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Internal and External Proton Load to Forest Soils in Northern Germany

Inst. of Soil Science and Forest Nutrition, Univ. of Göttingen, Büsgenweg 2, D-3400 Göttingen, Fed. Rep. Germany;

E. Matzner

Res. Centre Forest Ecosystems, Univ. of Göttingen, Büsgenweg 2, D-3400 Göttingen, Fed. Rep. Germany;

B. Ulrich

Inst. of Soil Science and Forest Nutrition, Univ. of Göttingen, Büsgenweg 2, D-3400 Göttingen, Fed. Rep. Germany.

^* Corresponding author.

ABSTRACT

Both external and internal proton sources contribute to total H⁺ load of the soil in terrestrial ecosystems. The most important internal net proton production (INP) processes are (a) accumulation of a surplus of inorganic cations over anions in organic matter, (b) net production and protolysis of organic acids, (c) dissociation of carbonic acid from root and decomposer respiration, (d) net nitrification of organic N, (e) uptake of a surplus of deposited NH⁺₄ over deposited NO^–₃ or nitrification of deposited NH⁺₄, (f) buffering of deposited H⁺ in the canopy and recharging of foliar buffer systems with subsequent H⁺-release to the soil. Processes (a) through (d) are proton production processes due to ecosystem state and site conditions; (e) and (f) are due to atmospheric deposition and must be regarded as part of acid deposition. The sum of the rates of all processes plus free acidity (H⁺) flux in throughfall constitute total proton load (TPL) to a soil. Rates of INP and TPL were evaluated for six experimental forests in northwestern West Germany, which had been monitored in long-term studies. Internal sources are relevant contributors to total proton load. At five of the six sites, however, H⁺ load due to atmospheric deposition exceeds 70% of TPL. Total acidity budgets show that soils in the aluminum-buffer-range become acid exchangers (3H₊ Al³⁺), i.e., they almost quantitatively transfer the incoming amount of acid (TPL) to deeper layers of the seepage conductor and eventually the hydrosphere. A comparison of the evaluated TPL rates with maximum rates of buffering by silicate weathering reported in the literature indicates that all carbonate-free soils will further acidify and reach the Al buffer range.

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