Rapid and significant lithium isotope response to afforestation in Icelandic topsoils

Abstract

Silicate weathering plays an important role in pedogenesis and the carbon cycle. Lithium (Li) isotopes are an effective tracer for the silicate weathering intensity and have been employed extensively to quantify both modern and past silicate weathering processes. The presence of vascular plants is believed to significantly influence weathering processes and pedogenesis, but how exactly plants and their ecosystems influence the behaviour of silicate weathering and Li isotopes in soils is poorly constrained. Here, we explored this question by measuring plant organs and sequentially leached fractions from soil chronosequences in Iceland spanning 25–63 years following afforestation. We found that Li isotopes were significantly fractionated within the plants during Li transport from the roots towards the leaves, and propose that 6Li preferentially crosses vacuole membranes in cells, while 7Li is enriched in the vascular system and accumulates in the leaves. This intra-plant fractionation could be most pronounced during plant growth when plants are subject to an excess nutrient supply. The Li isotope compositions in the exchangeable, carbonate, and oxide minerals of the afforested soils were ∼ 10 ‰ higher than in those of the heathland soils, and such differences can be attributed to both the input of heavy Li isotopes from litterfall decomposition and the effect of afforestation-mediated secondary mineral formation. Overall, this study suggests that silicate weathering processes traced by Li isotopes can respond significantly and rapidly (<25 years) to forest establishment, with important implications for paleoenvironmental reconstructions and the evolution of the critical zone.