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Spatial Variability of Canopy Throughfall and Groundwater Sulfate Concentrations under a Pine Stand

Inst. for Soil Science, Univ. of Hannover, Herrenhäuser Straβe 2, D-30419 Hannover, Germany;
Bergenerstraβe 18c, D-30625 Hannover, Germany.

^* Corresponding author.

ABSTRACT

Calculation of solute flux into groundwater by recharge must account for the spatial variability of water flux and recharge solute concentration. Solute concentration of the recharge can be characterized by sampling the uppermost groundwater. In an earlier study under pine (Pinus silvestris L.) on sandy soil, solute concentrations at the water table along a transect fluctuated in a recurrent pattern, probably caused by heterogenous solute input via canopy throughfall. To obtain more insight into the spatial variability of deposition processes and solute concentrations in recharge, transect sampling of the uppermost groundwater was repeated in 1989, and again in 1993. Solute deposition by throughfall was also measured in 1993. The data were analyzed by spectral and time series methods. Sulfate (SO₄) concentrations in canopy throughfall and uppermost groundwater showed recurrent fluctuations corresponding to canopy coverage of the ground. Sulfate maxima coincided with canopy edges. The dominant fluctuation frequency of SO₄ concentrations in 1993 was lower than in 1989 because of canopy enlargement by pine tree growth. Detailed analysis of the 1993 SO₄ concentrations revealed superimposed fluctuations with a frequency valid for the 1989 canopy edge distribution. This can be attributed to an actual patterned SO₄ retrieval from solid aluminum-hydroxo-sulfate (Al-OH-SO₄) phases, stored in the subsoil under previous higher SO₄ deposition rates.