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Microbial phenol oxidases and peroxidases mediate biogeochemical processes in soils, including microbial acquisition of carbon and nitrogen, lignin degradation, carbon mineralization and sequestration, and dissolved organic carbon export. Measuring oxidative enzyme activities in soils is more problematic than assaying hydrolytic enzyme activities because of the non-specific, free radical nature of the reactions and complex interactions between enzymes, assay substrates, and the soil matrix. We compared three substrates commonly used to assay phenol oxidase and peroxidase in soil: pyrogallol (PYGL, 1,2,3-trihydroxybenzene), l-DOPA (l-3,4-dihydroxyphenylalanine), and ABTS (2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid). We measured substrate oxidation in three soils across a pH gradient from 3.0 to 10.0 to determine the pH optimum for each substrate. In addition, we compared activities across 17 soils using the three substrates. In general, activities on the substrates followed the trend PYGL>. l-DOPA>ABTS and were inversely related to substrate redox potential. PYGL and ABTS were not suitable substrates at pH>5, and ABTS oxidation often declined with addition of peroxide to the assay. Absolute and relative oxidation rates varied widely among substrates in relation to soil type and assay pH. We also tested whether autoclaved or combusted soils could be used as negative controls for the influence of abiotic factors (e.g., soil mineralogy) on oxidative activity. However, neither autoclaving nor combustion produced reliable negative controls because substrate oxidation still occurred; in some cases, these treatments enhanced substrate oxidation rates. For broad scale studies, we recommend that investigators use all three substrates to assess soil oxidation potentials. For focused studies, we recommend evaluating substrates before choosing a single option, and we recommend assays at both the soil pH and a reference pH (e.g., pH 5.0) to determine the effect of assay pH on oxidase activity. These recommendations should contribute to greater comparability of oxidase potential activities across studies. © 2013 Elsevier Ltd.

Recent droughts in southwestern USA have led to large-scale mortality of piñon (Pinus edulis) in piñon-juniper woodlands. Piñon mortality alters soil moisture, nutrient and carbon availability, which could affect the root-associated fungal (RAF) communities and therefore the fitness of the remaining plants. We collected fine root samples at a piñon-juniper woodland and a juniper savannah site in central New Mexico. Roots were collected from piñon and juniper (Juniperus monosperma) trees whose nearest neighbors were live piñon, live juniper or dead piñon. RAF communities were analyzed by 454 pyrosequencing of the universal fungal ITS region. The most common taxa were Hypocreales and Chaetothyriales. More than 10% of ITS sequences could not be assigned taxonomy at the phylum level. Two of the unclassified OTUs significantly differed between savanna and woodland, had few like sequences in GenBank and formed new fungal clades with other unclassified RAF from arid plants, highlighting how little study has been done on the RAF of arid ecosystems. Plant host or neighbor did not affect RAF community composition. However, there was a significant difference between RAF communities from woodland vs. savanna, indicating that abiotic factors such as temperature and aridity might be more important in structuring these RAF communities than biotic factors such as plant host or neighbor identity. Ectomycorrhizal fungi (EM) were present in juniper as well as piñon in the woodland site, in contrast with previous research, but did not occur in juniper savanna, suggesting a potential shared EM network with juniper. RAF richness was lower in hosts that were neighbors of the opposite host. This may indicate competitive exclusion between fungi from different hosts. Characterizing these communities and their responses to environment and plant neighborhood is a step toward understanding the effects of drought on a biome that spans 19,000,000 ha of southwestern USA., (© 2015 by The Mycological Society of America.)