Modified dichromate method for determining low concentrations of extractable organic carbon in soil

Abstract

Fumigation‐extraction and rehydration methods for determining soil microbial biomass carbon (C) require analysis of extractable C levels in soils before and after treatment. In soils containing biomass C levels below 300 mg kg^‐1 or extractable C levels below 100 mg kg^‐1, it may be difficult to determine extractable C concentrations accurately. Classical dichromate oxidation methods measuring reduced chromium (Cr) determine accurately organic C concentrations from 50 to 1,000 mg C L^‐1. A method for C analysis for extracts containing less than 25 mg C L^‐1 is presented and is based on absorption of oxidized Cr at 340 nm. The calibration curve was linear from 0 to 25 mg C L^‐1 and curvilinear above 25 mg C L^‐1. Absorption at 590 nm of a similar digest provides a linear relation from 0 to 500 mg C L^‐1, but sensitivity was limited in the 0 to 50 mg C L^‐1 range. Seven soils were sampled and analyzed for biomass C using fumigation extraction (FE) and rehydration (RHD) techniques and the modified dichromate digestion method. Biomass C content ranged from 110 to 440 mg C kg^‐1 soil for FE and 150 to 570 mg C kg^‐1 soil for RHD method. Coefficient of variation for analyses were 0.7 to 1.3% which compares favorably to automated methods. The modified method is simple and accurate for soils containing low biomass C or low extractable C levels.