Effects of long-term nitrogen fertilization on the uptake kinetics of atmospheric methane in temperate forest soils

To determine whether repeated, long-term NH4+ fertilization alters the enzymatic function of the atmospheric CH4 oxidizer community in soil, we examined CH4 uptake kinetics in temperate pine and hardwood forest soils amended with 150 kg N ha(-1) y(-1) as NH4NO3 for more than a decade. The highest rates of atmospheric CH4 consumption occurred in the upper 5 cm mineral soil of the control plots. In contrast to the results of several previous studies, surface organic soils in the control plots also exhibited high consumption rates. Fertilization decreased in situ CH4 consumption in the pine and hardwood sites relative to the control plots by 86% and 49%, respectively. Fertilization increased net N mineralization and relative nitrification rates and decreased CH4 uptake most dramatically in the organic horizon, which contributed substantially to the overall decrease in field flux rates. In all cases, CH4 oxidation followed Michaelis-Menten kinetics, with apparent K-m (K-m(app)) values typical of high-affinity soil CH4 oxidizers. Both K-m(app) and V-max(app) were significantly lower in fertilized soils than in unfertilized soils. The physiology of the methane consumer community in the fertilized soils was distinct from short-term responses to NH4+ addition. Whereas the immediate response to NH4+ was an increase in K-m(app), resulting from apparent enzymatic substrate competition, the long-term response to fertilization was a community-level shift to a lower K-m(app), a possible adaptation to diminish the competitiveness of NH4+ for enzyme active sites. (C) 2004 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.