FACTORS CONTROLLING GHG EMISSIONS FROM FERTILIZER
Fertilizers, both inorganic and organic, are the source of nutrients for soil microbes and plant respiratory processes in GHG emissions from soil. The application and management of fertilizers can contribute significant amounts of GHGs to the atmosphere. The GHG emissions vary with climate, soil properties and management, and fertilizer application and management. Understanding the major controlling factors and the combination of factors that minimize emissions is critical in the development of effective mitigation. The important factors controlling the GHG emission are discussed below.
The climate of India varies from region to region due to its vast area and is home to diverse soil types. The tropical and subtropical climate of India results in a higher mean annual temperature (32°C-40°C) and precipitation (650 mm) than that of temperate countries. This higher mean annual temperature and precipitation increases the decomposition and mineralization of soil organic matter (SOM). The increased mineralization of SOM together with fertilizer application influences GHG emissions from soils. Further, the climate of a region directly affects soil temperature and moisture, which indirectly influences GHG emissions from fertilizer use. In general, GHG emissions are lower under drier climates than under wet, and also lower under cool than under warm temperatures.
7.4.2 Oil Properties
Soil physical and chemical properties include SOC content, mineral N content, water-filled pore space, pH, texture, drainage, temperature, moisture content, 02 status, porosity, microbial abundance, and activity (Lenka and Lai 2013; Lenka et al. 2017). The N20 fluxes correlate positively with soil nitrate and the nitrate/ammonium ratio and negatively with leaf litter and soil C/N ratio (Erickson et al. 2002). However, emission of CO, and CH4 is positively correlated with C/N ratio. Decomposition rate and N mineralization, the processes that control N availability, influence N20 fluxes from natural ecosystems (Werner et al. 2007). Low pH and soil moisture significantly suppress N20 emission. Water-filled pore space (WFPS) dictates the amount of aeration and oxygen concentration in the soil, and thus nitrification is the main process of N,0 emission at <60% WFPS. Denitrification becomes more important at soil water contents >60% WFPS due to decreased oxygen supply (Zaman et al. 2012; Butterbach-Bahl et al. 2013; Huang and Gerber 2015). Availability of other elector donors like Fe,+, Mn4+, NO,", and S042" in soil decrease emission of CH4 from paddy fields. There is no association between pH and CH4 production and consumption. CH4 emission is suppressed at low temperature (<25°C) and high soil oxygen concentration. CH4 and N20 emission are more present in fine-textured soil than coarse-textured soil because soils with dominant fine pores support the formation of CH4 and N20 under anaerobic conditions (Zaman et al. 2012; Oertel et al. 2016). Fertile soil with higher organic C and N shows higher N20 and CH4 emission than less fertile soils (Butterbach-Bahl et al. 2013; Lenka et al. 2017).
7.4.3 Fertilizer Application
Application of mineral N fertilizers into agricultural soils usually results in increasing N20 emissions. At N rates not exceeding or equal to those required for maximum yields, N rates tend to create a linear response in N20 emissions, with approximately 1% of applied mineral N lost as N20. Bhattacharyya et al. (2012) observed that N20 emissions increased with application of inorganic fertilizer and rice straw compared to a control in four years of paddy cultivation. They further reported cumulative N20 emissions were in the order of urea (1.0 kg ha'1) > rice straw + urea (0.84 kg ha'1) > rice straw + green manure (0.72 kg ha"1) > control (0.23 kg ha"1). In rice-wheat systems of Indo-Gangetic regions, urea application, either singly or in combination with rice straw, significantly increased the N,0 flux (Pathak et al. 2002; Bhatia et al. 2005). After nine years of experiment in soybean-wheat (Glycine max-Triticum aestivum), system application of inorganic fertilizer and organic manures increased the annual N20 emission in Vertisols of central India (Lenka et al. 2017).
Organic or chemical fertilizer application enhances CH4 production (Linquist et al. 2012b). Nitrogen fertilizers stimulate crop growth and provide more C substrates (via organic root exudates and sloughed-off cells) to methanogens for CH4 production (Malla et al. 2005; Banger et al. 2012). In addition to crop growth, N fertilizers alter the activities of methanotrophs in the soils (Bodelier and Laanbroek 2004; Schimel 2004). However, there are contradictory reports on the effects of N fertilizers on methanotrophs in rice soils. For example, Bodelier et al. (2000) reported that application of urea at 200-400 kg N ha-1 stimulated the activities of methanotrophs, which resulted in greater CH4 oxidation in the soil. In contrast, others have reported that N fertilizers suppress methanotrophs by attaching to the CH4 oxidizing enzyme “methane monooxygenase” of methanotrophs (Alam and Jia 2012; Serrano-Silva et al. 2014). Furthermore, due to similar genetic structure, methanotrophs can switch substrates from CH4 to ammonia when greater ammonium- N is available in soils (Alam and Jia 2012). These studies indicate the highly complex nature of the effects of N fertilizers on CH4 production and oxidation processes in rice soils (Bodelier and Laanbroek 2004).
