NEWLY DISCOVERED LINKS BETWEEN AGRICULTURE, ENERGY, HEALTH, FISHERIES, TOURISM, AND CLIMATE
After thousands of years of stability, the chemistry of the surface of the Earth is changing rapidly (Schlesinger
1997). Changes in the cycles of nitrogen and phosphorus are substantial and linked in complex ways to changes
in agriculture and energy (Vitousek et al. 1997a and 1997b, Carpenter et al. 1998a and 1998b).
Until the beginning of this century, microbes and lightning were the primary sources of fixed nitrogen (the form
usable by plants), and human contributions were negligible. These non-anthropogenic sources currently generate
approximately 140 TG fixed N/y. Humans contribute to nitrogen fixation by making fertilizers, burning fossil fuels,
and planting legumes widely. Human activities now produce more than an additional 140 TG fixed N/y. As a
result, the total amount of terrestrial nitrogen fixed each year has more than doubled.
Until the advent of extensive mining activity, new phosphorus was made available primarily through weathering
of rock. Mining and land disturbances have now more than tripled the rate of phosphorus mobilization (from
about 10 to more than 30 Tg/y) and the rate of phosphorus flow from the continents to the coastal oceans (from
about 8 to 22 Tg/y).
When nitrogen and phosphorus were only scantily available to the biological world, they served as limiting
factors that controlled the dynamics, biodiversity, and functioning of many ecosystems. Ecosystems now flush with
excess fixed nutrients are changing rapidly. Nutrients unused by crops and lawns, livestock waste and sewage,
and airborne nitrogen resulting from the burning of fossil fuels are disrupting a wide range of downstream and
downwind systems. Excess nutrients stimulate the growth of algae and can lead to eutrophication, harmful algal
blooms, loss of oxygen ("dead zones") in lakes and coastal waters, fish kills, loss of seagrass beds, degradation of
coral reefs, and loss of commercial and sport fisheries and shellfish industries (Carpenter et al. 1998a and 1998b).
In addition, the chemistry of the atmosphere is being altered by human-driven changes in the nitrogen cycle, with
serious implications for the greenhouse effect, smog, and acid precipitation. Nitrate contamination is also a
potential concern for human health, particularly in drinking water drawn from relatively shallow aquifers in
agricultural areas (USGS 1999a).
Scientific uncertainties include the controls on nitrogen fixation and denitrification processes in ocean waters;
triggers of harmful algal blooms; transport of nutrients across the landscape and among air, soil, and water;
evolutionary consequences of long-term nutrient enrichment; and controls of nutrient-retention processes in
ecosystems. Particularly important questions address the control points that could allow us to mitigate the flows or
effects of excess nutrients. For example, how can floodplains and shorelines be configured to minimize nutrient
flow to surface waters? Also, we need to understand the role of large reservoirs of nutrients in the control of
regional and global cycles. For example, what is the rate of phosphorus buildup in agricultural soils, and what are
the implications of this buildup for freshwaters and coastal oceans?