Understanding human influence on carbon (C) cycling is crucial for assessing stability of our ecosystems. Soil organic matter (SOM), which is the largest terrestrial store of C, provides nutrients for plants, water retention, and climate change mitigation. However, even after decades of researching SOM responses to global change, we have yet to reach consensus on its responses to global changes such as increasing temperatures and nitrogen (N) pollution. This variability may be attributed to distinct components of SOM responding differently to environmental change. Two SOM components that have distinct properties are the mineral-associated organic matter (MAOM), formed mostly of microbially-derived compounds associated with soil minerals, and the particulate organic matter (POM), formed mostly of only partially degraded structural plant inputs. We harnessed published data on responses of SOM components to global change and performed a systematic meta-analysis to assess the response of MAOM, POM, and bulk SOM to experimental N fertilization, warming, CO2 enrichment (eCO2), and changing precipitation regimes.
Preliminary results show diverse responses of C concentrations of POM, MAOM, and total SOM, that are dependent on the imposed global change factor. With N fertilization, there is a significant increase in all three pools, albeit larger for POM-C. There were more complex responses to other global change factors. Warming treatments had no effect on SOM-C, but significantly reduced MAOM- and POM-C. Similarly, eCO2 had negligible effects on SOM- and MAOM-C but significantly increased POM-C. Interestingly, additional precipitation significantly increased MAOM and decreased POM, leading to negligible effects on the total SOM pool. Lastly, for interacting global change factors, low sample sizes led to high variability, apart from significantly decreased POM-C in response to combined eCO2 and warming. These patterns have significant implications for how C cycling is studied. For N fertilization, data on total C stocks may be sufficient. However, for assessing effects of other global change factors on soil C storage, it is imperative that we consider different SOM pools. Our data suggest that global change will have a greater impact on freely cycling C, represented by the POM pool, which could lead to cascading effects on ecosystem function. In contrast, weaker effects on MAOM-C are expected given its stability, but significant changes in this pool have implications for long-term soil C storage and potential climate change mitigation. Our data suggest that assessing multiple SOM pools will provide a clearer understanding of ecosystem functioning under projected global changes.