Nitrogen deposition depletes the soil seed bank of a kermes oak thicket

Resumen de una presentación realizada en: I Simposio sobre Interacciones Planta-Suelo (ICA-CSIC, Madrid, 25-26 Febrero 2016)Aims: Nitrogen (N) deposition is a major driver of global change that can influence soil seed bank composition and abundance and seed germination. We investigated how eight years of simulated N deposition impacted the soil seed bank of a semiarid Mediterranean shrubland in Central Spain.Methods: The soil seed bank used in this study was collected from a kermes oak thicket located in the Nature Reserve El Regajal-Mar de Ontígola (Central Spain, 4o9’N, 3o29’W). Samples were collected on September 2014, following the spring/summer seed rain and prior to the onset of equinoctial rains. Consequently, we collected both transient and permanent seed banks without distinction. Three soil cores, 4.5 cm diameter and 4.0 cm deep, were collected from each of 24 plots that are fertilized since October 2007 (72 cores). Fertilization treatments corresponded to simulated N deposition rates of 0, 10, 20 and 50 kg N ha-1 year-1. Soils were incubated under semi-controlled conditions in a greenhouse and emerged plants were recorded.Results: During the course of the study, a total of 198 seedlings belonging to 21 species were recorded. When the number of germinated plants were compared among N treatments no significant differences were found (p<0.05). However, when the between-plot variations in soil variables were considered in the analysis, the highest N treatment had a significantly lower number of germinated seeds than the control (p<0.05). Soil organic matter, organic nitrogen and total nitrogen content masked the effects of nitrogen deposition on soil seed bank.Conclusions: N deposition can influence the emergence of soil seed bank species. However, the mechanism linked to this effect, alteration of seed emergence physiology or compositional shifts of the seed bank, remains unknown.Peer reviewe

Allelopathic potential of invasive Ulmus pumila on understory plant species

In Petriplate bioassay and Pot culture, we studied the allelopathic potential of leaf litter of Siberian elm (Ulmus pumila L.) invasive tree species on the germination and growth of three herbaceous plant native species (Dactylis glomerata L., Trifolium repens L. and Chenopodium album L.) commonly found in riparian understory communities. Two assays were done with leaf litter of U. pumila from riverine ecosystems (i). the effects of aqueous litter extracts in Petri dish assays and (ii). the effect of litter residues in pot experiments. The leaf litter extracts inhibited the radicle growth of D. glomerata and T. repens but did not effect the germination. However in pots, leaf litter inhibited the germination speed of C. album and growth of D. glomerata and T. repens. Our results showed the allelopathic potential of U. pumila litter, which reduced the growth of understory species (T. repens and D. glomerata). In the litter of U. pumila many phenolic compounds (hidroxibenzoic, ferulic, coumaric, protocatechuic, vanillic and rosmarinic acids, and quercetin) were detected.project POII10-0179-4700 of the Junta de Comunidades de Castilla La ManchaPeer reviewe

Mineral content in semiarid grassland systems affected by community structure and soil characteristics

This work was supported by the European Union project nº 8001-CT90-0021.Mineral element concentrations in plants and their relationships with botanical composition and soil characteristics were analyzed over two consecutive years in herbage samples from semiarid grassland systems (central-western Spain) affected by a topographic gradient. There were not significant differences in the mineral concentration of the plant community between sampling years suggesting that factors other than those related to interannual climatic variation are more important in determining the mineral concentration of plant community in semiarid grasslands. A significant effect of slope position on aboveground biomass production, soil characteristics and plant tissue nutrient contents was found. As compared to the upper and middle zones, the lower zones of slope showed greater aboveground biomass production, a lower proportion of forbs, a higher proportion of grasses, higher concentrations of N, Ca, Mg and Na in botanical groups, and lower concentrations of P and K in botanical components. Stepwise multiple regression analysis showed that most of the variation in N, P, K, Ca, and Mg concentrations in the plant community along the slope can be explained by botanical composition and its specific mineral content. Several soil properties (organic matter, total N, total C, coarse sand) proved to be significantly correlated with the N, Ca, Mg, and Na concentrations of grasses, legumes and forbs, indicating an increase in plant tissue concentrations with increasing eutrophic status in soil from the upper to the lower zones of the slope. The P and K concentrations of legumes and forbs varied according to the phosphorus or potassium contents in soil, which decreased from the upper and middle to the lower zones. The results showed that the mineral concentrations of botanical components were dependent on soil characteristics in different ways.Peer reviewe

Estimation of mineral content in natural grasslands by near infrared reflectance spectroscopy

