Efficient use of nitrogenous fertilisers can reduce N2O emissions from agricultural fields. In addition, by reducing the quantity of synthetic fertilisers required, improved management can also reduce CO2 emissions associated with their manufacture. In this article a variety of fertiliser management technologies are discussed in brief, followed by a discussion on their relative advantages and disadvantages.
Nitrous oxide mitigation in organic agriculture
Organic agriculture reduces emission of N2O due to the ban on the use of mineral nitrogen and the reduction in livestock units per hectare. A diversified crop rotation with green manure in organic farming improves soil structure and diminishes emissions of N2O, although the nitrogen provided by the green manure does contribute to N2O emissions. Soils in organic farming are more aerated and have significantly lower mobile nitrogen concentrations, which reduces emissions of N2O. Since organic crop systems are limited by the availability of N, they aim to balance their N inputs and outputs and their N use efficiency. Thus, their emissions are lower than those of conventional farming systems per unit of land area. However, with lower yields from organic farming, the emissions per unit of produce could be the same or higher. (Petersen et al., 2006).
Mitigation using nitrification inhibitors
Emission of N2O can be reduced by using nitrification inhibitors which slow the microbial processes that lead to N2O formation (figure 1; Robertson, 2004). The use of nitrification inhibitors such as: S. benzylisothiouronium butanoate (SBT butanoate) and S. benzylisothiouronium fluroate (SBT fluroate) increased yield of crop plants (figure 2), reduced emissions of N2O by 4-5%, and, because N2O is a more potent greenhouse gas than CO2, reduced global warming potential by 8.9-19.5% compared to urea treatment alone, thereby helping to mitigate N2O emission (Bhatia et al., 2010).
Nitrification and urease inhibitors can reduce the loss of N as N2O. The application of dicyandiamide (DCD) and Nitrapyrin to grassland reduced the emission of N2O from NH4+ based fertilisers by 64% and 52% respectively (McTaggart et al., 1994).
Slow release fertiliser application and manipulation technologies
Fertiliser application technology significantly influences nitrous oxide emissions. The various parameters of this technology are described below:
- The use of slow release fertilisers offers a cost effective mitigation option. Slow release of urea and NH4 based fertilisers can be achieved by using various coatings, chemical modifications, and changing the size of fertiliser granules (figure 3). For example, increasing the size of urea granules from conventional 0.01g to 1g decreased nitrification rates and was shown to be more effective than adding the nitrification inhibitor DCD (Skiba et al., 1997).
- A combination of increasing the size of pellet to 1g and adding DCD led to very slow nitrification rates, with 30% of the original N application still present 8 weeks after fertiliser application (Goose and Johnson, 1993).
- Global warming potential (GWP) due to N2O reduced from 231kg CO2e ha-1 on urea application to 200kg CO2e ha-1 under urea and SBT fluroate treatment under conventional tillage, whereas under zero-tillage it was reduced from 260kg CO2e ha-1 with urea alone to 210kg CO2e ha-1 with SBT fluroate (Bhatia et al., 2010). These reductions in global warming potential were 13.5% and 19.5% due to SBT fluroate compared to urea alone under conventional and zero-tillage, respectively.
Nitrogen management technology
Fertiliser nitrogen management practices significantly influence the emissions of nitrous oxide in agriculture. These practices are fertiliser type, timing, placement, and rate of fertiliser application, as well as coordinating the time of application with irrigation and rainfall events. Each direct nitrogen management practice influences nitrous oxide emissions.
Type of fertiliser: Nitrous oxide production can be affected by the form of fertiliser applied. Venterea et al (2005) observed that plots amended with anhydrous ammonia emit N2O at rates 2-4 times greater than from those amended with urea, ammonium nitrate, or broadcast urea. Tenuta and Beauchamp (2003) found that the relative magnitude of total emissions was greater from urea than from ammonium sulphate, which in turn was greater than that from calcium ammonium nitrate. Bouwman et al (2002) found that nitrate-based fertiliser resulted in significantly lower emissions of N2O than ammonium-based fertiliser. Snyder et al., (2007) demonstrated that slow, control release and stabilised N fertiliser can enhance crop productivity and minimize the N2O emissions. Emissions of N2O were significantly higher from a soil fertilised with urea compared to NH4NO3 (Mc Taggart et al., 1994). NH4NO3 was beneficial in reducing the volatilisation of NH3 and the emission of N2O. Another compound, NH4HCO3, when used as basal fertiliser, contributed less to N2O in contrast to urea.
