Ecosystem-specific patterns and drivers of global reactive iron mineral-associated organic carbon

<p>Reactive iron (Fe) oxides are vital for long-term soil or sediment organic carbon (SOC) storage. However, the patterns and drivers of Fe-associated organic carbon (Fe-OC) over global geographic scales under various ecosystem types remain controversial. Here, we provided a systematic assessment of the distribution patterns and determinants of Fe-OC content and its contribution to SOC (<span class="inline-formula"><i>f</i></span>Fe-OC) by assembling a global dataset comprising 862 observations from 325 sites in distinct ecosystems. We found that Fe-OC content across global ecosystems ranged from 0 to 83.3 g kg<span class="inline-formula">⁻¹</span> (<span class="inline-formula"><i>f</i></span>Fe-OC ranged from 0 % to 82.4 %), reflecting the high variability of the Fe-OC pool. Fe-OC contents varied with ecosystem type being greater in wetlands with a high molar ratio of Fe-OC <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="57ee8123d9c9aefcf23d9c7f6463c158"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-4761-2023-ie00001.svg" width="8pt" height="14pt" src="bg-20-4761-2023-ie00001.png"/></svg:svg></span></span> dithionite-extractable Fe (Fe<span class="inline-formula">_d)</span> compared with marine and terrestrial ecosystems. Furthermore, <span class="inline-formula"><i>f</i></span>Fe-OC in wetlands was significantly lower than that in other ecosystems due to rich organic carbon (OC). In contrast with climate variables and soil pH, the random forest modeling and multivariate analysis showed that the Fe-OC <span class="inline-formula">:</span> Fe<span class="inline-formula">_d</span> and SOC were the predominant predictors of Fe-OC content and <span class="inline-formula"><i>f</i></span>Fe-OC in wetlands and terrestrial ecosystems, whereas Fe<span class="inline-formula">_d</span> content was a primary driver in marine ecosystems. Based on upper estimates of global SOC storage in various ecosystem types, we further estimated that 83.84 <span class="inline-formula">±</span> 3.8, 172.45 <span class="inline-formula">±</span> 8.74, and 24.48 <span class="inline-formula">±</span> 0.87 Pg of SOC were preserved by association with Fe oxides in wetland, terrestrial, and marine ecosystems, respectively. Taken together, our findings highlighted the importance of reactive Fe oxides in global SOC preservation, and their controlling factors were ecosystem specific.</p>