Soil N2O production and emission is directly related to land use and soil management practices, since it is the biogenic product of microbial processes of denitrifi-cation and nitrification, as affected by physical-chemical characteristics of soil.
Bacterial denitrification is a respiratory reduction of nitrate and/or nitrite to gaseous NO, N2O and N2, coupled to phosphorylation electron transport. Many aerobic microorganisms use NO3~ as electron acceptor to derive energy from organic compounds when oxygen tension is low (heterotrophic denitrification), and, in this process, N2O is an obligatory intermediate.
Microbial nitrification is the oxidation of ammonium to nitrite and nitrate, and, in most soils, autotrophic bacteria are responsible of this process, even though some studies suggest heterotrophic nitrifiers may also contribute to nitrification and N2O production (Schimel et al. 1984; Anderson et al. 1993). N2O is not an obligatory intermediate of nitrification process, since when O2 supply is limited in soil (denitri-fication by nitrifiers), autothrophic bacteria can produce nitrous oxide by enzymatic reduction of nitrite. Autothrophic ammonium-oxidizing bacteria such as Nitrosomonas europea can use NO2~ as an alternative electron acceptor under anaerobic conditions, thus reducing NO2~ to N2O by the nitrite reductase enzyme (Firestone and Davidson 1989; Groffman 1991). The production of N2O may also be caused by other microorganisms, e.g., during dissimilatory reduction of nitrate to ammonium, nitrate reduction to nitrite, and nitrate assimilation (Smith and Zimmerman 1981; Bleakley and Tiedje 1982). The mechanism of N2O production by these bacteria and their role in N2O production in soil require extensive study. Many fungi lack N2O-reductase, so that N2O results as the major product of fungal denitrification (Shoun et al. 1992). Laughlin and Stevens (2002) demonstrated that fungi were responsible for most of the N2O production in a grassland soil. Spokas et al. (2006) also performed such inhibitor studies to elucidate potential mechanisms of N2O production following chloropicrin fumigation in soil. Their results suggest that 20% of the N2O production was from bacteria, while 70% of that was from fungi.
Typical key soil factors affecting the two microbial processes are: pH, temperature, total and mineral nitrogen content, labile organic matter availability, water content, soil aeration, redox potential. It is fundamental to distinguish between soil production and soil emission, since the N2O produced by the two microbial processes does not necessarily leave the soil, depending on soil physical characteristics. Nitrification and denitrification can be active simultaneously in soil, since it is a complex and heterogeneous system with aerobic and anaerobic microsites, which are not homogeneously distributed, and, consequently, N2O can be evolved via both these processes (Nielsen et al. 1996; Abbasi and Adams 2000). At the same time, in microsites where anaerobic conditions are extreme, denitrifi-cation process can utilize N2O produced by the same process or by nitrification, thus reducing the gas emission from soil.
Davidson (1991) presented a simplified model (hole in the pipe model) to describe the processes affecting production and emission of N2O from soils. The model identifies three levels of control. Level I is represented by all factors affecting the rates of nitrification and denitrification; level II is for N2O emissions depending on how soil physical-chemical parameters affect the ratio of end-products via both processes and on how fast; level III accounts for N2O diffusing to atmosphere through the soil gaseous phase.
Among all factors, SOM plays an essential role. Soil labile organic matter is the reaction substrate for denitrifiers, thus favouring O2 consumption and development of anaerobic microsites in soil, as well as enhancement of microbial activity. Many studies have found a significant positive correlation between N2O fluxes and SOM content (Bremner and Blackmar 1981; Robertson and Tiedje 1984; Iqbal 1992), whereas mineral soils produce less N2O (Duxbury et al. 1982). Several studies showed that in most mineral soil the key factor limiting denitrification is the availability of organic material, while others indicated that under soil anaerobic conditions, denitrifying activity is strongly regulated by the easily decomposable organic substances that are required to reduce nitrates (Burford and Bremner 1975; Paul and Beauchamp 1989; McCarty and Bremner 1992; Yeomans et al. 1992). Moreover, a reduction of the ratio N2O/N2 was observed with increase of easily degradable carbon materials in soil, since they appeared to promote a complete reduction of N2O to N2 (Elliot et al. 1990).
