| Resum: |
The nutrient balances of nitrogen (N), carbon (C) and phosphorous (P) in ecosystems are changing while its effects on the Earth's system are still a research enigma. Roots and mycorrhizae in the tropics are important in global C storage and nutrient cycling, which makes it necessary to understand how these belowground structures react to different nutrient levels. We collected soil samples at two depths (0 -15 cm and 15 - 30 cm) along a precipitation gradient in French Guiana, as well as a topographic gradient within one site, to analyze how depth, bulk density, nutrient levels and pH affected root densities, root diameter, presence of ectomycorrhizal fungi (EMF), root colonization percentage of arbuscular mycorrhizal fungi (AMF), relative abundance of (AMF) and AMF species diversity. Along the precipitation gradient we found that the root densities correlated negatively with N in the deeper soil, the abundance and species richness of AMF were positively related with N and negatively with P (congruent with the "trade balance model"), while EMF did not significantly correlate with any nutrient. With increasing depth, root densities and EMF presence decreased, while AMF relative abundance increased and AMF species richness was stable. No relations with pH were found except for the root density in the upper soil layer. Root density was not affected by bulk density, nor did it differ with topography. The root density and diameter did however respond to P in the top plots while it correlated to N in the base and slope plots. We conclude that variation in AMF, particularly at greater depth, can be explained by varying nutrient concentrations while EMF is more depending on tree species, the amount of nutrients in the vertical soil layers in a way limit root density and the nutrient triggering root response can vary along a topographic gradient since the gradient affects N:P ratios. |
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