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

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 < 0.001) and 0.3 Mg ha(-1) less N (P < 0.05). The C:N ratio was also consistently lower (P < 0.001) in irrigated soils. There was no evidence for an overall effect of soil order or region on the irrigation effect on C and N, but there was a significant negative relationship between the irrigation effect on soil C and aridity. There was no irrigation effect on soil C at the most arid sites, but as aridity decreased (precipitation >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 <6-12 years, to a difference of 8.2 Mg C ha(-1) in the top 0.5 m for sites irrigated for more than 12 years. There was no compelling evidence for continued long-term loss of C, which suggests that a new equilibrium may be reached with soil C stocks 5-10 Mg ha(-1) lower under irrigated than unirrigated grasslands. Lower C stocks in irrigated than unirrigated soils do not agree with current IPCC recommended default values and this discrepancy needs to be addressed.