# Biochar Affects Heavy Metal Uptake in Plants through Interactions in the Rhizosphere
## 1 Introduction  
Mining, smelting, land applications of sewage sludge and other human activities have led to widespread metal contamination of soils. Heavy metals such as Cd, Cu, Pb and Zn often coexist in contaminated soils and their mobility and bioavailability is of global concern due to uptake in plants and increasing human exposure through food [1]. Using lime or alkaline organic materials as soil amendments can increase soil pH and thereby reduce bioavailability of heavy metals [2, 3]. A wide range of studies have also highlighted biochar as a soil amendment that can reduce metal uptake in plants [4, 5, 6], in addition to its beneficial effects through enhancing soil C levels and improving related soil properties [7, 8]. The beneficial effects of biochar on metal availability in soil are partly due to its capacity to increase soil pH, and partly due to its surface properties and capacity for sorption of ions and fixation of heavy metals [9]. Thus, significant amounts of heavy metals/metalloids, such as As, Cd, Cu, Ni, Pb and Zn have been shown to be sorbed by a wide range of biochar products, which were produced from a variety of feedstock such as wood, grass, husks, manure and broiler litter, and charring conditions including medium to high temperature pyrolysis and low temperature hydrothermal liquefaction [10, 11, 12, 13]. Liming is far less complicated and less expensive than amendments with biochar, so the reasons for using biochar should, apart from general soil improvement, be related to additional immobilization of metals. To study such additional effects, low alkalinity biochars areparticularly useful [14]. When assessing bioavailability of metals, studies measuring plant uptake provide the most reliable data. However, as such studies are time-consuming and expensive, various extraction methods can provide relatively good estimates, or at least distinguish between efficient and less efficient means to affect bioavailability in comparative studies. Numerous extraction methods have been proposed, and some have been shown to be fair predictors of metal availability in soil [15; 16]. The rhizosphere is the first point of contact between soil and a root system. Still, responses to soil amendments with biochar have largely focused on above-ground biomass and crop yields [17]. Root system responses to biochar will be affected by changes in soil properties like pH, bulk density, water holding capacity, metal and metalloid availability and changes in soil microbial activity [18]. Rhizosphere soil is modified partly through the action of root exudates and is thus potentially a hotspot for altered mobility of heavy metals, as compared to bulk soil [19]. Root exudates frequently acidify the rhizosphere through excretion of H+ ions and organic acids, a mechanism that mobilizes and enhances uptake of several plant nutrients [20], including trace elements [21]. In addition, heavy metals are affected by the chemical and biological processes occurring in the rhizosphere [22], resulting in depletion or enrichment according to the sorption properties and soil diffusion rates of different the metals [23]. A better mechanistic understanding by biochar effects on metal availability, including the respective contribution of pH and sorption components, is necessary to devise successful remediation strategies.  
The objective of the present research is to characterize the availability of Cd, Cu, Pb and Zn in soils amended or not with biochar, focusing on differences between rhizosphere and non-rhizosphere (bulk) soil. Our approach featured measurements of extractable metals and pH in rhizosphere and bulk soil, and related these data to true bioavailability measured as metal uptake in plants.