Mitigation of N2O emissions through conservation agriculture

The two main processes of the N cycle that determine the production of N2O are nitrification and denitrification. Denitrification occurs under anaerobic conditions where nitrate is reduced to various N forms as follows:

Any management practice that creates anaerobic conditions including flooding, especially in heavy textured soils, when nitrate is present will lead to increased N2O emissions (Ball et al., 1999). These emissions can be reduced by aerating the soil, especially in coarse textured soils, as evidenced by reduced emissions in permanent raised-bed planted crops (Patiño-Zúñiga et al., 2009).

The other N cycle process that generates N2O is nitrification. This occurs under aerobic conditions with the oxidation of ammonia to NO2- and finally NO3-. If this soil is then flooded, denitrification can occur. N2O is also released from soils to the atmosphere during nitrification of ammonium and ammonium-producing fertilizers under aerobic conditions. This can be significant during fertilizer applications. Such emissions can be greatly reduced through the use of nitrapyrin, which inhibits nitrification of ammonium by soil microorganisms.

These two N-cycle processes are mainly influenced by factors such as soil temperature, soil moisture content, pH, supply of C and N compounds (Skiba et al., 1998; Lee et al., 2006) and soil electrical conductivity (Adviento-Borbe et al., 2006). These factors can be manipulated by tillage (Venterea et al., 2005), residue management, irrigation (Qian et al., 1997) and the application of N fertilizer (Smith et al., 1997). Increased soil organic matter can also result in increased N2O emission through the increase in N cycling in the soil as nitrification is stimulated (Butterbach-Bahl et al., 2004). However, zero tillage combined with residue retention results in a better soil structure, facilitating O2 diffusion and reducing the amount of anaerobic sites in the soil, and stimulating oxidation of CH4, but it remains to be seen how emissions of NO and N2O are really affected.

GHG emissions were studied in water-saving experiments with rice in China (Dittert et al., 2002). The results showed a significant drop in CH4 emissions with a more aerobic rice production system compared to a flooded rice system, but N2O emissions increased, especially after N fertilizer application. The researchers concluded that N fertilizer applications needed to be optimized to minimize N2O emissions, especially since this gas has a much greater heating potential than CH4 or CO2. Patiño-Zúñiga et al. (2009) observed in a laboratory incubation experiment that the N2O emission from conventional tillage with residue retention was 2.3 times larger compared to no tillage with residue retention. Jacinthe and Dick (1997) observed that the seasonal N2O emission from zero tillage was significantly lower than from conventional tillage (chisel tillage) under continuous maize, maize-soybean rotation and maize-soybean/ wheat-hairy vetch rotation in Ohio, USA. Kessavalou et al. (1998) demonstrated that the application of tillage during fallow increased the N2O flux by almost 100% relative to the no-tillage treatment. Robertson et al. (2000) reported that N2O emissions from zero tillage were similar to or slightly higher than from conventional tillage under maize-wheat-soybean rotations in the Midwest USA. Rochette et al. (2008) demonstrated in a 3-year study in East Canada that the average N2O emissions from zero tillage were more than double those from conventional tillage in a heavy clay soil. In a loam soil, the average emissions during the 3 years were similar in the two treatments. Rochette (2008) concluded that N2O emissions only increased in poorly drained, finely textured agricultural soils under zero tillage located in regions with a humid climate, but not in well-drained aerated soils. Six et al. (2004) compiled all available data of soil-derived GHG emission comparisons between conventional tilled and no-tillage systems for humid and dry temperate climates. They concluded that in both humid and dry climates, differences in N2O emissions between the two tillage systems changed over time. In the first 10 years, N2O fluxes were higher in zero tillage compared to conventional tillage, regardless of climate. After 20 years, however, N2O emissions in humid climates were lower in zero tillage than conventional tillage and were similar between tillage systems in the dry climate.

The key for the implementation of CA as a GHG mitigation strategy is the understanding of the integrated effect of the practice on all GHGs and developing the necessary component technologies and fertilization practices to reduce the emissions of N2O, since any gains in reduction of CO2 and CH4 emissions could be lost if these practices resulted in increased N2O emissions. Part of the conflicting results with zero-tillage and CA practices is related to development of the optimal implementation of the system. Years of research and development have been spent to optimize conventional tillage systems but a knowledge base for CA on all production-related components in different locations has yet to be developed. For example land levelling is a component technology that drastically increases the efficiency of zero tillage and CA in flood irrigated systems. In this case farmers level their fields using equipment on their farms. However, the use of laser land levelling results in an even better field level.

Well-levelled fields give much better results with CA whether they are planted on the flat or on beds; in particular, water productivity can be significantly improved.

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