Abstracts |
The essential constituent elements of land plants and animals move in biogeochemical cycles that begin with the uptake of nutrients from the soil, extending through allocations to plant biomass, especially photosynthetic tissues, and nutrient return to the soil in plant detritus. Decomposition of dead plant materials releases these nutrient elements for reuse or potential loss to stream waters. At any given time, this internal cycle of elements dwarfs the new sources of nutrients to the ecosystem from the atmosphere or rock weathering. Essential elements are strongly conserved within ecosystems, whereas nonessential or nonlimiting elements are potentially lost to stream water. For nitrogen, microbial transformations among forms in the soil result in small losses of nitrogen trace gases to the atmosphere. Animals influence the movement of materials within terrestrial ecosystems, and periodic fires result in large losses of nutrients. Human activities, resulting in acid rain, excess nitrogen deposition, rising CO2, and global warming, have all altered the cycle of essential elements in terrestrial ecosystems. |