primarily controlled by water regime, plant population, soil texture and interculture operations. Figure 8.1 summarizes the influence of various crop management practices on N2O emission.
Although all the factors mentioned earlier influence N2O emission from rice fields also, but exclusive conditions in a rice field need a special mention. Rice is cultivated under a wide variety of climate, soil and water regime. A majority of rice cultivation practices require standing water over the field for extended periods during their life cycle, which makes the saturated plow layer develop a reducing layer below an oxidizing layer at the soil surface. Below the water-saturated plow layer, there is an underground saturated soil layer in rice-growing season. In the plains, the underground saturated layer is usually located at 0.7-1.5 m below the surface layer and the its depth varies with a number of factors, such as topography, season, precipitation and irrigation. Flooding of rice fields during land preparation (as in case of wetland rice) or later (as done in many regions e.g. US, Australia, parts of Europe and in some Asian countries) initiates several physico-chemical and microbiological processes in soil, which are starkly different from other cultivated soils (DeDatta 1995). Flooded rice soils are characterized by limitation of O2, since the diffusivity of O2 through water is 10,000 times smaller than that in gas and thus anoxic conditions are easily created. Only in the first few millimeters of surface soil, partially aerobic conditions are found which is primarily due to the dissolved oxygen present in water (Ponnamperuma 1972).
N need for rice crop is primarily met through chemical fertilizers and urea has approximately 90% share in it (Vlek and Byrnes 1986). While ammonium sulphate is also used in many regions, but due to low use efficiency, NO3- fertilizers are seldom used. Thus, only 40% of added N is used by the rice crop (DeDatta et al. 1968). After application, NH4+ -N in fertilizers gets accumulated by fixation in soil and is lost as NH3 volatilization, leaching, run-off and nitrified in the oxidizing layer, at the water-and-soil interface, forming NO3- which moves downwards to the reducing layer and there it becomes denitrified subsequently (DeDatta 1995). Most research on denitrification in paddy fields focused on the submerged upper plow layer and little has been reported on denitrification in the underground saturated soil layer, but it has been reported that in regions with high N input in rice, denitrification in the underground saturated soil mitigates NO3- pollution in groundwater, but contributes N2O to atmosphere (Xing et al. 2002a). In the thin oxidized zone present at surface of soil, nitrifiers can transform NH4+ to NO3- via NO2-, which gets transported to lower anaerobic layers where it is transformed to N2O and N2 by denitrifiers (Zhu et al. 2003). Further down, where rice roots occupy much of the soil volume, a significant amount of O2 might be present through transportation by aerenchyma (Savant and DeDatta 1982). So, in the predominantly oxic zone, nitrification can also take place to produce N2O directly or to produce NO3-, which gets finally denitrified to N2O. Thus a coupling of nitrification and denitrification reactions in rice is evidenced indirectly by balancing N supply with N recovery from plant and soil (Fillery et al. 1984, 1986; Reddy and Patrick 1986). Rice plants will also affect nitrification and denitrification indirectly by immobilizing NH4+ and NO3- from the rhizosphere and also by supplying root exudates and dead root debris, which act as substrate for microorganisms. Savant and DeDatta (1982) have suggested mechanisms by which nitrification and denitrification occur near rice root. In upland rice cultivation, rice fields do not remain permanently flooded (except where permanent flooding is deliberately done) as the applied water drains from soil depending on soil type and extent of puddling done before transplanting. During drying cycles, N2O emission increases (Chen et al. 1997; Majumdar et al. 2000) as nitrification gets a boost due to diffusion of O2 into soil in absence of standing water and also because N2 formation gets reduced under less intense anaerobic condition (Granli and Bock-man 1994). During drying cycles, a significant amount of NO3- gets accumulated in soil due to nitrification of previously accumulated NH4+-N. This NO3- is lost by denitrification once the soil is flooded again and at the end of rice season, only limited amount of mineral N (NH4+ and NO3-) will remain due to different losses and plant uptake (DeDatta 1995). Permanently flooded rice fields are not considered a potent source of atmospheric N2O, because N2O is further reduced to N2 under the strict anaerobic conditions (Granli and Bockman 1994).
Topography of a rice field can influence soil water, thermal regime and soil organic matter content and hence might control N2O emission. Xu et al. (1997) have reported that the soil of low topography at Yingtan in China had higher total N and organic matter contents than those of higher topography and rice paddy field of low topography emitted more N2O than those of high topography, with the exception of N2O flux from middle plot of early rice. The exception may be due to the difference of water management between middle plot and the other two plots of early rice.
8.7 Production Versus Emissions of N2O from Crop Fields
Nitrous oxide emission does not match with the real time N2O production in soil as the gas is attenuated by further denitrification to N2 (Granli and Bockman 1994), entrapment in soil pores, dissolution in soil water and floodwater (Linke 1965), and leaching in case of rice fields (Nevison 2000). Since our immediate concern is N2O loading in the atmosphere, we limit N2O monitoring to soil surface emissions only, rather than soil profile measurements. Real time N2O production in soil is difficult to be estimated with precision, since soil is not a closed system per se and N2O gets diffused in many directions quickly, depending on soil porosity, compaction, water content etc. Depending on several simultaneous spatial measurements at a particular depth, N2O diffusion coefficient in soil could be determined (Li and Kelliher 2005). Soil profile N2O concentrations have been estimated by various researchers, which possibly gave us an idea only on the relative preponderance of gas production zones at the sampling point only and temporal and spatial trends of production, but not the exact amount of real time production.
