Nutrients are gained and lost by ecosystems in a variety of ways (Figure 18.2). We can construct a nutrient budget by identifying and measuring all the processes on the credit and debit sides of the equation. For some nutrients, in some ecosystems, the budget may be more or less in balance.
In other cases, the inputs exceed the outputs and nutrients accumulate in the compartments of living biomass and dead organic matter. This is especially obvious during community succession (see Section 17.4). Finally, outputs may exceed inputs if the biota is disturbed by an event such as fire, massive defoliation (such as that caused by a plague of locusts) or large-scale deforestation or crop harvesting by people. Another important source of loss in terrestrial systems occurs where mineral export (e.g. of base cations due to acid rain) exceeds replenishment from weathering.
The components of nutrient budgets are discussed below.
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