A particular nutrient atom may be taken up by a plant that is then eaten by a herbivore which then dies and is decomposed, releasing the atom back to the soil from where it is taken up through the roots of another plant. In this manner, nutrients may circulate within the community for many years. Alternatively, the atom may pass through the system in a matter of minutes, perhaps without interacting with the biota at all. Whatever the case, the atom will eventually be lost through one of the variety of processes that remove nutrients from the system (see Figure 18.2). These processes constitute the debit side of the nutrient budget equation.
Release to the atmosphere is one pathway of nutrient loss. In many communities there is an approximate annual balance in the carbon budget; the carbon fixed by photosynthesizing plants is balanced by the carbon released to the atmosphere as CO2 from the respiration of plants, microorganisms and animals. Other gases are released through the activities of anaerobic bacteria. Methane is a well-known product of the soils of bogs, swamps and floodplain forests, produced by bacteria in the waterlogged, anoxic zone of wetland soils. However, its net flux to the atmosphere depends on the rate at which it is produced in relation to its rate of consumption by aerobic bacteria in the shallower, unsaturated soil horizons, with as much as 90% consumed before it reaches the atmosphere (Bubier & Moore, 1994). Methane may be of some importance in drier locations too. It is produced by fermentation in the anaerobic stomachs of grazing animals, and even in upland forests, periods of heavy rainfall may produce anaerobic conditions that can persist for some time within microsites in the organic layer of the soil (Sexstone et al., 1985). In such locations, bacteria such as Pseudomonas reduce nitrate to gaseous nitrogen or N2O in the process of denitrification. Plants themselves may be direct sources of gaseous and particulate release. For example, forest canopies produce volatile hydrocarbons (e.g. terpenes) and tropical forest trees emit aerosols containing phosphorus, potassium and sulfur (Waring & Schlesinger, 1985). Finally, ammonia gas is released during the decomposition of vertebrate excreta and has been shown to be a significant component in the nutrient budget of many systems (Sutton et al., 1993).
Other pathways of nutrient loss are important in particular instances. For example, fire can turn a very large proportion of a community's carbon into CO2 in a very short time. The loss of nitrogen as volatile gas can be equally dramatic: during an intense wild fire in a conifer forest in northwest USA, 855 kg ha-1 (equal to 39% of the pool of organic nitrogen) was lost in this way (Grier, 1975). Substantial losses of nutrients also occur when foresters or farmers harvest and remove their trees and crops.
For many elements, the most important pathway of loss is in stream flow. The water that drains from the soil of a terrestrial community, via the groundwater, into a stream carries a load of nutrients that is partly dissolved and partly particulate. With the exception of iron and phosphorus, which are not mobile in soils, the loss of plant nutrients is predominantly in solution. Particulate matter in stream flow occurs both as dead organic matter (mainly tree leaves) and as inorganic particles. After rainfall or snowmelt the water draining into streams is generally more dilute than during dry periods, when the concentrated waters of soil solution make a greater contribution. However, the effect of high volume more than compensates for lower concentrations in wet periods. Thus, total loss of nutrients is usually greatest in years when rainfall and stream discharge are high. In regions where the bedrock is permeable, losses occur not only in stream flow but also in water that drains deep into the groundwater. This may discharge into a stream or lake after a considerable delay and at some distance from the terrestrial community.
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