Understanding the Spatio-Temporal Change in the Factors Controlling the Preservation of Soil Organic Carbon Under Contrasting Climatic Conditions- A Machine Learning Approach

In the context of global carbon cycling, soils serve as an important link between the sustenance of its co-existing vegetation and its subsequent decomposition back to the atmosphere, while preserving a part of it as Soil Organic Carbon (SOC). SOC consists of variably degraded soil organic matter and its gradual accrual leads to the preservation of three times more organic carbon than the atmosphere and vegetation combined. A multitude of factors such as the coexisting vegetation, change in climate, and physicochemical properties of the soil play a role in the preservation of the SOC among which, one is assigned to be the Predominant Preservation Factor (PPF). The PPF best explains the major controlling factor on the preservation of SOC in the soil. Past studies have been constrained to only modern age samples and hence temporal scale investigation needs to be done, as they represent fluctuations in the paleosol carbon preservation dynamics. This study uses analytical proxies such as δ13CSOM, δ13CSC, δ18OSC, clay, silt, sand, Al2O3, and Fe2O3 concentrations on samples (n = 111) from four distinct paleosol cores (35 m in length), across contrasting climatic regimes along the Ganga River Basin, coupled with Random Forest Regression analysis, to glean out the PPF behind SOC preservation in paleosol. The results indicate that, in high rainfall climatic regimes, high precipitation leads to the preferential leaching and reduction of Fe3+, subduing its SOC adsorption capacity and indirectly making Al2O3 to be the PPF. In contrast, in arid regions, in the absence of such a process, Fe2O3 plays the dominant role in the preservation of SOC. Interestingly, river flow dynamics may affect the vegetation type by promoting the deposition of clay, which leads to the proliferation of C4 grasses. C4 grasses, due to their faster germination capability and high root density, lead to more SOC accrual in the soil, thereby making vegetation type to be the PPF. Temporally, changes in the dominating factor have also been observed such that silt percentage can be the PPF as its abundance is high and fosters the preservation of SOC. Thus, it can be concluded that overall, the dominant role in the preservation of SOC is due to organo-minerallic interactions, while vegetation and grain size can affect the preservation potential on regional and temporal scales.