Conversion of cropland to forest increases soil CH4 oxidation and abundance of CH4 oxidizing bacteria with stand age

We investigated CH4oxidation in afforested soils over a 200-year chronosequence in Denmark includingdifferent 15 cm depth) were incubated and analyzed for the abundance of the soil methane-oxidizing bacteria(MOB) and ammonia-oxidizing bacteria (AOB) and archaea (AOA) based on quantitative PCR (qPCR) onpmoA and amoA genes. Our study showed that CH4oxidation rates and the abundance of MOB increasedsimultaneously with time since afforestation, suggesting that the methanotrophic activity is reflected inthe abundance of this functional group. The development of forest soils resulted in increased soil organic carbon and reduced bulk density, and these were the two variables that most strongly related to CH4oxidation rates in the forest soils. For the top mineral soil layer (0-5 cm) CH4oxidation rates did not differ between even aged standsfrom oak and larch, and were significantly smaller under Norway spruce. Compared to the other treespecies Norway spruce caused a decrease in the abundance of MOB over time that could explain thedecreased oxidation rates. However, the cause for the lower abundance remains unclear. The abundanceof ammonia-oxidizers along the chronosequence decreased over time, oppositely to the MOB. However, our study did not indicate a direct link between CH4oxidation rates and ammonia-oxidizers. Here, weprovide evidence for a positive impact of afforestation of former cropland on CH4oxidation capacity insoils most likely caused by an increased population size and activity of MOB. (C) 2014 Elsevier B. V. All rights reserved.