Do soil enzymes respond to silvicultural management?

Abstract

Soil enzymes mediate important soil processes and have been used to indicate changes due to management practices. In forest ecosystems, rotation length challenges the ability to assess the impacts of silvicultural practices on soil enzymes, which has led to these effects being understudied. The fluorometric microplate 4-methylumbelliferyl (MUF) enzyme assay using dry soil samples offers a promising method to explore soil enzyme activities over large time scales. Here, we investigated the impacts of silvicultural practices on soil enzymes, β-glucosidase (BG), β-glucosaminidase (NAG), and acid-phosphatase (AP), using the MUF enzymes assay method. We sampled across an extensive southeastern U.S. experimental pine plantation network which included the most common silvicultural practices - herbicide, thinning, fertilization, and intensification of fertilization (Intensity). We selected 264 plots and collected soil samples at 0–10, 10–20, 20–50, and 50–100 cm depth. Soil enzyme activities were extremely low below the 20 cm depth. Soil enzyme activities were spatially variable across soil depth and soil Orders, but did not show seasonal or other temporal variation. Soil pH, soil organic carbon, total nitrogen, and permanganate oxidizable carbon correlated positively with all enzymes analyzed at 0–10 and 10–20 cm depths and, in general, edaphic properties had higher importance in explaining enzyme variability than climate or stand properties. Compared to control plots (untreated), at the 0–10 cm depth, fertilization suppressed BG, NAG, and AP activities. Among managed forests, sites under Intensity management, at the 0–10 cm depth, showed higher NAG activity and NAG:AP ratio but lower BG:NAG. AP responded negatively to the frequency and doses of fertilization applied (Intensity treatment). The same trend was observed for BG, but it depended on how enzyme activity was expressed, particularly in relation to soil organic matter (SOM) content. Results from this study indicate that soil enzymes can be used to monitor impacts of silvicultural practices, but their effectiveness relies upon soil depth sampled, how their activity is expressed, and the choice of specific enzymes based on monitoring goals.