Biodegradation of low molecular weight organic acids in coniferous forest podzolic soils

Organic acids have been hypothesised to play a central role in long-term soil pedogenic processes such as podzolisation and also in short-term rhizosphere processes such as nutrient mobilisation. Their spatial and temporal patterns in soil, however, are poorly understood. This study was designed to quantify the rate of microbial decomposition of three low molecular weight organic acids (citrate, oxalate and acetate) as a function of soil depth (O, A, E, B and C horizons) in three podzolic coniferous forest soils and to assess their contribution to total soil CO2 fluxes. Soil solution extraction indicated that generally, most organic acids were present in the highest concentration in surface O horizons (10–100μM) declining with soil depth. Correspondingly, the rate of organic acid mineralisation was also greatest in the surface organic horizons. In contrast, the mineral horizons possessed low rates of biodegradation which appeared not to be due to a lack of microbial activity but moreover due to strong sorption to the solid phase preventing microbial uptake. Concentration-dependent mineralisation experiments indicated that decomposition could be adequately described by a single Michaelis–Menten kinetics equation. The results showed that generally most horizon samples had the greatest mineralisation capacity for citrate or oxalate (V max=<1–340nmolg−1 h−1) in comparison to acetate. The V max values typically declined with soil depth. K M values ranged from 10 to 7000μM, however, if adsorption was accounted for the K M values decreased (<1–5000μM) especially in B horizons. Calculations of the contribution of organic acids to total soil respiration indicated that between <0.1 and 47% of total CO2 production from an individual horizon could be accounted for by organic acid mineralisation alone. The findings are discussed in the context of soil forming and rhizosphere processes.