Methane oxidation in non-flooded soils as affected by crop production--invited paper

Methane is an important greenhouse gas, which contributes approximately 20% to global warming. The atmospheric CH4 concentration is increasing rapidly, resulting from an imbalance between CH4 production and consumption. The only known biological CH4 sinks are soils where methanotrophic bacteria consume CH4 by oxidizing it. For several reasons the CH4 uptake potential, particularly of arable soils and grassland, is only partly exploited, as several agricultural practices have adverse impacts on the activity of the CH4 oxidizing bacteria. The kind of land use in general has a remarkable influence with much higher oxidation rates under forest than under grassland or arable soil. Regular soil cultivation by ploughing and fertilization with ammonium or urea have been identified as main factors. Immediately after ammonium application the methanotrophic enzyme system is blocked, resulting in an inhibition of CH4 oxidation. In addition to this short-term effect a long-term effect exists after repeated ammonium fertilization, which is most likely caused by a shift in the population of soil microbes. Crop residues affect CH4 oxidation differently, depending on their C/N ratio: with a wide C/N ratio no effects are expected, whereas with a narrow C/N ratio strong inhibition was observed. Animal manure, particularly slurry, can cause CH4 emission immediately after application, whereas in the long run farmyard manure does not seem to have adverse impacts on CH4 oxidation. The methanotrophic activity decreased markedly with soil pH, although in many cases liming of acidified soils did not show a positive effect. Arable soils have a rather small pH range which allows CH4 oxidation, and the inhibitory effect of ammonium can partly result from a concomitant decrease in soil pH. Reduced tillage was identified as a measure to improve the methanotrophic activity of arable land, set aside of formerly ploughed soil points into the same direction. Plant growth itself is not primarily responsible for observed effects on CH4 oxidation, but secondary factors like differential pesticide treatments, changes in pH, or cultivation effects are more likely involved. Although for the overall CH4 fluxes the oxidation processes in agricultural soils are of minor importance, all available possibilities should be exhausted to improve or at least preserve their ability to oxidize CH4.