Chapter 12 Nutrient Cycling Coordinating Lead

An adequate and balanced supply of elements necessary for life, provided through the ecological processes of nutrient cycling, underpins all other ecosystem services. The cycles of several key elements—phosphorus, nitrogen, sulfur, carbon, and possibly iron and silicon—have been substantially altered by human activities over the past two centuries, with important positive and negative consequences for a range of other ecosystem services and for human well-being. In preindustrial times, the annual flux of nitrogen from the atmosphere to the land and aquatic ecosystems was 90–130 teragrams (million tons) per year. This was more or less balanced by a reverse ''denitrification'' flux. Production and use of synthetic nitrogen fertilizer, expanded planting of nitrogen-fixing crops, and the deposition of nitrogen-containing air pollutants have together created an additional flux of about 200 teragrams a year, only part of which is denitrified. The resultant N accumulation on land and in waters has permitted a large increase in food production, but at the cost of increased emissions of greenhouse gases and a frequent deterioration in freshwater and coastal ecosystem services, including water quality, fisheries, and amenity value. Phosphorus is also accumulating in ecosystems at a rate of 10.5–15.5 teragrams per year, which compares with the preindustrial rate of 1–6 teragrams of phosphorus a year, mainly as a result of the use of mined P in agriculture. Most of this accumulation is occurring in soils, which may then be eroded into freshwater systems, causing deterioration of ecosystem services. This tendency is likely to spread and worsen over the next decades, since large amounts of P have accumulated on land and their transport to water systems is slow and difficult to prevent. Sulfur emissions have been progressively reduced in Europe and North America but not yet in the emerging industrial areas of the world: China, India, South Africa, and the southern parts of South America. A global assessment of acid deposition threats suggests that tropical ecosystems are at high risk. Human-induced alteration of the iron and silicon cycles is less well understood, but it is believed, with medium certainty, to be a significant factor in altering the productivity of the ocean. This may be a significant benefit to the service of carbon sequestration. Human actions, many associated with agriculture, have increased the ''leakiness'' of ecosystems with respect to nutrients. Tillage often damages soil structure, and pesticides may decrease useful nontarget organisms, increasing nutrient leaching. Simplification of the landscape …