BIODEGRADATION OF LOW MOLECULAR WEIGHT ORGANIC ACIDS IN A LIMED FOREST SOIL

Abstract

The effect of liming (3.45 and 8.75 t ha-1 dolomite; 16 yr after application) on the biodegradation of three low molecular weight organic acids (citrate, oxalate and propionate) in forest soils was investigated. The concentration of organic acids in the soil solution followed the series propionate > citrate > oxalate with liming having no significant impact on soil solution concentrations (mean organic acid concentration = 8.7 ± 2.3 μM). Organic acid mineralization by the soil microbial community was rapid in surface organic horizons (mean half-life for citrate = 2–6 h), with biodegradation rate gradually declining with soil depth. Concentration-dependent biodegradation studies (0 to 350 μM) showed that the mineralization kinetics generally conformed well to a single Michaelis–Menten equation with Vmax values following the series oxalate > citrate > propionate (mean = 9.8 ± 1.0 nmol g-1 h-1) and KM values following the series oxalate = citrate > propionate (mean 168 ± 25 μM). The Vmax values declined with soil depth, which was consistent with a general reduction in microbial activity down the soil profile. Liming induced a significant increase in Vmax for citrate with no change for propionate and reduction in Vmax for oxalate. The latter was probably due to adsorption and precipitation of Ca-oxalate making it unavailable for microbial uptake. The higher adsorption/precipitation capacity for oxalate in the limed soils was confirmed by adsorption isotherms. Generally, liming increased soil microbial activity by approximately 10 to 35% with calculations based on soil solution concentrations indicating that organic acid mineralization constituted approximately 3 to 15% of the total soil respiration.