Liming of acidified forests changes leaf litter traits but does not improve leaf litter decomposability in forest streams

Liming, the spreading of Ca/MgCO₃ on terrestrial ecosystems, is often used as a way to promote the recovery of terrestrial and aquatic ecosystems affected by anthropogenic acidification. When liming is applied to a watershed, surface waters may be improved directly through water chemistry changes but also indirectly through possible changes in the quality of terrestrial organic matter inputs to the stream. The aim of this study was to evaluate the effect of liming on leaf litter chemical and structural traits and their consequences on another trait, leaf litter decomposability, giving the potential for a leaf litter to be decomposed.Beech (Fagus sylvatica, L.) leaf litter was collected in forested sites on acid bedrock (granite and sandstone), limed (9 or 21 years ago) or not (controls), as well as on two calcareous reference sites. Several leaf litter traits were determined for each collection site: carbon (C), nitrogen (N), phosphorous (P), calcium (Ca), magnesium (Mg), potassium (K), lignin and cellulose content, and leaf mass area and toughness. To compare leaf litter decomposability in aquatic ecosystem, leaf litters from all sites were placed in a reference forested headwater stream over a 128-day period, in fine and coarse mesh litter bags, and leaf mass loss dynamic as well as fungal biomass were evaluated.Liming significantly increased leaf litter Ca and Mg contents and decreased K content while other measured traits remained unchanged. Leaf litters collected in limed sites exhibited significantly lower decomposition rates in coarse mesh bags but similar decomposition rates in fine mesh bags. Liming had no significant effect on fungal biomass. In contrast, leaf litter collected on the calcareous, reference sites, showed significantly different trait profiles (e.g. higher Ca, Mg, K and P contents), leading to up to twice higher decomposition rates and fungal development.Our study shows that under our experimental conditions, increases in Ca and Mg leaf litter contents arising from acidic forests liming did not improve leaf litter microbial decomposition and even tended to reduce macroinvertebrate mediated decomposability. Concomitant changes in other leaf litter traits might be necessary to stimulate leaf litter decomposition. Liming with Ca/MgCO₃ might thus not improve functional recovery of stream ecosystems in forests affected by acidification through an increase of litter quality.