrigated sites.•On average irrigated soils had significantly less C and N than unirrigated soils.•C losses were greater at moister sites, and sites with a longer history of irrigation.•Loss of soil C under irrigation contrasts with IPCC default C stock change factors.