Sonia Chamizo

Dynamics of organic carbon losses by water erosion after biocrust removal

Abstract In arid and semiarid ecosystems, plant interspaces are frequently covered by communities of cyanobacteria, algae, lichens and mosses, known as biocrusts. These crusts often act as runoff sources and are involved in soil stabilization and fertility, as they prevent erosion by water and wind, fix atmospheric C and N and contribute large amounts of C to soil. Their contribution to the C balance as photosynthetically active surfaces in arid and semiarid regions is receiving growing attention. However, very few studies have explicitly evaluated their contribution to organic carbon (OC) lost from runoff and erosion, which is necessary to ascertain the role of biocrusts in the ecosystem C balance. Furthermore, biocrusts are not resilient to physical disturbances, which generally cause the loss of the biocrust and thus, an increase in runoff and erosion, dust emissions, and sediment and nutrient losses. The aim of this study was to find out the influence of biocrusts and their removal on dissolved and sediment organic carbon losses. One-hour extreme rainfall simulations (50 mm h-1) were performed on small plots set up on physical soil crusts and three types of biocrusts, representing a development gradient, and also on plots where these crusts were removed from. Runoff and erosion rates, dissolved organic carbon (DOC) and organic carbon bonded to sediments (SdOC) were measured during the simulated rain. Our results showed different SdOC and DOC for the different biocrusts and also that the presence of biocrusts substantially decreased total organic carbon (TOC) (average 1.80±1.86 g m-2) compared to physical soil crusts (7.83±3.27 g m-2). Within biocrusts, TOC losses decreased as biocrusts developed, and erosion rates were lower. Thus, erosion drove TOC losses while no significant direct relationships were found between TOC losses and runoff. In both physical crusts and biocrusts, DOC and SdOC concentrations were higher during the first minutes after runoff began and decreased over time as nutrient-enriched fine particles were washed away by runoff water. Crust removal caused a strong increase in water erosion and TOC losses. The strongest impacts on TOC losses after crust removal occurred on the lichen plots, due to the increased erosion when they were removed. DOC concentration was higher in biocrust-removed soils than in intact biocrusts, probably because OC is more strongly retained by BSC structures, but easily blown away in soils devoid of them. However, SdOC concentration was higher in intact than removed biocrusts associated with greater OC content in the top crust than in the soil once the crust is scraped off. Consequently, the loss of biocrusts leads to OC impoverishment of nutrient-limited interplant spaces in arid and semiarid areas and the reduction of soil OC heterogeneity, essential for vegetation productivity and functioning of this type of ecosystems.

Easy-to-make portable chamber for in situ CO2 exchange measurements on biological soil crusts

Commercial chambers for in vivo gas exchange are usually designed to measure on vascular plants, but not on cryptogams and other organisms forming biological soil crusts (BSCs). We have therefore designed two versions of a chamber with different volumes for determining CO2 exchange with a portable photosynthesis system, for three main purposes: (1) to measure in situ CO2 exchange on soils covered by BSCs with minimal physical and microenvironmental disturbance; (2) to acquire CO2-exchange measurements comparable with the most widely employed systems and methodologies; and (3) to monitor CO2 exchange over time. Different configurations were tested in the two versions of the chamber and fluxes were compared to those measured by four reference commercial chambers: three attached to two respirometers, and a conifer chamber attached to a portable photosynthesis system. Most comparisons were done on biologically crusted soil samples. When using devices in a closed system, fluxes were higher and the relationships to the reference chambers were weaker. Nevertheless, high correlations between our chamber operating in open system and measurements of commercial respiration and photosynthetic chambers were found in all cases (R2 > 0.9), indicating the suitability of the chamber designed for in situ measurements of CO2 gas exchange on BSCs.

Costras biológicas: " Ingenierías del suelo " en ecosistemas áridos y semiáridos

En regiones aridas y semiaridas, los espacios libres entre plantas suelen estar ocupados por una comunidad de algas, cianobacterias, liquenes, musgos y otros microorganismos, que se conocen como Costras Biologicas del Suelo (CBS). Las CBS controlan la disponibilidad de recursos en los ecosistemas regulando el balance de agua, reduciendo la erosion y aumentando la fertilidad del suelo. El objetivo de este trabajo es analizar la influencia de costras fisicas y biologicas, en diferentes estados de evolucion, sobre las propiedades del suelo subyacente y sobre la infiltracion, regimen de humedad y erosion. Para ello se han analizado las caracteristicas fisico-quimicas de la costra y material subyacente en dos ecosistemas semiaridos del SE espanol. La respuesta hidrologica y erosiva de estas costras se ha estudiado mediante simulaciones de lluvia y se han usado sensores de humedad para examinar su influencia sobre la humedad del suelo. A medida que aumenta el desarrollo de la costra, aumenta el contenido en C, N y agua util de la costra y del suelo subyacente, aumenta la infiltracion y disminuye la erosion. El registro de humedad durante ano y medio muestra que para potenciales muy negativos, las CBS favorecen la conservacion de agua en el suelo respecto a las costras fisicas o suelo bajo planta.