Phenolic compounds comprise a large class of plant secondary metabolites, ranging in complexity from simple organic acids, through complex polyphenolics such as tannins. They exert direct and indirect effects on animals, plants, and soil processes. While the potential effects of tannins on livestock have been documented, less is known about how phenolic compounds, entering soil through feces or crop residues, will affect formation and retention of organic matter or nutrient cycling. The objectives of this study were to quantify potential impacts of different kinds of phenolic compounds on the solubility and mobilization of major plant nutrients in order to evaluate their importance in soil management. We hypothesized that solubility of N would be decreased while that of important nutrients such as P, K, and Ca would be increased. We treated samples of North Dakota soil collected from pastures and cultivated land with solutions of increasing complexity including benzoic acid (BA), gallic acid (GA), or b-1,2,3,4,6-pentagalloyl-O-D-glucose (PGG) at four concentrations (1.25, 2.5, 5 or 10 mg compound gram -1 soil). We measured extractable nitrogen (N), phosphorus (P), potassium (K), calcium (Ca),magnesium (Mg) and manganese (Mn) in treatment supernatants and after a subsequent incubation in hot water (16 h, 80 C).
Results/Conclusions
Each treatment reduced the total amount of N extracted from soil in a compound by concentration dependent manner with BA exerting the largest effect (about 25%). Although PGG had little effect or even increased soluble N in treatment supernatants, all concentrations of PGG reduced the amount of N extracted with hot water. As a net effect, PGG reduced total soluble N. Consistent with earlier work, this study suggests both tannins and some low molecular weight organic acids can result in physico-chemical fixation of soluble organic-N in soil that might otherwise leach. Unlike N, GA and PGG increased extraction of P, relative to water, while BA had less effect. Extraction of the major cations, K, Ca, and Mg, was strongly increased by BA and GA but unaffected by PGG. Extraction of Mn was increased most by treatments of GA but less so by BA. The effects of PGG on Mn were consistent with GA when expressed on a molar basis. Whether they operate mainly at the root-soil interface of the individual plant or affect soil quality at the field scale as crop residues or manure, more research is needed to determine just how plant secondary compounds can be fruitfully harnessed in agro-ecosystems.