Depth-dependent dynamics of microarthropods in forest floors: interactions with temperature and phosphorus levels

The forest floor (FF) serves as the critical interface between the aboveground and belowground components of forest ecosystems. It plays a pivotal role in regulating water and energy exchange between the atmosphere and the soil, providing habitat for roots and diverse soil organisms, mitigating soil erosion, and promoting tree growth in forest ecosystems. Forest floors buffer harsh environmental conditions and insulate soil, thereby mitigating the effect of climate extremes on soil fauna. Conversely, the soil fauna is key for shaping the structure of FFs. Considering the recently documented decline in FFs across Europe, changes in the composition and activity of soil animal detritivores and their consequences for changes in the structure of FFs need closer attention. Unfortunately, the structure of decomposer animal communities across different layers of FFs and their variation with soil nutrient status and climatic factors has not been comprehensively investigated. We investigated the distribution patterns of two major decomposer microarthropod groups (Collembola and Oribatida)  across the different layers of the FF (Ol, Of/Oh and Ah) of 12 forest sites representing temperature and phosphorus gradients. A total of 58 Collembola  and 144 Oribatida species were recorded. Phosphorus as main factor neither significantly affected the abundance of Collembola nor that of Oribatida. The same was true for the effect of temperature on the abundance of Collembola, whereas the abundance of Oribatida varied significantly with temperature. Further, Oribatida richness significantly increased with increasing temperature but decreased with increasing phosphorus level. The effect of layer was highly significant for both Collembola and Oribatida. Specifically, the abundance, richness and biomass of both microarthropod groups was at a maximum in the Of/Oh layer followed by Ah and Ol layer. Collembola and Oribatida community structured varied with temperature and phosphorus levels but in both this depended on layer. Soil and litter carbon-to-nitrogen ratio, pH, Gram-positive bacterial phospholipid fatty acids (PLFAs) and thickness of Ol and Of/Oh layers were identified as major drivers. The results suggest that the distribution and community composition of Collembola and Oribatida are intricately linked to both biotic and abiotic factors in the FF. These findings highlight the critical influence of temperature, phosphorus and FF stratification on soil microarthropod communities, alongside additional soil chemical, microbial and physical characteristics of the FF. The differential responses of Collembola and Oribatida to temperature and phosphorus gradients underscore functional and ecological differences between these groups, with Oribatida displaying a stronger sensitivity to climatic and nutrient changes. Overall, the results emphasize the importance of maintaining the structural integrity of forest floors to support diverse and resilient soil fauna communities.