Sour orange fine root distribution after seventeen years of atmospheric CO2 enrichment

Long-term (more than 15 years) effects of elevated CO2 on belowground responses of trees have received limited attention in the literature. We report on a study in which sour orange trees (Citrus aurantium L.) grown in an Avondale loam in Phoenix, AZ, were exposed to ambient and elevated (ambient + 300 μmol mol−1) levels of atmospheric CO2 for 17 years. At study termination, soil cores were collected to determine how long-term CO2 enrichment affected horizontal (0.5, 1.0, 1.5, and 2.0 m from each tree) and vertical (0–105 cm in 15 cm increments) standing crop fine root dry weight and length. Overall, elevated CO2 increased both root length (35.6%) and root dry weight (39.1%) densities. There was no effect of CO2 concentration on horizontal root distribution patterns. However, significant CO2 by depth interactions were noted for both root length and root dry weight densities with differences occurring only in the top two depths. Elevated CO2 increased fine root length density by 64.5 and 57.2% at the 0–15 and 15–30 cm depths. Respective increases for root dry weight were 80.3 and 82.8%. These large responses occurred under water and nutrient supplies typical of orchard conditions and suggest that long-term citrus productivity can be enhanced under future levels of atmospheric CO2.