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Though typically discussed in the context of salinity, a shortage of sodium in tropical forests has been demonstrated in recent years, inhibiting soil biota with impacts on decomposition processes (Sodium Ecosystem Respiration hypothesis). So far, sodium limitation has been shown for soil fauna, whereas a potential role in saprotrophic fungi remains unknown. We tested the hypothesis that fungi have sodium demands at low levels resulting in reduced activity in tropical soils, analyzing fungal responses to sodium availability. Despite careful experimentation and the usage of different fungal strains we did not find evidence for fungal sodium demands using controlled nutrient media, not even in the absence of potassium. Likewise, there was no significant fungal response to sodium additions in litter samples, though decomposition rates were positively affected. These findings suggest that sodium is not essential for osmoregulation of tested fungal strains, which are hence not affected by sodium limitation. [ABSTRACT FROM AUTHOR]

Studies in temperate systems provide evidence that the abundance of arbuscular mycorrhizal fungal (AMF) depends on soil nutrient availability, which is mainly explained in the context of resource stoichiometry and differential plant biomass allocation. We applied this concept to an understudied ecosystem – tropical montane forest – analyzing root and AMF abundance along an elevational gradient with decreasing nutrient availability, combined with responses to nitrogen (N) versus phosphorus (P) additions. At three sites from 1000 to 3000 m above sea-level we analyzed fine root length, AMF root colonization as well as extraradical AMF biomass (neutral lipid fatty acid 16:1ω5, hyphal length and spore counts) in a nutrient manipulation experiment. We found a significant increase in root length as well as intra- and extraradical AMF abundance with elevation. Overall, P additions significantly increased, whereas N additions decreased AMF abundance, with differential though nonsystematic changes along the elevational gradient. Strongest effects were clearly observed at the intermediate site. These findings suggest a general dependency of roots and AMF on nutrient availability, though responses to N and P additions differed from previous studies in temperate systems. In the context of future nutrient depositions, results suggest diverging responses of AMF abundance depending on site characteristics. [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]

Our knowledge of soil biodiversity in agriculture in general is currently increasing rapidly. However, almost all studies have stopped with the quantification of soil biodiversity effects on crops at harvest time, ignoring subsequent processes along the agrifood chain until food arrives on our plates. Here we develop a conceptual framework for the study of such postharvest effects. We present the main mechanisms (direct and indirect) via which soil biodiversity can influence crop quality aspects and give examples of how effects at harvest time may become attenuated through postharvest operations and how biodiversity may also affect some of these operations (i.e., storage) themselves. Future research with a broader focus has the potential to unveil how soil biodiversity may benefit from what ends up on our forks. [ABSTRACT FROM AUTHOR]