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Root associated fungal (RAF) communities can exert strong effects on plant communities and are potentially sensitive to shifts in soil fertility. As increased atmospheric nitrogen (N) and phosphorus (P) deposition can alter the nutrient balance in natural ecosystems, we assessed the response of RAF communities to a fertilization experiment deployed on a highly diverse Andean forest. The stand level fine root fraction was sampled after 7 years of systematic N and P additions and RAF communities were characterized by a deep sequencing approach. We expected that fertilization will enhance competition of fungal taxa for limiting nutrients, thus eliciting diversity reductions and alterations in the structure of RAF communities. Fertilization treatments did not reduce RAF richness but affected community composition. At the phylum level fertilization reduced richness exclusively among Glomeromycota. In contrast, N and P additions (alone or in combination) altered the composition of several fungal phyla. The lack of a generalized response to long‐term fertilization among RAF lineages suggests that most of these lineages will not be directly and immediately affected by the increasing rates of atmospheric N and P deposition expected for this region by 2050. [ABSTRACT FROM AUTHOR]

Summary: Anthropogenic atmospheric deposition can increase nutrient supply in the most remote ecosystems, potentially affecting soil biodiversity. Arbuscular mycorrhizal fungal (AMF) communities rapidly respond to simulated soil eutrophication in tropical forests. Yet the limited spatio‐temporal extent of such manipulations, together with the often unrealistically high fertilization rates employed, impedes generalization of such responses.We sequenced mixed root AMF communities within a seven year‐long fully factorial nitrogen (N) and phosphorus (P) addition experiment, replicated at three tropical montane forests in southern Ecuador with differing environmental characteristics. We hypothesized: strong shifts in community composition and species richness after long‐term fertilization, site‐ and clade‐specific responses to N vs P additions depending on local soil fertility and clade life history traits respectively.Fertilization consistently shifted AMF community composition across sites, but only reduced richness of Glomeraceae. Compositional changes were mainly driven by increases in P supply while richness reductions were observed only after combined N and P additions.We conclude that moderate increases of N and P exert a mild but consistent effect on tropical AMF communities. To predict the consequences of these shifts, current results need to be supplemented with experiments that characterize local species‐specific AMF functionality. [ABSTRACT FROM AUTHOR]

In tropical ecosystems, interactions between arbuscular mycorrhizal fungi (AMF) and other organisms have been little studied, but may be of significant importance for understanding the role of AMF in decomposition processes and nutrient cycling. In this study, we used ingrowth cores to investigate the impacts of regular rotation of the cores, defaunation and nitrogen addition on AMF, microbial biomass and microarthropods in the fermentation/humus (F/H) and litter (L) layers of an Ecuadorian montane tropical rainforest. AMF were substantially reduced in the F/H layer (to 34% of initial), while in the L layer they remained constant during the experiment. Overall, microorganisms and microarthropods were largely independent of AMF hyphae and their exudates, however, defaunation strongly affected the recovery of their communities. Nitrogen addition increased the quality of litter material and beneficially affected microbial communities thereby increasing decomposition rates, but did not impact AMF abundance and microarthropod communities. These findings suggest that the cutoff of the carbon supply from the plant to the fungal mycelium was not compensated by switching resources in the F/H layer, underlining a strong association between AMF and living roots. While in the L layer, AMF likely competed with saprotrophic microorganisms for litter-derived resources at intermediate stages of decomposition pointing to indirect contributions of AMF to decomposition processes. Overall, the results support the view that root-derived resources are important in fueling soil food webs, but also indicate that in the studied montane rainforest these resources are only available close to roots and not channeled distant to roots via AMF. [ABSTRACT FROM AUTHOR]

Ecosystem processes and the organisms involved are generally limited by the availability of one or more elements in soil, an important phenomenon to consider for our understanding of ecosystem functioning and future changes. Especially in tropical forests, typically growing on nutrient depleted soils, nitrogen (N), phosphorus (P) or other limitations are assumed. However, large‐scale nutrient manipulation experiments revealed complex site‐specific patterns and several authors raised the need for novel approaches to reveal deeper mechanistic insights on limitation patterns, especially concerning soil microbial activity.In order to gain such deeper knowledge, based on a short review of previous small‐scale studies focusing on soil micro‐organisms, we developed an experimental approach which controls for common biases, including indirect treatment effects, addition of co‐elements or nutrient pulses. Using this technique, we tested the hypotheses that fungi growing in tropical forest soils are mainly limited by P and that N versus P limitations shift along an altitudinal gradient.Mesh bags of 38 μm filled with sand were amended with weak ion exchange resins loaded with N, P or potassium (K) and buried underneath the litter layer at three altitudinal sites in southern Ecuador. After a period of four months, the in‐growth of fungal hyphae was quantified, phospholipid fatty acids were analysed for a subset of samples, and chemical properties were determined.In line with the first hypothesis, hyphal abundance was increased in P‐amended mesh bags, indicating P limitation. However, this pattern was not significantly shifted along the altitudinal gradient. By contrast, N addition increased hyphal abundance at the lowest site, compared to significant reductions at 2,000 and 3,000 m—not only in fungi but also in bacterial abundance as indicated by PLFA analyses, contradicting common soil‐age hypotheses. Decreased nutrient immobilization and fungal in‐growth at higher elevations suggest slow microbial activity, including nitrification, which may have caused toxic ammonium accumulation.The experimental design offers a promising tool to provide more mechanistic and soil‐focused analyses specifically targeting microbes, which in this system strongly supported the hypothesis of primary fungal P limitation in tropical forest soils. A plain language summary is available for this article. [ABSTRACT FROM AUTHOR]

Abstract: Soil fungi and bacteria are the key players in the transformation and processing of carbon and nutrients in terrestrial ecosystems, yet controls on their abundance and activity are not well understood. Based on stoichiometric principles, soil microbial processes are expected to be limited by mineral nutrients, which are particularly scarce in often highly weathered tropical forest soils. Such limitation is directly relevant for the fate of soil carbon and global element cycles, but its extent and nature have never been assessed systematically across the tropical biome. Here, we address the relative importance of nitrogen, phosphorus, and other nutrients in limiting soil microbial biomass and process rates in tropical forests. We conducted an in‐depth literature review and a meta‐analysis of the available nutrient addition experiments in tropical forests worldwide. Our synthesis showed predominant and general phosphorus limitation of a variety of microbial processes across tropical forests, and additional nitrogen limitation in tropical montane forests. The apparent widespread microbial phosphorus limitation needs to be accounted for in the understanding and prediction of biogeochemical cycles in tropical forests and their future functioning. Other mineral nutrients or carbon may modify the importance of phosphorus, but more experimental studies are urgently needed. [ABSTRACT FROM AUTHOR]