Reply on RC1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Elevated carbon dioxide (<span class="inline-formula"><i>e</i></span>CO<span class="inline-formula">₂</span>) in the atmosphere increases forest biomass productivity but only where soil nutrients, particularly nitrogen (N) and phosphorus (P), are not limiting growth. <span class="inline-formula"><i>e</i></span>CO<span class="inline-formula">₂</span>, in turn, can impact rhizosphere nutrient availability. Our current understanding of nutrient cycling under <span class="inline-formula"><i>e</i></span>CO<span class="inline-formula">₂</span> is mainly derived from surface soil, leaving mechanisms of the impact of <span class="inline-formula"><i>e</i></span>CO<span class="inline-formula">₂</span> on rhizosphere nutrient availability at deeper depths unexplored. To investigate the influence of <span class="inline-formula"><i>e</i></span>CO<span class="inline-formula">₂</span> on nutrient availability in soil at depth, we studied various C, N, and P pools (extractable, microbial biomass, total soil C and N, and mineral-associated P) and nutrient cycling processes (enzyme activity and gross N mineralisation) associated with C, N, and P cycling in both bulk and rhizosphere soil at different depths at the Free Air CO<span class="inline-formula">₂</span> enrichment facility in a native Australian mature <i>Eucalyptus</i> woodland (EucFACE) on a nutrient-poor soil. We found decreasing nutrient availability and gross N mineralisation with depth; however, this depth-associated decrease was reduced under <span class="inline-formula"><i>e</i></span>CO<span class="inline-formula">₂</span>, which we suggest is due to enhanced root influence. Increases in available PO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">3</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="13pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="ddae2421316dad1d77c3146332f534f9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-505-2023-ie00001.svg" width="13pt" height="17pt" src="bg-20-505-2023-ie00001.png"/></svg:svg></span></span>, adsorbed P, and the C : N and C : P ratio of enzyme activity with depth were observed. We conclude that the influences of roots and of <span class="inline-formula"><i>e</i></span>CO<span class="inline-formula">₂</span> can affect available nutrient pools and processes well beyond the surface soil of a mature forest ecosystem. Our findings indicate a faster recycling of nutrients in the rhizosphere, rather than additional nutrients becoming available through soil organic matter (SOM) decomposition. If the plant growth response to <span class="inline-formula"><i>e</i></span>CO<span class="inline-formula">₂</span> is reduced by the constraints of nutrient limitations, then the current results would call to question the potential for mature tree ecosystems to fix more C as biomass in response to <span class="inline-formula"><i>e</i></span>CO<span class="inline-formula">₂</span>. Future studies should address how accessible the available nutrients at depth are to deeply rooted plants and if fast recycling of nutrients is a meaningful contribution to biomass production and the accumulation of soil C in response to <span class="inline-formula"><i>e</i></span>CO<span class="inline-formula">₂</span>.