Degradation kinetics of ptaquiloside in soil and soil solution

Abstract

Ptaquiloside (PTA) is a carcinogenic norsesquiterpene glycoside produced in bracken (Pteridium aquilinum (L.) Kuhn), a widespread, aggressive weed. Transfer of PTA to soil and soil solution eventually may contaminate groundwater and surface water. Degradation rates of PTA were quantified in soil and soil solutions in sandy and clayey soils subjected to high natural PTA loads from bracken stands. Degradation kinetics in moist soil could be fitted with the sum of a fast and a slow first-order reaction; the fast reaction contributed 20 to 50% of the total degradation of PTA. The fast reaction was similar in all horizons, with the rate constant k(1F) ranging between 0.23 and 1.5/h. The slow degradation, with the rate constant k(1S) ranging between 0.00067 and 0.029/ h, was more than twice as fast in topsoils compared to subsoils, which is attributable to higher microbial activity in topsoils. Experiments with sterile controls confirmed that nonmicrobial degradation processes constituted more than 90% of the fast degradation and 50% of the slow degradation. The lower nonmicrobial degradation rate observed in the clayey compared with the sandy soil is attributed to a stabilizing effect of PTA by clay silicates. Ptaquiloside appeared to be stable in all soil solutions, in which no degradation was observed within a period of 28 d, in strong contrast to previous studies of hydrolysis rates in artificial aqueous electrolytes. The present study predicts that the risk of PTA leaching is controlled mainly by the residence time of pore water in soil, soil microbial activity, and content of organic matter and clay silicates.

Ptaquiloside in Irish Bracken Ferns and Receiving Waters, with Implications for Land Managers

“…In October 2015, the ptaquiloside concentration in bracken fronds at Doolargy (27.6 µg¨g´1) was an order of magnitude higher than at Dooghill (3.33 µg¨g´1), yet ptaquiloside concentrations in the receiving water were lower at Doolargy (0.01 µg¨L´1) than at Dooghill (0.67 µg¨L´1), which implies that the dynamics of ptaquiloside release, transport and degradation between the bracken source and water recipient, to a high extent, govern the observed concentrations. The measures of ptaquiloside in the bracken plant provide evidence that there is a significant source of ptaquiloside in the environment in Ireland, but the risk that this may pose to drinking water depends on other factors, such as soil type and proximity to the water abstraction point, and there can be significant decreases in the passing of ptaquiloside to soil layers with groundwater [19], which can be attributed to both microbial degradation and abiotic hydrolysis [37]. Dooghill was a surface water abstraction site whereas the other sites were spring wells and classed as groundwater.…”

The naturally occurring carcinogen ptaquiloside is present in groundwater below bracken vegetation

“…Pterosin B is much more hydrophobic (log octanol/water partition coefficient [ K OW ] = 3.33) than PTA (log K OW = –0.63) and has a higher tendency to sorb to soil organic matter (log organic carbon partition coefficient [ K OC ] = 2.6) . Soil half‐lives of PTA in the range 8 h to 180 h have been reported at 25ė °C , whereas no significant degradation was observed within 28 d in sterile soil solutions , indicating a stabilizing effect by soil solution constituents. Further, the stability of PTA in aqueous solutions has been shown to be strongly dependent on pH (most stable at pH ∼5.5) and temperature .…”

“…Previously, UV detection was commonly used for quantification of PTA and pterosin B, and the lowest published high‐performance liquid chromatography (HPLC)–UV limit of detection was 151 nmol/L , though with a preconcentration step added occasionally . Few studies utilizing UV detection have detected PTA in natural waters , but data are nonconclusive as the interference by other compounds cannot be excluded.…”

Multiple Effects of Bracken Fern under in vivo and in vitro Conditions

“…Ptaquiloside leaching from fronds exposed to rain may subsequently contaminate soil and groundwater (Ayala-Luis et al, 2006;Engel et al, 2007;Ovesen et al, 2008;Jensen et al, 2008). There is no evidence to prove that water contamination with bracken fern can cause disease in human; nonetheless, a number of toxic compounds in fern are soluble in water and probably can be entered into water tanks (Evans et al, 1984).…”