Topographical effects on the interplay between soil organic carbon fractions, aggregate stability and water repellency at a small watershed scale

Abstract

Soil erosion is one of the main environmental problems worldwide and even though there were many efforts in the scientific community to understand soil erosion drivers, the complexity of positive and negative feedbacks and thresholds of these drivers on the extent of soil erosion still requires further research. This study examines the influence of terrain attributes (Position: Shoulder, Back-slope, and Foot-slope), Sub-Position (Ridge and Valley), and Transect – summarizing shape, slope, and length – on the interplay between soil organic carbon fractions (SOC) total, labile and very labile-, aggregate stability mechanisms (AS) by Le Bissonnais method, and soil water repellency (SWR) by water-drop penetration time, highlighted as important soil erosion drivers, within a no-tillage agricultural watershed. AS was predominantly stable, as assessed by micro-cracking (~3.4 ± 0.10 mm MWD) and mechanical breakdown tests (3.2 ± 0.12 mm MWD) mainly due to high SOC content (mean total SOC: 3.5 % ±0.59 g kg 1). Slaking tests (2.4 mm ± 0.58 mm MWD) revealed vulnerability in some areas, particularly influenced by terrain attributes and soil water repellency. SWR, categorized as slightly repellent (16.2 s ± 19.4), was positively correlated with slaking test showing a threshold where SWR increases no longer affected the slaking process. Slope position and shape, had significant effects on soil properties, especially on AS and SWR. In general Back-slope showed the lowest SOC and AS values, while the Foot-slope showed the opposite behavior. Valley showed low AS and SWR. Slope steepness and concavity affected SWR and SOC distribution probably due to its influence on soil water content. This study highlights the complex interplay between terrain attributes and soil properties, registering also thresholds between AS and SWR indirectly modulated by soil moisture. The magnitude of each soil property determined, among other factors by terrain attributes, plays a crucial role in soil erosion dynamics in agricultural landscapes.