Stable isotopes and changing paradigms on soil nitrogen and carbon biogeochemistry

Many perceptions on ecosystem biogeochemistry are based on flow diagrams in which element pools (boxes) are interconnected by abiotic and biotic mechanisms controlling transformations of chemical species and flows among the pools (arrows). Because of the ability of stable isotopes to integrate such processes over time and space, they have played a central role in our current understanding of nutrient cycling, particularly in the cases of N and C. Most fluxes and transformations involved in terrestrial nutrient cycling cross over or take place in soil compartments. We here review the development of new paradigms in soil nitrogen (and carbon) cycling research, to which stables isotopes contributed through three main approaches: (i) as integrators of nutrient input/output budgets from broad ecosystems compartments or "black-boxes", (ii) as tracers to unravel specific processes and end-member pools operating within these black-boxes, and (iii) as markers or indicators of nutrient use, availability and deficiency to plants. New challenges and future perspective to that respect are also discussed. Key words: nutrient cycling, isotope techniques, conceptual models, new perspectives. Carreira, J.A., Viñegla, B., Blanes, M.C., García-Ruíz, R. (2010). Isótopos estables y cambios de paradigma sobre la biogeoquímica del nitrógeno y del carbono en el suelo. Ecosistemas 19(3):14-23. Muchos de los modelos conceptuales e ideas desarrollados en el ámbito de la biogeoquímica se basan en diagramas de flujo que representan las reservas de nutrientes como "cajas" interconectadas por "flechas" que indican los procesos abióticos y bióticos que regulan su transformación y flujo entre reservas. El uso de isótopos estables permite integrar dichos procesos en el espacio y en el tiempo, y por ello han jugado un papel central en la comprensión de los ciclos de nutrientes como el N y el C en los ecosistemas, y en particular en el suelo. Aquí revisamos el proceso de cambio de paradigmas que ha tenido lugar respecto al reciclado de estos nutrientes en el suelo; un proceso en el que los isótopos estables han contribuido significativamente a través de tres aproximaciones principales: (i) como integradores en balances de entrada/salida que consideran como "cajas negras" a compartimentos amplios del ecosistema, (ii) como trazadores para revelar procesos intermedios que operan dentro de las "cajas negras" y el destino final de los nutrientes, y (iii) como marcadores o indicadores de uso, disponibilidad y estrés por nutrientes en las plantas. También se discuten los retos actuales y las perspectivas de futuro a este respecto. Palabras clave: ciclos de nutrientes, técnicas isotópicas, modelos conceptuales, nuevas perspectivas

Gas exchange at whole plant level shows that a less conservative water use is linked to a higher performance in three ecologically distinct pine species

Increasing temperatures and decreasing precipitation in large areas of the planet as a consequence of global warming will affect plant growth and survival. However, the impact of climatic conditions will differ across species depending on their stomatal response to increasing aridity, as this will ultimately affect the balance between carbon assimilation and water loss. In this study, we monitored gas exchange, growth and survival in saplings of three widely distributed European pine species (Pinus halepensis, P. nigra and P. sylvestris) with contrasting distribution and ecological requirements in order to ascertain the relationship between stomatal control and plant performance. The experiment was conducted in a common garden environment resembling rainfall and temperature conditions that two of the three species are expected to encounter in the near future. In addition, gas exchange was monitored both at the leaf and at the whole-plant level using a transient-state closed chamber, which allowed us to model the response of the whole plant to increased air evaporative demand (AED). P. sylvestris was the species with lowest survival and performance. By contrast, P. halepensis showed no mortality, much higher growth (two orders of magnitude), carbon assimilation (ca. 14 fold higher) and stomatal conductance and water transpiration (ca. 4 fold higher) than the other two species. As a consequence, P. halepensis exhibited higher values of water-use efficiency than the rest of the species even at the highest values of AED. Overall, the results strongly support that the weaker stomatal control of P. halepensis, which is linked to lower stem water potential, enabled this species to maximize carbon uptake under drought stress and ultimately outperform the more water conservative P. nigra and P. sylvestris. These results suggest that under a hotter drought scenario P. nigra and P. sylvestris would very likely suffer increased mortality, whereas P. halepensis could maintain gas exchange and avoid water-induced growth limitation. This might ultimately foster an expansion of P. halepensis to higher latitudes and elevations.This work was supported by the projects ECOLPIN (AGL2011–24296) and Remedinal 3 (S2013/ MAE- 2719) of the Madrid Government, by a FPU fellowship from the Spanish Ministry of Education, Culture and Sport (FPU13/03410) to DS and by EU Marie Curie (FP7–2013-IOF-625988) fellowship to EPSC

