Morfología Fluvial.-Physico-chemical and spectral characteristics of soil crusts in semiarid areas. An ecosystem condition index?

Abstract

ABSTRACT Physical and biological soil crusts occupy a wide extension in arid and semiarid areas all over the world. Numerous authors consider Biological Soil Crusts (BSC) as ecosystem engineers in arid lands because they control resource availability, through the regulation of the water balance, reduction of erosion and enhancement of soil fertility via nitrogen and carbon fixation. Evidences exist of the replacement of some types of BSC by other crust types over time and different types have different effects on resource availability. Therefore, the relative abundance and distribution of the different development stages of BSC can be considered as a soil quality indicator which includes information about the dynamic condition of the ecosystem, as well as its degree of maturity and conservation, as BSC extend slowly. The objective of this work is to verify if the physicochemical characteristics in the crust and the subjacent soil get better as BSC development increases, so the distribution of soil crusts (in different stages of development) in an ecosystem will be largely related to soil physicochemical conditions. Two semiarid areas in the province of Almeria were chosen: El Cautivo (Tabernas) and Amoladeras (Cabo de Gata) and the most representative physical and biological soil crusts were identified at both sites. The stage of development of BSC was considered. For each crust type, physical and chemical characteristics for the crust and the soil underneath were sampled and some spectral features of BSC were measured as an indicator of photosynthetic activity, using a portable spectroradiometer. Amoladeras appeared as an ecosystem with improved physicochemical characteristics in the two sampled fractions in comparison with El Cautivo (a badlands area), except water retention capacity, due to textural differences. Furthermore, the deepness of the absorption peak at 680 nm was higher in the BSC of Amoladeras than of El Cautivo. At both sites, water retention capacity, nitrogen and carbon content were higher in the BSC than in the physical crust both in the crust and the subjacent soil. These parameters, also at both sites, increased along with BSC development both in the crust and the in subjacent soil, except for the soil C and N content in Amoladeras.