THE C:N:P STOICHIOMETRY OF ORGANISMS AND ECOSYSTEMS IN A CHANGING WORLD: A REVIEW AND PERSPECTIVES

This study examined the literature in ISI Web of Science to identify the effects that the main drivers of global change have on the nutrient concentrations and C:N:P stoichiometry of organisms and ecosystems, and examined their relationship to changes in ecosystem structure and function. We have conducted a meta-analysis by comparing C:N:P ratios of plants and soils subjected to elevated [CO2] with those subjected to ambient [CO2]. A second meta-analysis compared the C:N:P ratios of plants and soils that received supplemental N to simulate N deposition and those that did not receive supplemental N. On average, an experimental increase in atmospheric [CO2] increased the foliar C:N ratios of C3 grasses, forbs, and woody plants by 22%, but the foliar ratios of C4 grasses were unaffected.

This trend may be enhanced in semi-arid areas by the increase in droughts that have been projected for the coming decades which can increase leaf C:N ratios. The available studies show an average 38% increase in foliar C:P ratios in C3 plants in response to elevated atmospheric [CO2], but no significant effects were observed in C4 grasses. Furthermore, studies that examine the effects of elevated atmospheric [CO2] on N:P ratio (on a mass basis) are warranted since its response remains elusive. N deposition increases the N:P ratio in the plants of terrestrial and freshwater ecosystems, and decreases plants and organic soil C:N ratio (25% on average for C3 plants), reducing soil and water N2 fixation capacity and ecosystem species diversity. In contrast, in croplands subjected to intense fertilization, mostly, animal slurries, a reduction in soil N:P ratio can occur because of the greater solubility and loss of N. In the open ocean, there are experimental observations showing an ongoing increase in P-limited areas in response to several of the factors that promote global change, including the increase in atmospheric [CO2] which increases the demand for P, the warming effect that leads to an increase in water column stratification, and increases in the N:P ratio of atmospheric inputs. Depending on the type of plant and the climate where it grows, warming can increase, reduce, or have no effect on foliar C:N ratios. The results suggest that warming and drought can increase C:N and C:P ratios in warm-dry and temperate-dry terrestrial ecosystems, especially, when high temperatures and drought coincide. Advances in this topic are a challenge because changes in stoichiometric ratios can favour different types of species and change ecosystem composition and structure.