Variability of soil methane production on the micro-scale: spatial association with hot spots of organic material and Archaeal populations

High temporal and spatial variability is a key problem when quantifying methane emissions from soils. Whereas the spatial variability on the landscape scale has been investigated in different studies, we investigated the spatial heterogeneity of CH4 production on 1 cm scale, as well as the role of organic material as a relevant factor. Undisturbed soil cores (dia. 6 cm) of two mineral and one peaty wetland soils (Typic Humaquept, Aeric Endoaquept and Limnic Haplohemist) from the cool-humid region in southwest Germany were anaerobically incubated for 3 months. The time course of the CH4 production rates was dependent on the water-table-level history of the incubated horizon and on the soil type. However, the absolute amounts of CH4 production differed largely between parallel cores from each soil type, although they were obtained within 1 m2. The native structures of the soil cores were determined by computed tomography. Fresh organic material was observed in all highly productive soil cores, whereas soil cores with low methanogenic activity included far less fresh organic material. The observed hot spots of fresh organic material were correlated to high amounts of Archaea, as analyzed by etherlipid analysis as well as by in situ hybridization using an Archaea-specific probe. The most dominant factor for the spatial variation in CH4 production on the micro-scale is the distribution of fresh organic material, which activates and possibly attracts methanogenic Archaea (methanogens).