# Biochar Affects Heavy Metal Uptake in Plants through Interactions in the Rhizosphere
## 4 Discussion
Due to the tubular nature of roots, exudates like ions, carboxylates and mucilage emanating from roots are rapidly diluted and buffered or consumed as the distance from the root surface increases. Thus, the extension of the rhizosphere is limited, and our ability to assess, e.g., the true concentration of metals therein depends on the width of rhizosphere that is sampled. Our sampling method is rather crude and allows for both the inclusion of aggregates that may be wider than the extension of the rhizosphere, and exclusion of particles and parts of aggregates belonging to a zone close to roots which then is then analyzed as part of the bulk soil. This may explain the rather small differences we observed in pH and metal concentrations between rhizosphere soil and bulk soil. More elaborate rhizosphere studies featuring mesh-based interfaces between roots and rhizosphere soil [50], permit amore precise sampling and a higher resolution of gradients forming in the rhizosphere. In such systems, the importance of plant characteristics (e.g., amount and lengths of root hairs) and soil properties (e.g., clay content and aggregation) may also be assessed. Weak extractants like organic acids or salts seem to be better predictors for metal availability in soil than stronger extractants like EDTA, even when soils are amended with biochar [51, 52], particularly for elements like Zn, Ni and Cu, and in soils with pH below 7. In our experiment, acetic acid extracted between 2 and 10 times more metals from soil than NH\({}_{4}\)NO\({}_{3}\). Again, the weakest extractant most readily distinguished biochar-treated and non-treated soil, and between rhizosphere and bulk soil. The extraction of metals is mainly caused by desorption of cations, while the formation of colloids and soluble metal-organic complexes, which are hardly taken up by plants, is strongly suppressed due to the high ionic strength of the 1M NH\({}_{4}\)NO\({}_{3}\) solution [28]. This can make NH\({}_{4}\)NO\({}_{3}\) extractions more precise for plant uptake predictions in near neutral to alkaline soils. While the extractable concentrations of metals in the rhizosphere give a momentary picture of metal availability to plants, the cumulative uptake of metals over the entire growth period is better reflected by the metal concentration in the plants. This uptake is distributed between roots and shoots, and as is commonly found, roots retain a large proportion of absorbed metals and translocate limited amounts to shoots. Low translocation ratios may partly stem from root surface adsorption of metals, contributing to an extension of the extraradical immobilization observed in rhizosphere soil [42]. [PERSON] and [PERSON] [53] described that release of phytosiderophores (PS) by roots is an important factor influencing the availability of trace elements in rhizosphere, suggesting that this mechanism may protect plants from heavy metal toxicity, reducing element transfer to plant tissues [54]. [PERSON] and [PERSON] [55] described that Graminaceous species synthesize and secrete Fe-chelating siderophores, the non-proteinaceous amino acids belonging to the mugineic acid family, that dissolve sparingly soluble Fe compounds in the rhizosphere. As biochar can be an important donor of Fe oxides, the presence of Fe from biochar can be an important factor inducing PS secretion by roots, reducing heavy metals uptake in tested soils.