Seasonal and altitudinal variability of litter decomposition across slope aspects in the Rila Mountains, Bulgaria

Elevation gradients provide a natural framework for studying environmental change over short geographical distances. As the altitude increases, the temperature declines, the variability in air humidity increases, and the growing season shortens. Furthermore, even under otherwise comparable environmental conditions, the slope aspect affects the microclimate of the ecosystems. These combined factors strongly influence ecosystem processes, including microbial activity and the rate of litter decomposition in mountainous forest ecosystems and thus affecting carbon and nutrient cycling.
To test the hypothesis on the relative roles of seasonality, elevation, and microclimate in controlling litter decomposition rates in forest ecosystems, we conducted a field experiment in Rila Mountains (Bulgaria) along an altitudinal gradient, throughout different stages of the vegetation period, and across contrasting slope exposures. We applied an updated Tea Bag Index (3.0) protocol as described in Middelanis et al. (2023) ensuring a time-series mass loss data by excavating the buried tea bags three times within a 90-days field experiment per season. The study was set along 100 m altitudinal gradients between 1500-1800 m a.s.l on north-facing and south-dominated slopes during three biologically active seasons – from early spring to late autumn. Enzyme activities and soil pH were measured at each sampling event (i.e., tea-bag incubation and excavation), thus providing repeated measurements that captured seasonal and microclimatic variation. In addition, soil physicochemical properties (soil organic carbon, total nitrogen, texture, bulk density, skeleton content, and ICP-based geochemistry) and stand mensuration were obtained to provide the sampling plots' characterization. Meteorological data, including air temperature and precipitation variables, were analyzed to characterize the regional meteorological conditions during the studied period. This integrated approach facilitated the analysis of the effects of multiple factors influencing litter decomposition in mountain forests. 
The study found that the labial fraction and the decomposition constant differed notably between tea types, with consistently higher decomposition rates and decomposable fractions for green tea compared to rooibos tea across all sites and seasons, reflecting differences in substrate quality. The decomposition rate varied between sampling plots, exposure sites, and seasons. No clear pattern in decay rate was identified across the elevation gradient during the study period, with the exception of the spring season, when a decline in decomposition rate was observed with increasing elevation. Strong seasonal patterns were exhibited, with maximum decomposition rates occurring in summer at south-facing sites, whereas north-facing sites demonstrated equal or higher decomposition rates in autumn. These trends are further supported by statistically significant variations in the seasonal activity of microorganisms observed between the two surveyed areas with contrasting slope aspects, as well as by significant disparities in enzyme activity across seasons. The study demonstrated that litter decomposition in mountain forests is shaped by multiple interacting factors. It provided a good basis for comparing the relative influence of seasonality, elevation, and microclimatic conditions, highlighting the importance of their combined effects. However, further long-term experiments are needed to refine these insights and better capture the temporal variability of decomposition processes.