ent cycling studies that include the entire soil profile indicate a considerable portion of the ecosystem's nitrogen and phosphorus stocks may be located in the soil (Jordan, 1985; Sanchez, 1979). However, additional research is required to determine more accurately the content and availability of these nutrients in the biomass versus in the soils.

The high efficiency of tropical forest nutrient cycles has long been recognized (Nye and Greenland, 1960; Sanchez, 1976). Agricultural systems generally operate in the same way, with one major exception: biomass is not removed from natural ecosystems, but crop harvests in agroecosystems can remove large quantities of biomass and constitute the main pathway of nutrient loss. In grain crops, about 40 percent of the carbon, 60 percent of the nitrogen, and two-thirds of the phosphorus in crops are removed with the harvest, while most of the potassium, calcium, and magnesium remain in the crop residues (Sanchez et al., 1989). In an agricultural or forestry system, nutrients lost through harvesting must be balanced with nutrient inputs in the form of fertilizers, manures, or biological nitrogen fixation.

In agricultural systems dominated by annual crops, the flow of nutrients from soil to crop occurs seasonally and must be extremely rapid if high yields are to be attained. As crop residues are returned to the soil, they are broken down by soil fauna and flora into simple components, which are then available for uptake by the next crop. Losses from the system can occur if crop residues are removed from the field, if soil is lost through erosion, or if soluble nutrients remain in the soil with no crop growth during periods of heavy rain. The use of crop or animal residues as fuel can be a major source of nutrient (and carbon) loss from the system.