Rapid human-caused increases in atmospheric carbon dioxide (CO2) levels during the next 50 years are almost certain to lead to global warming, influencing the growth and performance of vegetation worldwide. Plants and their ecosystems absorb roughly one-third of the CO2 emitted from fossil fuel combustion each year, and it is critical to understand whether plants can continue this important function as CO2 levels continue to rise. Realistic field experiments that test whether vegetation will continue to absorb extra C, and whether such C capture may be constrained by infertile soils or loss of diversity, number fewer than five in the world. The BioCON (Biodiversity, CO2, and Nitrogen) experiment is one of these, and is unique in testing interactions among atmospheric CO2, soil N supply, and plant species richness and composition, all of which have been simultaneously manipulated since 1998 in a temperate perennial grassland in Minnesota. This project will expose plants to CO2 levels expected to occur late in the 21st century; LTREB funds will help to maintain this experiment from 2007-2011 and to support a large number of measures of plants and soils relevant to both ecosystem science and atmospheric change.
This research will address issues of major importance to society regarding the potential impacts of human activities on environmental processes at local to global scales. It will determine whether lack of soil nitrogen constrains the long-term ability of ecosystems to capture and sequester carbon. Additionally, BioCON broadly enhances research and education, as a unique facility open to and encouraging new initiatives from interested scientists and students, and promoting teaching, training, and learning for students, teachers, journalists and others.
The global change experiment (BioCON) at the heart of our LTREB at the Cedar Creek Ecosystem Science Reserve, Minnesota, USA, is to our knowledge one of only three studies in the world capable of providing long-term evidence on joint effects of elevated atmospheric CO2 and N deposition on biodiversity and ecosystem function; the only such experiment involving either CO2 or N and biodiversity; and one of the two longest running biodiversity experiments. Several important findings have emerged from this work. (1) CO2 and N interact non-additively in influencing plant biodiversity. It is well known the elevated nitrogen pollution reduces plant biodiversity, but our long-term work shows that elevated CO2 eliminates about half of that adverse pollution impact. This resulted from multiple effects of CO2 and N on plant traits and soil resources that altered competitive interactions among species. As a result, elevated CO2 helped ameliorate the negative effects of N enrichment on species richness. If these results apply generally, this would be one positive effect of rising CO2 on plant ecosystems. However, as this interaction relied on the aggregate combination of many specific competitive interactions that could be particular to this set of species and this sit, it is impossible to know whether the result is a general one. This is problematic, as science funding agencies worldwide do not actively support this kind of research, and hence no other experiment on the planet directly addresses this question. (2) Nitrogen limitation of plant growth, which is common worldwide, constrains biomass responses to CO2 over the long-term. In 2007, the Intergovernmental Panel on Climate Change (IPCC) stated that the largest uncertainty in the global C cycle – and hence a key to predicting future climate change – involves the size of the so-called CO2 fertilization effect. Carbon is a key plant food and thus rising atmospheric CO2 can promote its uptake from the atmosphere and incorporation into plant biomass, all else being equal. We observe this in BioCON, as all of the grassland species in the experiment have higher photosynthetic productivity (which indicates increased biochemical scrubbing of carbon from the atmosphere) under elevated CO2 levels. However, during the most recent decade of BioCON, the stimulation of plant biomass production by elevated CO2 was only half as large in plots under naturally low nitrogen supply as in those fertilized with additional nitrogen. This is consistent with theory and results from other shorter term experiments. These results suggest that limited levels of fertility, which are typical of most soils, likely eliminate a large fraction of the capacity of vegetation to scrub CO2 out of the atmosphere. Our evidence base would be better if there were long-term experiments like BioCON in tropical rain forest, temperate forest, and tundra, to see how well responses there match with what we have found. But as such experiments do not exist, the results from BioCON must play an even more important role in addressing this issue for ecosystems everywhere. (3) Long-term evidence from BioCON and a companion study at Cedar Creek provide the strongest evidence that over time, the effects of plant diversity on biomass production become larger, becoming both non-saturating and increasingly linear. There is considerable evidence that plant diversity promotes biomass production, but there has been substantial uncertainty about the shape of the response curve, and particularly how it might change with time. Based on our two 15-year studies, we found that the positive effect of higher species diversity on productivity increased over time, largely because the difference between the highest diversity plots (16 species) and the next highest (8 or 9 species) shifted from neutral to highly positive. Thus, the initially saturating dependence of productivity on diversity became increasingly linear over time. Our analyses suggest that effects of diversity-dependent ecosystem feedbacks and interspecific complementarity accumulate over time, causing high-diversity species combinations that appeared functionally redundant during early years to become more functionally unique through time. Thus, even modest simplification of diverse ecosystems will likely have consequences for ecosystem function. Collectively, these findings have important implications globally. For instance, because of N limitation and biodiversity losses, global estimates of potential C sequestration in the face of rising CO2 may be currently considerably over-estimated. If this is true, atmospheric CO2 concentrations (and associated global temperatures) may increase more quickly than anticipated.
