Soil organic carbon response to global environmental change depends on its distribution between mineral-associated and particulate organic matter: A meta-analysis

Soil organic carbon (SOC), as the largest terrestrial carbon pool, plays an important role in global carbon (C) cycling, which may be significantly impacted by global changes such as nitrogen (N) fertilization, elevated carbon dioxide (CO2), warming, and increased precipitation. Yet, our ability to accurately detect and predict the impact of these global changes on SOC dynamics is still limited. Investigating SOC responses to global changes separately for mineral-associated organic carbon (MAOC) and the particulate organic carbon (POC) can aid in the understanding of overall SOC responses, because these are formed, protected, and lost through different pathways. To this end, we performed a systematic meta-analysis of the response of SOC, MAOC, and POC to global changes. POC was particularly responsive, confirming that it is a better diagnostic indicator of soil C changes in the short-term, compared to bulk SOC and MAOC. The effects of elevated CO2 and warming were subtle and evident only in the POC fraction (+5.11% and - 10.05%, respectively), while increased precipitation had no effects at all. Nitrogen fertilization, which comprised the majority of the dataset, increased SOC (+5.64%), MAOC (+4.49%), and POC (+13.17%). Effect size consistently varied with soil depth and experiment length, highlighting the importance of long-term experiments that sample the full soil profile in global change SOC studies. In addition, SOC pool responses to warming were modified by degree of warming, differently for air and soil warming manipulations. Overall, we suggest that MAOC and POC respond differently to global changes and moderators because of the different formation and loss processes that control these pools. Coupled with additional plant and microbial measurements, studying the individual responses of POC and MAOC improves understanding of the underlying dynamics of SOC responses to global change. This will help inform the role of SOC in mitigating the climate crisis.