Wood ash effects on growth and cadmium uptake in Deschampsia flexuosa (Wavy hair-grass).

Wood ash recycling to forests is beneficial because it regains nutrients and prevents acidification, but wood ash application is restricted due to its cadmium (Cd) content. We question if Cd in wood ash represents a problem, since decreases in Cd bioavailability due to ash-induced pH changes may counteract increased total Cd concentration. We studied effects of wood ash (0, 3, 9 and 30 t ha-1) and lime (pH increase equivalent to the wood ash treatments) on growth and Cd uptake in Deschampsia flexuosa. After four months, we measured plant biomass and Cd accumulation, and extracted Cd from the soil using three different methods; HNO3 (total), EDTA (chelator-based) and NH4NO3 (salt-based). Wood ash and lime strongly stimulated plant growth. Cd concentration in the plant tissue decreased with wood ash and lime addition, and correlated positively with the NH4NO3 extractable fraction of Cd in the soil. In contrast, HNO3 and EDTA extracted more Cd with increased wood ash application. We conclude that wood ash amendment increases soil pH, total Cd concentration, nutrient levels and stimulates plant growth. However, it does not increase Cd accumulation in D. flexuosa, as pH-driven decreases in Cd bioavailability leads to reduced plant Cd uptake. Finally, soil bioavailable Cd is best determined using NH4NO3-extraction.