Growth and nitrogen uptake in an experimental community of annuals exposed to elevated atmospheric CO2

Rising levels of atmospheric CO₂ may alter patterns of plant biomass production. These changes will be dependent on the ability of plants to acquire sufficient nutrients to maintain enhanced growth. Species-specific differences in responsiveness to CO₂ may lead to changes in plant community composition and biodiversity. Differences in species-level growth responses to CO₂ may be, in a large part, driven by differences in the ability to acquire nutrients. To understand the mechanisms of how elevated CO₂ leads to changes in community-level productivity, we need to study the growth responses and patterns of nutrient acquisition for each of the species that comprise the community. In this paper, we present a study of how elevated CO₂ affects community-level and species-level patterns of nitrogen uptake and biomass production. As an experimental system we use experimental communities of 11 co-occurring annuals common to disturbed seasonal grasslands in south-western U.S.A. We established experimental communities with approximately even numbers of each species in three different atmospheric CO₂ concentrations (375, 550, and 700 ppm). We maintained these communities for 1, 1.5, and 2 months at which times we applied a ¹⁵N tracer (¹⁵NH₄¹⁵NO₃) to quantify the nitrogen uptake and then measured plant biomass, nitrogen content, and nitrogen uptake rates for the entire communities as well as for each species. Overall, community-level responses to elevated CO₂ were consistent with the majority of other studies of individual- and multispecies assemblages, where elevated CO₂ leads to enhanced biomass production early on, but this enhancement declines through time. In contrast, the responses of the individual species within the communities was highly variable, showing the full range of responses from positive to negative. Due to the large variation in size between the different species, community-level responses were generally determined by the responses of only one or a few species. Thus, while several of the smaller species showed trends of increased biomass and nitrogen uptake in elevated CO₂ at the end of the experiment, community-level patterns showed a decrease in these parameters due to the significant reduction in biomass and nitrogen content in the single largest species. The relationship between enhancement of nitrogen uptake and biomass production in elevated CO₂ was highly significant for both 550 ppm and 700 ppm CO₂. This relationship strongly suggests that the ability of plants to increase nitrogen uptake (through changes in physiology, morphology, architecture, or mycorrhizal symbionts) may be an important determinant of which species in a community will be able to respond to increased CO₂ levels with increased biomass production. The fact that the most dominant species within the community showed reduced enhancement and the smaller species showed increased enhancement suggest that through time, elevated CO₂ may lead to significant changes in community composition. At the community level, nitrogen uptake rates relative to plant nitrogen content were invariable between the three different CO₂ levels at each harvest. This was in contrast to significant reductions in total plant nitrogen uptake and nitrogen uptake relative to total plant biomass. These patterns support the hypothesis that plant nitrogen uptake is largely regulated by physiological activity, assuming that physiological activity is controlled by nitrogen content and thus protein and enzyme content.