J. L. Rubio

Modified topsoil islands within patchy Mediterranean vegetation in SE Spain

The spatial distribution of some chemical and physical soil properties was determined in the microenvironment of individual plants of three matorral species (Rosmarinus officinalis L., Stipa tenacissima L., Anthyllis cytisoides L.) in an area of patchy vegetation in south-east Spain. The influence of isolated plants on the chemical and physical status of the topsoil decreased significantly from the plant axis to the bare inter-plant area, giving rise to “islands” of improved topsoil properties beneath each plant (i.e., increased organic matter content and aggregate stability, and decreased bulk density, penetration resistance, shear strength and rock fragment content and cover). The degree and lateral extent of topsoil modification was significantly more pronounced under Rosmarinus and Stipa plants than under Anthyllis shrubs, mainly because of the differential influences of species morphology (i.e., above-ground structure) and components (i.e., canopy and litter covers) on soil properties and erosion/sedimentation processes.

Mound development as an interaction of individual plants with soil, water erosion and sedimentation processes on slopes

In the Mediterranean region, semi-natural shrubland communities (named ‘matorral’) often present a discontinuous cover, where isolated perennial plants alternate with bare inter-plant areas. In such ecosystems, the patchy distribution of the vegetation is usually associated with microtopographic sequences of mounds that develop under isolated plants and break the overall slope continuity. In this study, the influence of three representative species of the Mediterranean matorral (Rosmarinus officinalis, Stipa tenacissima and Anthyllis cytisoides) on slope microtopography is determined and the processes that take part in the development of microtopographic structures beneath the plant canopy are identified. The influence of slope gradient, plant species and plant parameters on the shape and height of microtopographic structures is also studied. The shape of the microtopographic structures is described by using a two-dimensional microprofilemeter and mound height is determined by measuring in the field a ‘mound height index’ defined as the distance from the top to the bottom of the mound. The results obtained show that plant species play a major role in the shape and height of the microtopographic structures. Whereas terrace-type structures generally develop under Anthyllis shrubs, microtopographic forms associated with Rosmarinus and Stipa plants vary with slope gradient. The almost symmetric mound-type structures that develop under these two species on gentle slopes change into terrace-type structures as slope gradient increases. Moreover, statistically significant differences exist between the three species with regard to mound height. Mean values of mound height are 19·4, 14·6 and 4·3 cm under the canopy of Stipa, Rosmarinus and Anthyllis respectively. Plant parameters, essentially roughness, and slope gradient have a significant influence on mound height index. Four main processes were identified as affecting mound development in the studied field site: sedimentation, differential interrill erosion, differential splash erosion and bioturbation. Plant species interact in different ways with these processes according to their morphologies. Since Stipa and Rosmarinus plants are more efficient than Anthyllis shrubs in controlling water erosion, in retaining sediments and in modifying soil properties under their respective canopies, they give rise to higher microtopographic structures that facilitate water and nutrient storage by plants on slopes. Copyright

Temporal changes in soil aggregates and water erosion after a wildfire in a Mediterranean pine forest

Abstract The evolution of soil structure after a forest fire was studied on two zones representatives of a typical Mediterranean Pine forest. These zones were in opposite slope orientation but with similar topographical and pedological characteristics. Changes in soil macro-aggregation and water stable micro-aggregation were monitored seasonally during a year after the fire. The water erosion patterns were also studied from August 1993, immediately after the fire, to the end of 1996. The first 5 cm of soil depth were the most affected by fire temperature, showing clear differences on aggregate distribution and temporal variability between zones. In the north-facing soil, a substantial and gradual recovery on soil aggregation was observed mainly in the fraction greater than 5-mm diameter; this reached an increase of 27% in mass of aggregates. In the south slope, the evolution of aggregation was smooth and restricted to the fraction minor than 0.1 mm. These differences between zones are reflected also in their values on soil cohesiveness and compaction, being lesser in the south-oriented soils. Values of erosion parameters show that both zones have the higher soil losses during the immediate period (4 months) after the fire, being more important in the south zone. This zone produced more sediment and runoff (52.42% and 29.95%, respectively) than north-facing soils for the whole period studied.

Spatial patterns of soil temperatures during experimental fires

The main objective of this paper is to assess the spatial patterns of temperature distribution at the soil surface after a shrubland fire in a typical Mediterranean environment. The study was carried out by making experimental fires at a permanent field station (La Concordia, Valencia, Spain) in a typical Mediterranean forest slope. The set up consisted of nine plots (20 m long×4 m wide) with similar morphology, slope gradient, rock outcrops, soil (Rendzic Leptosol) and vegetation cover (Rhamno lycioidis–Quercetum cocciferae association). Two different fire severities were evaluated, high (F1) and moderate (F2), created by the addition of limited amounts of biomass. To measure soil temperatures, two complementary methods were used: thermocouples and thermosensitive paints. Results show that peak temperatures on the soil surface measured by the two systems (higher than 600 °C in most cases) are quite similar and there are no statistically significant differences between them. The mean values of soil surface temperatures measured with thermosensitive paints were 240, 239 and 218 °C for F1 plots and 418, 448 and 435 °C for F2 plots. Half of the F1 plots surface showed temperature values between 170 and 235 °C, and in the F2 plots, these values ranged between 322 and 543 °C. Geostatistics were applied to analyze and describe the spatial variation of soil temperatures at the soil surface. Results showed that there are two dominant spatial patterns of temperature distribution (spherical and linear). The spherical model varied approximately between 4 and 10 m, and its pattern is related mainly to the natural biomass distribution and the time of flame persistence. In the second, the linear pattern, the temperature rise from the lower part to the upper part of the plot seems to be controlled by the meteorology at the time of burning, mainly by wind speed and wind direction. The spatial patterns of soil temperatures during the studied experimental fires affect soil properties in different ways according to the fire severity. This fact could contribute to change the spatial dynamics of soil nutrients that will play an important role in the recovery of the burned vegetation.

