Comparison of methods for determining organic carbon content of urban soils in Central Ohio

Reliable methods are key to quantify organic carbon in urban soils. Samples from across the Ohio State University campus were divided into two datasets (a) pH ≤ 7.5 and (b) pH ≥ 7.8. Organic carbon was assessed using multiple methods, including heated acid dichromate digestion (OCDichromate), loss-on-ignition (LOI) at 400 °C and 550 °C, a dry combustion method (OCTC), and an acid pre-treatment (OCacid). Mean organic C, in g kg−1, were OCTC (39.8), LOI550 (39.8), LOI400 (33.2), OCDichromate (31.8) for soils with pH ≤ 7.5. LOI400 and OCDichromate were more accurate than OCTC and LOI550. Mean organic C, in g kg−1, were LOI550 (23.6) > OCacid (20.2) ≥ LOI400 (17.9), OCDichromate (16.0) for soils with pH ≥ 7.8. Similar to pH ≤ 7.5 soils, LOI400 and OCDichromate methods were more accurate measures of OC. The two loss-on-ignition methods and the dichromate method were strongly correlated with each other (r = 0.91) however the two dry combustion methods were weakly correlated with the others (0.45 ≥ r ≤ 0.67). The dichromate oxidation proved to have the lowest variance of the methods tested (29.7 g2 kg−2). Paired t-tests found no significant differences between the loss-on-ignition 400° and the acid-pretreatment methods, and no significant differences between the dichromate and inorganic subtraction method (p < 0.05). Dichromate oxidation method should be implemented to measure soil organic carbon in urban soils that have carbonates present, especially dolomite. The dichromate oxidation method was found to be simple, rapid, and reliably determines SOC without the need to remove SIC.