Organic acid behaviour in a calcareous soil: sorption reactions and biodegradation rates

Exudation of organic acids has been hypothesized to greatly improve the ability of plants to establish on nutrient poor soils such as those found in calcareous environments. An understanding of the behaviour of organic acids in these soils is crucial to the critical assessment of their potential capacity for nutrient mobilization. The aim of this study was therefore to study the sorption reactions and mineralization potentials of three organic acids commonly found in root exudates of calcicole plants (citrate, malate and oxalate). Using maize as a model plant, we studied the root exudation of organic acids into a calcareous soil and assessed the organic acid biodegradation rate in rhizosphere and bulk soil. Our results indicate that malate and citrate are rapidly biodegraded in this calcareous soil, in agreement with previous studies presented for non-calcareous soils. In contrast, oxalate appears to be resistant to microbial degradation possibly due to substrate protection by the formation and precipitation of Ca-oxalate. The rate of malate biodegradation was significantly enhanced in rhizosphere soil relative to that in bulk soil possibly reflecting the high degree of efflux of this organic acid from maize roots. The levels of free organic acids in the soil solution of calcareous soil surrounding maize roots, whilst higher than in the bulk soil, were extremely low.