Carbon flux response and recovery to drought years in a hemi-boreal peat bog between different vegetation types
Authors: Taylor Chris R , Keane Ben , Hartley Iain , Phoenix Gareth 
Published: March 2020

&lt;p&gt;Terrestrial ecosystems absorb 30% of anthropogenic carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;) emissions, slowing its rising atmospheric concentration and substantially inhibiting climate change. This uptake is believed to be due to elevated CO&lt;sub&gt;2&lt;/sub&gt; (eCO&lt;sub&gt;2&lt;/sub&gt;) 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&lt;sub&gt;2&lt;/sub&gt; 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&lt;sub&gt;2&lt;/sub&gt;, constituting a major gap in our understanding of how large areas of the biosphere will impact atmospheric CO&lt;sub&gt;2&lt;/sub&gt; over the coming decades.&lt;/p&gt;&lt;p&gt;In the first study conducted into the effect of eCO&lt;sub&gt;2&lt;/sub&gt; 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&lt;sub&gt;2&lt;/sub&gt; 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&lt;sup&gt;-2&lt;/sup&gt; y&lt;sup&gt;-1&lt;/sup&gt;), high N (14 g m&lt;sup&gt;-2&lt;/sup&gt; y&lt;sup&gt;-1&lt;/sup&gt;), and P (3.5 g m&lt;sup&gt;-2&lt;/sup&gt; y&lt;sup&gt;-1&lt;/sup&gt;)), have been exposed to either ambient or eCO&lt;sub&gt;2&lt;/sub&gt; (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&lt;sub&gt;2&lt;/sub&gt;.&lt;/p&gt;&lt;p&gt;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&lt;sub&gt;2&lt;/sub&gt; has only increased biomass production in the limestone grassland (by 17%; p&lt; 0.0001), and not in the acid grassland. There was also a significant effect of nutrient treatment (p&lt; 0.001) with biomass increasing under P and HN, indicating some co-NP limitation. Stable isotope tracing, using the fumigation CO&lt;sub&gt;2&lt;/sub&gt; 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&lt;sub&gt;4&lt;/sub&gt;) and respired CO&lt;sub&gt;2&lt;/sub&gt;. &amp;#160;In summary, our first two years of eCO&lt;sub&gt;2&lt;/sub&gt; 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&lt;sub&gt;2 &lt;/sub&gt;continues to rise.&lt;/p&gt;