Measurements of litter decomposition and litter traits
After the extraction of soil arthropods, litter was oven dried at 65 °C to weight constancy and weighed at 0.1 g. The same was done with litter that had not been exposed. The mass loss was then calculated as (pre-exposure – post-exposure)/ pre-exposure. Oven-dried litter was grinded using a ball mill. Carbon and Nitrogen concentrations were determined by using an elemental analyser (N1500, Carlo Erba, Milan, Italy). The changes in C and N concentrations were calculated in the same way as for mass loss. A decrease of C concentration reflects rapid decomposition. Inversely, an increase of N concentration reflects an accumulation of N in the litter due to decomposition, improving resource conditions for plants. We refrained from using changes of absolute C or N values to avoid bias from among-site variation of C or N. For instance, high-N sites would always show higher absolute changes of N than low-N sites. Subsequently, leaf carbon isotope was analyzed using an isotope ratio mass spectrometer (Isoprime100; Isoprime Ltd, UK) and the absolute atom percentage of carbon isotope was used for further analysis. Total phenolic concentration of initial litter was measured colorimetrically using the method of Santonja et al . (2015) with gallic acid as a standard. Leaf powder samples (250 mg) were suspended in 20 mL of a 70% aqueous methanol, shaken for 1 h and then filtered (0.45 µm filter). Filtered extracts (0.25 mL) were mixed with 4 mL of distilled water, 0.5 mL of saturated aqueous Na2CO3 and 0.25 mL of Folin-Ciocalteu reagent. After 60 min, phenolic concentrations were determined at 765 nm on the same UV/Vis spectrophotometer (Biomate 3, Thermo Electron Corporation®) and expressed as mg of gallic acid equivalent. g-1 DW.