Some key points about nutrient budgets in terrestrial ecosystems

The examples discussed above have illustrated that ecosystems do not generally have balanced inputs and outputs of nutrients. However, in many cases (as in the Hubbard Brook Forest) nutrients such as nitrogen are cycled quite tightly, and inputs and outputs are small compared to stored pools. For carbon too, fluxes may be small compared to storage, but note that tight cycling is not the rule in this case; the carbon molecules in respired CO2 will rarely be the same ones taken up by photosynthesis (because of the huge pool of CO2 involved).

We have also seen that nutrient budgets of a single category of ecosystem can differ dramatically, either because of internal properties (the age of trees in the pine forests in Section 18.2.3) or external factors (the dryness of the climate in the oak forests in Figure 18.4). Similarly, in a semiarid grassland in Colorado, nitrogen availability to grass plants adjacent to actively growing roots was greater in months when there was more rainfall (Figure 18.7).

Many other factors influence nutrient flux rates and stores. For example, the stoichiometry of elements in foliage (and thus in detritus when the leaves die) can influence decomposition rates and nutrient flux (see Section 11.2.4). There is a theoretical critical detritus C : N ratio of 30 : 1 above which bacteria and fungi are nitrogen-limited, when they then take up exogenous ammonium and nitrate ions from the soil, competing with plants for these resources (Daufresne & Loreau,

Figure 18.7 Nitrogen available to actively growing roots of the bunchgrass Bouteloua gracilis in shortgrass steppe ecosystems in relation to precipitation in the study period. The values for the six sampling periods are the averages of eight replicate plots. •, downslope plots; o, upslope plots (up to 11m further up the same hillslope). (After Hook & Burke, 2000.)

2001). When the C : N ratio is below 30 : 1, the microbes are carbon-limited and decomposition increases soil inorganic nitrogen, which may in turn increase plant nitrogen uptake (Kaye & Hart, 1997). In general, plants are most often nitrogen-limited and microbes carbon-limited, and whilst microbes are more significant in the control of nitrogen cycling, it is the plants that regulate carbon inputs which control microbial activity (Knops et al., 2002).

A quite different chemical property of foliage may have an equally dramatic effect. Polyphenols are a very widely distributed class of secondary metabolites in plants that often provide protection against attack; their evolution is usually interpreted in terms of defense against herbivores. However, the polyphenols in detritus can also influence the flux of soil nutrients (Hattenschwiler & Vitousek, 2000). Different classes of polyphenols have been found to affect fungal spore germination and hyphal growth. They have also been shown to inhibit nitrifying bacteria and to suppress or, in some cases, stimulate symbiotic nitrogen-fixing bacteria. Finally, polyphenols may restrict the activity and abundance of soil detri-tivores. Overall, polyphenols may tend to reduce decomposition rates (as they decrease herbivory rates) with important consequences for nutrient fluxes, but more work is needed on this topic (Hattenschwiler & Vitousek, 2000).

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