Soil methane oxidation and methanotroph responses to afforestation of pastures with Pinus radiata stands

Afforestation of pastures in New Zealand reduces methane (CH4) production from soil, while also stimulating oxidation of atmospheric CH4 by soil methanotrophs. However, neither the mechanisms by which soil CH4 oxidation is enhanced by afforestation, nor how long after forest planting tree-dependent responses in CH4 oxidation become detectable are fully known. Here, we investigated the effects of different-aged stands (5-20 y) of the exotic pine (Pinus radiata (D. Don)) on CH4 oxidation and methanotrophic community structure in soils, compared with adjacent, long-established pastures. Two of the pastures were on volcanic soils and two were on non-volcanic soils. Although the CH4 fluxes in soils from these young stands were not significantly different from those in the associated pastures, the rate of oxidation of added 13CH4 was higher in the pine soils. Both fluxes and 13CH4 oxidation rates were higher in the volcanic than the non-volcanic soils. Combined phospholipid fatty acid (PLFA) and stable isotope probe (SIP) analyses suggested that type II methanotrophs (PLFA C18:1ω7) were most active in all soils followed by uncultivable bacteria (C17:0ai). Molecular analysis of the methanotrophic community structure using pmoA (particulate methane monooxygenase) genes suggested that a particular type II methanotroph (TRF 35) was dominant in all soils, but more so in the pine than in pasture soils. A type I methanotroph (TRF 245) was more prevalent in the pasture than in associated pine soils, whereas TRF 128 (a type II methanotroph) was slightly more dominant in soils under pine. Cloning and sequencing data suggest TRFs 35 and 128, which differ from one another, belong to distant relatives of Methylocapsa sp; TRF 245 is related to Methylococcus capsulatus. Land-use change resulted in changes in soil bulk density, porosity, moisture contents and in methanotrophic community structure. Methane oxidation rates were most closely related to soil moisture, as well as to the methanotrophic community structure, and nitrate-N, extractable C and total C concentrations. Stepwise multiple regression also suggested a weak effect (P = 0.06) of stand age on CH4 oxidation rate. By contrast, the responses of the methanotrophic community structure to this land-use change were more readily detected by the specific molecular analyses, and indicated a predominance of type II methanotrophs in pine soils.