Dynamics of trace and rare earth elements during long-term (over four years) decomposition in Scots pine and Norway

Abstract

The temporal dynamics of 33 major, trace and rare earth elements (REEs) was studied in the Swedish Norway spruce-NSL (Picea abies) and Scots pine-SPL (Pinus sylvestris) litters with the aim to assess their release and accumulation dynamics. Litter bags (8×8cm) were incubated in paired monoculture stands of both the species for upto five years from 1979-1984 according to a randomized block design comprising 25 blocks (1×1 m) within an area of 625 m2. Decomposition rate was slightly higher for SPL (k = 0.315) compared to that of NSL (k= 0.217). During litter decomposition, until ~70% accumulated mass loss (AML), concentration of trace elements increased >50% in both litter types relative to initial concentrations. The pattern of concentration change was non-linear, and polynomial quadratic regression between concentration change and AML resulted in a significant relationships (adj R2= 0.20–0.97; P = 0.15–<0.0001). The changes in concentration and amount of trace elements resulted in two main types of dynamics; (i) both concentration and amount increased for Fe, Al, Ti, Cu, Mo, V, Zr, Sb, As, Cs, Pb, Th and U; (ii) concentration increased but amount decreased for Ni, Zn, Li and Sr. REEs’ amounts increased ~3-fold to 99-fold between the beginning and end of incubations suggesting accumulation during litter decomposition. Dynamics of different REEs were similar in their patterns of change in the two litters. Different REEs, had generally identical patterns of change during incubation, which is reflected in the high correlations among them (r2 = >0.95). A general upward convexity in the dynamics hints that if further incubated in the field, decomposing litter could have accumulated more REE in the organic matter. The results of this study can be useful for future studies in other ecosystems including metal-contaminated sites or element depleted sites. Plant litter accumulation, its decomposition and build-up of humic substances in the decomposing organic matter can act as sink for elements and can be used as a management tool for ecological amelioration of metal-contaminated sites as well as natural systems that are impoverished, especially recuperating ones. The study's findings have implications beyond such sites and can be useful in any research that seeks to understand the patterns of accumulation and release related to decomposition in different ecosystems.