Divergent controls on particulate and mineral-associated organic carbon formation and persistence

Identifying global controls on soil carbon (C) storage, as well as where soil C is most vulnerable to loss, are essential to realizing the potential of soils to mitigate climate change via C sequestration. However, we currently lack a comprehensive understanding of the global drivers of soil C storage, especially with regards to particulate (POC) and mineral-associated organic carbon (MAOC). To better understand global controls on these two C fractions, we synthesized climate, and net primary production (NPP), and soils data from 73 published studies and databases. This large dataset is representative of multiple land cover types, including broadleaved and coniferous forests, grasslands, shrublands, wetlands, tundra, and wetlands. We then applied structural equation modeling (SEM) to assess hierarchical, interactive controls on global POC and MAOC pools (i.e., g POC or MAOC per kg soil) in topsoils. Our SEM tested relationships between NPP and climate (i.e., mean annual temperature (MAT) and effective moisture, assessed as mean annual precipitation minus potential evapotranspiration), as well as the extent to which climate and NPP, along with soil texture and pH, govern POC and MAOC storage. We found that NPP is positively related to MAT and effective moisture. Additionally, POC storage is negatively related to both MAT and pH, while MAOC storage is positively related to NPP and effective moisture, but negatively related to soil % sand. Given that temperature and pH impose constraints on microbial decomposition, these results indicate that POC storage is primarily controlled by C output limitations. In contrast, strong relationships with variables related to plant productivity constraints and to mineral surfaces available for sorption indicate that MAOC storage is primarily controlled by climate-driven C input limitations and C stabilization mechanisms. Together, we demonstrate that divergent controls govern C storage in POC and MAOC, and that these controls are consistent across multiple ecosystem types.