There is increasing evidence that spatial and temporal dynamics of biodiversity and ecosystem functions play an essential role in biodiversity-ecosystem functioning (BEF) relationships. Despite the known importance of soil processes for forest ecosystems, belowground functions in response to tree diversity and spatio-temporal dynamics of ecological processes and conditions remain poorly described. We propose a novel conceptual framework integrating spatio-temporal dynamics in BEF relationships and hypothesized a positive tree species richness effect on soil ecosystem functions through the spatial and temporal stability of biotic and abiotic soil properties based on species complementarity and asynchrony. We tested this framework within a long-term tree diversity experiment in Central Germany by assessing soil ecosystem functions (soil microbial properties and litter decomposition) and abiotic variables (soil moisture and surface temperature) for two consecutive years in high spatial and temporal resolution. Tree species richness and identity had significant effects on soil properties (e.g., soil microbial biomass). Structural equation modeling revealed that overall soil microbial biomass was partly explained by (a) enhanced temporal stability of soil surface temperature and (b) decreased spatial stability of soil microbial biomass. Overall, spatial stability of soil microbial properties was positively correlated with their temporal stability. These results suggest that spatio-temporal dynamics are indeed crucial determinants in BEF relationships and highlight the importance of vegetation-induced microclimatic conditions for stable provisioning of soil ecosystem functions and services.

From February 2017 to December 2018, five soil samples from each monoculture and five-species mixtures were taken every two months to a standard depth of 10 cm using cylindrical steel soil corers with a diameter of 2.5 cm. The litter layer was removed prior to sampling and restored thereafter. The samples were taken in each of the five subplots per plot to account for spatial heterogeneity. In 12 campaigns, a total of 1,440 soil samples were taken, i.e., 120 samples per campaign. During the sampling, the soil samples were cooled and later transferred to a 4°C storing unit for a maximum of 5 days until further processing. After the sampling, all samples were sieved at 2 mm to remove stones, coarse roots, and large animals and to homogenize the soil.

The soil surface temperature was measured for each subplot, utilizing temperature loggers (HOBO Pendant® Temperature/Light 8K Data Logger, Onset Computer Corporation®, Bourne, USA) placed beneath the litter layer. On plots without a thick litter layer (i.e., ash monocultures), the loggers were placed on top of the soil without being covered. The temperature was logged every 30 min.

At the beginning of the experiment, one teabag (Lipton® rooibos tea - EAN 87 22700 18843 8, in PVC bags) per subplot was placed 5 cm deep into the soil and covered up. Based on the assumption that forest soil communities are adapted to complex substrates, we used rooibos tea that is known to be relatively recalcitrant. Before placing the tea bags in the soil, we oven-dried them at 40°C for at least 72 h until no further weight loss was detected to remove any water content. After drying and cooling down to constant weight, each bag was weighed and labeled with a unique ID to account for potential weight deviation caused by the manufacturing process. During each soil sampling event, the tea bags were removed and exchanged with a new one at the same position. The tea bag IDs and positions were recorded. After being taken out of the soil, the exposed bags were again dried in the laboratory at 40°C for at least 72 h until no further weight loss was detected. After the drying period, each tea bag was opened and their content weighed. We calculated the difference between the dried exposed tea bag content and its original weight, and subtracted the average (n = 100) weight of the nylon bag + the string + the plastic label of the bag to calculate the mass loss of the tea during the two months of exposure in the soil.