Soil represents a thin crust over the Earth created by the combined influence of abiotic (topography, climate, parent materials) and biotic (soil organisms) interactions (https://www.fao.org/soils-portal). A major contributor to the soil’s organic content is leaf litter, which plays a crucial role in the carbon cycle during the process of decomposition (Schmidt et al. 2011). Soil mesofauna, particularly soil microarthropods, are dominant inhabitants of soil systems and are found in all soil types. They have a significant impact on decomposition processes and, through various trophic interactions, create linkages in soil food webs (Coleman et al. 2017).

Microarthropods promote litter decomposition either by direct consumption or by the stimulation of microbial activity (Fujii and Takeda 2017). Dominant representatives of soil microarthropods are oribatid mites, which feed on a wide range of resources, including dead organic matter, lichens, fungi, algae, bacteria, and nematodes (Maraun et al. 2011, 2020). Recent studies also indicate that they show trophic plasticity by adjusting their diet to the specific type of food available, allowing them to occupy different types of habitats and ecosystems (Maraun et al. 2020).

While the fact that soil microarthropods show adaptations to their local environment is well documented (Menta et al. 2014, 2018), less information is available on how fast they start to colonize new microhabitats and utilize resources that are different from those they are already adapted to in their previous habitat. To fill this knowledge gap, we conducted an experiment with the aim to study the process of succession of microarthropods in defaunated litterbags filled with locally available leaves and those filled with leaf litter from a different location where the dominant tree habitat was different.

Specifically, we tested the patterns of colonization of soil microarthropods in litterbags filled with oak or beech leaves placed in stands of oak and beech trees. We hypothesized that the fauna would colonize leaves of the local dominant tree type faster; as a result, the rate of decay of oak leaves placed in a beech stand would be less rapid compared to the decay of oak leaves under a stand of oak, and vice versa. During the process of litter decomposition, different feeding resources become available to microarthropods, with fungi appearing within the first few days (Osono 2007) followed by bacteria (Aneja et al. 2006; Bray et al. 2012; Osono 2007). This should lead to changes in the microarthropod trophic composition during the progress of litter decay, regardless of local or distant microhabitats. We predicted that in the early stage of decomposition, the fauna would be dominated by primary decomposers, while by the end of the experiment, the share of fungal feeders and predators would have increased.

Among the microarthropods identified in this study, oribatid mites dominated all litterbags in both species composition and number of individuals. The succession of microarthropods during the process of litter decay was evident by the increasing number of taxa and individuals between the first and second sampling events. During this period, the overall number of Oribatida increased from 298 to 480 individuals, Mesostigmata from 19 to 274, and Collembola from 42 to 614. The increased biodiversity index and decreased evenness, associated with higher overall dominance in winter compared to summer, indicate an increase in certain types of resources that promote the dominance of specific taxa. Collembolans are known to be successful colonizers of litterbags; along with Oribatida, they increase their dominance as the litter decomposition process progresses (Miko 1995; Fujii and Takeda 2017). The increasing numbers of Mesostigmata are associated with an increased number of their prey, as true soil-inhabiting Mesostigmata are mostly predators (Coleman et al. 2017).

The community of oribatids changed during the process of litter decomposition. Gymnodamaeus bicostatus was present in all litterbags collected in June but was completely absent from litterbags in winter. Although the feeding preferences of this species are unknown, it is known to prefer moss- and lichen-dominated habitats and is frequently found on tree trunks (Weigmann 2006). Therefore, we can assume that it feeds on fresh or early-stage decaying plant tissue and is not adapted to the materials found in the later stages of decay, where persistent compounds are broken down by the enzymatic actions of fungi (Berg and McClaugherty 2008).

Cepheus dentatus was another species found in early decomposition stages and absent in winter in both oak- and beech-filled litterbags. This species is known as a primary decomposer and fungal feeder (Maraun et al. 2023); therefore, its absence from later successional stages is surprising. Eremaeus hepaticus, Eubelba dubinini, Phthiracarus longulus, and Suctobelbella duplex were absent from litterbags sampled in June, while they were frequent and numerous in winter. These species are typical inhabitants of forest soils and litter. Representatives of Suctobelbidae are known to be scavengers and predators, while Phthiracaridae are mostly primary decomposers (Schneider et al. 2004), and the majority of Eremaeidae are decomposers and fungal feeders (Maraun et al. 2023).

