G. Falsone

Soil properties under Norway spruce differ in spruce dominated and mixed broadleaf forests of the Southern Taiga

In natural forest, disturbance changes tree species composition which in turn affects soil properties. Two areas in the Central Forest State Biosphere Reserve, in the Russian Southern Taiga Zone, differed in the intensity of disturbance: Norway spruce was the dominant species at one site, while at the other spruce was mixed with broadleaves. The presence of broadleaves was due to large gaps in the canopy having been formed, which have triggered vegetation succession. At both sites, five plots were selected to evaluate how the presence of broadleaves influences the properties of the soils under spruce. Soil samples were taken close to spruce trees and the O, A and E horizons were analysed. A difference in the distribution of organic matter in the soil horizons was evident, with a higher concentration in the O and A horizons at the spruce dominated site, while a more homogeneous distribution was found under spruce at the site where broadleaves were abundant. The organic matter did not only differ in quantity, but also in quality as estimated by the C/N ratio, and therefore affected the CEC and element relative availability. No differences at the two sites were found for water-extractable and exchangeable elements, but the ratio between the exchangeable and the acid-extractable forms were different, suggesting a higher relative availability of the elements at the spruce dominated site, and thus potentially higher leaching. Both theoretical and empirical studies have suggested that podzolisation and accumulation of organic matter in the O horizon are related to stagnation of ecosystem processes and ecosystem decline. Our data suggest that the presence to windthrow sites and the inclusion of broadleaf species acts to slow or even reverse podzolisation even in spruce dominated sites.

Destabilization of aggregates in some typic fragiudalfs

Several mechanisms are responsible of the destabilization of soil aggregates in water: slaking, swelling, and dispersion of the clay and mechanical breakdown by abrasion. The aggregate resistance differs in relation to the dominant disruptive phenomenon, and by applying different methods to assess aggregate stability, a specific physical susceptibility of different horizons may be evidenced. Therefore, we evaluated the relative importance of the mechanisms of breakdown in some Typic Fragiudalfs, taking into account the specific horizon characteristics, to understand if the fragipan brittleness is related to a specific mechanism, and how the soil properties affect the resistance to fast wetting. The standard wet sieving test, which evaluates all destabilization mechanisms together, indicated a clear role of soil organic matter in protecting the aggregates, with greater losses in the deeper horizons (about 70%), but did not allow us to discriminate between fragipans and other horizons. When the losses caused by water abrasion were separated from the breakdown due to wetting phase using a kinetic approach, a high breakdown caused by fast wetting was found in deep horizons, but still no difference between fragipan and non-fragipans were visible, even though fragipan clods are known to be particularly sensitive to fast wetting. By excluding the effect of organic matter and prewetting the samples with ethanol, differences between these horizons appeared within the water saturation phase and fragipans and non-fragipans were found to be sensitive to different mechanisms. In fragipans, the relative percentage of slaking was higher, always above 30%, whereas for non-fragipans clay dispersion and swelling weighted more heavily. The complexity of the clay fraction did not allow us to relate the mineralogy to swelling or dispersion, but slaking was instead clearly related to clay arrangement and the consequent porosity characteristics (r2 = 0.62), and not to clay content only.

Aggregate Formation In Chloritic And Serpentinitic Alpine Soils

Variations in soil-water and mineralogical composition of clay particles are important factors governing the formation and stabilization of soil aggregates. We evaluated the effects of these factors on aggregate genesis in two soils with different mineralogical compositions. One developed from chlorite-rich schists and the other from serpentinite, but they have similar organic matter and Fe oxides contents. The <2-mm fractions were submitted to one cycle of wetting and drying, producing two classes of newly formed aggregates (2-5 and >5 mm in diameter). The aggregates differed in water stability, with those from serpentinitic soil being more stable. The amounts of water-dispersible clay (WDC) were lower in the more stable aggregates than in the original <2 mm fraction, and the WDC showed more positive electrophoretic mobility values, indicating its stabilization during aggregation. In the aggregates obtained from chlorite-rich soil, the WDC amounts were similar to those of the <2-mm fractions, and the electrophoretic mobility values were more negative. The aggregation and stabilization processes differed in the two soils and were affected by the clay dispersion/flocculation behavior. The variations in aggregate stabilization were therefore attributed to different clay-clay and clay-sand interactions caused by the mineralogical differences in the clay fractions involved. No differences were found when comparing the two size classes of newly formed aggregates from the same soil. This indicates that the same mechanism produced aggregates of a variety of sizes.