World population has quadrupled during the 20th century. This has been accompanied by a dramatic increase in agricultural productivity, based to a major extent on industrial nitrogen fixation by the Haber-Bosch process developed at the beginning of that century. From a global perspective, population growth was, and still is, larger than the growth in agricultural productivity per unit land area. Consequently, agriculture continues to expand into previously pristine ecosystems. Also, emerging and growing demands for new products, such as biofuels from sugar cane, maize or palm oil, result in land-use changes. Increasing affluence in many parts of the world boosts demands for meat and milk products, driving a conversion of large areas of tropical forests into grasslands and soya bean fields. Land-use change has also taken place in temperate and boreal regions, where pristine grasslands have been converted to cropland, and wetlands have been drained to establish commercial forestry, pastures or cropland. Such changes often involve the clearance of forest vegetation, drainage, tillage of soil, and the introduction of new crops and animals, often combined with the use of mineral fertilizers and pesticides. Each activity affects plant and animal compositions and density, microclimate, hydrology and soil properties. Consequently, cycles of water, C, N, P and other nutrients are disturbed. If sustainable, a new type of land use may lead to different equilibria of pools and fluxes of nutrients. Where land use is not sustainable, it will be transitory and often results in abandonment and secondary forest succession (Hirsch et al, 2004). During transition from one type of land use to another, factors affecting N2O emissions may change. Emissions may also be different from those in the pristine situation, once a new equilibrium has been reached.
N2O is the product of microbial transformations of a range of N compounds in soil. Related processes and pathways are discussed in Chapter 2. In the context of land-use change, it is important to remember that the major control of N2O emissions is the amount of mineral N (ammonium [NH4+] and nitrate [NO3~]) turned over by the soil microbial population (described by the hole-in-the-pipe model - see Chapter 5). Microbial turnover of N is a function of total N in the system, microbial activity and, importantly, the competition between plants and microbes for N. Any N taken up by plants no longer has the potential to be turned into N2O before it is returned again to the soil. Advances in understanding the competition between plants and the soil microbial population, and how it affects turnover in N-limited and more fertile soils, has led to a new paradigm of N mineralization (Schimel and Bennett, 2004), which may help us to understand the effect of land-use changes on N2O emissions. Batjes (1996) estimated that between 133 and 140Pg of organic N are stored in the organic matter of soils globally. Organic N becomes available to plants and microbes in successive steps, whereby the first ones play a minor role in most agricultural systems. The first of these steps is the depolymerization of organic matter by extracellular enzymes (Chapin et al, 2002). No N2O is produced in this process. The resulting monomers (amino acids, amino sugars, etc.) are suggested to be the main forms of N taken up by plants and microbes when N availability is low. Plants access these compounds with the help of mycorrhizal fungi (Hobbie and Hobbie, 2008). With increasing N availability, amino acids are mineralized to NH4+, which is again taken up quickly by plants and microorganisms. Again, no N2O is produced in the process. Where heterotrophic nitrifiers oxidize organic N to NO3~ (heterotrophic nitrification of organic N), N2O may be produced (Bateman and Baggs, 2005). The presence of N-fixing plants or external inputs of organic and mineral fertilizers in an ecosystem can substantially increase N availability, lead to increasing concentrations of NH4+ in soil and reduce the competition between plants and microbes. Autotrophic nitrifiers will flourish, derive energy from oxidizing NH4+ to NO3~ and generate some N2O as a byproduct. When N availability increases even further, the proportion of NH4+ transformed into NO3~ increases, as does the concentration of NO3~, which may be taken up by plants or reduced stepwise by O2-limited microorganisms to N2O or N2 - the denitrification process. There are more pathways of N2O production, discussed elsewhere (for example Bothe et al, 2007, and in Chapter 2). In summary, the number of possible N transformations in the course of which N2O may be produced, as well as the total amounts of N2O generated, increase with increasing N availability. Understanding the influence of land-use changes on N availability enables us to infer consequent effects on N2O emissions.
A major factor governing N availability in ecosystems is the C:N ratio of plant litter and soil organic matter, which depends on the vegetation and can change with land use. In a tropical forest, for example, the C:N ratio of litter can be 25, whereas it reaches values of 60 to 70 in pastures replacing it (Wick et al, 2005). Larger C:N ratios are associated with smaller rates of mineral N release, transformation and ultimately N2O emission. This has to do with the C:N ratio of the microbial decomposer community and its C use efficiency (Manzoni et al, 2008). Soil microbial biomass has a relatively stable C:N ratio of around 8.6 across different ecosystems (Cleveland and Liptzin, 2007). Let us assume that from one unit of C in organic matter, decomposers assimilate 0.4 and respire 0.6 units (Chapin et al, 2002). Then, N is in excess of the microbial requirement for a C:N ratio of 8.6 and excreted as NH4+ when the C:N ratio of the decomposed organic matter is below 21.5 (8.6:0.4). The proportion of excreted N increases strongly with decreasing C:N ratio. Springob and Kirchmann (2003) determined an increase in N release by about a factor of five when C:N ratios in soil organic matter dropped from 15 to 10. Equally, large C:N ratios result in small rates of mineral N release during decomposition and, subsequently, in smaller rates of mineral N turnover and N2O formation.
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