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Responses of arbuscular mycorrhizal fungi to nutrient additions in Southern Ecuador

Subject Area Ecology and Biodiversity of Plants and Ecosystems
Term from 2014 to 2019
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 244578176
 
Final Report Year 2018

Final Report Abstract

Within the DFG funded project „Responses of arbuscular mycorrhizal fungi to nutrient additions in Southern Ecuador”, which was part of the “Platform for Biodiversity and Ecosystem Monitoring and Research in South Ecuador”, we elucidated long-term effects of nitrogen (N) and phosphorus (P) additions on the abundance and diversity of arbuscular mycorrhizal fungi (AMF), its functional role in this special ecosystem, and also broadened current knowledge on nutrient limitations and demands of soil microbial organisms in tropical forests. The observed responses in AMF abundance to nutrient shifts were highly context dependent and affected by several parameters, e.g. tree species identity, soil nutrient status, elevation, the duration of nutrient additions and the AMF structure observed. Still, an overall detrimental effect of N additions was found, also supported by decreasing diversity, whereas P additions exerted partly positive effects. Beside shifts in abundance and richness, communities showed a significant shift following N and P inputs at different elevational sites. This community shift appeared to get more pronounced within the observed years, whereas shifts in AMF abundance were diminishing over time, pointing towards a highly resilient mycorrhizal interaction. Potentially, shifts in AMF species composition may compensate for detrimental effects on abundance by adaptations to changing conditions. Several pot experiments aiming to elucidate the effects of shifting communities and decreased AMF abundance failed, which may be an interesting hint that successful AMF colonization of tree seedlings requires an intact mycorrhizal network, as only found in soil surrounded by mother trees. Further experiments by mycorrhizal exclusion from soils revealed a potential role of AMF in NH4 uptake, and the positive effect of P additions might be explained by fungal P limitation in these nutrient poor soils: In a small-scale element addition experiment fungi showed increased demands for P, compared to the control and other elements, whereas N again showed negative impacts. To compare these fungal responses indicating P limitation in the tropical forest sampled here, we compared our results to other nutrient manipulation experiments in the tropics by meta-analytical methods. A global meta-analysis revealed primary P limitation of soil microbial organisms in tropical forests, while N limitation only was apparent in high montane areas characterized by younger soils. Interestingly, also other elements showed up as important limiting factors for certain microbial processes, though hardly considered in nutrient cycling models, e.g. molybdenum or zinc. Finally, in the study area we further looked at other elements, mostly sodium (Na) and potassium (K). As previously shown in other tropical sites with low oceanic inputs, litter decomposition rates in the lower montane site (1000m a.s.l.) proved to be Na limited, however, an involvement of fungi or microbes was precluded based on several field- and lab-based experiments. In future studies, it will be especially interesting to find further hints why AMF are the predominant mycorrhizal form in this tropical montane forest, which is characterized by slow decomposition rates and highly organic soils, a system typically rather characterized by ectomycorrhizae. Furthermore, the functional impacts of observed community shifts are still unknown and methodologically challenging, though such knowledge will provide relevant information for expected changes in future nutrient cycling.

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