Biodegradation of 14C-labeled low molecular organic acids using three biometer methods

Abstract

Low molecular weight organic acids (LMWOA) are produced in soil by various biological and chemical processes and can exhibit substantial metal complexing and dissolution capacity. The reactivity of these compounds in the soil environment is dependent on their non-complexed concentration in the soil solution. Adsorption of LMWOA has been shown to reduce their concentration in the soil solution; however, little is known about the reduction of LMWOA concentration due to microbial degradation. To examine the extent of microbial degradation in reducing LMWOA concentration in the soil solution, three-biometer methods were used: a soil biometer flask, an in-situ field biometer and a soil column biometer. Four soil horizons were used with each method. To each soil sample, 2.0×10 −6 moles of organic acid containing 3.7×10 4 Bq total activity was applied. The 14C-radiolabeled aliphatic and aromatic acids studied included oxalic, malonic, succinic, and phthalic acid. Evolved 14CO 2 was trapped in 0.5 mol l −1 NaOH and measured using liquid scintillation counting. Labeled acids degraded rapidly within the first 5 days for the Ap1, Ap2, and BA horizons, with a generally slower rate of 14CO 2 evolution being observed for the Bt1 horizon. The % degradation of labeled acid was substantially greater for the soil biometer flask method, compared to the field and soil column biometer methods. The average % degradation for the soil biometer flask was 67% for all soil horizons and organic acids, compared to 14% for the field biometer and 13% for the soil column biometer. Results indicate that substantial microbial degradation of organic acids can occur within a relatively short time period and the biometer method selected can influence the % acid degraded. Based on primary results, the soil column biometer method better approximated microbial degradation under field conditions, as evaluated using the field biometer.