Currently, fertilizer-N use efficiency by agricultural crops is estimated to be approximately 50 per cent (Smil, 1999; Howarth et al, 2002; Ladha et al, 2005). In other words, on average half of the amount of the fertilizer-N applied cannot be recovered in the crop or in the soil and has to be considered to have been lost from the cropping system. Denitrification (N2O and N2), volatilization (NH3) and leaching (NO3) are the major pathways of N losses (Schlesinger, 1997; Mosier et al, 2001), causing a cascade of environmental and human health problems (Galloway et al, 2003). Therefore there is the potential for major improvements in N use efficiency through adopting fertilizer, soil water and crop management practices that focus on maximizing crop N uptake, with minimum N fertilizer losses and the optimum use of indigenous soil N (Ladha et al, 2005). Through optimizing N use efficiency, a total reduction in N2O emissions will be achieved as well as a further reduction in N2O emissions per kg yield or per kg crop N.
Improving fertilizer-N efficiency can be achieved through various management practices, which are often part of so-called best management practices (BMPs). BMPs try to reduce N input without a reduction in yield (McSwiney and Robertson, 2005; Adviento-Borbe et al, 2007). In intensively managed maize (Zea mays) cropping systems, using BMPs such as increasing planting densities, splitting N fertilizer applications and using higher N fertilizer rates, grain yields increased whereas emissions of N2O did not increase even though fertilizer-N input had increased by 40 to 92 per cent compared to conventional cropping systems (Adviento-Borbe et al, 2007).
Improved fertilizer-N use efficiency can also be obtained by using site-specific N management practices. Here the application of synthetic fertilizer-N is matched with the natural variability of available soil N and the specific demand of N by the crop at that particular site that is required to achieve maximum yield potential (Raun and Schepers, 2008). The amount of fertilizer-N to apply during the growing season, adjusted for each m2, can be based on optical sensors linked to an in-season estimate of yield. Using this technology, an improvement of 15 per cent in fertilizer-N use efficiency and grain yield of wheat was obtained compared to conventional management practices (Raun et al, 2002). Slow-release fertilizers allow for better matching of the supply with the crop demand during the growing season (Rao, 1987).
Management practices aimed at reducing NH3 volatilization and leaching can also help to increase fertilizer use efficiency. Fertilizer application timing and mode influence the NH3 volatilization and the efficiency of plant uptake, hence the availability of N for nitrification and denitrification. Timing and matching the N application with plant needs is important, because any prolongation of the period in which NH4+-based fertilizers can leach or undergo nitrification or NO3~-based fertilizers can be denitrified, without competition from plant uptake, is likely to increase emissions of NO and N2O (Ortiz-Monasterio et al, 1996; Smith et al, 1997; Chantigny et al, 1998).
However, some of these options may have an impact on processes further downstream along the N cascade. For example, subsurface application or injection of N fertilizers, to reduce NH3 volatilization, leads to higher N2O emission and leaching than broadcasting synthetic fertilizers and animal manure (Ellis et al, 1998; Kessavalou et al, 1998; Smith et al, 1998; Flessa and Beese, 2000).
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