Biodegradation of isoproturon in agricultural soils with contrasting pH by exogenous soil microbial communities

Abstract

Soil pH is a limiting factor for biodegradation of isoproturon (IPU) in the field. IPU dissipation is hampered in soil with low pH. Efficacy of two IPU-mineralizing microbial communities in IPU dissipation was investigated during 189 d in microcosms of three agricultural soils with contrasting pH (Marsdorf, pH 3.8; Neumarkt, pH 5.8 and Dürneck, pH 7.3). The microbial communities enriched from an acidic soil (MC-AS) and a neutral soil (MC-NS), respectively, were established on carrier material, namely expanded clay particles, and introduced to the tested soils at a carrier-soil ratio of 1%. IPU was applied to the soil twice, on day 0 and day 133. The effect of inoculation size, sorption–desorption and pH on biodegradation of IPU were studied. IPU mineralization, extractable residues and non-extractable residues were analyzed with uniformly ring-labeled [14C]-isoproturon. Both microbial communities resulted in significantly enhanced IPU mineralization (52%–60%) and low concentrations of IPU residues in soil Dürneck (pH 7.3). The acquired IPU-mineralizing activity was persistent in the soil for at least 133 d. Initially a 9-fold higher IPU mineralization rate was attained in soil Neumarkt (pH 5.8) by inoculating MC-AS. However, no difference between treated and untreated soils was detected after IPU re-application. Both communities had negligible effect on the fate of IPU in soil Marsdorf (pH 3.8), where biodegradation of IPU was inhibited, with ≈2% IPU mineralized over 189 d. Lowering the carrier-soil ratio to 0.1% sharply reduced the IPU-mineralizing capacity of MC-AS in soil Neumarkt, however, this effect of inoculation size was less pronounced for both microbial communities in soil Dürneck. The low Kd of IPU (1.3–2.0 mL g−1) indicate that IPU bioavailability is not a limiting factor of IPU degradation in the soils. Relationships between pH and maximal IPU mineralization rendered by MC-AS or MC-NS in soil closely approximate those observed in mineral salts medium, suggesting that pH is an important factor influencing biodegradation of IPU by the exogenous microbial communities. We propose that MC-AS, which has a broad pH tolerance for IPU degradation, is a promising candidate for accelerating IPU dissipation in acidic soils. The effect of inoculant density on IPU degradation is microbial community-soil specific. Using degrader microorganisms according to their physiological requirements and properties of the targeted soils may maximize the effectiveness of IPU dissipation.