Measurements of CO2 and N2O Fluxes from Soil 921 State of the Art on Soil Gases Measurements

Although agricultural soils are important source of anthropogenic CO2 and N2O, no alternative soil managements have been developed to limit their fluxes, particularly for N2O. Since pedo-climatic conditions are key factors, a monitoring activity at territorial scale is needed, not only for testing different soil managements but also to obtain data from Mediterranean soil-crop systems. Freibauer (2003) has pointed out already that large uncertainties are present in the GHGs inventory for Mediterranean croplands due to lack of extensive monitoring activities.

Among alternative soil managements, minimum tillage, green and animal manure have been largely studied. Research on the use of compost in agricultural soils has been mostly focused on nutrition and environment aspects, i.e., OM quantity and quality, accessibility of organic contaminants and heavy metals, crops yield, soil microbial response. Fewer studies were devoted on GHGs emissions from soils following compost addition. Moreover, no examples are up-to-date present in literature for the use of catalysts in soils to structurally modify SOM and increase carbon fixation.

Monitoring GHGs fluxes from soil presents some difficulties. In the case of CO2, it is difficult to discriminate among the different biogenic sources of CO2 (see Sect. 9.1.2). The separation between SOM-derived and plant-derived CO2 is essential to evaluate the real capacity of soil as source or sink of atmospheric CO2. Also soil N2O fluxes result from complex interaction among biological, physical and chemical factors, within a large spatial and temporal variability. Thus, the evaluation of soil fluxes of both gases is made difficult by methodological limitations (Kuzyakov 2006; Groffman et al. 2006), and high spatial and temporal variability in field-scale, particularly for N2O (Clemens et al. 1999; McSwiney and Robertson 2005; Wagner-Riddle and Thurtell 1998).

Models are increasingly used to quantify C and N gas fluxes at territorial scale, especially when agricultural policies are to be developed. However, there are few long-term data sets, particularly for N gas fluxes, to be used in model validation (see also Chap. 2). The current IPCC (2007a, b) methodology for producing national inventories of N2O from agricultural land is based on the study of Bouwman (1994) and it assumes a default emission factor (EF) of 1.25% for soil-added nitrogen. This approach does not account for climate, management practices, irrigation, soils and crop types, and other variables. Moreover, the data considered by Bouwman (1994) were mainly referred to croplands under temperate climatic conditions. Thus, more experiments are required to obtain a correct evaluation of N2O emissions from agricultural lands under different climatic regimes at regional and national scale.

Due to such shortcomings, new experimental designs for soil gases monitoring must be planned to obtain data with large time resolution. Spatial and temporal variabilities depend on the physical-chemical factors that affect all soil biological processes inducing the production of CO2 and N2O. Much of the challenge arises from the fact that small areas (hotspots) and brief periods (hot moments) often account for high fluxes. In the last decades several experiments were conducted to understand the factors controlling the CO2 and N2O fluxes from soils (oxygen content, nitrogen availability, soil moisture and texture, and so on). However, the complex regulation of these factors, including soil management practices, creates hotspots and hot moments that are difficult to quantify and model. Due to technical restrictions, most attention was focused in determining the hotspots, particularly for N2O production and emissions. N2O hotspots in soils involve the interaction among patches of organic matter and physical factors controlling oxygen diffusion in soil, and transport and residence time of N2O in soil pores. Thus, a series of plant and soil factors, e.g., rooting patterns and soil structure at small (0.1-10 m) scales, topography, hydrologic flow paths and geology at larger (>1 km) scales, need to be considered to understand the spatial distribution of hotspots. Currently, soil N2O emissions predicting models are calibrated on the basis of spatial variability. However, their reliability to predict temporal variations is seriously undermined due to the very few data available in literature to calibrate these models over time.

The hot moments concept has been known since long time but hardly investigated by continuous monitoring, particularly in the small time scale, since few experiments are based on high-time-resolution measurements systems [dynamic chambers, Tunable Diode Laser (TDL) associated with eddy covariance technique]. The large part of data produced up-to-now, are referred to manual chamber measurements limited in temporal resolution.

Despite the increasing popularity of the eddy covariance technique to assess ecosystem C exchange and, recently, also N exchange by means of TDL, classical static or dynamic chamber methods remain the most useful tools. This is due to some limitations of the eddy covariance technique for C exchange. Mainly, micrometeorological techniques are only able to obtain the total CO2 fluxes and cannot partition total flux into its individual sources (Buchmann 2002). Conversely, chamber methods allow CO2 fluxes to be measured directly from soil. Moreover, the eddy covariance technique has large purchase and installation costs, particularly for TDL equipment, even though this has the additional advantage to provide soil exchange also for N2O and CH4 gases. Some studies have simultaneously used eddy covariance and chamber methods to separate net ecosystem CO2 exchange from soil respiration (Lavigne et al. 1997; Dore et al. 2003), as well as to correct the fluxes obtained by eddy correlations during night periods (Anthoni et al. 1999; Law et al. 1999; Dore et al. 2003).

Factors affecting time variation of soil gas fluxes are different: management, root exudates, drying and rewetting, etc. Factors affecting time variation of soil gas fluxes are different: management, root exudates, drying and rewetting, etc. Contrary to current understanding that daily CO2 dynamics are attributed to day-night variation of soil temperature, the process is also associated to fast decomposition processes that release easily decomposable substrates and enhance CO2 production, thus resulting in diurnal CO2 dynamics (Kuzyakov 2006). In agricultural ecosystems, pulse emissions of N2O are also frequently associated with fertilizer additions, organic treatments and following re-wetting after periods of prolonged drought (Davidson et al. 1993; Ranucci et al. 2011).

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