Near infrared reflectance spectroscopy (NIRS) was tested to predict nitrogen and mineral concentration (N, P, K, Ca, Mg, Na, Mn, Fe, Cu and Zn) in natural grassland samples. The samples were taken from different community types according to the topographic gradient, at different maturation stages and during a period of four consecutive years. A subset of 95 samples was selected on the basis of the spectral variation. Chemical values from the calibratíon sample set were regressed on the corresponding spectral data using a stepwise multiple regression analysís. Another subset of 75 samples was used as the validation set. Standard errors of prediction and correlation coefficients, respectively, were: 0.71 and 0.97 (N), 0.22 and 0.73 (P), 1.83 and 0.84 (K), 0.83 and 0.92 (Ca), 0.15 and 0.92 (Mg), 3.94 and 0.66 (Na), 44 and 0.84 (Mn), 19 and 0.75 (Fe), 1.01 and 0.77 (Cu), 3.9 and 0.79 (Zn).This work was supported by the European Uníon (Contract N 8001-CT90-0021). Thanks are due to L.G. Criado, C. Estévez and M. Hernández for their assistance in the field work.Peer reviewe

Decomposition and biological colonization of native and exotic leaf litter in a Central Spain stream

Riparian invasion by exotic trees may lead to changes in the quality of leaf-litter inputs to freshwater ecosystems. Leaflitter inputs are especially important in headwater streams, where aquatic food webs largely depend on the organic matter provided by the terrestrial vegetation. In a headwater stream of Central Spain, North of Guadalajara Province (Tagus basin) we compared the decomposition and biological colonization of leaf litter among two exotic (Ailanthus altissima Mill. Swingle and Robinia pseudoacacia L.) and two native trees (Fraxinus angustifolia Vahl. and Populus alba L.) co-existing in riparian zones. We hypothesized a lower biological colonization of exotic leaves due to organisms have not co-evolved with the exotic trees according to the Novel Weapons Hypothesis (NWH) and, consequently, lower decomposition rates of exotic leaves. To test our hypothesis, litter bags with different mesh sizes (coarse (C) and fine (F)), were used to distinguish between the total and microbial decomposition. The litter bags were placed in the stream and recovered after 2, 20, 39, 62 and 82 days of incubation. After each collection, we assessed the amount of remaining litter, analysed the nitrogen (N) and phosphorous (P) in the litter, identified the macroinvertebrates and quantified the fungal biomass. Consistent with our hypothesis, we found greater fungal biomass buildup on native leaves than on exotic leaves. However, decomposition rates (k) were species-specific, being the ranking in C-bags (A. altissima = F. angustifolia > P. alba > R. pseudoacacia) slightly different than in F-bags (A. altissima > F. angustifolia > P. alba > R. pseudoacacia). The k in both the C-bags and F-bags were correlated with leaf traits, such as lignin, acid detergent fibre (ADF) and lignin:P. There was not a clear effect of the exotic species on macroinvertebrate variables, but the contribution of macroinvertebrates to litter decomposition was lower in exotic litter than in native litter, as shown by a lower ratio between k in C-bags and k in F-bags (kC : kF). Our results supported that invasive species might impact both biological colonization and decomposition rates in stream ecosystems by the addition of different quality leaf litter when compared to native vegetation. The differences in the biological colonization may be explained by the NWH, but differences in the decomposition rates were better explained by litter quality

DEGRADACIÓN DE LA HOJARASCA EN UN PASTIZAL OLIGOTRÓFICO MEDITERRÁNEO DEL CENTRO DE LA PENÍNSULA IBÉRICA

The decomposition of plant litter from legume and grass species was studied in a Mediterranean grassland of the center of the Iberian Peninsula. As study variables, the yearly evolution of the remaining dry biomass in litter from buried litterbags and their chemical composition (total nitrogen and organic carbon) were considered. The results indicated that litter biomass was lost faster in the shrub legume (k = 0.55 year–1) that in the other two plant litter types (clover and grass litter), being the grass litter more resistant to decomposition (k = 0.34 year–1). The loss of organic carbon and nitrogen from the litter was also different among the litter types, the grass litter showing the smallest variations nitrogen content variations throughout the study period. However there were not remarkable differences in litter organic carbon at the end of the experiment. The nitrogen input to the ecosystem through litter decomposition was higher for the grass litter than for legumes and it was different depending on the considered zone of the topographic gradient studied. The contribution of the clover species to that nitrogen input was higher in the lower zones of the gradient whereas the grass species showed higher values in the upper parts of the gradient, where their relative abundance was also higher.Se estudia la descomposición de la hojarasca de gramíneas (bromas madritensis L. y B. tectorum L.), leguminosas herbáceas (Trifolium angustifolium L., T. arvense L., T. cherleri L. y T. glomeratum L.) y leguminosas arbustivas (Lygos sphaerocarpa (L.) Heyw.) en un pastizal oligotrófico mediterráneo del centro de la Península Ibérica. Como variables de estudio se consideran la evolución de la biomasa seca remanente y el contenido de nitrógeno total y de carbono orgánico en las hojarascas de gramíneas y leguminosas, a lo largo de un año de enterramiento. Los resultados indican que la velocidad de descomposición de la hojarasca de la leguminosa arbustiva (k = 0.55 año-1) es mayor que la de las otras dos hojarascas y que la hojarasca de las gramíneas es la más resistente a la descomposición (k = 0.34 año-1). La pérdida de carbono orgánico y nitrógeno total con el tiempo difiere también entre las hojarascas. Así, la hojarasca de las gramíneas sufre variaciones menores en el contenido en nitrógeno a lo largo del periodo de estudio que las otras hojarascas. No existen diferencias significativas en el contenido de carbono orgánico de la hojarasca al final del periodo de estudio. La entrada de nitrógeno en el sistema por medio de la descomposición de la hojarasca siempre es mayor para las gramíneas, pero difiere dependiendo de la zona del gradiente geomorfológico considerado. La contribución de las leguminosas a la liberación de nitrógeno al suelo es mayor en las zonas bajas de ese gradiente mientras para las gramíneas es mayor en la zona alta, donde su abundancia relativa es también mayor

