Phenol oxidases are a group of extracellular enzymes that catalyze the oxidation of soil organic matter. In particular, phenol oxidases are involved in litter decomposition and nutrient mineralization. Despite their ubiquity, little is known about these enzymes in tropical soils, or the abiotic factors that moderate their activity. We assayed phenol oxidase activity in surface and subsurface soils from 13 profile pits, representing a steep gradient in manganese (Mn) oxide concentrations. Fresh and sterilized (autoclaved) soils were assayed to determine the abiotic contribution to activity. Assays were conducted in bicarbonate buffer at pH 8.5, using 5 mM L-DOPA substrate, with absorbance determined after 1 and 4 hours. We also determined exchangeable, amorphous, and crystalline metal oxide concentrations, soil pH, organic matter, and effective cation exchange capacity.
Results/Conclusions
Our results demonstrate that phenol oxidase activity is mediated by mineral soil components. We found only minor differences in activity between fresh and sterile soils, indicating a considerable abiotic contribution to substrate oxidation. Similarly, apparent phenol oxidase activity in sterile subsurface soil was correlated linearly with Mn oxide concentration, further supporting our hypothesis of abiotic influence to phenol oxidase activity. In some surface soils, activity increased after sterilization, potentially due to a release of manganese oxide from complexation with organic matter. Our results have important implications for understanding carbon and nutrient dynamics in tropical soils. At the ecosystem scale, phenol oxidases contribute to the transformation of soil organic matter, making them important drivers of decomposition and turnover. Understanding the abiotic factors that regulate phenol oxidase activity is thus a crucial step towards developing a predictive framework of nutrient dynamics in tropical soils.