Productivity of agro-ecosystems

The growth of agricultural production worldwide has led to an increasing role of agro-ecosystems within the global carbon cycle. The objective of this chapter is to estimate the contribution of agricultural crops in this total carbon flux. In addition to arable land, grasslands are used for agricultural purposes. They are mainly used for grazing by cattle and sheep, at stocking densities that vary enormously, with the extensively managed rangelands of Australia or Latin America on one end of the scale and the “green carpets” of the Netherlands on the other. Grasslands cover about one-third of the vegetated area and contribute about one-fifth to the global carbon cycle. The data presented in this chapter is on the production of major crop groups in terms of carbon and nitrogen. The decadal time trend is analyzed, the relation with nitrogen fertilizer production is indicated, and productivity is discussed in relation to other growth-limiting factors. It is shown that application of nitrogen fertilizer alone accounts for 60% of the agricultural net primary production. The study presented in this chapter is largely based on production statistics. A complementary agro-ecological study, which relates agricultural productivity to the availability and use of natural resources, would be most welcome. Such a study would require a geographically explicit approach, in which each production situation is characterized by the prevailing ecosystem, its climatic conditions, soil type and soil fertility, and the availability of water resources for irrigation.