Distinct, direct and climate‐mediated environmental controls on global particulate and mineral‐associated organic carbon storage

Abstract

Identifying controls on soil organic carbon (SOC) storage, and where SOC is most vulnerable to loss, are essential to managing soils for both climate change mitigation and global food security. However, we currently lack a comprehensive understanding of the global drivers of SOC storage, especially with regards to particulate (POC) and mineral‐associated organic carbon (MAOC). To better understand hierarchical controls on POC and MAOC, we applied path analyses to SOC fractions, climate (i.e., mean annual temperature [MAT] and mean annual precipitation minus potential evapotranspiration [MAP‐PET]), carbon (C) input (i.e., net primary production [NPP]), and soil property data synthesized from 72 published studies, along with data we generated from the National Ecological Observatory Network soil pits (n = 901 total observations). To assess the utility of investigating POC and MAOC separately in understanding SOC storage controls, we then compared these results with another path analysis predicting bulk SOC storage. We found that POC storage is negatively related to MAT and soil pH, while MAOC storage is positively related to NPP and MAP‐PET, but negatively related to soil % sand. Our path analysis predicting bulk SOC revealed similar trends but explained less variation in C storage than our POC and MAOC analyses. Given that temperature and pH impose constraints on microbial decomposition, this indicates that POC is primarily controlled by SOC loss processes. In contrast, strong relationships with variables related to plant productivity constraints, moisture, and mineral surface availability for sorption indicate that MAOC is primarily controlled by climate‐driven variations in C inputs to the soil, as well as C stabilization mechanisms. Altogether, these results demonstrate that global POC and MAOC storage are controlled by separate environmental variables, further justifying the need to quantify and model these C fractions separately to assess and forecast the responses of SOC storage to global change.

Investigating the complementarity of thermal and physical soil organic carbon fractions

“…Overall, our study confirms results from recent studies conducted at regional and global scales showing that physical fractions have different drivers (King et al, 2023 ;Hansen et al, 2024) and strongly supports the idea that it is relevant and informative to consider SOC fractions (either physical or thermal) instead of TOC alone. However, it is difficult to compare our results directly with those of these recent studies.…”

“…The higher proportion of POC in forest than in grassland and cropland soils (0.13, 0.17, and 0.27 in croplands, grasslands, and forests, respectively) was also observed in Lugato et al (2021) who used results from 352 physical fractionations obtained using a fractionation protocol similar to ours on samples from the LUCAS soil monitoring network. It was also observed by Hansen et al (2024) on a global dataset combining physical fractions obtained with different protocols. Other studies showed similar results for POC, but comparisons are less straightforward as the physical fractionation protocols used are significantly different from those used in our study.…”

“…Notably, we observed that land cover and soil variables such as exchangeable calcium and iron oxides were explanatory variables of primary importance in explaining the quantities of POC and MAOC respectively. These variables were not considered as explanatory variables in the path analyses developed by Hansen et al (2024) which likely explains the low explained variations provided by these path analyses. Conversely, we did not consider NPP as explanatory variable which was found to be a significant driver of MAOC quantities by Hansen et al (2024).…”

“…These variables were not considered as explanatory variables in the path analyses developed by Hansen et al (2024) which likely explains the low explained variations provided by these path analyses. Conversely, we did not consider NPP as explanatory variable which was found to be a significant driver of MAOC quantities by Hansen et al (2024). We therefore consider that new data that will probably arrive in the next few years will enable us to refine the drivers of physical and thermal fractions at different spatial scales.…”

Enhanced Mineral Preservation Rather Than Microbial Residue Production Dictates the Accrual of Mineral‐Associated Organic Carbon Along a Weathering Gradient

“…However, controls on the formation and accrual of MAOC are only partially resolved currently. It is not entirely clear what environmental variables (processes) predominate the accumulation patterns and capacity of MAOC in soils (Begill et al, 2023;Cotrufo et al, 2019;Hansen et al, 2024). Answering this question will contribute to our understanding of the mechanisms underlying MAOC formation and potentially provoke management strategies to enhance MAOC sequestration for climate change mitigation.…”