Nitrogen concentration dominates litter quality effects on earthworm performance across species and soils

Background/Question/Methods

Earthworms drive important ecosystem functions like decomposition and nutrient mineralization. Factors controlling their performance remain however largely unknown. We conducted a microcosm experiment with two common endogeic earthworm species (Aporrectodea caliginosa and Octolasion tyrtaeum), two different soils (from a beech dominated forest and a mixed stand) to test litter quality effects on earthworm performance. The soil was incubated with the litter of six different tree species spanning a gradient in litter quality (Fagus sylvatica, Acer platanoides, Acer pseudoplatanus, Carpinus betulus, Tilia spp. and Fraxinus excelsior). 

Results/Conclusions

Earthworm weight gain was significantly lower in soil from beech dominated forest than in soil from the mixed forest. Regression analyses of earthworm log response ratios and litter quality measures (C, N, P, Ca, Mg, and lignin content; mg g^-1 dry weight) informed later path analyses to separate inter-correlated explanatory variables. Path analysis revealed that litter N concentration was the most significant predictor of earthworm weight gain across soil types and earthworm species. Notably, microbial biomass, being regarded as a major food resource for earthworms, was on average highest in the mixed soil, but was not affected by litter type indicating endogeic earthworms to not mainly depend on soil microbial biomass, but also on litter as food resource. Our results suggest that litter N concentration is a good predictor of endogeic earthworm performance and thus provide important information on the nutrient limitations of essential decomposers.