# Biochar Affects Heavy Metal Uptake in Plants through Interactions in the Rhizosphere
## 4 Discussion  
Biochar has mainly been given attention in recent years due to its capacity for long-term storage of C in soil, and for the beneficial effects it has on soil properties that are important in agriculture [8]. An increasing number of reports have focused on aspects of mitigation of pollution for both metals/metalloids [30; 31; 32] and organic pollutants [33; 34; 35]. Relatively few studies have however addressed the potential reduction in bioavailability of metals to plants in a complementary way that accounts for both biological and chemical availability [36; 31; 37]. The present results confirm that biochar amendments can have profound effects on plant uptake in polluted soil, and that the changes that biochar mediates in soil are also apparent when estimating bioavailability by non-stringent soil extraction. A novel finding is that biochar results in a reduced bioavailability of the four metals studied, and that this reduction occurred at a pH which similar or very close to that of the bulk soil. Our results also represent a rare case showing that biochar can reduce extractability and plant uptake of metals while changes in soil pH remain negligible. By contrast, most reported attempts to sequester heavy metals in soil using biochar are accompanied by increases soil pH, which then accounts for at least part of the reduced metal uptake in plants [38; 39; 40; 41]. Reducing heavy metal uptake without substantially increasing pH is important, as increases in soil pH beyond 7-8 lead to reduced availability of several plant nutrients, and may thus hamper plant growth, whether plants are used for sequestration, remediation or production. Another novel finding in our study is the demonstration of Cu, Pb and Zn accumulation in rhizosphere soil in the presence and absence of biochar, and the finding that Cd behave differently from the three other metals we studied. Root-induced changes of rhizosphere pH play a major role in the bioavailability of the many pH dependent nutrients, but also potentially toxic metals [42]. It has been shown that when acidifying their rhizosphere, plants increase the solubility of toxic compounds [43; 44], as observed in our study. Amongst the processes that are responsible for the differentiation of the rhizosphere relative to the bulk soil, the exudation of C-compounds [45] has been much studied for its major impact on soil biogeochemistry. Apart from mobilization mediated by acids, metal accumulation in rhizosphere soil has also been observed, and partly attributed to immobilization by root mucilage [46]. Carboxylic acids remain the most investigated component of root exudates due to their potential effect on metal bioavailability via complexation processes. Amongst the range of carboxylates exuded into the rhizosphere, malate, citrate and oxalate seem to have the most pronounced effects on metal complexation [47]. The effects of biochar on plant heavy metal uptake are generally dependent on soil pH [48], but may be inconsistent and vary for different metals. In our study, Cd seemed to be the most pH independent element, in agreement with e.g., [PERSON] et al. [49] who described that biochar application resulted in large reductions of plant Cd concentrations (32-40%) in both acid, neutral, and alkaline soils. Metal availability in the rhizosphere may vary considerably, depending on soil characteristics like pH, organic C and clay content, but it may also vary depending on plant species and plant nutrition. Plant nutrient uptake may result in acidification or alkalinization, depending, e.g., on P availability and N-fertilization. The latter may show striking differences between acidifying fertilizers like urea and ammonium salts, while nitrate-based fertilizers cause a weak alkalinization. These effects are accentuated in rhizosphere soil, as they are driven by ion exchange during root uptake.