Abstract
Roots are among the major controls of nutrient and C cycles and together with mycorrhizal fungi they are assumed to play a key role especially in P nutrition of forest ecosystems. Current publications emphasized that size distribution of fine roots reflects the crucial impact of roots on biogeochemical cycles. However, we know hardly anything about the spatial distribution of fine root size classes and their specific surface as well as distribution of mycorrhizal fungi among different size classes in undisturbed soils. We used a novel method based on epifluorescence microscopy to analyze fine roots in undisturbed soil samples. We expected that based on these analyses proposed differences between P-rich and P-poor soils get clearer than based on routine methods for fine root analysis. This was examined at two European beech forests in Germany at silicate rock that differ in P supply by the parent material. We analyzed the fine root frequency, size distribution and surface based on resin impregnated cross sections taken from undisturbed soil samples. Fine roots were classified according to their size and morphology of associated ectomycorrhiza with epifluorescence microscopy. More than 82% of the absorbing surface was associated with roots having a diameter smaller than 100µm. The fine root surface area present per square meter of soil was 388m² at the P-poor site and 220m² at the P-rich site. In addition, percentage of mycorrhization of fine root surface was 47% at P-poor site and only 38% at P-rich site. The biggest root length density and the highest absorbing fine root surface with mycorrhization and abundance of extramatrical mycelia was observed in forest floor at the P-poor site and in subsoil at P-rich site. Our results confirm that beech trees adapt their root traits according to P availability showing higher absorbing surface at the P-poor site compared to the P-rich site. In contrast, at the soil-profile scale, rooting density and mycorrhization increased with P availability thereby showing an efficient way of root and mycorrhizal fungi placement. Overall, distinct differences in fine root traits, between the P-rich and P-poor site were most evident for fine roots smaller than 100μm.
Highlights
Roots are less accessible for researchers than above-ground plant compartments and are widely assumed to bear still a lot of unidentified possibilities of adaptation (Finér et al, 2011)
As hypothesized we found clear differences between the traits of root/mycorrhizal associations at both sites, which would not have been identified by routine methods for fine root quantification: We found higher root length density (RLD) on the P-poor than on the P-rich site and a higher portion of mycorrhizal root surface
The results of our quantitative micro morphological cross sections analyzes of undisturbed soil samples, are in agreement with the idea that we could show that beech forest nutrition strategies are reflected in the distribution of fine roots and the associated mycorrhization of European beech forests
Summary
Roots are less accessible for researchers than above-ground plant compartments and are widely assumed to bear still a lot of unidentified possibilities of adaptation (Finér et al, 2011). At sites with low stocks in soil P, beech forests maintain their P supply by recycling of P from organic sources, while the acquisition of P from mineral resources controls P nutrition at high soil P stocks (Raghothama and Karthikeyan, 2005). Till it is unknown, if fine root spatial and size distribution as well as mycorrhization reflect nutrition strategies (Lopez-Bucio et al, 2000; Plassard and Dell, 2010; Hauenstein et al, 2018). We assume that European beech trees of the above-mentioned recycling and acquiring systems show different rooting density, depth distribution, and mycorrhization depending on P availability
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