Dynamics of heavy metals during litter decomposition in fire-affected boreal forests

Russian boreal forests represent a globally significant carbon stock, been suffering from frequent surface fires that modify natural cycles of elements, including heavy metal (HM). The behaviour of HM, exerting various ecosystem effects, is not well understood, especially in northern larch forest ecosystems affected by fires. The dynamic of Fe, Pb, Mn, Zn, Cu, Co, Ni and Cr was studied in the 850-day field decomposition experiment in a natural unburned larch stand (Larix gmelinii (Rupr.)) and adjacent burned forest on the Russian Far East. We observed mass loss, HM release/accumulation and correlation of HMs with soil properties. The litter decomposed slower in the burned site, with pronounced differences in the late decomposition stage. The concentrations of HMs except Mn had increased by the end of the experiment in both forest sites. Among all the HMs, Fe, Cr and Ni showed pronounced accumulation in burned stand compare to the unburned forest. Fire does not modify the patterns of HM release/accumulation but significantly alters the final values. In unburned forest, soil pH and water content strongly influenced only Fe dynamics, whereas, on burned site, soil properties correlate with the group of HMs. Our experiment showed that HM dynamics are coupled with the mass loss only in the late stages of litter decomposition. We found that fire’s legacy effect in natural larch forests could last over 15 years, creating favourable conditions for significant accumulation of Fe, Pb, Cr and Ni.