Microsuction cups

The literature shows that many fewer data are available on dissolved than on solid-phase heavy metals at the soil-root interface. To date, several approaches have been used to acquire soil solution samples from the rhizospheric environment, includmg water displacement, water extraction, centrifugation, and microsuction cups. However, because of technical constraints, no single method appears to sample unaltered rhizosphere solution. 

Contrary to the results for labile Ni, the gradients of relative concentrations of dissolved Ni obtained with the model including exudation (Model 3) matched water-extractable Ni data better (Fig. 2b). The curves produced by the other models were more discrepant. Water extracts may not represent real soil solutions and freezing of soil samples might have introduced additional artifacts. However, gradients of Ni concentration in soil solution were not available. Water-extractable Ni data in Fig. 2b are presented for orientation but caimot be used for proper model corroboration. All simulated curves were opposite to the trend implied by measured water-extractable Ni. Accumulation of solutes near the root is known for excluders or for situations where convective flow toward the root exceeds plant demand. This was not the case in our experimental system. In future experiments, the use of microsuction cups will enable comparison of simulated and measured dissolved Ni (Wenzel et al., 2001). 

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