Interactive effects of drought and N fertilization on the spatial distribution of methane assimilation in grassland soils PETRA A. STIEHL-BRAUN *, ADRIAN A. HARTMANN *, ELLEN KANDELER w , NINA BUCHMANN * and PASCAL A. NIKLAUS * 1 *Institute of Plant, Animal and Agroecosystem Sciences, ETH Zurich, Universitaetsstrasse 2, CH-8092 Zurich, Switzerland, wInstitute of Soil Science and Land Evaluation, Soil Biology Section, University of Hohenheim, Emil-Wolff-Strae 27, 70599 Stuttgart, Germany Abstract Soil methanotrophic bacteria constitute the only globally relevant biological sink for atmospheric methane (CH 4 ). Nitrogen (N) fertilizers as well as soil moisture regime affect the activity of these organisms, but the mechanisms involved are not well understood to date. In particular, virtually nothing is known about the spatial distribution of soil methanotrophs within soil structure and how this regulates CH 4 fluxes at the ecosystem scale. We studied the spatial distribution of CH 4 assimilation and its response to a factorial drought  N fertilizer treatment in a 3-year experiment replicated in two grasslands differing in management intensity. Intact soil cores were labelled with 14 CH 4 and methanotrophic activity mapped at a resolution of  100 mm using an autoradiographic technique. Under drought, the main zone of CH 4 assimilation shifted down the soil profile. Ammonium nitrate (NH 4 NO 3 ) and cattle urine reduced CH 4 assimilation in the top soil, but only when applied under drought, presumably because NH 4 1 from fertilizers was not removed by plant uptake and nitrification under these conditions. Ecosystem-level CH 4 fluxes measured in the field did show no or only very small inhibitory effects, suggesting that deeper soil layers fully compensated for the reduction in top soil CH 4 assimilation. Our results indicate that the ecosystem-level CH 4 sink cannot be inferred from measurements of soil samples that do not reflect the spatial organization of soils (e.g. stratification of organisms, processes, and mechanisms). The autoradiographic technique we have developed is suited to study methanotrophic activity in a relevant spatial context and does not rely on the genetic identity of the soil bacterial communities involved, thus ideally complementing DNA-based approaches. Keywords: 14 C labelling, atmospheric methane, carbon isotopes, climate change, grazing effects, scaling, soil aggregate structure, soil micro-organisms Received 26 May 2010 and accepted 4 February 2011 Introduction Atmospheric methane (CH 4 ) concentrations have more than doubled since preindustrial times, with current concentrations around 1.8 ppm (IPCC, 2001). While many sources exist, both natural and anthropogenic, only two processes remove significant amounts of CH 4 from the atmosphere. Chemical oxidation in the atmo- sphere constitutes the dominant sink for CH 4 , but atmospheric CH 4 is also oxidized by terrestrial soils, a process essentially driven by soil methanotrophic bac- teria (Le Mer & Roger, 2001; Dunfield et al., 2007). However, the available estimates of the global soil CH 4 sink are associated with large uncertainties, on one hand because direct field measurements of soil CH 4 fluxes are scarce, on the other hand because the ecology of methanotrophic bacteria of well-drained oxic upland soils is only poorly understood to date, complicating the modelling and scaling of soil fluxes. A major factor limiting the soil sink for atmospheric CH 4 is restricted diffusion of CH 4 through soils (Striegl, 1993). Soil gas diffusive conductance depends on soil texture, and many estimates of the soil CH 4 sink (in- cluding the one in IPCC, 2007) base on correlations with soil texture (Do ¨rr et al., 1993; Boeckx et al., 1997). At any given site, the temporal variability of soil CH 4 uptake is generally well predicted by soil moisture, at least in loamy soils (Adamsen & King, 1993; Price et al., 2004). Soil water limits soil gas diffusion into aggregates by blocking small pore channels and by flooding macro- pore networks (Young & Ritz, 2000), and therefore generally decreases soil CH 4 uptake. Correspondence: Pascal A. Niklaus, tel. 1 41 44 635 3413, fax 1 41 44 635 5711, e-mail: pascal.niklaus@ieu.uzh.ch 1 Present address: Institute of Evolutionary Biology and Environ- mental Studies, University of Zurich, CH-8057 Zurich, Switzer- land. Global Change Biology (2011) 17, 2629–2639, doi: 10.1111/j.1365-2486.2011.02410.x r 2011 Blackwell Publishing Ltd 2629