<p>Terrestrial ecosystems absorb 30% of anthropogenic carbon dioxide (CO<sub>2</sub>) emissions, slowing its rising atmospheric concentration and substantially inhibiting climate change. This uptake is believed to be due to elevated CO<sub>2</sub> (eCO<sub>2</sub>) stimulating plant photosynthesis and growth, thus increasing carbon (C) storage in plants and soil organic matter. However, nitrogen (N) limitation can reduce ecosystem C uptake capacity under eCO<sub>2</sub> by as much as 50%. Phosphorus (P) limitation in ecosystems is almost as common as N-limitation and is increasing due to ongoing deposition of N from anthropogenic activities. Despite this, we do not know how P-limited ecosystems will respond to eCO<sub>2</sub>, constituting a major gap in our understanding of how large areas of the biosphere will impact atmospheric CO<sub>2</sub> over the coming decades.</p><p>In the first study conducted into the effect of eCO<sub>2</sub> on P-limited ecosystems with manipulated nutrient availability, the Phosphorus Limitation And ecosystem responses to Carbon dioxide Enrichment project (PLACE), investigates the effects of eCO<sub>2</sub> on C cycling in grasslands, which are a critical global C store. Turf mesocosms from P-limited acidic and limestone grasslands, where N and P inputs have been manipulated for 20 years (control, low N (3.5 g m<sup>-2</sup> y<sup>-1</sup>), high N (14 g m<sup>-2</sup> y<sup>-1</sup>), and P (3.5 g m<sup>-2</sup> y<sup>-1</sup>)), have been exposed to either ambient or eCO<sub>2</sub> (600 ppm) in a miniFACE (mini Free Air Carbon Enrichment) system. Long-term P addition has alleviated P limitation while N additions have exacerbated it. The two contrasting grasslands contain different amounts of organic versus mineral P in their soils and, thus, plants may have to use contrasting strategies to acquire the additional P they need to increase growth rates under elevated CO<sub>2</sub>.</p><p>We present data from the first two growing seasons, including above and below ground productivity, and C, N and P cycling through plant, soil and microbial pools. Aboveground harvest data from the second year have shown eCO<sub>2</sub> has only increased biomass production in the limestone grassland (by 17%; p< 0.0001), and not in the acid grassland. There was also a significant effect of nutrient treatment (p< 0.001) with biomass increasing under P and HN, indicating some co-NP limitation. Stable isotope tracing, using the fumigation CO<sub>2</sub> signal has shown the fate of newly assimilated C and its contribution to gaseous C flux to the atmosphere in the form of methane (CH<sub>4</sub>) and respired CO<sub>2</sub>. &#160;In summary, our first two years of eCO<sub>2</sub> treatment suggests that productivity of limestone and acidic grassland respond differently and that these responses depend on nutrient availability, indicating the complexity of predicting P-limited ecosystem responses as atmospheric CO<sub>2 </sub>continues to rise.</p>
Sporadic P limitation constrains microbial growth and facilitates SOM accumulation in the stoichiometrically coupled, acclimating microbe–plant–soil model
