ABSTRACT
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.
