METHANE

2.4.1 Emission overviews and biological processes

Methane is a critical GHG with a GWP of 34 times stronger than that of CO2 in 100-year horizon (IPCC, 2013). CH4 contributes to 17% radioactive forcing by LLGHGs (WMO, 2019). The atmospheric CH4 nowadays is 259% of that in preindustrial time and the increment mainly attributes to anthropogenic emissions (WMO, 2019). Anthropogenic sources account for 60% of the atmospheric CH4 abundance including cattle farming, rice agriculture, fossil fuel exploitation, landfills and biomass burning (WMO, 2019). In the decade of 2001 to 2011, emissions from synthetic fertilizers containing both CH4 and N2O accounts for the third most agricultural emissions, among which synthetic fertilization induced CH4 emissions grew faster (3.5%) than

other agricultural contributors (FAO, 2014). It is projected that synthetic fertilizer would be the second largest CH4 source in the next decade (FAO, 2014).

The methanogenic archaea are the main CH4 biological source (Peters and Conrad, 1995; Angel, Claus and Conrad, 2011). In the soil, CH4 is generated from methanogenesis in anaerobic layer and travels from soil to atmosphere by diffusion, aerenchyma structures and ebullition (Serrano-Silva et al., 2014; Marushchak et al., 2016). Diffusion is the primary gas translocation pathway in well aerated agricultural soils. The amount of methanogens in aerobic soil is often less than that in anoxic soils (Peters and Conrad, 1995; Angel, Claus and Conrad, 2011).

The primary biological process that consume CH4 and integrates carbon cycle is CH4 oxidation (Dunfield et al., 2003). It is an aerobically enzymatic reaction catalyzed by monooxygenase (MMO) (Dunfield et al., 2003). Methane is predominantly oxidized by anaerobic bacteria (methanotrophs) in both aerobic and anaerobic soil conditions during its translocation (Curry, 2009; Knittel and Boetius, 2009). Methane oxidizing bacteria (methanotrophs) are the most well-known biological CH4 sink (Shrestha et al., 2010). They contain two types and consume CH4 as a carbon and energy source (Shrestha et al., 2010). Type I methanotrophs are more dynamic, rapidly proliferate and be active under favorable conditions such as in rhizospheres and even more on roots (Shrestha et al., 2010). Type II are more adapted methanotrophs that fit less favorable conditions such as in soils other than on roots, where is less abundant with CH4 (Shrestha et al., 2010). Both types of methanotrophs are found in various soils including forest, rice, landfill and agriculture (Jensen et al., 1998; Wise, McArthur and Shimkets, 1999; Eller and Frenzel, 2001; Reay et al., 2001). The abundance of either type is the result of the competition of available CH4. Type I methanotrophs dominate low CH4 and high O2 conditions while those of type II prevail the opposite situations (Graham et al., 1993). Besides, studies found that atmospheric CH4 oxidizer can oxidize CH4 at lower concentration (<1 nM) than cultured methanotrophs (Henckel, Friedrich and Conrad, 1999). Apart from methanotrophs, ammonia oxidizing bacteria are also able to oxidize CH4 due to its enzymatic system, but they are not able to gain energy from the process (Seghers et al., 2003).

It is also reported that ammonia oxidizing bacteria is enriched in minimally fertilized soils (Hermansson and Lindgren, 2001).

2.4.2 Fertilization on methane flux

Nitrogen fertilizer is found a prohibitor to CH4 oxidation as soil CH4 sink ability is constrained under fertilization (Hütsch W., Webster and Powlson, 1993; Hütsch W., 1996; Kravchenko et al., 2002; Seghers et al., 2003). The inhibition mechanisms differ in respect of short and longterm N fertilization. In short term fertilization, the inhibition of CH4 oxidation is caused by the interference of NH4+ which impact the methanotrophic enzyme system (Boeckx and Van Cleemput, 1996;

Tlustos et al., 1998). NH4+ is a competitive inhibitor of CH4 monooxygenase due to its low substrate specificity (Seghers et al., 2003). It is also an inhibitor for high affinity CH4 oxidation methanotrophs while NO3- does not affect high affinity CH4 oxidation (Seghers et al., 2003). Long-term N fertilization leads to a shift of microbial community composition (Seghers et al., 2005) and the influence is significantly pronounced on roots other than that of rhizospheres’ soils (Shrestha et al., 2010). It is reported that addition of N fertilizer promotes activities of ammonia oxidizing bacteria (Hermansson and Lindgren, 2001) thus leading to the competition for CH4 between methanotrophs and the ammonia oxidizing bacteria (Seghers et al., 2003). The microbial shifting diminishes atmospheric CH4 oxidizing capacity of agricultural soils (Seghers et al., 2003). The chronical effect is observed in an agricultural grassland that annually fertilized with NH4NO3

(Mosier et al., 1991). It also indicates that low affinity methanotrophs do not present or enrich in fertilized mineral soils (Seghers et al., 2003). However, N fertilizer addition meanwhile improves N substrates for methanotrophs’ growth and development, which might contribute to CH4 oxidation (Hahn, Arth and Frenzel, 2000; Paul et al., 2000; Schimel, 2000; Dan et al., 2001).

2.4.3 Other influential factors

Various factors influence methane generating and consuming including soil temperature, soil moisture, PH, agricultural management and other factors (Topp and Pattey, 1997; Le Mer and Roger, 2001; Kammann et al., 2009). Soil temperature impact both methanogenic and methanotrophic activities due to the sensitivity of underlying enzymatic process (Steinkamp, Butterbach-Bahl and Papen, 2001; Butterbach-Bahl and Papen, 2002). In general, the optimum CH4 production temperature ranges from 35 to 40℃ by slurry incubation experiment (Fey, Chin and Conrad, 2001; Conrad, 2002) while temperature around 25℃ and low salinity create a favorable environment for CH4 consumption (Serrano-Silva et al., 2014). Noteworthy, temperature influence is more pronounced under 15 °C because higher temperature leads to gas diffusion limits

and draught effects rather than temperature sensitivity (Steinkamp, Butterbach-Bahl and Papen, 2001)

Gas diffusion which is a major limiting factor coordinating activities of methanogens and methanotrophs was governed by soil moisture (Khalil and Baggs, 2005). Hence, soil moisture conditions control CH4 flux rate under both high and low soil moisture due to either physiological water stress or O2 and CH4 diffusion burdens (Khalil and Baggs, 2005). Higher soil water contents are favored by methanogens since it maintains a habitable environment (Andersen et al., 1998;

Peng et al., 2008; Prem, Reitschuler and Illmer, 2014). While CH4 consumption is ambiguous under the combined influence of both temperature and soil moisture. Some studies found that increasing water content at higher temperature results in a negative effect on CH4 consumption than lower temperatures (1-10℃). In other words, CH4 consumption respond to temperature declines with moisture abundance (King and Adamsen, 1992; Khalil and Baggs, 2005; Shukla, Pandey and Mishra, 2013). Moreover, declination of soil moisture may result in an increased evapotranspiration that promotes the enzyme activity of CH4 oxidizing bacteria thus increasing CH4 consumption (Blankinship et al., 2010).

Agricultural soils reside both methanogens and methanotrophs, the CH4 flux from the agricultural soil is the net result of both methane production and methane oxidation (Peters and Conrad, 1995;

Angel, Claus and Conrad, 2011) while upland soils are usually CH4 sinks rather than sources (Maljanen et al., 2010).