Climate change predictions derived from coupled carbon-climate models are highly dependent on assumptions about feedbacks between the biosphere and atmosphere. One critical feedback occurs if C uptake by the biosphere increases in response to the fossil-fuel driven increase in atmospheric [CO 2] (''CO2 fertilization''), thereby slowing the rate of increase in atmospheric [CO 2]. Carbon exchanges between the terrestrial biosphere and atmosphere are often first represented in models as net primary productivity (NPP). However, the contribution of CO 2 fertilization to the future global C cycle has been uncertain, especially in forest ecosystems that dominate global NPP, and models that include a feedback between terrestrial biosphere metabolism and atmospheric [CO 2] are poorly constrained by experimental evidence. We analyzed the response of NPP to elevated CO 2 (Ϸ550 ppm) in four free-air CO 2 enrichment experiments in forest stands. We show that the response of forest NPP to elevated [CO 2] is highly conserved across a broad range of productivity, with a stimulation at the median of 23 ؎ 2%. At low leaf area indices, a large portion of the response was attributable to increased light absorption, but as leaf area indices increased, the response to elevated [CO 2] was wholly caused by increased light-use efficiency. The surprising consistency of response across diverse sites provides a benchmark to evaluate predictions of ecosystem and global models and allows us now to focus on unresolved questions about carbon partitioning and retention, and spatial variation in NPP response caused by availability of other growth limiting resources.CO2 fertilization ͉ global change ͉ leaf area index ͉ net primary productivity A nalysis and prediction of the effects of human activities, particularly the combustion of fossil fuels, on climate and the biological, physical, and social responses to changing climate require an integrated view of the complex interactions between the biosphere and atmosphere. Carbon cycle models are now being coupled to atmosphere-ocean general circulation climate models to achieve a dynamic analysis of the relationships between C emissions, atmospheric chemistry, biosphere activity, and climatic change (1-3).Exchanges between the terrestrial biosphere and atmosphere are represented in models using empirical and theoretical expressions of net primary productivity (NPP), the net fixation of C by green plants into organic matter, or the difference between photosynthesis and plant respiration. Because the photosynthetic uptake of carbon that drives NPP is not saturated at current atmospheric concentrations (4), NPP should increase as fossilfuel combustion adds to the atmospheric [CO 2 ]. Increased C uptake into the biosphere in response to rising [CO 2 ] (''CO 2 fertilization'') can create a negative feedback that slows the rate of increase in atmospheric [CO 2 ] (3, 5). Hence, assumptions regarding CO 2 fertilization of the terrestrial biosphere greatly affect predictions of future atmospheric [CO 2 ] (3)...

There is considerable uncertainty about how rates of soil carbon (C) and nitrogen (N) cycling will change as CO # accumulates in the Earth's atmosphere. We summarized data from 47 published reports on soil C and N cycling under elevated CO # in an attempt to generalize whether rates will increase, decrease, or not change. Our synthesis centres on changes in soil respiration, microbial respiration, microbial biomass, gross N mineralization, microbial immobilization and net N mineralization, because these pools and processes represent important control points for the below-ground flow of C and N. To determine whether differences in C allocation between plant life forms influence soil C and N cycling in a predictable manner, we summarized responses beneath graminoid, herbaceous and woody plants grown under ambient and elevated atmospheric CO # . The below-ground pools and processes that we summarized are characterized by a high degree of variability (coefficient of variation 80-800%), making generalizations within and between plant life forms difficult. With few exceptions, rates of soil and microbial respiration were more rapid under elevated CO # , indicating that (1) greater plant growth under elevated CO # enhanced the amount of C entering the soil, and (2) additional substrate was being metabolized by soil microorganisms. However, microbial biomass, gross N mineralization, microbial immobilization and net N mineralization are characterized by large increases and declines under elevated CO # , contributing to a high degree of variability within and between plant life forms. From this analysis we conclude that there are insufficient data to predict how microbial activity and rates of soil C and N cycling will change as the atmospheric CO # concentration continues to rise. We argue that current gaps in our understanding of fine-root biology limit our ability to predict the response of soil microorganisms to rising atmospheric CO # , and that understanding differences in fine-root longevity and biochemistry between plant species are necessary for developing a predictive model of soil C and N cycling under elevated CO # .

In recent years, increased awareness of the potential interactions between rising atmospheric CO2 concentrations ([ CO2 ]) and temperature has illustrated the importance of multifactorial ecosystem manipulation experiments for validating Earth System models. To address the urgent need for increased understanding of responses in multifactorial experiments, this article synthesizes how ecosystem productivity and soil processes respond to combined warming and [ CO2 ] manipulation, and compares it with those obtained in single factor [ CO2 ] and temperature manipulation experiments. Across all combined elevated [ CO2 ] and warming experiments, biomass production and soil respiration were typically enhanced. Responses to the combined treatment were more similar to those in the [ CO2 ]-only treatment than to those in the warming-only treatment. In contrast to warming-only experiments, both the combined and the [ CO2 ]-only treatments elicited larger stimulation of fine root biomass than of aboveground biomass, consistently stimulated soil respiration, and decreased foliar nitrogen (N) concentration. Nonetheless, mineral N availability declined less in the combined treatment than in the [ CO2 ]-only treatment, possibly due to the warming-induced acceleration of decomposition, implying that progressive nitrogen limitation (PNL) may not occur as commonly as anticipated from single factor [ CO2 ] treatment studies. Responses of total plant biomass, especially of aboveground biomass, revealed antagonistic interactions between elevated [ CO2 ] and warming, i.e. the response to the combined treatment was usually less-than-additive. This implies that productivity projections might be overestimated when models are parameterized based on single factor responses. Our results highlight the need for more (and especially more long-term) multifactor manipulation experiments. Because single factor CO2 responses often dominated over warming responses in the combined treatments, our results also suggest that projected responses to future global warming in Earth System models should not be parameterized using single factor warming experiments.