Phenol oxidases are a group of extracellular enzymes that catalyze the oxidation of lignin and phenolic compounds in soil organic matter. Despite their importance, little is known about these enzymes in tropical soils, particularly the extent to which minerals like manganese (Mn) oxides contribute to the measured activity. We assayed phenol oxidase activity in surface and subsurface (A and B) horizons from 15 soil profiles representing a steep Mn gradient under a lowland tropical forest in Panama. Fresh and sterilized (autoclaved) soils were assayed to determine the abiotic contribution to activity. Assays were conducted using 5 mM l-DOPA substrate in bicarbonate buffer at pH 8.5. We also determined amorphous metal oxide concentrations, soil pH, organic matter, and effective cation exchange capacity (CEC). We found little difference in phenol oxidase activity between fresh and autoclaved soils, indicating a considerable abiotic contribution to substrate oxidation. Furthermore, phenol oxidase activity was similar in surface and subsurface soils, despite markedly lower organic matter at depth. Phenol oxidase activity was correlated positively with amorphous Mn oxide concentrations (extractable Mn determined by acidic ammonium oxalate extraction), suggesting a potential mechanism of abiotic oxidation. These results have important implications for understanding carbon dynamics in tropical soils, because they suggest that (1) a portion of organic matter turnover is mediated by abiotic oxidation via Mn oxides, and (2) conventional measures of phenol oxidase activity reflect abiotic oxidation rather than the activity of enzymes synthesized by the microbial community.

苯酚氧化酶是一组胞外酶，它们催化土壤有机质中木质素和酚类化合物的氧化。尽管它们很重要，但对于热带土壤中的这些酶知之甚少，尤其是矿物质（如锰（Mn）氧化物）对所测活性的贡献程度。我们在巴拿马低地热带森林下，从15个土壤剖面（代表陡峭的Mn梯度）分析了表层和地下（A和B）层中的苯酚氧化酶活性。分析新鲜和无菌（高压灭菌）的土壤，以确定非生物活性的贡献。使用5 mM l进行测定-在pH 8.5的碳酸氢盐缓冲液中的DOPA底物。我们还确定了非晶态金属氧化物的浓度，土壤的pH值，有机质和有效的阳离子交换容量（CEC）。我们发现新鲜土壤和高压灭菌土壤之间的酚氧化酶活性差异不大，表明非生物性对底物氧化的贡献很大。此外，尽管深层有机物含量明显降低，但表层和地下土壤中的酚氧化酶活性相似。苯酚氧化酶活性与无定形Mn氧化物浓度（通过酸性草酸铵萃取确定的可萃取Mn）呈正相关，表明非生物氧化的潜在机制。这些结果对于理解热带土壤中的碳动力学具有重要意义，