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Sphagnum cell walls exhibit a high cation exchange capacity whereby nutrient cations (e.g., NH4+, Ca2+, Mg2+, K+) are taken up from the environment in exchange for protons. This acidifying mechanism causes cell walls to express a pH as low as 3.3, a condition that regenerates and remains active in live plants. Cell wall bound polyuronic acids rich in galacturonic residues, which can constitute between 10–30% of plant dry weight in Sphagnum, appear responsible for this behavior. This exchange mechanism allows Sphagnum to succeed under conditions of low nutrient availability and also acidifies the environment, inhibiting the establishment or growth of many vascular and nonvascular plant competitors and influencing succession in fen and bog ecosystems. During succession in Sphagnum-dominated ecosystems, pH can drop from >7 to <4. However, this is caused only in part by the cell wall cation exchanger. Sphagnum also produces organic acids, which form the primary source of acidifying protons in bogs.
