Efficiency in Elemental

In the previous section, we discussed the stoichiometric patterns in nutrient limitation. However, there is considerable variation in published relationships between biomass accrual and nutrient supply such that the relationship between nutrient availability and production is often not linear. We now turn our attention to examining how primary producers differentially accumulate biomass under nutrient-limiting conditions. The nutrient use efficiency (NUE) concept describes the amount of producer biomass generated per unit of nutrient taken up by organisms. Where conclusions about limitation are categorical, NUE is a continuous variable that reflects the relative influence of nutrient availability on net production.

NUE is normally expressed as the ratio of producer growth to the amount of nutrients assimilated by the organism. More explicitly, NUE is the ecosystem-scale expression of the ratio between individual photosynthetic nutrient use efficiencies (PNUE; flux of nutrient relative to carbon fixation) and individual growth rates (y) such that NUE = PNUE/y. Relationships between net photosynthesis and organism N content demonstrate that PNUE can vary by an order of magnitude (25-200). Combined with variation in organism growth rates, it is not surprising that NUE varies considerably among ecosystems.

In stoichiometric terminology, variation in NUE indicates that ecosystems are nonhomeostatic, or plastic, with respect to nutrient use. Why should biomass production become less efficient with increased nutrient availability? A frequent explanation is that strong selective pressures for greater efficiency at low-nutrient concentrations drive the observed pattern. However, there is no obvious explanation for why efficiency would be reduced under high nutrient conditions from either a competition or natural selection perspective. The likely explanation relates to the multiplicity of factors that can limit primary production. For example, under conditions of rapid biomass accumulation, light limitation of producer growth through self-shading increases, leading to smaller producer growth per unit of nutrient absorbed (i.e., reduced producer NUE) if resident organisms are plastic in their elemental composition. Another explanation presented in the literature relates to trophic structure and the strength of top-down control of primary producer biomass. When top-down control of primary producers is strong, as in systems with an even number of trophic levels, rapid consumption maintains primary producers in a state of rapid growth with consequent increased nutrient content (as described elsewhere in this encyclopedia).

Nutrient use efficiency has largely been a primary producer concept though there is no conceptual barrier to its application to heterotrophic organisms. In part, this may stem from the historical perspective that higher trophic level organisms are strongly homeostatic in their elemental composition (and thus have constant NUE). However, by now it should be apparent that hetero-trophic stoichiometry is not fixed. This observation has recently led to a carbon-based corollary of NUE. Carbon use efficiency (CUE) has been defined as secondary production divided by net primary production. Many people will recognize this as the stoichiometric repackaging of trophic transfer efficiency (based on C rather than energy). In lakes, CUE has been demonstrated to vary by more than two orders of magnitude (0.002-0.4%). We are unaware of similar data from terrestrial systems. Stoichiometric theory predicts that consumers will use nutrients more efficiently when C:nutrient ratios are high. Consumers tend to use carbon inefficiently under these conditions because they will need to release excess carbon as they accrue biomass. All else being equal, it follows that CUE and NUE should be inversely related, a prediction that has yet to be assessed as far as we are aware.

Up to this point, we have focused our discussion on ecosystems where nutrients enter biotic pools through

Figure 1 The relationship between measured assimilative nitrogen uptake and calculated nitrogen demand for biomass production. The line indicates where equal values would fall. Adapted from Webster JR, Mulholland PJ, Tank JL, etal. (2003) Factors affecting ammonium uptake in streams: An interbiome perspective. Freshwater Biology 48: 1329-1352.

Calculated nitrogen demand from NPP and microbial production (g N m-2 d-1)

Figure 1 The relationship between measured assimilative nitrogen uptake and calculated nitrogen demand for biomass production. The line indicates where equal values would fall. Adapted from Webster JR, Mulholland PJ, Tank JL, etal. (2003) Factors affecting ammonium uptake in streams: An interbiome perspective. Freshwater Biology 48: 1329-1352.

autotrophic production. However, heterotrophic microbial production is also an important avenue through which nutrients must flow in most, if not all, ecosystems. For example, in headwater streams heterotrophic production often exceeds photoautotrophy. As a result, nutrient assimilation through microbial heterotrophs has received considerable attention. Lotic ecosystem ecologists have a rich history of measuring ecosystem-scale metabolic rates (respiration and gross primary productivity) and nutrient uptake. Recently, stoichiometric relationships have been combined with metabolic measurements to predict nutrient demand. In the Lotic Intersite Nitrogen experiment (LINX), algal and bacterial C:N ratios and carbon use efficiencies (though not expressed as such) were combined with estimates of heterotrophic respiration and gross primary production to predict nitrogen uptake in several stream ecosystems. Using whole-stream 15N-NH4 experiments and an oxygen mass balance approach for measuring ecosystem metabolism, these researchers found that predicted and measured rates of N uptake were similar and straddled the 1:1 line, though considerable variation existed (Figure 1). Given the number of assumptions in this analysis, the observed variance was not surprising and they concluded that continued application and improvement of this approach holds promise for illuminating links between carbon and nutrients as they move from inorganic to biotic pools.

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