Ectomycorrhizal weathering, evidence from the field?

Abstract

Ectomycorrhizal fungi have been put forward as important agents in soil mineral weathering. Indeed, many laboratory experiments confirm the ability of these fungi to enhance mineral dissolution. But field evidence is still lacking. Here we present field measurements of apatite weathering in a vegetation gradient in southern Norway. A natural occurring lead contamination, probably present since the last glacial, caused a gradient from bare soil, via sparse grass to healthy spruce forest. Because of the absence of a host tree, ectomycorrhizal fungi were absent in the bare soil and grass patches. We measured apatite content and soil solution chemistry in soil profiles in the different vegetation zones of the gradient. To our surprise apatite loss for each soil depth could be described by the same, simple proton-based, dissolution function over the whole vegetation gradient: r = K.(H+ )n . The parameters n and K increased with soil depth. The reaction order (n) increased from 0.18 in the top soil to 0.6 at 30-40 cm depth. It indicates changes in weathering mechanisms with depth, but constant over the vegetation gradient. Lower n and K values could indicate apatite surface interactions with simple and complex organic complexants. In the vegetation gradient, we found considerable amounts of spruce litter over the whole gradient, including on top of the bare soil. Our data indicate that it is the breakdown of dead organic matter, rather than ectomycorrhizal actions as major source or organic weathering agents.