Antonio Rodríguez

Labile carbon pools and biological activity in volcanic soils of the Canary Islands

It is important to assess the mineralisation of soil organic carbon (SOC) to predict the short-term response of biosphere carbon reservoirs to changing environmental conditions. We investigated the labile (easily-mineralisable) SOC in volcanic soils, where the bioavailability of SOC is typically affected by physico-chemical stabilisation mechanisms that are characteristic of these soils. Ten soils were selected that represent the most typical soil types (mainly Andosols) and natural habitats (xe- rophytic scrubland, laurel forest and pine forest) in the Canary Islands, a volcanic archipelago. Over two years we measured several physico-chemical SOC fractions with different degrees of bioavailability: water-soluble carbon in fresh soil samples (WSC) and in the saturated extract (WSC se ), hot water- extractable carbon (HWC), potassium sulphate-extractable carbon (PSC), microbial biomass car- bon (MBC), particulate organic carbon (POC), humic substances carbon (HSC), and total organic carbon (TOC), and performed CO 2 emission incubation assays. We related these measurements to the potential C inputs of plant litter and roots and to the activity of certain hydrolytic enzymes (CM-cellulase, β-D-glucosidase, and dehydrogenase) that are involved in carbon turnover. In vitro carbon mineralisation measurements from short assays (ten days) were fitted with simple first-order kinetics to investigate SOC. This procedure was simple and allowed us to obtain estimates both for potentially mineralisable SOC and for the heterogeneity of the substrates that were consumed during incubation. The investigated volcanic soils had large labile SOC concentrations in which simple carbohydrates predominated and that were mainly derived from roots and aboveground non-woody residues. Among the analysed physico-chemical SOC fractions, HWC (3.1 g kg -1 on average at 0-30 cm depth in Andosols) was the most correlated with C 0 (1.2 g kg -1 ) and therefore best represents po- tentially mineralisable SOC. PSC (0.77 g kg -1 ), which represents an SOC pool of low bioavailability,La medida de la susceptibilidad del carbono organico del suelo (SOC) a la mineralizacion es esencial para predecir la respuesta a corto plazo de los reservorios biosfericos de carbono a los cambios en las condiciones ambientales. En este trabajo abordamos el estudio del SOC labil (facilmente mineralizable) en suelos volcanicos, donde la biodisponibilidad del SOC se ve caracteristicamente afectada por mecanismos de estabilizacion fisico-quimica propios de estos suelos. Con este fin seleccionamos diez suelos representativos (sobre todo Andosoles) de los principales habitats naturales (matorral xerofitico, monteverde y pinar) en las Islas Canarias, un archipielago de origen volcanico. Durante dos anos medimos diversas fracciones de SOC a las que se atribuye un distinto grado de biodisponibilidad: carbono soluble en agua en muestras frescas de suelo (WSC) y en el extracto saturado (WSCse), carbono extraible en agua caliente (HWC), carbono extraible con sulfato potasico (PSC), carbono ligado a la biomasa microbiana (MBC), carbono organico particulado (POC), carbono de sustancias humicas (HSC), y carbono organico total (TOC), y realizamos ensayos de incubacion de las emisiones de CO2. Relacionamos estas medidas con los posibles aportes de carbono procedentes de la hojarasca y las raices, y la actividad de enzimas hidroliticas (CM-celulasa, β-D-glucosidasa, y deshidrogenasa) implicadas en el ciclado del carbono. La medida in vitro de la mineralizacion en ensayos cortos (10 dias) se ajusto a un modelo cinetico simple de primer orden, un procedimiento sencillo que nos permitio obtener no solo una estimacion del SOC mas inmediatamente mineralizable, sino tambien de la heterogeneidad de los sustratos consumidos durante la incubacion. Los suelos volcanicos investigados mostraron una gran riqueza de SOC labil, en los que predominaron los carbohidratos simples procedentes principalmente de aportes organicos radiculares y de restos vegetales no-lenosos aereos. Entre las fracciones fisico-quimicas de SOC analizadas, el HWC (3.1 g kg-1 de promedio a 0-30 cm de profundidad en Andosoles) fue el que mejor se correlaciono con el C0 (1,2 g kg-1) y por tanto el que mejor represento el SOC inmediatamente mineralizable. El PSC (0,77 g kg-1), que representa un reservorio de baja biodisponibilidad, se encuentra protegido de la mineralizacion por su adsorcion a la alofana en Andosoles silandicos.Gypsisols, mainly distributed in arid lands, support a key economic activity and have attracted a lot of scientific interest due to their particular physical and chemical properties. For example, Gypsisols show a high erodibility, low fertility and a variable water holding capacity that can be attributed to different gypsum particle sizes. This study aims to describe some representative Gypsisols from the middle Ebro Basin. Five representative soil profiles (mainly Gypsisols by WRB) were selected and sampled at different positions along a hillside where soils where developed on gyprock. Furthemore, it links micromorphological properties with soil water retention. Soils have a dominant loamy texture, more rarely stoney. Gypsum is abundant in all soil profiles, ranging from 6 to 84% with minimum values in Ah horizons and maximum in By and Cy. The soils have a low level of salinity and a very low cation exchange capacity (CEC). The soil organic matter (SOM) is medium or abundant in the Ah horizons, otherwise it is low. Soil aggregate stability (SAS) is related significantly and positively with SOM and porosity, which is also positively related with moisture retention at field capacity and saturation humidity. However, there is no significant correlation between porosity and permanent wilting point (PWP). Soil water retention is dependant on the gypsum percentage and textural class. Low levels of gypsum have no influence on water retention, but high gypsum levels (> 60%) enhance the field capacity (FC) and decrease PWP, especially when the gypsum is microcrystalline. Gypsum levels between 40 and 60% also increase available water contents (AWC) due to a decrease in PWP. There is a positive and significant correlation between PWP and FC in Gypsisols, except for those which are loamy and have gypsum values over 40%. The higher available water capacity (AWC) than expected is related to microcrystalline gypsum, predominant in the studied soils. These high AWC values are counteracted by an increasingly irregular pore space not suitable for root growth. All these cited characteristics result in a low fertility, influenced by the weather and the human impact, which deforested the highest part of these mountains in the Middle Ages.

Enzyme Diversity in Andosols of the Canary Islands (Spain)

The loss of biodiversity usually accompanies the degradation of the terrestrial ecosystems. Besides the loss of species, such a phenomenon affects the functional diversity of the ecosystem, that is, the diversity of the ecological processes taking place inside it, and the performance of the main soil functions. This work includes the results obtained at Andosols located at nine experimental plots in the National Parks of Garajonay and El Teide (Canary Islands, Spain). The measurement of the enzyme diversity by means of the Shannon seems to provide a good estimation of the variety of functions and ecological processes in which the soil microbiota is involved. Its temporal stability can allow obtaining a good approximation to its value from single measures, in contrast to the usual long monitoring time needed in the studies of the soil enzyme activities.