The Nitrogen System

Abstract

Although both nitrogen and sulfur are components of protein, and, thus, are essential elements for plant nutrition, there are some fundamental differences in the cycles of these two elements in terrestrial ecosystems that must be considered in our analysis of soil acidification. In contrast to the sulfur system in which SO ²⁻₄ adsorption can be an important factor in the accumulation of sulfur in the ecosystem, there is little tendency for inorganic nitrogen to accumulate in most forest soils. The most oxidized form of nitrogen, the nitrate (NO ⁻₃ ) anion, is only weakly adsorbed on soil surfaces and is, thus, readily leached. The ammonium ion (NH ⁻₄ ) can be retained on cation exchange sites. However, the quantities of exchangeable NH ⁺₄ on exchange sites in forest soils is usually small because (1) high biological demand by plants and microorganisms in nitrogen-limited ecosystems (which are very common in forests of North America and Scandinavia) causes NH ⁺₄ to be rapidly taken up and (2) any NH ⁺₄ that is not incorporated into biological tissue is likely to be oxidized by microorganisms to nitrate. Although this oxidation is commonly thought to be inhibited under acid conditions, recent reports of high nitrification rates in very acid forest soils under heavy loadings of acid deposition (van Breemen et al. 1982; van Breemen and Jordens 1983) testify to the capacity of the system to oxidize NH ⁺₄ . Because large pools of inorganic nitrogen are seldom, if ever, found in forest ecosystems, we can safely generalize that, for nitrogen, biological rather than chemical mechanisms account for ecosystem accumulation.