Penetration resistance of biological soil crusts and its dynamics after crust removal: Relationships with runoff and soil detachment

Abstract

Soil sealing and crusting are common, widespread processes in arid and semiarid soils that strongly affect runoff and erosion. In these soils, biological soil crusts (BSCs) are an essential surface component known to have an important role in water processes, and are also strong erosion protection agents. However, these crusts are also highly vulnerable to disturbances, which seriously affect their pedological and ecological roles. The aims of this study were to examine whether penetration resistance (PR) depends on BSC developmental stage and how removal of BSCs affects PR, under dry and wet soil conditions, and to analyse the relationship between PR and runoff and soil detachment in these soils. This research was conducted in two semiarid ecosystems representative of crusting processes in SE Spain (El Cautivo and Las Amoladeras, Almeria province). Four pairs of plots containing the main BSC types identified at both sites were selected and two treatments were considered, one in each plot in the pair: i) unaltered BSC and ii) BSC removal by scraping. Two intense rainfall simulation experiments were performed one after another in these plots. Runoff was measured during both, and water samples were collected for further determination of sediment yield. PR was measured in undisturbed BSC and scalped soil on dry soil prior to rainfall, on wet soil after the first and second rainfalls, and on dry soil seven months after BSC removal. Crust PR was higher at Las Amoladeras (3.13±0.54kgcm−2) than at El Cautivo (1.52±0.74kgcm−2) due to soil compaction from trampling by livestock in the former. PR varied with BSC development, with less developed cyanobacteria BSCs showing higher resistance (under dry and wet conditions) than more developed lichen BSCs. BSC removal was followed by the formation of a new seal (physical crust) upon raining, which however, did not result in increased PR. No significant relationship was found between PR and runoff, but there was a weak negative relationship between PR and soil detachment, because in BSCs, which had higher PRs, soil detachment rates were much lower than in recently scalped soils. Seven months after BSC removal, PR was significantly higher in scalped soils than for undisturbed BSCs. Our results show that PR was not a good indicator of hydrological and soil detachment response of BSCs or the soil sealing during rain immediately after BSC removal, but it was a good indicator of physical soil crust formation in scalped soils over time.