Flash Forward: Predicting Soil Response to Extreme Erosion Events

Abstract

Soil is a critical natural resource - the foundation of conventional agriculture and terrestrially foraged food, battery for below-ground carbon and nutrient storage, filter for water, and support system for most other ecosystem services. Fundamental questions remain regarding soil sustainability under rapidly-changing climatic and environmental conditions, including massive human impacts and climate change. Accelerated rates of soil erosion threaten the stability of ecosystems, nutrient and carbon cycles, clean water reserves and global food supplies if the processes that produce soil cannot keep pace. Weathering of underlying bedrock substrate helps dislodge rock fragments and prepares particles for movement and incorporation of minerals into the mobile regolith layer, but soil is produced from below and above. The flux of weathered minerals into the near-surface zone acts from below, but the incorporation of dust, transported soil and organic matter near the surface of the soil amends soil from above. Thus, the total amount of soil represents the balance between these soil formation mechanisms and organic matter degradation from physical erosion and chemical transformations in the soil. Sustainability of soil over long timescales requires a positive balance of inputs and outputs. A mismatch of erosion rate and soil production rate can persist for a while before badlands are created, but the buffering timescale depends on the thickness of the existing soil stocks. Fortunately, over millennial timescales, the rate of soil production is thought to keep pace with the rate of surface erosion through negative feedbacks between soil thickness and the rate at which soil is produced from the underlying mineral substrate. This paradigm in the Earth Sciences holds that an underlying mechanism lowers the rate of soil production when soil is thick and increases the rate of soil production when soils are thin. This dynamic balance lends support to two observations: first, soil covers >90% of Earth‘s ice-free surface despite natural soil erosion rates that vary by three orders of magnitude, and second, the thickness of soils on Earth exists within a relatively narrow range even in old and deeply weathered landscapes. However, the actual coupling mechanism between soil thickness and soil production is still unknown, and the environmental controls governing the maximum soil depth or maximum production rate in a given landscape are also yet to be discovered. Unfortunately, a new view is emerging that previously reported rates of soil production were subject to an artifact of the method and that increases in erosion may not give rise to a concomitant, matched rise in soil production. Instead, rates of soil production seem to be set by soil ‘age‘ and the aridity index, i.e. the amount of water left from precipitation after evapotranspiration is accounted for. These controls on soil production are either set by landscape position or features of the Earth system that may be swinging out of balance due to climate change or climate change-induced erosion processes. If correct, this result would have drastic consequences for soil sustainability in the context of anthropogenically accelerated soil erosion. If we fast-forward with current rates of erosion and new soil production rates, how long until soils themselves are endangered?