In contrast, Ceratoppia bipilis, which shares the same feeding guild (decomposer and fungivore; Maraun et al. 2023), constantly dominated all types of litterbags throughout the entire experiment. The clear separation of the studied microarthropod fauna by season provides further proof of the involvement of different taxa in decomposition processes (Miko 1995; Skubala and Maslak 2010; Hågvar and Steen 2013). The presence of more diverse and numerous soil animals at later stages of decay is further supported by our investigation, which showed higher overall abundance and species richness at the final stage of the experiment. Seasonal differences in microarthropod composition are also affected by seasonal differences in moisture and temperature, which lead to the accumulation of different feeding resources (Cordes et al. 2022).

The type of habitat was not a significant factor in shaping the soil microarthropod community. Species that showed preferences for either oak or beech habitats were usually not numerous and were represented by only one or two individuals.

The succession of microarthropods in the litterbags is demonstrated by the trophic composition of species, with an increased dominance of predatory taxa and the appearance of lichen-feeding species. In fact, representatives of predatory Mesostigmata, such as Discourella sp. and Trachytes arcuatus, only appeared in litterbags collected in winter, followed by lower numbers of Uropodina, which include both predatory and fungivorous species (Coleman et al. 2017). The presence of lichens at later stages of decomposition further supports colonization by microarthropods by releasing the nutrients and promote the abundance of soil fauna participating in litter breakdown (Lang et al. 2009; Concostrina-Zubiri et al. 2021; Jegede et al. 2023). Lichen-feeding oribatids were present in small numbers: Cymbaeremaeus cymba was only found in winter, and Mycobates parmeliae was more frequent in winter, with only one individual of the latter species being found during the first sampling event. The increased dominance of fungivorous mites is also not surprising, considering the colonization of fungi in decomposing litter (Coleman et al. 2017).

The significant difference in abundance and species numbers between microarthropods in litterbags with oak leaves (as opposed to beech leaves) under oak trees supports our assumption that the well-established community under oak trees is adapted to oak litter decay. Surprisingly, no significant difference was observed between the litterbags under the beech stands. This may be the result of our study site represented by the mixed forest dominated by oak, with beech being restricted to the upper edge of the forest slope. The formation of distinct oribatid communities in specific habitats is often attributed to their low dispersal abilities (Cordes et al. 2022), but one would expect the side-by-side placement of litterbags filled with oak or beech leaves to bypass this limitation.

The experiment showed a continuous decline of beech litter mass under both oak and beech stands. However, this was not the case with oak litter; the oak leaves maintained their shape and structure throughout the whole experiment, whereas the beech leaves were almost unrecognizable by the end of the study. The data show that the process of mass loss slowed down between the first and second sampling events in litterbags with oak litter placed under oak trees, but continued to decline in oak litterbags placed under beech trees. Beech litter is known to decompose faster than that of many other trees (Jacob et al. 2010), while oak litter shows slow decomposition rates compared to other deciduous leaf litter (Miko 1995; Pardo et al. 1997). One suggested reason for this difference is the less dense canopy of oak compared to beech forests (Miko 1995). We suggest another reason for the faster mass decline of beech litter may be the structural differences between oak and beech leaves. Studies show that the structural thickness and density of oak leaves are higher than those of beech (Poorter et al. 2009; Aranda et al. 2004). Moreover, oak leaves have been shown to have a thicker adaxial cuticle and epidermal cell wall compared to beech leaves (Bahamonde et al. 2018); this could make oak leaves less permeable to microorganisms, resulting in a slower decomposition process.

In contrast to our findings, Lorenz et al. (2004) observed different results in their litterbag experiment in the Mannheim-Käfertal Forest after 0.5 and two years of decomposition. The authors report a greater dry mass decline for oak than for the beech litter. This differing pattern may be explained by a higher C/N ratio and higher tannin content in fresh beech litter compared to oak leaves, indicating a lower quality of beech litter in terms of decay in that specific environment.