ABSTRACT.RTF
- 1. ABSTRACT It is widely accepted that afforestation adversely affects soil chemical properties. A significant decrease in pH, base cation status and acid neutralising capacity (ANC), coupled with increasing acidity have been observed to occur in acid coniferous forest soils both in the Northern and Southern Hemispheres. International and local studies in Southern Africa have observed declining productivity over successive rotations in pine plantations. Increasing acidity loads in the Northern Hemisphere and their adverse effects on forest health have generated a search for chemical indices that would assist in pursuing thresholds beyond which forest vitality is damaged. A common and widely used chemical index is the Ca/Al ratio. The most common parameter adopted is the Ca/Al molar ratio in soil solutions. Recent studies in Southern Africa (Swaziland, Kwazulu- Natal) and in the Sabie area (Eastern Escarpment) have indicated that some forest compartments (Pinus sp.) may experience Ca/Al molar ratios dropping below unity. The value of unity is widely used and proposed as a threshold beyond which tree vitality may be adversely affected. This study was conducted to determine the state of surface (0-20cm) soils underlying pine plantations of different ages in the Sabie area on the Eastern Escarpment of Southern Africa. The pine plantations were selected in areas overlying a uniform bedrock (shale), high altitude, high rainfall, north facing aspect and confined to 3 pine species (P. elliottii, P. patula and P. taeda). A composite sample was taken from each of 22 forest compartments varying in age (between 22 and 67 years) and rotation (first, second and third). In addition, the results of previous investigations of forest soils in the Eastern Escarpment region were considered in order to expand the array of forest compartment ages and the size of the overall data set used for statistical analysis, so that comparisons could be made. Prior to analysis the soils were air dried and sieved (2mm). Characterising the soil 1
- 2. and investigation of the acidity status was conducted by analysis of pH (in water and KCl), organic carbon, acid neutralising capacity and exchangeable acidity and basic cations. All the pine forest surface soils are highly acidic. The acidification of natural soils under grassland is enhanced by afforestation and increases with increasing period of afforestation, predominantly within the first two decades. The pH levels (3.7-4.1 in KCl) dropped markedly in surface soils in the first two decades following afforestation. KCl-extractable acidity was observed to decline in a similar way for the first two decades. There is some indication of further acidification with plantation age beyond two decades, although the effect is not significant. Base saturation levels do not exceed 25%, and exhibit a general drop in respect to afforestation of grassland soils. The depletion of basic cations (BC) from the exchange sites is evident as shown by a decline in the BC/acidity ratio, to values below unity, predominantly in the first decade following afforestation. High levels of organic carbon were detected in the surface soils of the pine plantations studied (3.7-10%). The presence of organic matter is important in the context of Al complexation, which was confirmed by a significant correlation (r2 =0.59) between CuCl2-extractable Al and organic carbon. Soil solution composition was estimated based on the analysis of saturated paste extracts. The analysis included determination of pH (in the paste and in the extract), electrical conductivity (EC), dissolved organic carbon (DOC), monomeric Al, monomeric silica and major cations and anions. In addition, geochemical modelling (MINTEQA2) was used to calculate ion activities and saturation indices. Similarly to the solid phase, the soil solution was highly acidic with pH (saturated paste) levels ranging from 4.36 to 4.91. An important observation, and of utmost importance for future studies, was that of declining ionic strength of the soil solution with increasing plantation age (r2 =0.59). Additional interesting observations were made of unexpectedly high levels of fluoride in solution (1.9-12 mg/l), which altered the detection of Al3+ and Al(OH)X 3-X 2
- 3. concentrations in solution, and the calculation of alumino-silicate mineral solubility equilibria. The colorimetric method used for the detection of monomeric Al in solution probably excludes, to a large extent, F-complexed Al thus, posing great difficulty in the calculation of F-free monomeric Al activity in solution. Consequently, a more realistic approach was considered. In the process of calculating monomeric Al activities in solution (using MINTEQA2), the fluoride concentrations were thus, excluded. The inclusion of F in geochemical speciation calculations has indicated that the majority of Al species in solution are complexed to F and in addition, produced inconsistencies with respect to saturation indices of gibbsite, kaolinite and jurbanite. Evaluating the dissolution of Al in the absence of F offers the possibility that the dissolution of Al is controlled by Al-OM complexes (under pH 4.7) and by gibbsite (above pH 4.7). The Ca/Al molar concentration ratio in solution was observed to drop in the soils following afforestation, and to reach, in some cases, levels below unity thus suggesting, that at some future time, forest health may be adversely affected. Ca/Al activity ratios calculated in the presence of F, were more than five orders of magnitude higher than activity ratio calculated in the absence of F. High concentrations of NH4 + were detected in solution (4.8-36.5 mg/l) while other soluble forms of nitrogen were detected in significantly lower concentrations (NO2 - 4 mg/l; NO3 - - mostly undetected). The Abundance of NH4 + -N coupled with smaller concentrations of NO2 may be the cause of the method used for the preparation of the saturated soil pastes and consequently interruption of biochemical processes such as nitrification taking place in the soil environment. Alternetively, the reason may be the loss of nitrogen from the F and H horizons (forest floor) to the upper mineral horizon in the form of NH4 + -N, whereas the loss of nitrogen from the A horizon is mainly in the form of NO3-N. High levels of dissolved organic carbon (DOC) were detected (53-193mg/l) and conform to the abundance of organic matter. A considerable anion deficit with respect to the charge balance of major ions in solution was assumed to be accounted for by negatively charged dissolved organic matter. 3
- 4. Comparing the Ca/Al ratio of the soil solution to the basic cations/acidity ratio of the exchange sites reveals similar trends (r2 =0.37 and r2 =0.39, respectively) with plantation age. As with acidity and pH, the Ca/Al ratio drops markedly in the first two decades following afforestation. Thus, strengthening the argument that the most prominent chemical changes in surface soils of these pine plantations occur within the first 20 to 30 years after the grasslands soils have been planted to trees. 4