Degradation of phthalate and di-(2-ethylhexyl)phthalate by indigenous and inoculated microorganisms in sludge-amended soil

The metabolism of phthalic acid (PA) and di-(2-ethylhexyl)phthalate (DEHP) in sludge-amended agricultural soil was studied with radiotracer techniques. The initial rates of metabolism of PA and DEHP (4.1 nmol/g [dry weight]) were estimated to be 731.8 and 25.6 pmol/g (dry weight) per day, respectively. Indigenous microorganisms assimilated 28 and 17% of the carbon in [C-14]PA and [C-14] DEHP, respectively, into microbial biomass. The rates of DEHP metabolism were much greater in sludge assays without soil than in assays with sludge-amended soil. Mineralization of [C-14]DEHP to (CO2)-C-14 increased fourfold after inoculation of sludge and soil samples with DEHP-degrading strain SDE 2. The elevated mineralization potential was maintained for more than 27 days. Experiments performed with strain SDE 2 suggested that the bioavailability and mineralization of DEHP decreased substantially in the presence of soil and sludge components. The microorganisms metabolizing PA and DEHP in sludge and sludge-amended soil were characterized by substrate-specific radiolabelling, followed by analysis of C-14-labelled phospholipid ester-linked fatty acids (C-14-PLFAs). This assay provided a radioactive fingerprint of the organisms actively metabolizing [C-14]PA and [C-14]DEHP. The C-14-PLFA fingerprints showed that organisms with different PLEA compositions metabolized PA and DEHP in sludge-amended soil. In contrast, microorganisms with comparable C-14-PLFA fingerprints were found to dominate DEHP metabolism in sludge and sludge-amended soil. Our results suggested that indigenous sludge microorganisms dominated PEEP degradation in sludge-amended soil. Mineralization of PEEP and PA followed complex kinetics that could not be described by simple first-order equations. The initial mineralization activity was described by an exponential function; this was followed by a second phase that was described best by a fractional power function. In the initial phase, the half times for PA and DEHP in sludge-amended soil were 2 and 58 days, respectively. In the late phase of incubation, the apparent half times for PA and DEEP increased to 15 and 147 days, respectively. In the second phase (after more than 28 days), the half time for DEEP was 2.9 times longer in sludge-amended soil assays than in sludge assays without soil. Experiments with radiolabelled DEHP degraders suggested that a significant fraction of the (CO2)-C-14 produced in long-term degradation assays may have originated from turnover of labelled microbial biomass rather than mineralization of [14C]PA Or [14C]DEHP. It was estimated that a significant amount of DEHP with poor biodegradability and extractability remains in sludge-amended soil for extended periods of time despite the presence of microorganisms capable of degrading the compound (e.g., more than 40% of the DEHP added is not mineralized after 1 year).