The cycles of the key nutrient elements nitrogen (N) and phosphorus (P) have been massively altered by anthropogenic activities. Thus, it is essential to understand how photosynthetic production across diverse ecosystems is, or is not, limited by N and P. Via a large-scale meta-analysis of experimental enrichments, we show that P limitation is equally strong across these major habitats and that N and P limitation are equivalent within both terrestrial and freshwater systems. Furthermore, simultaneous N and P enrichment produces strongly positive synergistic responses in all three environments. Thus, contrary to some prevailing paradigms, freshwater, marine and terrestrial ecosystems are surprisingly similar in terms of N and P limitation.

/pdf/global-analysis-of-nitrogen-and-phosphorus-limitation-of-1j2o5ic67f.pdf

Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems.

We review the current understandings of the frequency, spatial distributions, mechanisms, and ecological consequences of fire in riparian zones. Riparian zones are well known for influencing many ecological processes at local to landscape scales, and fire can have an important ecosystem-scale influence on them. Riparian zones differ from surrounding uplands in their biophysical templates, moisture regimes and disturbance regimes; as a consequence the characteristics and effects of fire are different than in adjacent uplands. Fire impacts on riparian zones vary proportionally with the severity and extent of burning in the catchment and are affected by stream size. Riparian zones can act as a buffer against fire and therefore as a refuge for fire-sensitive species. However, under some circumstances, such as dry pre-fire climatic conditions and the accumulation of dry fuel, riparian areas become corridors for fire movement. Fire incursion into riparian zones creates canopy gaps and drier conditions, which allow subsequent build up of dead wood and establishment of fire adapted species. In concert, this increases fuel loads and the probability of another fire. Secondary effects of riparian fire include altering nutrient fluxes and cycling, increasing sediment loads, and stimulating erosion. We conclude that riparian fires are potentially important in shaping ecological characteristics in many regions, but this is poorly quantified. A better understanding of riparian fire regimes is essential to assess the effects of fire in helping shape the complex ecological characteristics of riparian zones over the longer-term.

Fire in the Riparian Zone: Characteristics and Ecological Consequences

Elevated CO2 is expected to lower plant nutrient concentrations via carbohydrate dilution and increased nutrient use efficiency. Elevated CO2 con- sistently lowers plant foliar nitrogen, but there is no consensus on CO2 effects across the range of plant nutrients. We used meta-analysis to quantify elevated CO2 effects on leaf, stem, root, and seed concentrations of B, Ca, Cu, Fe, K, Mg, Mn, P, S, and Zn among four plant functional groups and two levels of N fertiliza- tion. CO2 effects on plant nutrient concentration depended on the nutrient, plant group, tissue, and N status. CO2 reduced B, Cu, Fe, and Mg, but increased Mn concentration in the leaves of N2 fixers. Elevated CO2 increased Cu, Fe, and Zn, but lowered Mn concentration in grass leaves. Tree leaf responses were strongly related to N status: CO2 significantly decreased Cu, Fe, Mg, and S at high N, but only Fe at low N. Elevated CO2 decreased Mg and Zn in crop leaves grown with high N, and Mn at low N. Nutrient concentrations in crop roots were not affected by CO2 enrichment, but CO2 decreased Ca, K, Mg and P in tree roots. Crop seeds had lower S under elevated CO2 .W e also tested the validity of a ''dilution model.'' CO2 reduced the concentration of plant nutrients 6.6% across nutrients and plant groups, but the reduction is less than expected (18.4%) from carbohydrate accu- mulation alone. We found that elevated CO2 impacts plant nutrient status differently among the nutrient elements, plant functional groups, and among plant tissues. Our synthesis suggests that differences between plant groups and plant organs, N status, and differences in nutrient chemistry in soils preclude a universal hypothesis strictly related to carbohydrate dilution regarding plant nutrient response to elevated CO2.

CO 2 effects on plant nutrient concentration depend on plant functional group and available nitrogen: A meta-analysis

Elevated CO2 is expected to lower plant nutrient
 concentrations via carbohydrate dilution and increased nutrient use efficiency.
 Elevated CO2 consistently lowers plant foliar nitrogen, but
 there is no consensus on CO2 effects across the range of
 plant nutrients. We used meta-analysis to quantify elevated
 CO2 effects on leaf, stem, root, and seed concentrations of
 B, Ca, Cu, Fe, K, Mg, Mn, P, S, and Zn among four plant functional groups and two
 levels of N fertilization. CO2 effects on plant nutrient
 concentration depended on the nutrient, plant group, tissue, and N status.
 CO2 reduced B, Cu, Fe, and Mg, but increased Mn
 concentration in the leaves of N2 fixers. Elevated
 CO2 increased Cu, Fe, and Zn, but lowered Mn concentration
 in grass leaves. Tree leaf responses were strongly related to N status:
 CO2 significantly decreased Cu, Fe, Mg, and S at high N,
 but only Fe at low N. Elevated CO2 decreased Mg and Zn in
 crop leaves grown with high N, and Mn at low N. Nutrient concentrations in crop
 roots were not affected by CO2 enrichment, but
 CO2 decreased Ca, K, Mg and P in tree roots. Crop seeds had
 lower S under elevated CO2. We also tested the validity of a
 “dilution model.” CO2 reduced the concentration of plant
 nutrients 6.6% across nutrients and plant groups, but the reduction is less than
 expected (18.4%) from carbohydrate accumulation alone. We found that elevated
 CO2 impacts plant nutrient status differently among the
 nutrient elements, plant functional groups, and among plant tissues. Our synthesis
 suggests that differences between plant groups and plant organs, N status, and
 differences in nutrient chemistry in soils preclude a universal hypothesis strictly
 related to carbohydrate dilution regarding plant nutrient response to elevated
 CO2.

/pdf/retraction-note-to-co2-effects-on-plant-nutrient-3i91kdvp4z.pdf

Retraction Note to: CO2 effects on plant nutrient concentration depend on plant functional group and available nitrogen: a meta-analysis

We used a simple model of carbon-nitrogen (C-N) interactions in terrestrial ecosystems to examine the responses to elevated CO2 and to elevated CO2 plus warming in ecosystems that had the same total nitrogen loss but that differed in the ratio of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN) loss. We postulate that DIN losses can be curtailed by higher N demand in response to elevated CO2, but that DON losses cannot. We also examined simulations in which DON losses were held constant, were proportional to the amount of soil organic matter, were proportional to the soil C:N ratio, or were proportional to the rate of decomposition. We found that the mode of N loss made little difference to the short-term (,60 years) rate of carbon sequestration by the ecosystem, but high DON losses resulted in much lower carbon sequestration in the long term than did low DON losses. In the short term, C sequestration was fueled by an internal redistribution of N from soils to vegetation and by increases in the C:N ratio of soils and vegetation. This sequestration was about three times larger with elevated CO 2 and warming than with elevated CO2 alone. After year 60, C sequestration was fueled by a net accu- mulation of N in the ecosystem, and the rate of sequestration was about the same with elevated CO2 and warming as with elevated CO2 alone. With high DON losses, the ecosystem either sequestered C slowly after year 60 (when DON losses were constant or proportional to soil organic matter) or lost C (when DON losses were proportional to the soil C:N ratio or to decomposition). We conclude that changes in long-term C sequestration depend not only on the magnitude of N losses, but also on the form of those losses.

/pdf/terrestrial-c-sequestration-at-elevated-co2-and-temperature-cwffps9yso.pdf

B. Moldan

Papers

Disturbance and element interactions.