N2O production potential of soils could be more realistically determined in undisturbed soil cores under controlled conditions in laboratory where the entire N2O emitted could be sampled from a definite quantity of soil. Though N2O diffusion and leaching could be minimized in a small incubation chamber, real time entrapped or dissolved N2O cannot be sampled without disturbing the soil core. Entrapped N2O could be sampled by soil shaking and sampling incubation chamber headspace, but only with loose soil samples, which are not ideal representative of the field condition.
Leaching of N2O in groundwater could also pose a problem in the quantification of real time production in soil. The importance of N2O leaching in crop fields has been identified (IPCC 2001), but a few field experiments have been done to quantify this loss. A very recent study on dissolved N2O in paddy agro-ecosystems in China has indicated that the dissolved N2O/NO3- ratios in leachate and groundwater in paddy ecosystem could be significant, but is far lower than the current IPCC default value (Xiong et al. 2006).
Appreciable amount of N2O may remain dissolved in floodwater in rice fields after emission from soil and thus escapes sampling for surface emissions. Dissolved N2O needs to be estimated for a better quantification of N2O emissions from soil. An equation had been proposed by Moraghan and Buresh (1977) to calculate dissolved N2O for laboratory incubation systems only, which is based on Henry's Law:
Y = a. x. (solution volume/atmosphere volume)
Where Y = amount of dissolved N2O (mg) in a closed system a = solubility of N2O (cm3 N2O dissolved per cm3 of water)
x = amount of N2O (mg) in the flask atmosphere (i.e. flask headspace).
Solubility of N2O in water at different temperatures is known (Linke 1965) and can be used in this formula. The solubility of 0.7gN2O-NL-1 at 25°C (Wilhelm et al. 1977) can be used for estimating N2O dissolution at NTP. It might be interesting to study the applicability of this equation for estimating floodwater N2O in rice by tallying measured and calculated dissolved N2O in a flooded rice by static chamber method. Minami and Fukushi (1984) have described a method to effectively measure N2O dissolved in floodwater and released from floodwater surface. They have found dissolved N2O concentration in the range of 0-0.38 ^gL-1 in the floodwater. But, floodwater N2O has rarely been estimated in N2O monitoring activities in spite of availability of this technique.
Appreciable amount of N2O might remain entrapped in the soil pore spaces (Lindau and DeLaune 1991), but its extent will be governed by soil texture, pore geometry, soil water content and pressure of standing water in case of rice. During field drying in a rice field, N2O flux might increase due to withdrawn water pressure, which, otherwise, would push back some N2O inside soil. More the time of N2O entrapment, more is the chance for its conversion to N2 by denitrification. One would desire this to happen, as this would reduce atmospheric load of N2O from the rice fields.
Although much research has been done on the source strength of crop fields with regard to N2O, a little focus has been given on its possible role as a sink of atmospheric N2O. The N2O uptake capacity of flooded rice soils comes to light, when published results on N2O emission from rice fields are carefully examined (Majumdar 2005). Available literature on N2O emissions from other agricultural crops indicates that negative fluxes from aerobic soils are not very uncommon and an excellent review on this subject is available (Chapuis-Lardy et al. 2007).
Simulation of greenhouse gas emissions from agro-ecosystems was taken up after a few decades of understanding of the subject. It has assumed a great significance since emissions from agro-ecosystems are highly variable, both temporally and spatially and it is tedious to monitor all types of agro-ecosystems to come to a finer adjustment of regional and global estimates of greenhouse gas emissions. Among few models available for simulating N2O emissions, two important models are discussed briefly here. The DAYCENT ecosystem model (a daily version of Century model) has been applied to simulate soil organic carbon levels, crop yields, and annual trace gas fluxes including N2O for various soils (Del Grosso et al. 2001). This model utilizes a N gas sub-model to simulate N2O and NOx emissions from nitrification and denitrification as well as N2 emissions from denitrification. Significantly, results from DAYCENT simulation indicated that conversion to no tillage at the national scale could mitigate ~20% of US agricultural emissions or ~1.5% of total US emissions of greenhouse gases (Del Grosso et al. 2005).
The DeNitrification-DeComposition (DNDC) model is a process-based model that focuses on N2O, CO2, NO, CH4 and NH3 emissions and has been applied to estimate N2O emissions from agricultural fields (Li et al. 1992; 1994). Comparison of DNDC with the IPCC methodology for developing a national inventory of N2O emissions has been done for arable lands in China and it was found that although total N2O emissions were similar in both estimates, but geographical patterns of emissions were quite different (Li et al. 2001). Only recently, DNDC model has been modified to enhance its capacity of predicting greenhouse gas (GHG) emissions from rice ecosystems (Li et al. 2004). According to the simulation fir rice, shifting water management from continuous flooding to mid-season drainage increased N2O fluxes by 0.13-0.20TgN2O-Nyr-1.
Although the role of soils in N2O emissions is long known (Arnold 1954), monitoring of N2O from crop fields has been started effectively from late 1970's to early 1980's (Delariche et al. 1978; Denmead et al. 1979; Freney et al. 1981; Smith
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