Cation Turnover

The importance of cation (K, Mg, Ca) supply in ecosystems was apparent already in the N cycle. Cation supply occurs primarily from the chemical weathering of primary minerals (Chap. 2.3.1) or dust particles entering the ecosystem from the atmosphere. Examples of input of dust are the formation of loess in the post-glacial period (Schachtschabel et al. 1998), the supply of dust from the Sahara to the Amazon delta (Lloyd et al. 2001), and buffering of sulfur-containing emissions by industrial dusts in the 1960s which delayed acidification of soils, as the ionic charge of deposited material was neutral.

Cations are taken up by the roots. The return of cations into the soil is not only via litter, but also through leaching from the canopy. Canopy leaching is a consequence of ammonium uptake from air pollution and a consequence of buffering during the input of protons (leaching).

In soils, accumulation of cations in the organic deposits (see Fig. 3.2.4) and a dense root network in the organic layer enable direct re-supply of cations from litter into plants, particularly in nutrient-limited conditions. Cations are also released from the organic matrix and transported together with organic acids (dissolved organic carbon, DOC; see Chap. 2.3). Cation uptake by roots and cation losses caused by leaching of DOC decrease the concentration of cations in the soil solution of the upper layers and bleached horizons are formed, especially in nutrient-poor soils (eluvial E horizons) if the losses exceed weathering. Alkaline saturation increases only in deeper soil layers (B and C horizons; see also Chap. 3.2.2.3).

The turnover, i.e. the cycle from the uptake to the release, is very different for individual elements. In a spruce stand on granite (Horn et al. 1989) the calcium turnover is twice that of potassium, and exceeds that of magnesium fivefold. As leaching into the groundwater occurs for all elements at about the same magnitude, different amounts of Ca, K and Mg must be supplied by weathering of primary minerals in the soil profile (Table 3.3.5). Since weathering of granite is slow, this leads to decreased soil pH and to long-term forest damage particularly on acidic rocks (see Chap. 3.5.1).

Leaching of cations from an ecosystem into the groundwater and lateral transport into other ecosystems may have far-reaching consequences for nitrogen cycles of "supplier" and "receiver" systems. Two interactions are possible occur (Fig. 3.3.14): (1) Nitrate (or sulfate) is leached and carries cations down the slope leading to an increased total turnover; this is the "classic" assumption. (2) DOC is leached and cations accumulate in the groundwater. As DOC is microbio-logically mineralised during transport there is secondary cation accumulation down the slope. Thus the conditions for nitrification improve along the transport path. Local nitrification and absence of nitrate leaching improve biomass production and thus the material cycles. The transport of DOC appears to dominate in the boreal climate (Hogberg 2001) on acid substrates (granite), effecting relocation of cations, e.g. on a slope (Guggenberger and Zech 1993; Kaiser and Guggenberger 2000).

Table 3.3.5. Cation flows in a spruce forest (mmol rn 2 a

Calcium

Potassium

Magnesium

Soil balance Atmospheric input Leaching from canopy Weathering Release from litter Leaching into groundwater Available in soil

Plant balance Plant uptake Incorporation into wood Leaching from canopy Litter

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