Typical soil managements, such as nitrogen fertilization and irrigation, are responsible for large N2O fluxes. The great application of mineral-N fertilizers in traditional croplands, such as NO3~, NH4+, NH4NO3 and NH3, is a key controller of microbial processes involved in N2O evolution from soil (Bremner and Blackmar 1980; Duxbury et al. 1982; Dambreville et al. 2006). Irrigation is a fundamental crop management in arid lands characterized by cyclic water deficit, as in the Mediterranean regions. Several authors detected peaks of N2O fluxes from crop soils following irrigation events, evidently as a result of enhanced denitrifying activities under restricted aeration state (Teira-Esmatges et al. 1998; Sanchez et al. 2001; Vallejo et al. 2004), while large emissions occur when irrigation is performed simultaneously or soon after N supply (Webster and Dowdell 1982; Ranucci et al. 2011).
9.1.3 Alternative SOM Management in Cultivated Lands
Different management practices can be listed for soil C sequestration in croplands, although their applicability might differ according to soil type and region (1) conservation tillage (zero or minimum tillage); (2) cover crops in rotation; (3) green manure of cover crops; (4) cultivating crops with deep-root systems; (5) developing and cultivating plants with high lignin content, especially in residues and roots; (6) applying non-toxic exogenous organic matter (animal manure, compost). At present, few studies have been performed on the effect of this alternative soil management on soil CO2 and N2O fluxes.
Conservation tillage systems, including zero and minimum tillage, leave more surface residues because the soil is not turned over, thus creating less degradation and biological risk for soil erosion and, therefore, preserve SOM. Many studies on soil under long-term management involving CT and no-tillage (NT) practices have demonstrated that tillage causes a substantial decrease of SOM content and mineralization of carbon (Elliott et al. 1994; McCarty et al. 1995; Six et al. 1999). Johnson et al. (2005) summarized 44 studies regarding conservation tillage and showed that the rate of SOC storage in NT compared to CT has been significant, but variable, averaging 0.4 ± 0.61 Mg C ha-1 year-1.
A counter-indication, often observed under no or minimum tillage practices, is the increase of surface residues with increased water retention, a major distribution of anaerobic microsites, and a reduced gas diffusivity that may contribute to enhance N2O emissions (Situala et al. 2000; Forte et al. 2009). Robertson et al. (2000) measured an increase of 7.7% in soil N2O emission, as compared with CT. An increased N2O emission with NT represented a small offset (3.6%) of the SOC gain that occurred during 10 years of NT. Bremner and Blackmar (1980) observed increased soil N2O fluxes shortly after mechanical disturbance by tillage and ascribed the phenomenon to release of N2O-rich soil air. On the other hand, other works found larger denitrification rates and N2O emission from undisturbed soil than for ploughed ones (Aulakh et al. 1984; Linn and Doran 1984; Staley et al. 1990), while Elmi et al. (2003) did not find any difference at all for denitrification and N2O emission between no-tilled soil and soil cultivated by conventional and reduced tillage systems. The greater microbial activity involved in soil emission of N2O might be dependent on the enhanced water-holding capacity of surface soil, as indicated by studies on no-till soils (Doran 1980). US research indicated that no-tilled soils had, on average, 1.4 times greater surface moisture than conventionally tilled soils. Other factors determining large fluxes, as the consequences of lack of soil disturbance in reduced tillage, are the reduction of macro pores (Lal 1997), increased soil aggregation (Doran 1980) and reduced aeration (Dowell et al. 1979).
