Mobility and sequestration of Zn in Schoenoplectus californicus shoots decomposition

S. ARREGHINI; L. DE CABO; R. SERAFINI; M. V. CASARES; A. F. DE IORIO

Barcelona, España

Congreso; 3rd Wetland Pollutant Dynamics and Control - WETPOL2009; 2009

The aim of the present work is to assess Znadsorption/precipitation and release dynamic during S. californicus shoots decomposition.Dry shoots of S. californicus were disposed into plastic bags of 2mm mesh. Litterbags weresubmersed in non polluted stream water (0.018ppm Zn), kept in darkness and at ambient temperature.Treatments were carried out by triplicate with shoots containing the following Zn concentrations:T1:19.6ppm, T2:106.3ppm and T3:324.5ppm. Litterbags from each treatment were removed at 8, 15,33, 68, 148 and 369 days from the beginning of the experiment. Shoots were dried at 70º C andweighted. Zinc concentration in water and detritus were determined. During the first eight days detritus acted as a source of Zn attributed to Zn release and tolixiviation of adsorbed or soluble metal, especially in T3. T1 and T2 continued acting like a source ofZn for two months. There was a continued release of metal to water all over the lixiviation phase (0-68 days). In fact, there was a 70 to 90% reduction in metal concentration in plants. However, T3shoots acted like a sink for Zn. During decomposition there was also an increment in detritus surfaceand consequently in potential binding sites. So, if ion strength increased, there would be a majorchemical affinity for ions and especially for those with charge of high density like Zn2+. This wouldfavour a reduction in its concentration and total content in water. In T3, at the end of the lixiviationphase, the metal released was lower than 50%. It is possible to conclude that an important fraction ofmetal was associated with more resistant structures. Usually, plants with high levels of metals in theirtissues develop detoxification mechanisms which promote synthesis of proteins that associate metalsto cell walls. During decomposers phase (following lixiviation phase) in T1 and T2 zinc concentration in waterand zinc total content in detritus maintained almost constant. However, Zn concentration in detritusshowed a slight increase. It can be inferred that at low to moderate zinc concentration, detritus actedlike a sink for metal. T3 showed a reduction in Zn total content in detritus and an increase in Znconcentration in water. This would indicate that during decomposition phase and at high levels of Zn,detritus acted like a source of metal (Figure 1). Although Zn is released to water, a fraction remains inthe refractory biomass during decomposition. Du Laing et al. (2006) found that Zn concentration inshoots of Phragmites australis incubated in situ for 12 months was lower than 100 μg Zn.g-1. In T1and T2 and during decomposers phase Zn concentration reached that level (between 40 and 70 μgZn.g-1). That would explain the reason why plants began to act like a sink for metal. In T3, Znconcentration was higher (932μg.g-1). Furthermore, plants would continue releasing metal even afterdecomposition phase.The high metal release velocity suggests that S. californicus shoots with Zn levels higher than300μg.g-1, would release metal to water. In this case the appropriate strategy would be to harvest thestanding biomass. Nevertheless, when metal levels in shoots show low translocation, it would beconvenient to allow shoots in situ decomposition so they can act as a sink for metals.