Biological Cycles

Abstract

Soil fertility depends on processes of both decay and synthesis. In natural ecosystems, a large and diverse population of living organisms well attuned to their environment accomplishes the conservative cycling of energy and nutrients. Turnover of the more easily decomposed fractions of soil organic matter takes only a few months to a few years, but turnover of recalcitrant fractions occurs over hundreds or thousands of years. Farming systems have reversed the natural accumulation of carbon in soils; the energy gap, created by the export of large amounts of biomass off-farm, is inexorably depleting the reserves of soil organic matter that have been built up over millennia. In natural ecosystems, pathogens are held in check by competition between many species in the food web. Agroecosystems are much simpler, less diverse, usually less well adapted to the environment and prey to outbreaks of weeds, pests and disease. Modern farming systems have replaced dependence on biological cycles by application of artificial fertilizer, chemical herbicides and pesticides. However, mineralization of organic matter, especially the more-easily decomposed or labile organic matter, still contributes a big share of crop nutrients. Nitrogen, in particular, is often released in excess of crop requirements and contaminates the groundwater, streams and lakes.