Decomposition and colonization by micro-arthropods of two litter types in a tropical montane rain forest in southern Ecuador

The decomposition of litter of two tree species Graffenrieda emarginata (Melastomataceae), Purdiaea nutans (Cyrillaceae) and the mixture of both was investigated in a tropical montane rain forest in southern Ecuador at two different altitudes (1850 and 2280 m). The two litter types differed strongly in nitrogen concentration, suggesting that G. emarginata (1.21% N) decomposes faster than P. nutans (0.73% N). To study the effect of soil micro-arthropods on the decomposition process, litterbags with mesh-size of 48 μm, excluding soil micro-arthropods, and 1 mm, allowing colonization by soil micro-arthropods, were used. Litter mass loss was measured after 2, 6 and 12 mo exposure in the field; further, microbial biomass and micro-arthropod colonization of the litter were investigated after 2 and 12 mo. Generally, litter decomposed faster at 1850 m than at 2280 m (60% and 76% dry mass remaining after 12 mo, respectively); G. emarginata and mixed litter decomposed faster than P. nutans litter. After 12 mo mixed litter decomposed faster (65% of dry mass remaining) than both individual litter species (70% and 71% of dry mass of G. emarginata and P. nutans litter remaining, respectively) indicating that non-additive effects contributed to litter decomposition. Microbial biomass increased during the experiment and was higher at 1850 m than at 2280 m. The most abundant micro-arthropods in both litter types were oribatid mites followed by Collembola, Gamasina, Uropodina and Prostigmata + Astigmata. Micro-arthropods were generally more abundant at 1850 m suggesting higher biotic activity at lower altitudes. Soil micro-arthropods contributed little to decomposition processes indicating that litter decomposition is mainly due to micro-organisms.