Role of phenolic substances from decomposing forest litter in plant-soil interactions

Phenolic substances are an important constituent of forest leaf material. Within living plant tissue they occur as free compounds or glycosides in vacuoles or are esterified to cell wall components (Harborne 1980). Major classes of phenolic compounds in higher plants are summarized in Table 1. ‘Phenolics’ are chemically defined as substances that possess an aromatic ring bearing a hydroxyl substituent, including functional derivatives (esters, methyl esters, glycosides, etc.). ‘Polyphenols’ are those phenols with two or more hydroxyl-groups. The term ‘polymeric phenol’ will be used for compounds composed of two or more rings (e.g. condensed tannins, flavenoids). Phenolics in plant litter are to a small extent water soluble, are released by rain water and can be detected in small amounts in throughfall and stem flow (Bruckert et al. 1971). However, their main origin is their release from decomposing leaf litter deposited on the soil surface and from decomposing fine roots. Phenolics act as intermediates in the formation of humic compounds in the soil environment (Stevenson 1982). By condensation and polymerization reactions mediated by soil microbes (Bailly & Raboanary 1986), simple low molecular weight phenolic compounds together with amino acids and proteins are chemically altered into the more stable higher molecular weight fulvic acids, humic acids and finally into humins (Martin & Haider 1980). After reaching the soil, the effects of phenolics on the functioning of forest ecosystems are many and drastic. Soil phenolic substances directly affect bacteria, the development of mycelia and spore germination of saprotrophic and mycorrhizal fungi, and the germination and growth of higher plants (Hartley & Whitehead 1985). The process of humification is also largely directed by the polyphenol content of litter (Duchaufour 1983). Thus, site quality characteristics such as mineralization rate, nitrification rate and soil productivity are highly influenced by phenolics, released from living, senescent and decomposing plant tissue.