Responses of roadside soil cation pools to vehicular emission deposition in southern California

Abstract

Roadside soils are heavily loaded with reactive nitrogen due to vehicular emissions, and these loadings likely acidify near-road soils. Cationic metals are mobilized from acidified soils during exchange reactions. Depletion of soil cation pools is well documented in forest soils, but poorly understood in roadside soils despite heavy nitrogen loadings to roadside environments and the continued growth of road networks. This study considers soil acidification impacts across gradients of soil chemistry, road network density, climate, and geology in southern California, clarifying acidification processes in near-road environments. Although there is no direct relationship between soil pH and road proximity, several elements (Al, Be, K) are depleted in near-road environments. The depletion of these elements occurs despite more sediment surface area and organic matter in near-road soils. In contrast, the weathering of road construction materials (i.e., concrete) replenishes soil Ca and Mg pools and likely buffers soil pH. These observed near-road patterns in soil chemistry point to the potential for mobilization of metal contamination from roadside environments. Improved understanding of metal dynamics in roadside soils is essential to the effective management of road-sourced nutrients and storm water, particularly as expanding road networks continue to affect wider areas.