La Distribució dels carbonats en els sòls de la Depressió Central Catalana

Soils from different calcareous substrates are studied in order to search the evolution of the carbonate fraction. There is a close relationship between soil-type and lithologic material. From hard limestones, red-brown (fersiallitic) soils are formed. In general they have lost the carbonates almost completely but, in some cases, they have secondary lime in different forms. These accumulations of calcium carbonate are associated with an increase in the carbonate content of the finer fractions, clay and fine silt. On calcareous sandstones, completely carbonate-leached red-brown soils coexist with carbonatic ones, in the same ecological conditions. In the latter, as in soils formed from calcarenite substrate, carbonates concentrate in the sand fractions. The total amount of carbonates in the profile depends on its content in the parent material but, in any case, the carbonate leaching rate is high: 40-50 %. On calcareous clays and calcilutites, the soils are highly calcareous, mainly in the silt fraction, and they have the lower carbonate leaching rate, about 10 %. Soils from gypsic materials have lost most of the gypsum and thus the carbonate is relatively concentrated. Two general types of soils with respect to carbonate evolution are distinguished: a) Carbonate-leached soils, mostly occurring in fissures in the rock layers, which are postulated to be relicts. b) Calcareous soils, in which carbonates are mainly removed from the top horizons and from the finer granulometric fractions. The dynamics of these soils is supposedly in agreement with the current climatic conditions. The overall evolution of the soils in the studied area is characterized by the presence of calcium carbonate, which comes either from an incomplete carbonate leaching of the calcareous substrates, from carbonate deposition of previously leached red soils or from the relative accumulation of carbonates in soils from gypsipherous materials

Contribució al coneixement mineralògic d'uns sòls laterítics de l'Alt Volta

The mineralogical composition of samples of a soil which has developed on top of granitic material in the Ouagadougou region (Upper Volta) is studied using the techniques of: X-ray diffraction, infrared spectrometry and differential thermal analysis. The results obtained indicate the presence in the profile of kaolinite as the only neoformation of clay mineral, grains of quartz and small quantities of anatase and illite as inherited materials, iron oxides: hematite, goethite and amorphous gels such as hisingerite. These minerals are found in the samples in a clayey-ferruginous material which contains quartz grains and pisolites in which the goethite is mainly concentrated. No outstanding changes are observed in the profile except for the disappearance of the pisolitique structures at a depth of 1'10 m, although different coloured mottles continue to be present. On account of the above mineralogical composition, these soils could be included in the sub-class of ferralytic soils, ferralytics with kaolinite group (DUCHAUFOUR, 1977), in which this clay mineral dominates and weathering has almost totally destroyed the clays 2:1, thus not succeding in forming gibbsite. These soils do not correspond to the actual climate of the region studied, and so they could form part of the abundant relict soils of the region (BOULET, 1970), whose origin would date from the most humid and warm periods of African Pleistocene

Els Efectes del foc en pinedes de la Catalunya central

Es descriu un cas representatiu de sistemes forestals fortament modificats pel foc, corresponent a l'incendi de l'estiu de 1986 a l'àrea de Jorba (Anoia). S'han mostrejat parcel•les cremades i no cremades al cap de 2 anys del foc, pròximes i de similars característiques fisiogràfiques. Els sòls han perdut completament els horitzons orgànics, s'ha format una crosteta superficial, i s'han generalitzat els processos erosius. Les pèrdues totals de carboni orgànic del sàl són de 8 t/ha i les de nitrogen de 250 kg/ha. El fòsfor total ha augmentat en 35 kg/ha a causa de la combustió de la vegetació. La regeneració de la vegetació ha estat deficitària. Les espècies rebrotadores presenten percentatges de rebrotada del 50 % en exposicions al solell. La germinació dels pins (Pinus halepensis) és molt pobra i són plantes herbàcies les que expliquen una gran part del recobriment existent: 50-70 % al cap de dos anys de l'incendi.A representative case of Mediterranean forest strongly affected by fire is described. It corresponds to the wild fire produced in summer 1986 in the area of Jorba (Central Catalonia; NE Spain). Burned and unburned plots have been sampled two years after the fire. The plots have been selected to have similar physiographic characteristics. The soils have lost completely the forest floor layers and surface sealing has appeared; consequently, erosion processes are widespread. Total soil organic carbon losses are 8 t/ha and nitrogen losses are 250 kg/ha. Total phosphorus has increased in 35 kg/ha due to vegetation combustion. Plant regeneration has been poor. Sprouting species have resprouting percentages of 50 % in South-facing slopes. Pinus halepensis germination is very low, whereas the herbaceous vegetation explains most of the measured plant cover, e.g. 50-70 % two years after the fire

Carbon sequestration in Mediterranean ecosystems: critical aspects related to plant respiration, wildfires and nitrogen budget

