Jacques Herbauts

Influence of logging traffic on the hydromorphic degradation of acid forest soils developed on loessic loam in middle Belgium

Modifications of soil physical parameters (bulk density, pore space pattern and clay dispersibility) induced by compaction in wheel-rutted areas due to mechanized forest exploitation was studied in loamy soils developed under beech stands in the loessic belt of middle Belgium. The consequences of surface waterlogging on pedological processes (especially the geochemical behaviour of iron and aluminium) were also investigated. The results show that rutted areas have: (1) a significant decrease of total porosity in the eluvial upper layers of the soil (0-30 cm depth), associated with an important lowering of the transmission pore volume and a decrease in macroporosity to about 6% of the total soil volume, i.e. to less than the current threshold value for root viability (10%); (2) a reduction of macropore space to about 9.5% in the lower illuvial horizons (30-50 cm depth); (3) a decrease of redox potential due to temporary waterlogging, inducing (a) the occurrence of reduced forms of iron, (b) a strong leaching of iron oxyhydroxides from the E(g) to the B(1g) horizon, (c) an increase of organically complexed iron forms in both the Es and B(tg) horizons, and (d) high solubility of Goethite and even of ferromagnesium minerals in the eluvial horizons; (4) an increase of clay dispersibility in both the E(g) and B(tg) horizons. These results provide clear evidence that on loessic materials soil compaction due to logging operations leads to rapid soil degradation through active hydromorphic processes.

Ring width and element concentrations in beech (Fagus sylvatica L.) from a periurban forest in central Belgium

The Foret de Soignes is a beech high forest located near Brussels (Belgium), established on a strongly acidic soil and subjected to atmospheric pollution and recreational pressure. We investigated variations in ring width and mineral element concentrations (N, P, K, Ca, Mg, Mn, Al) over the last 95 years in five 135-year-old trees, variations in ring width in four 40-year-old trees and tested associations with climatic parameters. Growth curves showed a striking increase in mean sensitivity in the last 20 years in the old trees but not in the young ones, starting with the 1976 summer drought. Mean sensitivity is a statistical measure of the mean relative variability between adjacent ring widths within a tree, which is correlated to susceptibility to climatic stress (Fritts, H.C., 1976. Tree Rings and Climate, Academic Press, London, 567pp). May rainfall and, to a lesser extent, soil water recharge (i.e. pooled rainfall from October to May) correlated significantly with ring width. However, two of the five growth depressions in the last 20 years could not readily be accounted for by adverse climatic conditions. Element concentration profiles were suggestive of a decrease in the availability of Mn, Mg and Ca, but no trend of increasing N was obvious. The possible causes of the recent increase in sensitivity of old beech trees are discussed in terms of interactions between ageing and extreme climatic events, aggravated by soil compaction due to logging traffic.

Evaluation of the mobility and discrimination of Ca, Sr and Ba in forest ecosystems: consequence on the use of alkaline-earth element ratios as tracers of Ca

A comprehensive understanding of Ca cycling in an ecosystem is desirable because of the role of this element in tree mineral nutrition and its status as a major base cation on the soil exchange complex. The determination of the origin of Ca in forests is particularly indicated in regard of important changes linked to acid inputs and intensive logging. Natural strontium isotopes are increasingly used as tracers of Ca in forest ecosystems for qualitative and quantitative assessments. Nevertheless this method is limited to relatively simple systems with two sources of nutrients. Some recent studies coupled Sr/Ca or Sr/Ba ratios to Sr isotopic measurements in order to solve more complex systems. Such method has however associated with it some uncertainties: this approach assumed that Ca, Sr and Ba behave similarly throughout the ecosystem and does not take into account the Ca biopurification processes occurring in some tree’s organs which can alter element ratio. The present work focuses on two deciduous species covering large areas in Europe: European beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.). In order to test the similarity of behaviour between Ca, Sr and Ba, their concentrations were measured extensively in the major compartments of two forest ecosystems. In parallel, the discrimination process inside tree organs was studied in 23 stands for beech and 10 stands for oak. We found that Sr and Ca behave similarly in all soil and tree compartments. By contrast, Ba and Ca appear to have contrasting behaviours, especially in streams, soil solution and soil exchange complex (no correlations between element concentrations). Sr/Ba and Ba/Ca ratios must therefore be used with care as tracer of Ca. The Ca biopurification is absent in roots and slight in bole wood but is large in bark, twigs and leaves. The discrimination factors (DF) between wood and leaves are characteristic of the two species studied and do not change significantly as a function of the soil Ca status (acidic or calcareous soils). Therefore, strontium–calcium DF can be used as a correction factor of the Sr/Ca ratio of leaves when this ratio is used in connection with Sr isotopic ratios. This correction allows to solve systems of tree nutrition with more than two sources of Ca.

