Stimulation of soil N cycling after two years of Free Air CO2 Enrichment increases nitrous oxide emissions in a temperate forest, UK.
Authors: Sgouridis Fotis , Cotchim Suparat , Ullah Sami 
Published: March 2020

&lt;p&gt;Increasing atmospheric CO&lt;sub&gt;2&lt;/sub&gt; concentrations in temperate forests may affect soil nitrogen (N) cycling processes due to the increased demand&amp;#160; for nitrogen availability by trees to support CO&lt;sub&gt;2&lt;/sub&gt; uptake through photosynthesis. This in turn can affect the emission of nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) from the forest soil leading to a potential trade-off between the enhanced canopy CO&lt;sub&gt;2&lt;/sub&gt; uptake and soil N&lt;sub&gt;2&lt;/sub&gt;O emission. The Birmingham Institute of Forest Research (BIFoR) established a Free-Air CO&lt;sub&gt;2&lt;/sub&gt; Enrichment (FACE) facility in a mature oak forest in Staffordshire, UK, which became operational in 2017. In April 2018 and again in May 2019, two years after the start of fumigation with 550 ppm CO&lt;sub&gt;2&lt;/sub&gt;, we collected soil samples (0 &amp;#8211; 15 cm depth) from the three elevated CO&lt;sub&gt;2&lt;/sub&gt; (eCO&lt;sub&gt;2&lt;/sub&gt;) and three control plots. Soils were amended in the laboratory with 98 at % &lt;sup&gt;15&lt;/sup&gt;N-NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; and &lt;sup&gt;15&lt;/sup&gt;N-NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; . Gross N mineralisation and nitrification were estimated by the isotope dilution technique, while N&lt;sub&gt;2&lt;/sub&gt;O emission from nitrification (&lt;sup&gt;15&lt;/sup&gt;N-NH&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;+&lt;/sup&gt; treatment) and denitrification (&lt;sup&gt;15&lt;/sup&gt;N-NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; treatment) were estimated by the &lt;sup&gt;15&lt;/sup&gt;N Gas-Flux method. Additionally, C/N ratio and &amp;#948;&lt;sup&gt;15&lt;/sup&gt;N and &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C were measured in unamended eCO&lt;sub&gt;2 &lt;/sub&gt;and control samples via EA-IRMS. Whilst gross N mineralisation and N&lt;sub&gt;2&lt;/sub&gt;O emission were only marginally higher in eCO&lt;sub&gt;2&lt;/sub&gt; plots compared to controls after one year of fumigation, there was a significant stimulation of N cycling after the second year that led to more pronounced differences. Gross N mineralisation rates doubled in the eCO&lt;sub&gt;2&lt;/sub&gt; plots (mean: 4.09 &amp;#956;g N g&lt;sup&gt;-1&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;, P &lt; 0.05) compared to the control plots (mean: 2.02 &amp;#956;g N g&lt;sup&gt;-1&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;), while a similar twofold increase was observed for gross nitrification rates (mean eCO&lt;sub&gt;2&lt;/sub&gt;: 1.63 &amp;#956;g N g&lt;sup&gt;-1&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;; &lt;sup&gt;&amp;#160;&lt;/sup&gt;mean control: 0.70 &amp;#956;g N g&lt;sup&gt;-1&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;, P &lt; 0.05). N&lt;sub&gt;2&lt;/sub&gt;O emission from both denitrification (mean: 0.03 ng N g&lt;sup&gt;-1&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;) and nitrification (mean: 0.02 ng N g&lt;sup&gt;-1&lt;/sup&gt; d&lt;sup&gt;-1&lt;/sup&gt;) were generally low but of similar magnitude and more than double than in the control plots. C/N ratio was conservative between eCO&lt;sub&gt;2&lt;/sub&gt; and control plots as a result of proportional increase of C and N contents in the eCO&lt;sub&gt;2 &lt;/sub&gt;plots. The observed stimulation in N cycling was further corroborated by the significantly enriched &amp;#948;&lt;sup&gt;15&lt;/sup&gt;N signal (-0.66 &amp;#8240;) in eCO&lt;sub&gt;2 &lt;/sub&gt;plots compared to the controls (-1.38&amp;#8240;). Moreover, the eCO&lt;sub&gt;2&lt;/sub&gt; samples had a more depleted &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C signal (-28.37 &amp;#8240;) compared to the controls (-27.99 &amp;#8240;), as a result of the additional carbon supplied through fumigation (CO&lt;sub&gt;2&lt;/sub&gt; &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C ~ -28 &amp;#8240;). Following the first year of CO&lt;sub&gt;2&lt;/sub&gt; fumigation, there were indications of soil N limitation despite the high rates of atmospheric N deposition (22 kg N ha&lt;sup&gt;-1&lt;/sup&gt; y&lt;sup&gt;-1&lt;/sup&gt;). However, after only 2 years of the FACE experiment there is strong evidence of a shift in key soil N processes to sustain the enhanced nutrient demands to support enhanced canopy CO&lt;sub&gt;2&lt;/sub&gt; uptake. Further research is underway in BiFOR-FACE to elucidate whether carbon quantity and/or quality drives the stimulation of soil N cycling and what are the long-term implications of the trade-off between enhanced CO&lt;sub&gt;2&lt;/sub&gt; sequestration and a potential increase in N&lt;sub&gt;2&lt;/sub&gt;O emissions.&lt;/p&gt;