The holes in the pipe

Soil WFPS is an important regulatory factor for both the amount and form of gaseous N loss.

The wetter the soil, the less opportunity there is for gas diffusion and the greater the potential for complete reduction to N2 (Davidson, et al., 2000). Ordinarily there is a shift in trace gas emissions from predominantly NO release under aerobic conditions to N2O under anaerobic conditions (Figure 2-4) (Davidson, 1991; Davidson, et al., 2000; Davidson & Verchot, 2000;

Weitz, et al., 2001). Whilst rainfall is the prime regulator of soil O2, microbial respiration can also lead to oxygen depletion in soil microsites. Where soils are relatively aerobic (i.e. WFPS

<65%), nitrification is typically the primary NO and N2O forming process but as soils become wetter, the relative importance of denitrification increases. At high soil moisture (i.e.

WFPS>80%), N2 is usually the dominant end product from denitrification. Peak NO and N2O emissions ordinarily occur at intermediate soil moisture (i.e. 60% <WFPS< 80%), as the presence of aerobic and anaerobic microsites allow simultaneous nitrification and

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denitrification. Abundant and frequent rainfall in the humid tropics should translate therefore to high WFPS placing tropical soils in the denitrification zone of emissions where N2O and N2 dominate (Veldkamp, et al., 1998; Veldkamp & Keller, 1997; Koehler, et al., 2012).

However, high rates of evaporation and transpiration may rapidly deplete moisture and limit the length of time that soils are anaerobic. The periodicity of wetting and drying cycles therefore affects redox status and production of trace gases through alternate nitrification and denitrification (Liptzin, et al., 2011; Pett-Ridge, et al., 2006).

Figure 2-4: Conceptual model of the relationship between WFPS(%) and NO, N2O and N2 emissions from nitrification and denitrification (after Davidson (1991) in Bouwman et al 1998)).

In the presence of an oxygen-limiting environment, denitrification still requires both a suitable electron donor and available nitrogen oxides to proceed. The electron donor (usually labile carbon) is a pre-requisite of heterotrophic respiration and an important regulator of

denitrification, (Nobre, et al., 2001; Garcia-Montiel, et al., 2003). In old-growth forests of the humid tropics, plant and microfauna inputs are perhaps the highest of any world biome.

Hence one might expect soil carbon pools to be greater here than in any other region.

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However, the same climatic controls (i.e. warm year-round temperatures and a lack of

moisture stress) that favour high rates of net primary production also result in high CO2 efflux via soil respiration (Schlesinger, 1977; Raich & Schlesinger, 1992; Pregitzer & Euskirchen, 2004). Consequently, even within tropical forests that appear to have plentiful litter returns, there is evidence of C limitation to microbial N processing (Nobre, et al., 2001; Hall &

Matson, 2003; Neill, et al., 2005; van Haren, et al., 2010).

In the absence of disturbances such as deforestation, tropical soils are generally not assumed to be N-limited, (Parsons, et al., 1993; Martinelli, et al., 1999; Jenny, 1950; Vitousek &

Sanford, 1986). Under the nitrogen saturation model, N status is the principal factor

determining the magnitude of losses from soils (Aber, et al., 1998). Therefore, the response of tropical soils to anthropogenic N will depend on factors such as current soil nutrient status, size of the nitrifier and denitrifier community and hydrological pathways (Hall, et al., 2004;

Lohse & Matson, 2005; Hall & Matson, 2003; Koehler, et al., 2012). Under N-limiting conditions, there may be additional capacity for the microbial community to assimilate nitrate rather than, for example, denitrify. However, where microbial process rates are limited by other nutrients, anthropogenic N additions might be expected to increase denitrification rates and N2O emissions. For example, on Mount Kinabalu, Sabah, at elevations between 700-3100m, N-rich sedimentary soils generally responded with greater NO and N2O emissions to nutrient additions than N-poor ultrabasic soils indicating possibly greater assimilation where N was limiting (Hall, et al., 2004). Similarly, in Hawaii where P, or P and N in combination, limited microbial production, N fertilisation resulted in large losses of NO and N2O after both one-time and long-term applications, (Hall & Matson, 2003). In general, limited capacity for assimilation of additional N will increase denitrification and N2O emissions with one recent study reporting a doubling of N2O emissions following the 10-year addition of 125 kg N ha^-1

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y^-1 to lowland Panamanian rainforests (Koehler, et al., 2012). Ultimately, physical and chemical characteristics such as hydrological pathways and nutrient status may control the response of tropical soils to increasing anthropogenic N (Lohse & Matson, 2005). However, much is to be gained by increasing the number of nutrient addition experiments undertaken to determine resource limitations and fertilisation response in this region.

Until recently a lack of evidence to the contrary, coupled with the fact that denitrification genes are widespread among evolutionary distinct species of denitrifiers, has led to the assumption that the denitrifying population (be it temperate or tropical) exerts little control over denitrification rates. Advances in molecular methods now permit characterisation of the population through DNA and mRNA analyses and have begun to open up this area of

research. One area of interest is the effect of community structure on ecosystem function.

For example, not all microorganisms carry each of the necessary genes to reduce NO3

-sequentially to N2. Some bacteria and most (possibly all) fungi appear able to reduce NO3- to N2O but many organisms facilitate only one step in the 4-step sequence (Shoun, et al., 1992;

Takaya & Shoun, 2000; Hayatsu, et al., 2011; Shoun & Tanimoto, 1991). It is theoretically possible, therefore, that a community lacking in sufficient organisms carrying the N2O reductase gene may produce more N2O or have higher N2O/(N2O+N2) ratios than a

community where N2O reducers are abundant, (Henry, et al., 2006; Richardson, et al., 2009;

Toma, et al., 2011; Philippot, et al., 2009). This structural population control of

denitrification is exemplified by fungi: fungal denitrifiers appear to lack the N2O reductase though there is increasing evidence of their importance to soil denitrification, (Laughlin &

Stevens, 2002; Yanai, et al., 2007; Ma, et al., 2008; Toma, et al., 2011). Whilst there is no evidence that fungi can denitrify to N2, the reduction of nitrate may be coupled to the reaction of nitric oxide with organic nitrogen to form N2 through the process of codenitrification,

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(Spott, et al., 2011). Publications on fungal denitrification in temperate or boreal soils have been gathering momentum over the past 15 years, however apparently only one study has compared the contribution of fungi and bacteria to denitrification within tropical forest and agricultural soils (Yanai, et al., 2007). Working in Indonesian Borneo, Yanai et al. (2007) found N2O emissions from the arable peat soil to be largely from fungal rather than bacterial nitrification and/or denitrification. Furthermore, emissions of N2O were much higher in agricultural sites relative to the natural forest, though the authors stopped short of relating these land-use differences to changes in microbial community and relative abundance of fungal denitrifiers. Data on fungal denitrification lags behind that of bacterial denitrification, however, there is evidence that fungal denitrification is significant across a range of

environments. For example, fungi have been shown to be important denitrifiers in birch forests on drained Swedish peat (Rütting, et al., 2013), coniferous forests in The Netherlands (Laverman, et al., 2000), Irish grasslands (Laughlin & Stevens, 2002) and semi-arid riparian soils in Arizona (McLain & Martens, 2006). The challenge of ongoing research is to employ recent advances in molecular methods to relate community composition and possibly the fungal community in tropical soils to ecosystem functions such as N2O emission.