No detectable upper limit when predicting soil mineral-associated organic carbon stabilization capacity in temperate grassland of Central Argentina mountains

Abstract

Soil organic carbon (SOC) is the major active organic carbon (OC) reservoir within terrestrial ecosystems. SOC is stored in different soil organic matter fractions, among which mineral-associated organic matter is considered the more stable. It has been proposed that the OC stored in this fraction (i.e., MAOC-mineral associated organic carbon) depends on the proportion of fine soil particles. This idea has been challenged by studies that found soils with significantly higher MAOC values than predicted by several models. We assessed the soil MAOC contents in central Argentina mountain grasslands. These soils store high OC contents, and thus may constitute a valuable case to evaluate the soils' saturation behavior. We compared the actual soil MAOC content of these mountain grasslands with the predictions from different MAOC saturation models, and we explored if there were indications of saturation behavior by: a) plotting the MAOC against the total SOC contents, and b) by plotting the OC contents in the fine fraction against the fine soil particles content. We found a positive relationship between MAOC and the fine particle content in the soils studied. However, we also found that even the model with the highest upper limit of MAOC stabilization underestimated the MAOC contents of these grasslands. In addition, we found the absence of an inflection point when plotting the MAOC against total SOC contents. Finally, we observed a weak but significant negative relationship between the concentration of OC in the fine fraction and the proportion of the fine fraction of the soil, with high OC loading values in the coarser soils. These last patterns could be indicators of non-saturation behavior. Our results provide additional evidence that many of the models currently used for predicting the OC saturation deficit of soils may be underestimating the potential MAOC stabilization capacity of certain soils. Additionally, our findings contribute to the evidence challenging the ideas of OC saturation on a fine soil particles basis. We highlight some lines of research that should be developed to confirm the existence of a MAOC saturation behavior, as well as to continue exploring the drivers of OC accrual in different systems.