The spatial distribution of soil hyphae in structured spruce-forest soils

The external mycelium forms the major part of the absorbing surface of mycorrhized tree roots. Because the macro pore space of acid forest soils is selectively depleted of mobile nutrient cations, it is ecologically important, whether soil hyphae grow into the soil aggregates or not. Seedlings of Norway Spruce (Picea abies (L.) Karst.) with defined mycorrhiza were grown in unsterilized soil cores taken from the A and B-horizon of a limed and an unlimed cambisol on triassic sandstone in the Northern Black Forest, Germany. Water-tension treatments were 10, 30, 160 and 900 hPa. On ground and polished vertical cuts stained with acridine orange, we identified and measured the location of hyphae and characterized their micropedological environment using an image analyzing system. Mean length density varied between 17 m/cm³ and 100 m/cm³ and was independent of aeration parameters. The percentage of hyphae completely embedded in the soil matrix varied between 30% and 8% and decreased significantly with increasing CO₂ concentration in the soil air. Of the hyphae in the soil matrix, 70% were located in a 50 m shell around the macro pores. Pair correlation functions show, that the majority of soil hyphae occur in clusters with diameters below 100 m. Between 60% and 80% of randomly chosen circles with 250 m diameters were completely devoid of hyphae. The inefficient opening up of the intra-aggregate space by soil hyphae is explained by the very slow oxygen diffusion between air-filled macro pores and the intra-aggregate space and mechanical restrictions for hyphae growth.     

Changes in soil structure and aeration due to liming and acid irrigation

Biologic activity is one of the main factors controlling the floating equilibrium between loosening and compacting forces in humic forest soils. Therefore it can be expected that both acid deposition and compensatory liming indirectly influence the soil structure and soil aeration. To measure macro-pore structure, CO₂ concentrations in the soil air, and respiration rates we used naturally structured soil cores from the Höglwald forest-ecosystem research plots with experimental acid deposition and liming, but standardized water supply. Results are integrated in an one-dimensional soil-aeration model. Compared to the control plot, in the top soil of the limed plot both gaseous diffusion coefficients and respiration rates are increased. Since the CO₂ concentration in the soil air is decreased at the same time, the soil aeration status of the whole profile can be regarded as stabilized. On the acid irrigated plot, gas-diffusion coefficients are not significantly changed with regard to the control. In the top-mineral soil, in contrast to the working hypothesis, they are tendencially increased. In the case of liming, a stimulation of biologic activity and a positive feedback on the soil structure could be attributed to an increased earth-worm activity due in turn to decreased acidity and enhanced feeding conditions. The acid irrigation leads to increased Al³⁺ activity, which can stabilize the soil structure.