The relation between spruce monoculture and incipient podzolisation in ochreous brown earths of the Belgian Ardennes

SummaryThe effects of coniferous monoculture on the distribution of C, Fe and Al in the upper horizons of brown ochreous earths of the Belgian Ardennes were investigated by comparing 5 soil profiles developed under an 80 year old spruce stand with another 5 profiles developed under a climactic broad-leaved forest (beechwood).Organic carbon, Fe and Al were extracted with 0.1N NaOH/Na-tetraborate solution buffered at pH 9.7: recent studies have shown that this extraction is particularly appropriate for the detection of incipient podzolisation in brown earths-brown podzolic soils intergrades.Even if most of the classic podzolisation ‘indexes’ fail to illustrate differences, nevertheless our results show that fulvic acids and organo-ferric complexes are present in significantly greater amounts in the upper part of the cambic (B)1 horizon of the soils developed under conifers. Moreover, this podzolic tendancy is confirmed by the weathering patterns of the clay minerals in the A1(B) horizons developed under spruces, i.e. a more pronouced weathering of chloritic layers than those observed in the beechwood soil, with a correlative genesis of more abundant smectite-like minerals. One may therefore conclude that the change in the humus type (moder to mor) after the planting of spruce trees, has been sufficient, within the local climatic and edaphic context, to promote incipient podzolisation.

Changes in humus microbiological activity induced by the substitution of the natural beech forest by Norway spruce in the Belgian Ardennes

Abstract Different microbiological methods, including determination of enzymatic activity and potential nitrogen availability, and microbial biomass estimation were used to evaluate soil modifications induced by the massive planting of Norway spruce on acid soils of the climatic deciduous forest of high Belgium. Investigations were made on the surface humic layers (holorganic and hemiorganic horizons) of ochreous brown earths (Dystrochrepts) in two neighbouring forest stands, a natural beech high-forest (Fagus sylvatica L.) and a 92-year-old spruce plantation (Picea abies (L.) Karst.). Results show that spruce monoculture has a significant depressive effect on soil microbiological activity, clearly shown by the lowering of microbial biomass and the decrease of both FDA hydrolytic activity and nitrogen potential availability.

Change of the Origin of Calcium in Forest Ecosystems in the Twentieth Century Highlighted by Natural Sr Isotopes

Publisher Summary Calcium (Ca) is essential in forest ecosystems, both as nutrient for trees and for its role in the neutralization of acid inputs. The decline in bioavailable Ca reserve in the soil, caused by acid deposition produced by industrial activity, was reported for a lot of forest sites in Europe and North America. The understanding of the causal mechanisms requires a precise knowledge of the supplying sources of Ca. The measurements of natural Sr isotopes can be used to determine accurately the origin of Ca in the vegetation. By means of this method, this chapter highlights the extreme dependence of some forest ecosystems to the Ca input by atmospheric precipitation. The use of this isotopic tracer permits ,also to characterize the Ca exchanges between the ecosystem components. Finally, the coupling of this method with dendro-chemistry adds an important piece to the knowledge of the soil acidification story. Change of the Sr isotopic ratio in the tree rings of several Belgian sites and the comparison with similar trends in other regions suggest that the soil acidification in industrial regions started in the beginning of the twentieth century, that is, earlier than it is generally admitted.

The Effect of Seasonal Variations, Covariations with Minerals and Forage Value on Itchgrass’ Foliar Silicification from Sudanian Benin

Silica (SiO 2) in forage grasses has been found in reducing cell-wall digestibility. This study investigates whether: (i) the seasonal variability affects the silica and minerals accumulation and forage values of leaves of R. cochinchinensis and (ii) silica concentration is correlated with minerals and fodder value. In an itchgrass population selected in the W Biosphere Reserve, leaves were collected on 90 marked plants from May to October 2003 and 2004, at 15 days intervals except May, June and October. Some 300 g of fresh blades from the 3 rd most recently expanded leaves were oven dried and analyzed for dry mass, SiO 2, ash, N, Na, Ca, P, K, and Mg. Digestible Nitrogen Matter (DNM) and Fodder Energetic Value (FEV) were calculated using the Demarquilly formula. Apart from SiO 2, ash and forage value, data were log-transformed to restore homoscedasticity before statistical analyses. SiO 2 ranges from 5.69 % to 9.95 %, i.e. varying 1.4 fold between May and October, reaching 1.75 fold at mid-September. SiO 2 is positively related to Ca but negatively to K, P, N, DNM and FEV. The negative correlations suggest that SiO 2 concentration in R. cochinchinensis could be reduced with a significant increase in energy and accumulation of important nutrients such as N, P and K. Therefore, leaf silicification and nutritive value relationship should be conclusive in the case of itchgrass.