The microbial processes of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are two important nitrate reducing mechanisms in soil, which are responsible for the loss of nitrate (NO3−) and production of the potent greenhouse gas, nitrous oxide (N2O). A number of factors are known to control these processes, including O2 concentrations and moisture content, N, C, pH, and the size and community structure of nitrate reducing organisms responsible for the processes. There is an increasing understanding associated with many of these controls on flux through the nitrogen cycle in soil systems. However, there remains uncertainty about how the nitrate reducing communities are linked to environmental variables and the flux of products from these processes. The high spatial variability of environmental controls and microbial communities across small sub centimeter areas of soil may prove to be critical in determining why an understanding of the links between biotic and abiotic controls has proved elusive. This spatial effect is often overlooked as a driver of nitrate reducing processes. An increased knowledge of the effects of spatial heterogeneity in soil on nitrate reduction processes will be fundamental in understanding the drivers, location, and potential for N2O production from soils.

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The oxygen control of denitrification and its emission of NO/N2O/N2 was investigated by incubation of Nycodenz-extracted soil bacteria in an incubation robot which monitors O2, NO, N2O and N2 concentrations (in He+O2 atmosphere). Two consecutive incubations were undertaken to determine (1) the regulation of denitrification by O2 and NO2(-) during respiratory O2 depletion and (2) the effects of re-exposure to O2 of cultures with fully expressed denitrification proteome. Early denitrification was only detected (as NO and N2O) at

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Abstract. Remote sensing and inverse modelling studies indicate that the tropics emit more CH 4 and N 2 O than predicted by bottom-up emissions inventories, suggesting that terrestrial sources are stronger or more numerous than previously thought. Tropical uplands are a potentially large and important source of CH 4 and N 2 O often overlooked by past empirical and modelling studies. To address this knowledge gap, we investigated spatial, temporal and environmental trends in soil CH 4 and N 2 O fluxes across a long elevation gradient (600-3700 m a.s.l.) in the Kosñi-pata Valley, in the southern Peruvian Andes, that experiences seasonal fluctuations in rainfall. The aim of this work was to produce preliminary estimates of soil CH 4 and N 2 O fluxes from representative habitats within this region, and to identify the proximate controls on soil CH 4 and N 2 O dynamics. Area-weighted flux calculations indicated that ecosystems across this altitudinal gradient were both atmospheric sources and sinks of CH 4 on an annual basis. Montane grasslands (3200-3700 m a.s.l.) were strong atmospheric sources, emitting 56.94 ± 7.81 kg CH 4 -C ha −1 yr −1 . Upper montane forest (2200-3200 m a.s.l.) and lower montane forest (1200-2200 m a.s.l.) were net atmospheric sinks (−2.99 ± 0.29 and −2.34 ± 0.29 kg CH 4 -C ha −1 yr −1 , respectively); while premontane forests (600-1200 m a.s.l.) fluctuated between source or sink depending on the season (wet season: 1.86 ± 1.50 kg CH 4 -C ha −1 yr −1 ; dry season: −1.17 ± 0.40 kg CH 4 -C ha −1 yr −1 ). Analysis of spatial, temporal and environmental trends in soil CH 4 flux across the study site suggest that soil redox was a dominant control on net soil CH 4 flux. Soil CH 4 emissions were greatest from habitats, landforms and during times of year when soils were suboxic, and soil CH 4 efflux was inversely correlated with soil O 2 concentration (Spearman's ρ = −0.45, P < 0.0001) and positively correlated with water-filled pore space (Spearman's ρ = 0.63, P < 0.0001). Ecosystems across the region were net atmospheric N 2 O sources. Soil N 2 O fluxes declined with increasing elevation; area-weighted flux calculations indicated that N 2 O emissions from premontane forest, lower montane forest, upper montane forest and montane grasslands averaged 2.23 ± 1.31, 1.68 ± 0.44, 0.44 ± 0.47 and 0.15 ± 1.10 kg N 2 O-N ha −1 yr −1 , respectively. Soil N 2 O fluxes from premontane and lower montane forests exceeded prior model predictions for the region. Comprehensive investigation of field and laboratory data collected in this study suggest that soil N 2 O fluxes from this region were primarily driven by denitrification; that nitrate (NO