Surface morphology of quartz grains from tropical soils and its significance for assessing soil weathering

The surface morphology of quartz grains can indicate the degree of weathering of soil material. We have compared two methods of assessing the relative weathering of soils on the basis of differences in the surface morphology of quartz sand grains in a catena of soils in Rwanda. One method is based on the presence or absence of surface features indicative of weathering or freshness, while the other uses the frequency and size of dissolution etch pits. A ranking of relative weathering could be obtained using the first technique for the slightly and somewhat weathered soils but not for the weathered soils. On the other hand, weathering trends and differences between the horizons studied were detected in weathered soils using the second method. The introduction of more specific definitions of the weathering classes used in the latter method leads to clear improvement of the inter-observer reproducibility of the weathering classification. The surface features on the quartz grains suggest that the soil at the summit is less weathered than the other soils of the sequence. Quartz grains from the well-drained soils on the slopes, which are subjected to more intense leaching and thus to stronger chemical weathering, have more triangular etch pits and chatter marks. In the imperfectly drained soils in the valley bottom quartz grains are less etched because dissolution is inhibited by the oversaturation in silica of the drainage waters.     

Comparison of Nitrate Leaching under Silver Birch (Betula Pendula) and Corsican Pine (Pinus Nigra SSP. Laricio) in Flanders (Belgium)

In a forest in Flanders (Belgium), situated in a region of intensive livestock production, comparable stands of Corsican pine and silver birch were studied for (1) NH₄ ⁺ and NO₃ ^- concentrations in throughfall water and soil solution and (2) depositions and leaching of NH₄ ⁺ and NO₃ ^- to groundwater. In each stand, throughfall collectors and porous cup lysimeters at three depths (0.1m, 0.5m and 1m) were installed in three replicated sets. Throughfall concentrations of ammonium and nitrate were significantly different for both species as well as soil solution concentrations of nitrate at all depths. Under pine, nitrate concentrations of the soil solution at 1m depth clearly exceeded the Belgian critical level for drinking water (50 mg.1^-1). Under birch, this level was only sporadically exceeded. During the sampling period, the depositions of NH₄ ⁺-N and NO₃ ^--N reached respectively 21.6 kg/ha and 6.3 kg/ha under birch and 81.3 and 15.2 kg/ha under Corsican pine. Nitrate-N leaching under silver birch amounted 25.4 kg/ha whereas 56.4 kg/ha was measured under Corsican pine.