COS 43-6
Plant competitive interactions under soil nutrient transport limitation
Competitive interactions determine the performance of plant individuals in communities, and species coexistence. Models such as Tilman's resource ratio hypothesis predict that species can exclude competitors by reducing nutrient concentrations to levels where these will fail to maintain positive population growth. However, this mechanism of competitive exclusion should be less effective under conditions of limited soil nutrient transport.
Here, we present a study in which we factorially manipulated soil inorganic nitrogen forms (“nutrient mobility”) and plant species diversity. These treatments were applied to a total of 224 microcosms that contained eight plant individuals arranged in pairs of two in the quarters of a pot. Nutrient mobility was manipulated using a commercial nitrification inhibitor, together with fertilization with NH4+ or NO3-. The biodiversity treatment consisted of eight temperate grassland species (four grasses and four non-legume forbs) which were combined to communities with 1, 2 or 4 species.
Plant individual growth was monitored over six months, using allometric relations. Nutrient transport between plants was assessed by labelling the soil of one quarter in each microcosm with 33P, 32P, and 15N, and tracing isotope incorporation into the labelled plant and its neighbors.
Results/Conclusions
Reduced nitrogen mobility increased the isotopic labelling of plants grown in the quarters to which the label was applied (P<0.05). Also, labelling was lower in plants grown in more diverse mixtures (P<0.05), and these effects were species-dependent.
At the individual level, no effect of the nutrient mobility treatment was found on growth. However, at the community-level, increased soil nutrient mobility led to higher productivity (P<0.05). Also, some species showed higher yields when grown in higher diversity mixtures (P<0.05).
These initial results suggest that the nutrient mobility treatment affected plant growth and the uptake of labelled nutrients. However, the analyses available at the time of writing of this abstract do not unequivocally indicate reduced competition between neighbor plants. So far, the available data are in line with theoretical models indicating that reduced mobility alleviates competition. Lower labelling of plants in high diversity suggests that more nutrients were transferred to neighbors, or that the applied label was diluted in a larger total amount of nutrients that was taken up in high diversity communities. Further, the biodiversity effects indicate that species interacted below-ground (they were separated above-ground), creating community-level effects, most likely due to enhanced complementarity of resource-niches in more diverse mixtures.