Investigating the effects of bryophytes on carbon cycling in a temperate forest ecosystem from stable isotope composition

Nonvascular plants such as bryophytes are often overlooked; however, they are important players at the soil-atmosphere interface and affecting water exchange, nutrient fluxes or carbon storage. Bryophytes also act as soil stabilizers in a variety of ecosystems and thus contribute significantly to the mitigation of soil erosion. Nevertheless, these stabilizing effects are not completely understood, with two distinctions: Firstly, bryophytes form a physical protective barrier that prevents direct drop impact on soil. Secondly, they fix carbon and thus contribute to soil carbon storage, which in turn enhances soil aggregation. Both factors result in a reduction of soil erosion, although it is unclear to what extent. As bryophytes showed a high impact on carbon assimilation and soil carbon storage in boreal environments, gaining an understanding of these effects in a temperate forest can be a key factor in assessing the overall state of that ecosystem.

In this study, we used the stable carbon isotope ratio (δ¹³C) analysis as an approach to evaluate the effect of bryophytes on soil organic carbon (SOC). Furthermore, we investigated the influence of SOC on aggregate size. Soil substrates and bryophyte species were sampled in temperate forests in southern Germany with different kinds of parent material. Five sites were located in Schönbuch Nature Park next to Tübingen and one site in the Black Forest close to Freiburg. Each study site consisted of three treatments: bryophyte-covered patches, bare undisturbed soil and partial disturbed soil from forest management. In this context, it was hypothesized that there is a significant contribution of bryophytes to SOC, which is reflected by a change in isotopic signatures and aggregate sizes. Consequently, bryophyte-covered soils are assumed to exhibit higher SOC contents, form larger soil aggregates and for this reason be more resistant to soil erosion, and the contribution of bryophytes to SOC can be estimated based on the differences in the δ¹³C of bryophytes compared to C₃ plants.

Preliminary results of two study sites revealed a distinct positive correlation between SOC and aggregate size, whereby a contribution of bryophytes to SOC could not yet be established based on δ¹³C values. This could be due to the different ecological structure of the two studied sites, the similarity of carbon isotope signatures of C₃ plants and bryophytes, the alteration of isotope signature as a result of decomposition, and the combination of all these factors. Laboratory and data analysis of the four remaining sites is currently ongoing, so further results will be presented at EGU 2022.