Effect of solar and artificial UV radiation on photosynthetic performance and carbonic anhydrase activity in intertidal macroalgae from southern Spain

The effect of UV radiation on photosynthetic rate and carbon uptake, estimated as carbonic anhydrase activity, was studied by incubating intertidal algae, Fucus spiralis (Phaeophyta) and Ulva olivascens (Chlorophyta), under both artificial and solar radiation. In these experiments, treatments excluding UV-B or UV-A + UV-B radiations were applied. Thalli of U. olivascens and F. spiralis showed completely different responses when exposed to artificial radiation and three different light treatments (PAR + UV-A + UV-B, PAR + UV-A, and only PAR). After exposure for 24 h, photosynthetic activity increased in F. spiralis, although the presence of UV-B slightly reduced the increase, whereas U. olivascens showed an inverse pattern, with decreasing photosynthetic rates in all the treatments and with the lowest decrease in the presence of UV-B radiation. Surprisingly, the presence of UV-B stimulated carbon uptake, though this effect was more clear in the case of F. spiralis, while a similar inhibitory effect of UV-A radiation was found in both species. Thalli of U. olivascens and F. spiralis were also exposed to solar radiation for daily and yearly periods. The photosynthetic rate was coupled to incident irradiance in F. spiralis, whereas in U. olivascens a significant decrease in the presence of both UV-A or UV-A + UV-B was found. A significant increase of carbon uptake in the presence of UV-B radiation was observed in U. olivascens, while no clear pattern was found in F. spiralis under any of the light treatments. During an annual cycle, carbon uptake in U. olivascens decreased in winter, compared with the initial levels, in the PAR and PAR + UV-A treatments. The high level of carbon uptake was maintained until the end of spring, when higher levels of incident energy might promote photoinhibition (in the presence of UV-B). The absence of a general response to UV radiation in the macroalgae studied may be due to their different photoprotection mechanisms, physiological-morphological characteristics, and acclimation to environmental conditions.

Morphostatic Speciation within the Dagger Nematode Xiphinema hispanum-Complex Species (Nematoda: Longidoridae)

This article belongs to the Special Issue The Systematics, Morphological, and Molecular Characterization of Economically Important Plant–Parasitic Nematodes.Dagger nematodes of the genus Xiphinema include a remarkable group of invertebrates of the phylum Nematoda comprising ectoparasitic animals of many wild and cultivated plants. Damage is caused by direct feeding on root cells and by vectoring nepoviruses that cause diseases on several crops. Precise identification of Xiphinema species is critical for launching appropriate control measures. We deciphered the cryptic diversity of the Xiphinema hispanum-species complex applying integrative taxonomical approaches that allowed us to verify a paradigmatic example of the morphostatic speciation and the description of a new species, Xiphinema malaka sp. nov. Detailed morphological, morphometrical, multivariate and genetic studies were carried out, and mitochondrial and nuclear haploweb analyses were used for species delimitation of this group. The new species belongs to morphospecies Group 5 from the Xiphinema nonamericanum-group species. D2-D3, ITS1, partial 18S, and partial coxI regions were used for inferring the phylogenetic relationships of X. malaka sp. nov. with other species within the genus Xiphinema. Molecular analyses showed a clear species differentiation not paralleled in morphology and morphometry, reflecting a clear morphostatic speciation. These results support the hypothesis that the biodiversity of dagger nematodes in southern Europe is greater than previously assumed.This research was supported by Spanish Ministry of Science, Innovation and Universities, grant number RTI2018-095345-B-C21, LITHOFOR (“Modulating role of LITHOlogy in the response of Mediterranean FORest ecosystems to climate change: growth, edaphological processes and future predictions”) and the Humboldt Research Fellowship for Postdoctoral Researchers awarded for the first author.Peer reviewe