Understanding changes in soil organic carbon

Knowledge of how soil organic carbon (SOC) content changes within a landscape and over time will help improve our understanding of both how agricultural practices contribute to climate change and how climate change affects agriculture. Changes to both shallow and deep SOC result from belowground transformations such as microbial decomposition and addition of plant material, and from physical transport over the ground surface by erosion and deposition. The interactions between the two types of processes—transformation and transport— have been strongly perturbed by agricultural practices, but quantifying these interactions at the watershed scale is a challenge.

Agricultural practices have dramatically accelerated soil erosion and altered SOC dynamics. Soil erosion not only redistributes surface SOC but influences the biogeochemical transformations belowground. To study these complex interactions, a team of researchers from the University of Illinois and Purdue University (graduate student Tingyu Hou and professor Timothy Filley, Earth, Atmospheric and Planetary Sciences) developed a 3‐D computational model that couples hydrological, biogeochemical, and geomorphological processes to simulate, for the first time, the coevolution of landscape and SOC dynamics at fine scales in space and time.

The model was applied to an agricultural watershed under a corn and soybean rotation. This watershed is a study site within the NSF-funded Intensively Managed Landscapes Critical Zone Observatory. The results show that in this setting, physical transport rather than biogeochemical transformation is the dominant driver affecting both the below ground profile as well as the stocks of SOC. The majority of upland, erosional sites were found to be a net local atmospheric carbon sink, and the majority of flood plain, depositional sites a net atmospheric carbon source. Interestingly, mechanical mixing by tillage was found to increase the SOC stock at erosional sites but reduce the stock at depositional sites. The study provides a better understanding of SOC dynamics, and the model could serve as an important tool for managing soil carbon.

Yan, Q. P. Le, D. K. Woo, T. Hou, T. R. Filley, and P. Kumar (2018) 3-D Modeling of the Coevolution of Landscape and Soil Organic Carbon. Water Resources Research, 55, 1218-1241.