Fungal turnover in soil and roots in time and space

Soil and root-associated fungi are important for ecosystem nutrient cycling and health. We found that symbio-trophic fungal groups decreased, while pathotrophic and saprotropic groups increased with land transformation from lowland rain forests to oil palm plantations. Root fungal species richness increased only moderately with increasing plant diversity but fungal species composition showed drastic turnover among different land use systems. Changes in soil properties and root nutrient concentrations were related to fungal turnover, but differently for soil- or root-associated fungal assemblages. In Phase 3, we plan to investigate changes of fungal assemblages and root nutrients at the landscape level (Landscape Assessment). Since knowledge on functions of tropical fungi is scarce, we will expose mesh bags supplemented with different nutrient sources as bait and identify colonizing fungi.

Soil fungi are playing key roles in biogeochemical processes and in soil health (Cardoso et al. 2013, Tedersoo et al. 2014). They are important components of ecosystems because they drive nutrient cycling as decomposers (Treseder & Lennon 2015), regulate species composition as pathogens (van Agtmaal et al. 2017) and provide benefits to plants as symbiotrophs (Pena 2016). Consequently, it is important to understand how deforestation and land-use intensification in the tropics affect soil fungal communities, influence mutualistic interactions with roots and how these alterations impact ecosystem functions and services. Our initial hypothesis was that land transformation from highly diverse tropical rain forests into rubber or oil palm monoculture plantations resulted in a drastic loss of fungal species. In contrast to this assumption, we found only relatively moderate differences in soil fungal species richness among forests, intensively managed mono-culture plantations and jungle rubber (i.e., extensive agro-forestry); however, the fungal community composition was strongly changed (Brinkmann et al. 2019) and we observed strong legacy effects of intensive land-use on the soil fungal community composition (Ballauff et al. 2020). Using quantitative management data provided by the socioeconomic groups (C07 Qaim), we developed a scale for land-use intensity (Brinkmann et al. 2019) based on the concept proposed by Blüthgen et al. (2012). We found that fungal communities clustered according to the intensity of land use and showed changes in the ecological functions of fungal groups: the abundances of symbiotrophic fungi decreased, while those of saprotrophic and pathotrophic fungi increased in managed systems compared with rain forest (Brinkmann et al. 2019). In particular, a strong increase in Fusarium oxy-sporum, a potential pathogen, was detected (Brinkmann et al. 2019), especially in plots with low root vitality and low arbuscular mycorrhizal colonization (Sahner et al. 2015). Obvious symptoms of oil palm diseases were not observed (Edy Nur, personal communication), but our findings suggest that tropical land transformation may threaten the balance of ecosystem health functions (Brinkmann et al. 2019). Furthermore, we observed different turn-over rates of soil and root-living fungi along a land-use gradient (Ballauff et al. 2021).

To gain deeper insights into the processes driving changes in fungal community composition at the level of roots and soil on the one hand and the consequences of these changes for ecosystem functions on the other hand, we address the following questions:

  • What are the consequences of different soil fungal communities for litter degradation and N cycling in oil palm plantations compared with rain forest?
To investigate this question, we generated 15N labelled leaf and root litter of different tree species. Mesh bags, which permit the ingrowth of microbes (fungi, prokaryotes), and mesh bags, which also permit ingrowth of fine roots, have been exposed in oil palm and rainforest plots and were sequentially collected. Analyses of biomass, 15N, and fungal communities will be conducted
  • What are the drivers of spatial variation of fungal diversity across landscapes?
We hypothesized that at the landscape level, turnover of fungal communities in soil is driven by changes in soil properties and that of root-associated fungi by root diversity and root nutrient composition (contributes to Focus 1 - Hypotheses 1, Focus 2- Hypotheses 1 and 2, and Focus 3- Hypotheses 1, 2, 3).

We will use the Landscape Assessment to investigate fungal community structures and drivers at a larger scale. We hypothesize that root-associated fungal communities are driven by root nutrients and host species composition, while soil fungal communities are mainly driven by soil properties. To test this hypothesis, we will sample defined soil volumes, determine soil and root mass (together with Irawan Bambang) and use aliquots of soil and roots for fungal DNA barcoding (2 x 124 samples). Aliquots of roots will be used for C, N and elemental analyses (ICP-OES) of root nutrient concentrations (124 samples) as before (Sahner et al. 2015). We offer support to measure nutrient concentration in leaves (costs covered by Z01). We will provide root DNA to B14 Gailing for analyses of plant diversity in the samples. We will obtain information on soil nutrients and pH (A05 Corre/Veldkamp). We will use the data to elucidate the contribution of plant diversity, root nutrient diversity and soil properties as drivers for root and soil fungal communities. We will provide our data for synthesis activities in Focus 2 and 3.

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