Collembola are among the most abundant and diverse microarthropods that affect litter decomposition, control microbial functioning and support invertebrate predators. Rainforest conversion and expansion of plantations in tropical regions are associated with changes in microclimate and biodiversity decline, but information on the impact of tropical land use on Collembola communities and their seasonal fluctuations is very limited. Here, we investigated seasonal fluctuations in density and community composition of Collembola in rainforest, and in rubber and oil palm plantations in Jambi province (Sumatra, Indonesia), a region with moderate seasonality (wet and dry seasons) that experienced one of the strongest deforestations globally during the last decades. Samples were taken at the end of the wet season (March), the beginning of the dry season (June), the end of the dry season (August) and the beginning of the wet season (November). Collembola density in the litter was at a maximum at the beginning of the wet season, whereas in soil it generally varied little. Densities of euedaphic (soil-adapted) and epedaphic species (upper litter-adapted) fluctuated less with season than hemiedaphic (lower litter-adapted) and atmobiotic species (aboveground-adapted). Collembola community composition changed with season in each of the land-use systems, with the differences being more pronounced in litter than in soil. Differences in community composition among all three land-use systems were most pronounced at the beginning of the dry season. Water content, pH, fungal and bacterial biomarkers, C/N ratio and root biomass were identified as factors related to seasonal variations in species composition of Collembola in different systems. Our results show that Collembola density and community composition often fluctuate in a similar way across land-use systems, indicating that they are driven by common environmental factors, with water content, pH and food availability being of prominent importance. Notably, differences in Collembola community composition between rainforest and plantation systems were most pronounced in the dry season (June, August), indicating that the conversion of rainforest into plantation systems aggravates detrimental effects of low moisture on soil animal communities. Fluctuations in Collembola communities were generally stronger in litter than in soil, reflecting that Collembola in litter are less buffered against climatic variations than those in soil. Overall, the results document the sensitivity of tropical soil animal communities to seasonal climatic variations, which intensifies the effects of the conversion of rainforest into plantation systems on soil biodiversity.

Samples were taken within 50 m x 50 m plots established at each study site with minimum distance of 500 m, but usually more than 1 km between plots. Each land-use system was replicated four times resulting in a total of 12 plots (3 land-use systems x 4 replicates). From each plot at each sampling date, one randomly positioned sample was taken resulting in 48 soil cores in total. Each sample measured 16 cm x 16 cm taken to a depth of 5 cm of the mineral soil. Litter and soil were separated in the field and processed separately (96 samples in total). Then, samples were transported to the laboratory for extraction of soil animals. Animals were extracted by heat for 4–8 days until the substrates were completely dry  and stored in 70% ethanol until further processing. Environmental variables were measured in composite samples of each litter and soil (five cores per plot within a radius of ca. 2 m around the soil animal sample), including abiotic factors (pH, water content, C/N ratio) and biotic factors (microbial community composition in litter and soil as indicated by phospholipid fatty acids) including the sum of phospholipid fatty acid (PLFA) marker/relative marker lipids of Gram-positive bacteria (i15:0, a15:0, i16:0, i17:0), Gram-negative bacteria (2OH 12:0, 2OH 14:0, 16:1ω7, cy17:0, 2OH 16:0, cy19:0, 2OH 10:0), fungi (18:2ω6,9), algae (20:5ω3), arbuscular mycorrhizal fungi based on the neutral lipid fatty acid (NLFA) 16:1ω5c and root biomass. Litter and soil pH (CaCl2) was measured using a digital pH meter (Greisinger GPHR 1400A, Regenstauf). Aliquots of litter and soil material were dried at 65°C for 72 h, milled and analyzed for total C and N concentrations using an elemental analyzer (Carlo Erba, Milan, Italy). Water content (Wet weight, proportion of dry weight) of litter and soil were determined gravimetrically.

Collembola were sorted into morphological groups under a dissecting microscope (Stemi 508, Zeiss, Jena, Germany) at 50x magnification based on basic morphological characters (body shape, morphology of furca, antennae, number of eyes). Several individuals of each morphological group from each sample were subsequently cleared with Nesbitt solution on a heating plate (50^⁰C) for 3-10 min. Then, the animals were mounted on slides with Hoyer’s solution. Collembola were identified to species level using a compound microscope (Axiovert 35, Zeiss) at maximum 400x magnification using the checklist for Indonesian Collembola  and additional articles containing keys for Collembola of southeast Asia, particularly Indonesia