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Abstract "Heft 45"

Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des
Instituts für Bodenkunde und Waldernährungslehre

der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: P. Trüby

Heft 45

Friderike Oehler

Key factors determining the distribution of fungual hyphae in forest soils

Freiburg im Breisgau 2006

ISSN 0344-2691


Soils developed from periglacial cover sediments on silicic bedrock are widespread in German mountain areas. During the last decades, these soils have been submitted to intense acidification processes caused not only by natural agents, mainly carbonic acid, but also by anthropogenic acid deposition. Among the major consequences are a decrease in effective cation exchange capacity and a depletion of nutrient cations. However, the scenario of extensive forest dieback, forcasted in the 1980s, has not occurred. By contrast, many forests on acidic, nutrient-poor soils do not show symptoms of nutrient deficiency.

A possible explanation was discovered in the short-term release of nutrient cations from the weathering coarse soil (> 2 mm), which is commonly not included in analyses of soil nutrient availability. Based on observations in podzols of boreal forests, it was further hypothesised that microorganisms and fungi could act as additional agents of weathering, thereby contributing to forest nutrition in nutrient depleted soils. With the aim of testing this hypothesis, the spatial distribution of soil fungal hyphae within, and in relation to, the structured soil environment has been investigated.

The research was carried out for two skeleton-rich acidic forest sites on a bedrock of biotite rich, fine-grained paragneiss in the central Black Forest. The coarse soil uniformely contained significant quantities of available nutrient cations, but the fine soil of the sites differed much in nutritional properties. While the Haslach site presented a moderate to high base saturation and effective cation exchange capacity in depths of > 40 cm, the Conventwald site exhibited a very poor nutrient availability. The soil at the nutrient poor site was also coarser textured than at the nutrient rich site. The research was focussed upon the B/C horizon at a depth of 1.20 m, where the coarse soil content was highest and few investigations had formerly been carried out. Only a few sample areas were analysed for the A/B and B horizon of the Conventwald site.
Undisturbed soil samples were impregnated with polyester resin and prepared as polished soil sections for epifluorescence microscopy. Best differentiation of fungal hyphae cross-sections and soil structural elements was achieved using acridine orange staining. However, an automatised recognition of hyphae had to be disregarded because of insufficient optical contrast. Hyphae data were thus recorded manually. Sample areas of 1100 * 824 um2 size were established at random points on the polished sections. Within these sample areas the positions of observed fungal cross-sections were recorded by (x,y) Cartesian coordinates. The structural elements pore, matrix, fine particle (> 20 um) and coarse particle (> 2 mm) were categorised and processed using a Geographical Information System.

The identification of fungal cross-sections using a 50x/0.85 NPL Fluotar objective was subject to uncertainties, in particular with regard to small hyphae < 2 um. Hyaline hyphae cross-sections were hard to discriminate against clay or small silt particles, while small melanine hyphae cross-sections were similiar to Fe-Mn-concretions of the same size. For this reason, a classification system for the probability of correct identification was introduced. This allowed the recognition of possible errors in the hyphal data, and also the countering of uncertainties either by weighting the respective results or by analysing scenarios based on different levels of reliability. Being smaller in their mean diameter, observations of hyaline hyphae tended to be less reliable than those of melanine hyphae or sclerotia.
Three different approaches were used to describe the spatial distribution of fungal hyphae in their natural soil environment. Firstly, hyphal densities per square millimeter were described using summary statistics. Secondly, the patterns formed by the positions of hyphal cross-sections on the plane of their observation windows were analysed using spatial statistical methods. The empty space and the pair correlation function test if point patterns exhibit a regular, random or clustered distribution and estimate the intensity of clustering. Thirdly, the soil environment around hyphae was analysed at different scales by measuring the area proportions and edge lengths of the soil structural elements in the immediate, close or wide vicinity of the hyphae and in the soil in general. From this “landscape analysis”, preferences of hyphal growth were deduced for the B/C horizon of the nutrient-poor and the nutrient-rich site.

Hyphal density decreased substantially from the A/B to the B/C horizon. Mean hyphal density at the B/C horizon was higher for the nutrient poor site (4.9 hyphae/mm2) than for the nutrient rich site (3.4 hyphae/mm2). At both sites hyphal density varied greatly within a distance of a few millimeters. Most abundant were hyaline hyphae, while melanine hyphae and sclerotia showed a significant presence only in the A/B horizon and were hugely underrepresented in the B and B/C horizons. Mean diameter was largest for sclerotia and smallest for hyaline hyphae.

A slight to moderate clustering of fungal hyphae could be observed for all investigated horizons. However, clustering was much more pronounced if only hyphae of a high reliability were considered. A strong aggregation of fungal hyphae could be observed for small interpoint distances of ˜ 50 um). To a certain degree this was attributed to hyphal branching. A major influence on hyphal growth, however, was exerted by large sand grains and coarse particles, in the presence of which the intensity of hyphal clustering increased. Thus, the coarser structured B/C horizon exhibited a stronger clustering of hyphae than the upper horizons. For the same reason, hyphae seemed slightly more clustered in the B/C horizon of the nutrient-poor site than at the nutrient-rich site, although the difference was not significant.

Hyphal objects identified with moderate or low confidence were disregarded from further analysis, because they were shown to be probably not hyphae but clay minerals which were erroneously taken for hyaline hyphal cross-sections. “Landscape analysis” of the microscopic pictures demonstrated that fungal hyphae in the B/C horizon of both sites grew preferentially in macropores, often in close vicinity to the fine soil (< 20 um), a fact that is in accordance with the literature. At the nutrient-rich site both fine and coarse particles were avoided by fungal hyphae. At the nutrient-poor site, however, the fungal hyphae colonised also the margins of fine particles and they were often found at the boundaries of coarse particles. It was thus concluded that the frequency with which certain soil compartments were found in the neighbourhood of fungal hyphae could be explained mainly by soil texture, and maybe to a lesser extent by nutrient availability in the fine soil. The latter, however, could not be clearly confirmed because of the low hyphal density at a depth of 1.20 m.

No evidence of fungal tunnelling through mineral particles was observed in the present research. A superficial weathering of coarse particles by acid exudations at hyphal tips might occur in the B/C horizon of the Conventwald site, but due to the low hyphal density only a minimal contribution to forest nutrition might be achieved, even if the observed hyphae are assumed to be mycorrhizal. An increased branching of hyphae that randomly grow along nutrient rich coarse particles could not be confirmed.

A substantial bias in the results might have been induced by the scale of observation. Given the high variability of hyphal density and considering that only a diminutive area of a few millimeters could be analysed using the described methodology, important agglomerations of fungal hyphae were possibly missed. For the same reason, predictions for the whole forest cannot be made. Thus, further research will certainly enhance the presented results. More attention should be paid to the B horizon, where radical and hyphal densities are higher and coarse particles are also present. Automatising the analysis of undisturbed polished soil sections would allow the collection of data on larger scales.

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