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

Abstract Irrigation has been used for millennia to increase the quantity and reliability of food and fibre production in areas where lack of precipitation limits plant growth. Despite the importance of irrigation, our understanding of the impact of irrigation on soil carbon (C) and nitrogen (N) stocks is still limited, particularly for grazed grasslands. In this study, we analysed soil profile C and N data collected to 0.6 m depth from 124 paired irrigated and unirrigated grassland sites across different regions and soils in New Zealand. Averaged across all sites, soil C and N stocks were significantly lower in irrigated than adjacent unirrigated soils. Cumulative differences increased with depth in the profile to a maximum at 0.5 m, where irrigated soils had 5.21 Mg ha−1 less C (P  575 mm or aridity index >0.85) there were increasingly significant C losses from irrigated soils relative to adjacent unirrigated soils. There was evidence that the effect of irrigation increased with irrigation duration – from no significant effect at any depth for sites irrigated for