Nutrients cycle at different rates according to element identity and ecosystem type
The time it takes a nutrient molecule to cycle through an ecosystem, from uptake by organisms to release to subsequent uptake, can vary substantially depending on the element in question and the ecosystem where the cycle is occurring.
In general, nutrients that limit primary production are cycled more rapidly than nonlimiting nutrients. For example, nitrogen and phosphorus may cycle through the photic zone of the open ocean over a period of hours or days, while zinc may cycle over geologic time scales associated with sedimentation, mountain building, and erosional processes. Nutrient cycling rates also vary with climate because of the effects of temperature and moisture on the metabolic rates of the organisms associated with production, decomposition, and chemical transformations of nutrients.Biogeochemists measure rates of nutrient cycling by estimating the mean residence times of elements in some component of an ecosystem:
The mean residence time is the amount of time an average molecule of an element spends in a pool before leaving it. The pool of an element is the total amount found within a physical or biological component of the ecosystem, such as soil or biomass. The inputs include all possible sources of the element for that ecosystem component. This approach to estimating mean residence time assumes that pools of nutrients do not change over time and that the mean residence time reflects the overall rate of nutrient cycling. It is most commonly used for estimating rates of nutrient turnover in soil organic matter, which reflect rates of nutrient input and subsequent decomposition. Decomposition rates, as we have seen, are related to climate and the chemistry of plant litter.
Given that both inputs of plant litter and decomposition rates control the mean residence times of nutrients in soil, and that both are subject to climatic control, what differences would we expect to see among ecosystems with similar plant
growth forms (e.g., forests) in different climates? Relative to boreal and temperate forests, tropical forests have higher net primary productivity, and therefore higher litter input rates.
Does this difference result in differences in the mean residence times of nutrients? A comparison of mean residence times for organic matter and for several nutrients indicates that nutrient pools in the soils of tropical forests are much smaller than those in boreal forests (TABLE 22.3). The turnover rates of nitrogen and phosphorus are more than 100 times faster in tropical forest soils than in boreal forest soils. Temperate forests and chaparral have turnover rates that fall in between but are closer to those in the tropics.TABLE 22.3 Mean Residence Times of Soil Organic Matter and Nutrients in Forest and Shrubland Ecosystems
| Mean resident time (yr) | ||||||
| Ecosystem type | Soil organic matter | N | P | K | Ca | Mg |
| Boreal forest | 353 | 230 | 324 | 94 | 149 | 455 |
| Temperate coniferous forest | 17 | 18 | 15 | 2 | 6 | 13 |
| Temperate deciduous forest | 4 | 5 | 6 | 1 | 3 | 3 |
| Chaparral | 4 | 4 | 4 | 1 | 5 | 3 |
| Tropical rainforest | 0.4 | 2 | 2 | 1 | 1.5 | 1 |
Source: W.
H. Schlesinger. 1997. Biogeochemistry: AnAnalysis of Global Change, 2nd ed. Academic Press: San Diego, CA, and references within.
The main reason for this trend in mean residence times is that the influence of climate on rates of decomposition is greater than its influence on primary productivity. Boreal forest soils often have permafrost layers, which cool the soils and lower rates of biological activity. The permafrost also blocks the percolation of water through the soil, creating wet, anoxic soil conditions.
Furthermore, the litter produced by boreal forest trees is rich in secondary compounds that slow rates of decomposition in the soil.
The variation in mean residence times among specific nutrients is related to their chemical properties (e.g., solubility). Some nutrients, such as potassium, occur in more soluble forms, and thus are lost from soil organic matter more quickly than others such as nitrogen, some of which is found as insoluble organic compounds.
In Chapter 25, we will return to nutrient cycling at a much larger spatial scale as we consider global cycles of carbon, nitrogen, phosphorus, and sulfur in the context of human alterations of these cycles.