In a meta-analysis, Banger et al. (2012) reported that N fertilizers increased CH4 emissions in 98 of 155 data pairs in rice soils. The response of CH4 emissions per kgN fertilizer was significantly (P < 0.05) greater at < 140 kg N ha-1 than > 140 kg N ha-1, indicating that substrate switch from CH4 to ammonia by methanotrophs may not be a dominant mechanism for increased CH4 emissions. On the contrary, decreased CH4 emissions in intermittent drainage by N fertilizers indicate stimulation of methanotrophs in rice soils. The effects of N fertilizer-stimulated methanotrophs in reducing CH4 emissions were modified by continuous flood irrigation due to the limitation of oxygen to methanotrophs. Greater response of CH4 emissions per kg N fertilizer in urea than ammonia sulfate probably indicated the interference of sulfate in the CH4 production process.
Seasonal flux of CH4 increased by 94% following application of fertilizer-N (urea). Wide variation in CH4 production and oxidation potentials was observed in rice soils tested. CH4 oxidation decreased with soil depth, fertilizer-N and nitrification inhibitors, while organic amendment stimulated it (Adhya et al. 2000). Nitrification inhibitors also have a considerable impact on CH4 emission through their inhibitory effect on CH4 oxidation due to higher conservation of ammonium in soil (Adhya et al. 2000; Jain et al. 2000; Wang et al. 2014). Exposure of soil to NH4 leads to an increase in the population of nitrifiers relative to methanotrophs and thus the overall CH4 oxidation is reduced, as nitrifiers oxidize CH4 less efficiently than methanotrophs (Chan and Parkin 2001; Serrano-Silva et al. 2014).
Apart from the quantity of fertilizer, the quality, mode, and time of fertilizer application also influence N,0 and CH4 emissions from soil. N,0 emissions from agricultural land may increase by 35%-60% by 2030 from N fertilizer and manure production (Bruinsma 2003). However, the rate of growth could slow down because of efficient use of fertilizers (Bruinsma 2017) and management- related factors, including N application rate per fertilizer type, fertilizer application technique, application timing, tillage system, irrigation, incorporation of crop residues, and composition. Many of these factors also affect CH4 and CO, emissions. N,0 production or consumption is generally controlled by nutrient availability in soil. It is, therefore, believed that fertilizer application increases the availability of nutrients, and increases global N,0 emissions by 10% (Bouwman et al. 1995). Butterbach-Bahl et al. (2013) mentioned the availability of reactive N as the major driver of N,0 soil emissions. Moreover, Palm et al. (2002) also attributed higher N,0 fluxes in cropping systems to N fertilization, and higher N,0 fluxes from tree-based systems to litter fall N, which are both components of nutrient availability.
7.4.4 Soil Management Practices
Soil management practices like tillage could have a large influence on the emission of N,0 and CH4 from fertilizer applied in soil. Tillage is the major mechanism by which soil is exposed to oxidation and thus loss of soil C. Intensity of tillage before sowing a crop is known to profoundly affect soil aggregation, water-holding capacity, water-filled pore space, and the concentration of oxygen and C02 in soil air, which directly affect the quantity of GHG emissions from soil. No-till (NT) accumulates a mulch of crop residue on the soil surface, which can result in higher contents of soil water and labile SOC fractions. Wetter soil conditions with available substrate due to no-tillage management may increase emissions of N20 (Ball et al. 1999; Venterea et al. 2005; Liu et al. 2006). However, some studies have shown lower N,0 emissions for NT soil or no difference between tillage systems, even with higher soil moisture content in NT. Contradictory findings may be a result of different cropping systems and soil types. Rochette (2008) recently summarized published experimental results showing that NT only increased N,0 emissions in poorly aerated soils.
Tillage, residue, and fertilizer management also affect nitrate-N concentration, water content, aeration, and available C (Myrbeck 2014; Wang et al. 2015; Wiese et al. 2016), which, in turn, can impact N loss through denitrification and N,0 emissions (MacKenzie et al. 1997; Mangalassery et al. 2014; Krauss et al. 2017). However, Hao et al. (2001) found that removing residues decreased N,0 emissions from an irrigated soil in southern Alberta, particularly on plots tilled in autumn after harvest. N,0 flux from agricultural soils depends on a complex interaction between climatic factors, soil properties, and soil management.