The addition of exogenous organic materials (EOM) may be a tool to conserve organic matter and maintain or enhance soil fertility (Smith 2004), while also being effective in mitigating the rise of atmospheric carbon dioxide (CO2) concentration (Follett 2001; Lal 2008). The management of organic residues has received much interest in recent years, as a means to increase the potential carbon sink of cultivated soils (Six et al. 1999) and improve control of nitrogen dynamics (Jenkinson 1985; Jarvis et al. 1989). The ultimate function of organic residues is to turn agricultural soil into sink for OC by enhancing the persistent pool of soil OC or the microbially stable humified matter, humic acids and humin in particular (Piccolo 1996).
Transformation of organic wastes (sewage sludge, green waste, industrial and organic waste, animal manure) into compost is becoming increasingly popular, thus reducing the use of artificial fertilizers, and the amount of waste added to landfill sites. Compost is considered to be an environmentally safe, agronomically advantageous and relatively cheap organic amendment that stimulates soil microbial activity and crop growth (Garcia et al. 1994; Pascual et al. 1997; Van-Camp et al. 2004). Composting decreases the volumes of waste and their potentially dangerous organisms, becoming an important way to recycle organic matter from wastes. Composting or anaerobic digestion of animal manure and slurry together with straw, green wastes or other OM, in vulnerable areas, may also be useful to balance nutrients excess from nitrogen-rich areas to deficient areas. Soil amendment with organic N fertilizers has the advantage to recycle an already biologically fixed N, instead of industrially fixing additional N by the energy-intensive Haber's production of new mineral fertilizers. Hence, compost is a good supplier of N at low cost, considering that the average N content in the compost varies between 12 and 22 g kg-1, in urban organic-waste compost and sewage sludge compost, respectively. More than 90% of total nitrogen content in compost is in organic form.
Treatments with different types of EOM (compost from green waste and from sewage sludge, biosolids) enhance organic matter and total N in soil (Ros et al. 2006; Mantovi et al. 2005; Zaman et al. 2004). According to Ayuso et al. (1996), this may be attributed to a direct effect of organic N in compost that is only slowly mineralized in soil (Castellanos and Pratt 1981). Ros et al. (2006) showed a significant increase of OC and ON in soils after 12 years of compost application, together with an increase in microbial activity, due to the combined effect of highsubstrate C availability and direct microorganisms addition. Previous long-term field studies based on comparable organic amendments have shown similar effects. Zaman et al. (2004) found a noticeable effect on Corg in plots treated with sewage-sludge compost in a 37-year field experiment, while Canali et al. (2004) observed an increase in soil Corg content with dried poultry manure in a 6-year field experiment. In a 3-year field experiment, Madejon et al. (2003) reported a noticeable effect on Corg in plots treated with either organic waste compost or agricultural compost (olive mill wastewater mixed with other agricultural wastes). Ros et al. (2003) also observed an increase in Corg in a 2-year trial with organic waste compost. Moreover, organic fertilizer applications improve soil properties: aggregate stability and mineral nutrition of crops (Clark et al. 1998), pH stabilization, cationic exchange capacity (CEC) and water infiltration (Stamatiadis et al. 1999).
The effects of soil treatment with EOM, including compost, on CO2 and N2O fluxes from field soil are very complex, and the few existing experiments showed contrasting results. Studies conducted in field and by laboratory incubation, showed a general stimulation of microbial growth due to increased substrate-C availability, though a direct effect from compost-added microorganisms is also possible (Pascual et al. 1997; Garcia et al. 1998; Stamatiadis et al. 1999; Garcia-Gil et al. 2000; Ros et al. 2003). Basal respiration (CO2 release) is considered a valuable indicator for C availability to sustain microbial activity (Insam et al. 1991). In a long-term study (12 years) aimed to compare effects of different composts, mineral fertilizers and compost plus mineral fertilizer (Ros et al. 2006), an increase of basal respiration and metabolic quotient (qCO2 = CO2/Cmic) was observed in soils that had received compost. Adani et al. (2009) studied the effect of two rates (50 and 85 Mg ha1) of compost application to a soil cropped with maize. A stable soil respiration was found for compost-amended soils (CO2 flux of 0.96 ± 0.11 g CO2 m-2 h-1 and 1.07 ± 0.10 g CO2 m-2 h1, respectively, for 50 and 85 Mg ha-1), whereas respiration increased in control with time.
