Carbon and nitrogen storage in a typic Albaqualf as affected by assessment method

Accurate quantification of soil organic carbon (OC) and nitrogen (N) concentrations are necessary to ascertain the effects of land use, crop rotation systems, and management practices on soil C and N sequestration potential. Soil OC and total N were determined by various methods in a Typic Albaqualf under native tallgrass prairie and agricultural soil cropped to a rice (Otyza sativa L.)-soybean (Glycine max L.)-wheat (Triticum aestivum L.)/soybean rotation that has been annually cultivated for 15, 26, and 44 years. Two wet-oxidation, methods, the Walkley-Black (WB) and modified Walkley-Black (mod WB), and high-temperature combustion using a Carlo-Erba and LECO analyzer were used to determine the effects of assessment method on soil OC concentration, while the high-temperature combustion method using the Carlo-Erba and LECO analyzer were used to determine the effects of assessment method on total soil N concentration. Soil OC and total N concentrations determined by high-temperature dry combustion using the Carlo-Erba and LECO analyzers did not differ significantly. Soil OC concentrations determined by the modWB method were generally significantly higher than those from the WB or high-temperature combustion methods. Despite significant linear correlation (r > 0.74; p < 0.001) between soil OC concentrations by wet-oxidation and dry-combustion methods, assessment methodology significantly affected interpretations regarding changes in soil OC storage over time. The results of this study indicate that the choice of assessment methodology is a critical decision for the accurate quantification of soil OC concentration, content, and change over time.