Postfire Effects on Soil Properties and Nutrient Losses

The effects of an August, 1992, wildfire on nutrient losses by water erosion have been studied. The fire affected an area of 9498 ha of pine forest and shrub, located in Sierra Calderona (Valencia, Spain). In the burned area, six stations of erosion measurement were set up immediately after the end of the fire. The results obtained in these stations are reported. Topographical, edaphological and vegetation characteristics of each station are described. Fourteen episodes of erosive rain with production of runoff and sediments, between August,1992, and November, 1993, were studied, mainly in relation to changes in the soil chemical characteristics. Data show that the highest soil and nutrient losses were produced in the period immediately after the fire. However, fire intensity is the factor that determines the soil status with respect to its response to nutrient losses.

Synthesis of inorganic-organic hybrid materials from TEOS, TBT and PDMS

Inorganic-organic hybrid materials have been synthesized by reaction of tetraethoxysilane (TEOS), titanium tetrabutoxide (TBT) and silanol-terminated polydimethylsiloxane (PDMS). The hydrolysis and polymerization reactions of TEOS and PDMS in presence of TBT have been followed by means of FT-IR spectroscopy. Hydrolysis reactions have been characterized by Si—O—C and Ti—O—C bonds and polymerization reactions by Si—O—Si, Si—O—Ti and Ti—O—Ti bonds. The instantaneous hydrolysis of TEOS, the condensation reactions between Si—OH groups forming crosslinked and linear Si—O—Si structures and the copolimerization reactions between Si—OH groups and PDMS molecules have been observed. The reaction between Ti—OH and Si—OH groups or PDMS gives Si—O—Ti bonds. Si—O—Ti bonds are formed during the addition step of TBT and they show a decrease along the reaction time up to gelling. By another hand, the concentration of crosslinked and linear Si—O—Si structures depend on the TBT concentration. High TBT concentration favors the formation of crosslinked chains. TBT influences the hydrolysis and copolymerization reactions between hydrolyzed TEOS and PDMS molecules.

Nitrogen fertilization in citrus

SummaryThe absorption rate of nitrogen (N) fertilizer labelled with15N was studied during the flowering and fruit set periods inCitrus mitis Bl., trees grown in sand culture.The absorbed N was concentrated preferentially in ovaries, young fruits, new flush leaves and new flush twigs, but N supplied to these organs comes mostly from the N reserve in old leaves, old twigs and stem, and roots, and less than 3% of the absorbed N reached ovaries and young fruits. Nitrogen translocation to young fruits was higher during the fruit set period. Spring leaves were an essential organ since they received a large portion of the absorbed N and were quickly converted into a supplying organ. Old leaves, old twigs and stem, and roots supplied a great deal of N to developing organs; during fruit set, their N supply was replenished with N coming from fertilizer.During flowering, trees absorbed 30 mg per 1000g of whole tree (dry weight) and per day. During fruit set, this amount increased considerably.

Desertification in the Mediterranean Region. A security issue

Proceedings of the NATO Mediterranean Dialogue Workshop on Desertification in the Mediterranean Region. A Security Issue Valencia, Spain 2-5 December 2003

Testing three Mediterranean shrub species in runoff reduction and sediment transport

Runoff yield and sediment delivery are compared in three shrub species (Medicago arborea, Atriplex nummularia, and Psoralea bituminosa) with natural (matorral) vegetation in an experimental plot set. The planted species are arranged in discontinuous rows perpendicular to the slope, acting as barriers to the direction of the surface water flow. The study was carried out from 1988 to 1995, in which 56 erosive rain events were monitored in runoff production and in soil losses, together with their intrinsic characteristics (duration, intensity and rainfall). Growth of the three shrub species (height, vegetation cover, and stem diameter) and the influence on soil properties during this period were recorded. Because of certain characteristics of the study area (shallow soils, high levels of carbonates, stoniness) and the climate (an especially intense period of drought since 1990), the growth of these plants was very irregular. M. arborea gave the best protection against water erosion reducing sediment production by more than 58% compared to the bare soil. However, the protection afforded by this species never reached that obtained with natural vegetation, which reduced sediment production by almost 75%.

FT-IR Study of the Hydrolysis and Polymerization of Tetraethyl Orthosilicate and Polydimethyl Siloxane in the Presence of Tetrabutyl Orthotitanate

Abstract In this work, we have used FT‐IR spectroscopy to study the hydrolysis and polymerization reactions of tetraethyl orthosilicate (TEOS) and polydimethyl‐siloxane (PDMS) in the presence of tetrabutyl orthotitanate (TBOT). These reactions are used for obtaining SiO2–PDMS–TiO2 organically modified silicates (Ormosils). In order to obtain semi‐quantitative information about such reactions, a deconvolution procedure of the FT‐IR spectra has been done by use of a computer program. Hydrolysis reactions have been characterized by Me–O–C (Me = Si, Ti) bonds, and polymerization reactions by Me–O–Me bonds. Instantaneous hydrolysis of TEOS has been observed, together with condensation reactions between Si–OH groups, which give crosslinked and linear Si–O–Si structures. The TBOT is also hydrolyzed, but the high acid concentration inhibits condensation reactions between Ti–OH groups. The PDMS also condenses mainly with Si–OH groups and probably with Ti–OH, finally forming Me–O–PDMS bonds. The formation of Si–O–Si crosslinked structures and also Me–O–PDMS structures continues until the end of reaction. The gelling time is dependent on TBOT concentration in the reaction medium and, therefore, polycondensation reactions are dependent on TBOT concentration.