La producció primària neta (NPP) de la biosfera està limitada pels baixos nivells actuals de CO2 atmosfèric (Ca). Per conseqüent, l'augment de Ca degut a l'activitat humana, considerat la causa principal del canvi climàtic, s'hauria de traduir en un augment de la NPP i, per tant, en un augment del segrestament de Ca a la biomassa. Els augments de NPP i del segrestament de Ca ja són detectables en boscos boreals i tropicals. L'augment de Ca produeix també una disminució de la respiració vegetal ?un 17 % de mitjana?, fet que podria provocar que el segrestament de Ca fos més gran que no s'havia previst. Aquests resultats han fet que alguns autors ofereixin visions força optimistes sobre la capacitat dels ecosistemes terrestres per compensar l'excés de Ca. No obstant això, l'augment de Ca no es tradueix només en un augment de la NPP (diferent per a cada espècie), sinó també en canvis en la morfologia de la planta (també diferents per a cada espècie), que comportaran canvis en els patrons d'ocupació de l'espai, que fan difícil predir en quin sentit canviarà la NPP a llarg termini en l'ecosistema sencer. No és gens clar que aquestes previsions optimistes siguin aplicables als ecosistemes terrestres mediterranis, en els quals la sequera ?principal factor limitant de la NPP? s'espera que s'agreugi en un futur, com a conseqüència del canvi climàtic. L'agreujament de la sequera es pot traduir en un augment del risc d'incendi, fet que pot conduir els ecosistemes mediterranis a una situació de gran inestabilitat, en cas que la periodicitat i la intensitat dels focs superin llur capacitat de recuperació (per germinació o rebrot). L'incendi implica pèrdues importants de N i P per volatilització; també provoca un fort augment de la disponibilitat de nutrients (sobretot N) i, doncs, un augment de la fertilitat del sòl. Moltes espècies mediterrànies, i en concret l'alzina (Quercus ilex), tenen una gran capacitat d'acumular N en òrgans basals, probablement per facilitar un rebrot ràpNet primary production (NPP) in the biosphere is limited by the current low atmospheric CO2 concentration (Ca). Hence, the increase in Ca due to anthropogenic activities, which is thought to be the main cause of climate change, is expected to result in an increase in NPP, and therefore in an increased Ca sequestration in the biomass. Increases in NPP and Ca sequestration in the biota are already detectable in boreal and tropical forests. Increased Ca results in a decrease in plant respiration ?on average, about 17%?, and therefore the increase in Ca sequestration could be higher than previously assumed. This has led some authors to publish highly optimistic views about the capacity of terrestrial ecosystems to compensate for the excess of Ca. Nevertheless, increased Ca results not only in an increase in NPP (different for every plant species), but also in changes in plant morphology (also different for every plant species); these will result in changes in the patterns of space occupation by plants, which make it difficult to predict how the NPP will change in the long term, at a whole-ecosystem level. It is not clear that such optimistic views can be applied to Mediterranean terrestrial ecosystems, in which drought ?the main constraint for NPP? is expected to increase in the future as a result of climate change. Increased drought is expected to lead to a greater risk of wildfires, which can generate a highly unstable situation in the ecosystem if the periodicity and intensity of fire events surpasses its capacity to recover (either by resprouting or seeding). Wildfires cause significant losses of N and P by volatilization. They also result in a strong increase in the availability of nutrients (mainly N), and hence an increase in soil fertility

Reassessing global change research priorities in mediterranean terrestrial ecosystems : how far have we come and where do we go from here?

Aim: Mediterranean terrestrial ecosystems serve as reference laboratories for the investigation of global change because of their transitional climate, the high spatiotemporal variability of their environmental conditions, a rich and unique biodiversity and a wide range of socio-economic conditions. As scientific development and environmental pressures increase, it is increasingly necessary to evaluate recent progress and to challenge research priorities in the face of global change. - Location: Mediterranean terrestrial ecosystems. - Methods: This article revisits the research priorities proposed in a 1998 assessment. - Results: A new set of research priorities is proposed: (1) to establish the role of the landscape mosaic on fire-spread; (2) to further research the combined effect of different drivers on pest expansion; (3) to address the interaction between drivers of global change and recent forest management practices; (4) to obtain more realistic information on the impacts of global change and ecosystem services; (5) to assess forest mortality events associated with climatic extremes; (6) to focus global change research on identifying and managing vulnerable areas; (7) to use the functional traits concept to study resilience after disturbance; (8) to study the relationship between genotypic and phenotypic diversity as a source of forest resilience; (9) to understand the balance between C storage and water resources; (10) to analyse the interplay between landscape-scale processes and biodiversity conservation; (11) to refine models by including interactions between drivers and socio-economic contexts; (12) to understand forest-atmosphere feedbacks; (13) to represent key mechanisms linking plant hydraulics with landscape hydrology. - Main conclusions:(1) The interactive nature of different global change drivers remains poorly understood. (2) There is a critical need for the rapid development of regional- and global-scale models that are more tightly connected with large-scale experiments, data networks and management practice. (3) More attention should be directed to drought-related forest decline and the current relevance of historical land use

La Distribució dels carbonats en els sòls de la Depressió Central Catalana

Soils from different calcareous substrates are studied in order to search the evolution of the carbonate fraction. There is a close relationship between soil-type and lithologic material. From hard limestones, red-brown (fersiallitic) soils are formed. In general they have lost the carbonates almost completely but, in some cases, they have secondary lime in different forms. These accumulations of calcium carbonate are associated with an increase in the carbonate content of the finer fractions, clay and fine silt. On calcareous sandstones, completely carbonate-leached red-brown soils coexist with carbonatic ones, in the same ecological conditions. In the latter, as in soils formed from calcarenite substrate, carbonates concentrate in the sand fractions. The total amount of carbonates in the profile depends on its content in the parent material but, in any case, the carbonate leaching rate is high: 40-50 %. On calcareous clays and calcilutites, the soils are highly calcareous, mainly in the silt fraction, and they have the lower carbonate leaching rate, about 10 %. Soils from gypsic materials have lost most of the gypsum and thus the carbonate is relatively concentrated. Two general types of soils with respect to carbonate evolution are distinguished: a) Carbonate-leached soils, mostly occurring in fissures in the rock layers, which are postulated to be relicts. b) Calcareous soils, in which carbonates are mainly removed from the top horizons and from the finer granulometric fractions. The dynamics of these soils is supposedly in agreement with the current climatic conditions. The overall evolution of the soils in the studied area is characterized by the presence of calcium carbonate, which comes either from an incomplete carbonate leaching of the calcareous substrates, from carbonate deposition of previously leached red soils or from the relative accumulation of carbonates in soils from gypsipherous materials