Carbon flux response and recovery to drought years in a hemi-boreal peat bog between different vegetation types

Terrestrial ecosystems absorb 30% of anthropogenic carbon dioxide (CO₂) emissions, slowing its rising atmospheric concentration and substantially inhibiting climate change. This uptake is believed to be due to elevated CO₂ (eCO₂) 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 eCO₂ 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 eCO₂, constituting a major gap in our understanding of how large areas of the biosphere will impact atmospheric CO₂ over the coming decades.

In the first study conducted into the effect of eCO₂ on P-limited ecosystems with manipulated nutrient availability, the Phosphorus Limitation And ecosystem responses to Carbon dioxide Enrichment project (PLACE), investigates the effects of eCO₂ 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^-2 y^-1), high N (14 g m^-2 y^-1), and P (3.5 g m^-2 y^-1)), have been exposed to either ambient or eCO₂ (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 CO₂.

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 eCO₂ 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 CO₂ signal has shown the fate of newly assimilated C and its contribution to gaseous C flux to the atmosphere in the form of methane (CH₄) and respired CO₂.  In summary, our first two years of eCO₂ 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 CO₂ continues to rise.