Does long-term center-pivot irrigation increase soil carbon stocks in semi-arid agro-ecosystems?

This study was conducted to determine the effect of center-pivot irrigation management on soil organic carbon and soil inorganic carbon stocks in semi-arid agroecosystems. We sampled two locations in the Central Great Plains, USA: an area near Imperial, Nebraska and a more arid area near Otis, Colorado. At both locations, three different agroecosystems were sampled: a non-cultivated native grassland (NG), a center-pivot irrigated (CP) and an adjacent dryland (DL) cultivated system. All sampled soils were classified as Aridic, Pachic or Calcidic Argiustoll. Soil organic (SOC) and inorganic (SIC) carbon was analyzed at 0–5, 5–20, 20–50 and 50–75 cm depths. Although significantly greater SOC stocks were found in the surface (0–20 cm) soil layers of CP compared to DL systems at both locations, when the entire 0–75 profile was considered, irrigation significantly increased SOC only at the Imperial site (on average 7626 g m− 2 versus 6637 g m− 2 at Imperial and 6769 g m− 2 versus 5698 g m− 2 at Otis). Soil inorganic C was significantly greater in the cultivated versus the grassland soils, but did not differ between dryland and irrigated management systems. Over the entire profile, differences in SIC at depth were more important than the changes of SOC storage by irrigation management in the 0–20 cm depth. Further, though highly variable, SIC accounted for nearly 50% of the total soil C storage in the cultivated systems at Otis and 30% at Imperial. Large proportions of SIC in these cultivated semi-arid lands are most likely due to cultivation-induced changes in soil properties, water regime and carbonate-forming ion concentrations. These factors must be considered to accurately assess management-induced effects on soil carbon.