Composted organic matter is rich in humic substances, and soil addition with water-extractable humic substances from compost was shown to enhance root uptake of nitrate and ammonium, through several mechanisms. Cacco et al. (2000) suggested that humic substances are directly involved in switching on nitrate transport genes in roots. Panuccio et al. (2001) suggested that water extractable humic substances specifically stimulated only NH4+ uptake. Such improved nitrogen uptake by roots may lead to reduced mineral nitrogen transformation in N2O, via denitrification and/or nitrification in anaerobic soil microsites. Conversely, it has been reported that organic fertilizers rich in readily available C compounds, such as volatile fatty acids (Paul and Beauchamp 1989) and hydrosoluble materials, stimulate soil biological N-processes such as nitrification (Muller et al. 2003) and denitrification (Loro et al. 1997; Rochette et al. 2000). These two microbial processes may easily induce N2O production if anaerobic microsites are favoured in soils under minimum tillage and irrigation. Using a combination of organic residues with low and high C/N ratios, it may be possible to control N mineralization rates from organic amendments. Application of high C/N ratio organic residues may be a management tool to reduce N2O emissions from amended soil. For example, Baggs et al. (2000) and Velthof et al. (2003) found that N2O emissions from soil amended with organic materials of high C/N ratio were reduced due to N immobilization. Similar results were obtained by Ram et al. (2009), comparing low-nutrient green waste compost with feedlot manure rich in N. A similar study was performed by Meijide et al. (2007), who evaluated the influence of mineral and organic N fertilizers on nitrification and denitrification processes, and consequently on N2O emissions. Their field experiment was carried out on an irrigated sandy loam soil under Mediterranean conditions during maize (Zea mays L.) growing season. The use of digested slurries mitigated N2O emission by 25% in relation to untreated pig slurry. Denitrification appeared as the most important process responsible for N2O emissions when organic fertilizers were applied to soil, while nitrification was most important in the case of inorganic fertilizer.
Manure application in modern cropping systems is known to sustain or increase SOC, while improving nutrient management and general soil quality. Manure addition may not be entirely beneficial, as increased CH4 and N2O emissions can occur.
A study, where composted pig manure and ammonium nitrate were compared for 7 years (Dambreville 2006), showed that 14 months after the last application pig compost increased potential denitrifying activity (+319%), N mineralization (+110%) and organication (+112%), whereas the N2O/(N2O + N2) ratio resulted lower than the mineral fertilization. Fingerprints and analyses of clone libraries showed that the structure of the denitrifying community was affected by the fertilization type.
Proper management, such as avoiding excess manure application and synchronizing application time with crop uptake, will ensure the most positive effects of manure addition on SOC storage and GHG emission (Johnson et al. 2005). Soil N2O emissions are enhanced by spreading animal manure as a slurry, since 60-70% of N in slurry is present as NH4+, urea and uric acid, while solid manure and crop residues have larger content of less labile organic N materials. Moreover, slurry application of manure to soil surfaces can favour temporary anaerobic conditions leading to peaks in N2O emissions (Vallejo et al. 2004; Mcswiney and Robertson 2005). In soils where the availability to microbial activity of labile organic material is limited, manure may produce more N2O than mineral N fertilizers (Christensen 1983, 1985; Benkiser et al. 1987; Bowman 1990; Van Cleemput et al. 1992) and a combined application of manure and mineral fertilizers can lead to amplified N2O emission rates.
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