Fertiliser N timing: Synchronous timing of N fertiliser application with N demand from plants is an important factor in determining the emissions of N2O from row crop cultivation. Crop nitrogen intake capacity is generally low at the beginning of the growing season, increasing rapidly during vegetative growth and dropping sharply as the crop nears maturity. Prior to spring crop planting results in increased soil N with poor plant N uptake, and therefore, it results in increased N2O emissions. About 30% of the US area cropped to corn is fertilised in autumn (CAST, 2004). Therefore, large emissions of N2O could potentially be avoided by fertilising in spring rather than autumn. Hultgreen and Leduc (2003) showed that emissions of N2O were lower following spring N fertiliser application compared to autumn application.
Fertiliser N placement: Placement of N fertiliser into the soil near the zone of active root uptake may reduce surface N loss and increase plant N use resulting in a reduction in N2O emissions (CAST, 2004). Liu et al (2006) found that injection of liquid urea, ammonium nitrate at a deeper level in soil profile (10-15cm) resulted in 40-70% lower emission of N2O compared to shallow injection (5cm) or surface application. Hultgreen and Leduc (2003) reported that the N2O emissions were reduced when urea was broadcast in mid-row rather than side-banded.
Fertiliser N rate: The emission of N2O correlates well with fertiliser N rate (Drury et al., 2008). Millar et al (2010) also report that increasing the amount of N applied to soil resulted in increasing emissions of N2O.
Global warming potential in a no-N treatment of conventional transplanted rice was 1,419kg CO2e ha-1, whereas GWP under traditional nutrient application of NPK was 6,730kg CO2e ha-1 (Pathak, 2010). The loss in yield was not significant.
Millar et al (2010) suggested that the incentive for nitrous oxide emission reduction by application of lower nitrogen application rates within a profitable range ultimately could be financially remunerated through a carbon or nutrient market. That would bring economic and environmental advantages to compensate for lost productivity benefits due to the use of higher nitrogen application rates.
Coordination with irrigation and rainfall events: application of fertiliser immediately after rain will increase N use efficiency of plants and mitigate N2O emissions. Losses of N through leaching, volatilisation, and denitrification in a farmer’s rice field (which had received 67.5kg N ha-1 after rain) decreased up to 40.5kg N ha-1 compared to total amount of loss which was 80.3kg N ha-1 with the farmer’s practice of alternate flooding. The exception was when there were mid-season drainage or alternate flooding and drainage cycles, in which case it increased (Pathak, 2010). The N management regime also reduced global warming potential (GWP) by 1 to 9%.
Nitrogen fertilisation is a significant input cost for farmers worldwide, and therefore, some of the approaches, such as split applications of fertiliser to better match plant uptake needs, are in common use. On the other hand, chemical inhibitors are relatively expensive, so they are less widely used, but nevertheless have gained some acceptance as suggested by the number of positive yield studies in the United States (figure 2).
Besides costs, lack of knowledge and education are barriers. Research is needed to determine the best management practices for specific crops and local conditions.
Improved nitrogen fertiliser management has many environmental benefits such as:
- Reductions in N2O emissions can be achieved by relatively simple adjustments in the farming practices, such as using fertiliser in larger granules and applying it in more frequent, smaller applications, yet high productivity can be maintained.
- Increase in farm N use efficiency will reduce leaching of NO3– to ground water.
- Making crops more N-use efficient will decrease the need for inorganic N fertilisers and thereby reduce emissions from fossil fuel associated with their manufacture.
- The use of chemical inhibitors of N2O emissions may leave unacceptable residues, and they may not be effective in certain types of soil.
- The present prices of chemical inhibitors of N2O emission are quite high, so they aren’t affordable to many farmers, and they are not commercially available in many regions.
As presented above in the individual sections, the several N management approaches have a high potential for reducing greenhouse gas emissions. However, costs for nitrification inhibitors are high, and reducing rates of N applications can have negative impacts on productivity. On the other hand, relatively simple changes, such as increasing particle size of the fertilisers and changing the timing of applications can minimise emissions with little or no additional cost or loss of productivity.
See the individual sections above for details about financial requirements and costs.
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