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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]

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: Previous research from the tropics indicates that AMF may be well adapted to organic soils and even represent the dominant mycorrhizal form, though the extraradical part of the symbiosis was omitted as in most other tropical studies. Our study aims at characterizing the extraradical part of arbuscular mycorrhizal fungi (AMF) in a highly organic tropical montane forest soil in Southern Ecuador. Based on recent studies on the interaction of AM fungal hyphae and litter we hypothesized that within the organic layer AM hyphae grow in close contact with decomposing material. To test this idea, AM fungal hyphal distribution in the organic layer was determined by directly staining roots and decomposing leaves and extracting hyphae from the remaining particulate organic material. AM and non-AM fungal hyphae were analyzed, as well as root colonization patterns. Our results showed that AMF indeed represented the dominant mycorrhizal form with an average root colonization of 43%. The extraradical AM hyphal length ranged from 2 to 34 m g−1 soil with a mean of 10.4 m g−1 soil (equals 3.1 m cm−3 soil), and therefore exceeded root length about 13-fold. As hypothesized, 29% of AM extraradical hyphae were closely attached to decomposing leaves. These hyphae were mainly located at the leaf surface, though in some parts leaf veins and inner leaf tissues were colonized. More than half of AM hyphal biomass was detected on the root surface, a pattern potentially driven by the predominant Paris-type AMF. Non-AM fungal hyphae colonized decomposing material to a significantly greater extent, though hyphal length attached to roots was equal. This study supports the adaptation of AMF to highly organic soils in the tropics and the existence of a widespread extraradical mycelium, which is not readily detectable by standard methods. The close association with decomposing leaves most likely improves direct nutrient uptake from decomposed material and points to a potential indirect contribution of AMF to the decomposition process. [Copyright &y& Elsevier]