Nitrogen fertilization suppresses soil phenol oxidase enzyme activity in no-tillage systems

Phenol oxidase is associated with the carbon cycle and its presence in soil environments is important to the formation of humic substances. Little effort has been made to integrate the response of phenol oxidases with soil management. We investigated phenol oxidase activity on a Maury silt loam (fine, mixed, mesic Typic Paleudalfs) soil after 33 years of imposed tillage and N fertilization treatments. Particle size fractions were investigated independently to help identify the location of the enzyme. Phenol oxidase activity was 1.7 times greater (P < 0.01) in no-tillage (NT) compared with moldboard plow (MP) in the control treatment (0 kg N ha^-1), consistent with the known effects of tillage. The phenol oxidase was located primarily in the silt fraction, followed by the clay and sand in the NT. In NT, N fertilization (336 kg N ha^-1) had a marked negative effect on soil phenol oxidase activity, showing a 38% decrease (P < 0.01) despite the increase in soil organic carbon (SOC). In contrast, MP plots were relatively insensitive to applied N rate. Phenol oxidase activity was related negatively to dissolved inorganic nitrogen (DIN) (r = -0.49, P < 0.1), SOC (r = -0.49, P < 0.1) and dissolved organic carbon (r = -0.51, P < 0.1) in NT. This research provides new information about the response of phenol oxidase enzymes to long-term N fertilization in NT and MP systems. These findings suggest that manipulating the application rates of fertilizer N in soils under NT will make it possible to impact phenol oxidase activity.