Soil fertility and the carbon:nutrient stoichiometry of herbaceous plant species

Biomass allocation to plant aboveground and belowground compartments may change in response to changes in soil nutrient fertility (e.g., nitrogen (N) and phosphorous (P) availability). It is not clear however whether and how changes in soil fertility could influence carbon(C):nutrient stoichiometry of wild plants distributed along environmental gradients. Here we use ecological stoichiometric theory to test two hypotheses: (1) C:N (or C:P) ratios of different plant compartments (i.e., roots, stems and leaves) increase when soil N (or P) availability decreases, (2) the relative availability of N compared to P in soils predictably influences C:nutrient ratios (e.g., high soil N:P ratios are associated with high plant C:P ratios). Data from 72 wild plant species were collected along a gradient of soil development determined by the temporal progression of four primary ecological successions across Europe. We found significant changes in soil N and P availability and content along the four vegetation successions, and our hypotheses were partially confirmed: (1) stem and root C:N ratios increased with decreases in soil N availability, and (2) stem C:P ratios increased with increases in soil N:P ratios. We also found that plant functional group identity strongly influenced plant C:nutrient stoichiometry whereby plant C:N ratios were significantly lower in legumes and C:P ratios were significantly higher in grasses. Our study suggests that changes in (1) soil N and P availability and content, and (2) soil N:P ratios can influence plant C:nutrient stoichiometry. However, we also bring evidence that plant functional group identity can be more important than soil fertility in influencing plant C:nutrient stoichiometry. Further studies could address how changes in soil fertility interact with multiple plant functional traits to better understand patterns of plant species distribution along environmental gradients.