Fertility and pH effects on polyphenol and condensed tannin concentrations in foliage and roots

Plant secondary compounds such as polyphenols and tannins are purported to influence nutrient cycling by affecting organic matter degradation, mineralization rates, N availability and humus formation. In turn, site conditions have been shown to affect plant phenolic concentrations. In a greenhouse sand culture system we investigated the influence of fertility and pH on foliar and fine root polyphenol and tannin concentrations of two pines and three ericaceous species that grow in the pygmy forest of coastal California. Higher fertility levels increased plant growth up to 10-fold while N concentrations increased 1.2-2.4 fold in foliage and roots. Concentrations of foliar total phenols (TP) and condensed tannins (CT) ranged between 130-300 mg g(-1) and 100-320 mg g(-1) on a dry weight basis, respectively. Root TP and CT concentrations were lower than corresponding foliar levels, ranging between 60-150 mg g(-1) for TP and 80-190 mg g(-1) for total CT. As fertilizer levels decreased, concentrations of foliar TP and CT increased 1.2-2.0 fold. Effects of fertility on root TP and CT concentrations were not always significant and not as great as those seen in the foliage (0-33% increase under low versus high fertility). Effects of pH level on plant growth and chemical composition were less consistent, and often confounded by an interaction with fertility. Foliar C:N, TP:N and CT:N ratios, which may be predictors of litter quality, were significantly higher under low fertility than high fertility. These ratios were generally higher at pH 6.5 than pH 3.5. Similar trends were obtained from a greenhouse soil study which included lime and fertilizer treatments. Our results indicate that changes in soil nutrient supply have a greater influence on secondary compound production than do changes in solution pH.