Abstract
Heavy metals in soil pose a constant risk for animals and humans when entering their food chains, and limited means are available to reduce plant accumulation from more or less polluted soils. Biochar, which is made by pyrolysis of organic residues and sees increasing use as a soil amendment to mitigate anthropogenic C emissions and improve agronomic soil properties, has also been shown to reduce plant availability of heavy metals in soils. The cause for the reduction of metal uptake in plants when grown in soils enriched with biochar has generally been researched in terms of increased pH and alkalinity, while other potential mechanisms have been less studied. We conducted a pot experiment with barley using three soils differing in metal content and amended or not with 2% biochar made from Miscanthus x giganteus, and assessed plant contents and changes in bioavailability in bulk and rhizosphere soil by measuring extractability in acetic acid or ammonium nitrate. In spite of negligible pH changes upon biochar amendment, the results showed that biochar reduced extractability of Cu, Pb and Zn, but not of Cd. Rhizosphere soil contained more easily extractable Cu, Pb and Zn than bulk soil, while for Cd it did not. Generally, reduced plant uptake due to biochar was reflected in the amounts of metals extractable with ammonium nitrate, but not acetic acid.
Highlights
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]
The addition of 2% biochar to soil did not increase plant biomass compared to unamended control treatments for non-polluted and weakly polluted soil (Table 2)
Plants grown without biochar grew very poorly and only reached a biomass of 8–10% of plants in the corresponding biochar amended treatment
Summary
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]. A wide range of studies have 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]. 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 are
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