Herb litter mediates tree litter decomposition and soil fauna composition

Leaf litter from forest understorey species has very different characteristics (e.g. a higher nutrient content) which are regarded to be more palatable and digestible for the decomposer community compared to litter from the overstorey trees. Consequently, the decomposition of tree litter may show different dynamics when mixed with litter from herb species. To study the effect of understorey litter on tree litter decomposition, we set up a litterbag experiment in a temperate mixed deciduous forest in Belgium. We investigated the effect of spring (Anemona nemorosa) and summer (Aegopodium podagaria) herb litter on the litter decomposition of five native tree species that differ in litter quality. The spring and summer withering herb litters were added above the tree litters. After 742 days of in situ incubation, mass loss (40%–100%) and the decomposition rate (0.27–1.68 yr−1) of the litter from the five tree species were positively correlated with litter quality. The addition of summer herb litter resulted in increased mass loss and decomposition rates (k) of the underlying recalcitrant tree litter. In addition, summer herb litter increased nutrient concentration in the short-term as well as the abundance of soil fauna during litter decomposition. In contrast, spring herb litter slightly inhibited tree litter decomposition. Structural equation models showed that the effect of understorey litter on decomposition rates of tree litter acted via nutrient leaching towards the tree litter layer rather than via the alteration of soil fauna (detritivores and omnivores) abundance. Our results show that litter from understorey vegetation can alter nutrient availability in recalcitrant litters (Fagus sylvatica and Quercus robur) via increased nitrogen (N), calcium (Ca), magnesium (Mg), phosphorus (P) and potassium (K) concentrations by 9%, 21%, 22%, 41%, and 315% respectively after the summer herb litter addition. The effect on tree litter decomposition strongly depends on herb species identity and future research should focus on the generality of the observed patterns. We conclude that herb litter has a major role in accelerating carbon sequestration and nutrient cycling in temperate forests.