Decomposition and biological colonization of native and exotic leaf litter in a stream

Riparian invasion by exotic trees may lead to changes in the quality of leaf-litter inputs to freshwater ecosystems. Leaflitterinputs are especially important in headwater streams, where aquatic food webs largely depend on the organic matterprovided by the terrestrial vegetation. In a headwater stream of Central Spain, North of Guadalajara Province (Tagus basin)we compared the decomposition and biological colonization of leaf litter among two exotic (Ailanthus altissima Mill. Swingleand Robinia pseudoacacia L.) and two native trees (Fraxinus angustifolia Vahl. and Populus alba L.) co-existing in riparianzones.We hypothesized a lower biological colonization of exotic leaves due to organisms have not co-evolved with the exotictrees according to the Novel Weapons Hypothesis (NWH) and, consequently, lower decomposition rates of exotic leaves. Totest our hypothesis, litter bags with different mesh sizes (coarse (C) and fine (F)), were used to distinguish between the totaland microbial decomposition. The litter bags were placed in the stream and recovered after 2, 20, 39, 62 and 82 days ofincubation. After each collection, we assessed the amount of remaining litter, analysed the nitrogen (N) and phosphorous(P) in the litter, identified the macroinvertebrates and quantified the fungal biomass. Consistent with our hypothesis, we foundgreater fungal biomass buildup on native leaves than on exotic leaves. However, decomposition rates (k) were species-specific,being the ranking in C-bags (A. altissima = F. angustifolia > P. alba > R. pseudoacacia) slightly different than in F-bags (A.altissima > F. angustifolia > P. alba > R. pseudoacacia).La invasión de las riberas por árboles exóticos puede provocar cambios en la calidad de las entradas de hojarasca a los ecosistemas de agua dulce, especialmente importantes en arroyos de cabecera, donde las redes tróficas dependen en gran medida de los aportes de materia orgánica proporcionada por la vegetación terrestre. En un arroyo de cabecera del Centro de España, Norte de la provincia de Guadalajara (Cuenca del Tajo), comparamos entre la descomposición y colonización biológica de la hojarasca de dos árboles exóticos (Ailanthus altissima Mill. Swingle y Robinia pseudoacacia L.) y dos nativos (Fraxinus angustifolia Vahl. y Populus alba L.), que coexisten en zonas de ribera. Hipotetizamos que habrá una menor colonización biológica de las hojas exóticas debido a que los organismos del arroyo no han co-evolucionado con los árboles exóticos de acuerdo con la Hipótesis de las Nuevas Armas (NWH) y, por tanto, menores tasas de descomposición de éstas. Para testar nuestra hipótesis, se sumergieron en el arroyo bolsas de descomposición con distinta de luz de malla, gruesa (C) y fina (F) –usadas para distinguir entre descomposición total y microbiana– y se recuperaron después de 2, 20, 39, 62 y 82 días de incubación. Después de la recolección pesamos la cantidad de hojarasca remanente, analizamos el nitrógeno (N) y fósforo (P) de la hojarasca, identificamos los macroinvertebrados y cuantificamos la biomasa fúngica. De acuerdo con nuestra hipótesis, encontramos mayor biomasa fúngica acumulada en hojas nativas que en exóticas. Sin embargo, la tasa de descomposición (k) fue específica de la especie, siendo el ranking en las bolsas C (A. altissima = F. angustifolia > P. alba > R. pseudoacacia) ligeramente diferente al de las bolsas F (A. altissima > F. angustifolia > P. alba > R. pseudoacacia). Ambas k (en bolsas C y F) se correlacionaron con lignina, fibra ácido detergente (ADF) y lignina:P. No hubo un efecto claro de las especies exóticas sobre las variables de macroinvertebrados, pero la contribución de los macroinvertebrados a la descomposición fue menor en la hojarasca exótica que en la nativa, como muestra el menor ratio entre k en bolsas C y F (kC : kF). Nuestros resultados apoyan que las especies invasoras pueden impactar tanto la colonización biológica como las tasas de descomposición de la hojarasca en los ecosistemas de arroyos mediante el aporte de hojarasca de distinta calidad respecto de la vegetación nativa. Las diferencias en colonización biológica pueden ser explicadas por la NHW pero las diferencias en descomposición son mejor explicadas por la calidad de la hojarasca.Ministerio de Economía y CompetitividadJunta de Comunidades de Castilla-La ManchaComunidad de Madri

Integración de impactos ecológicos causados por plantas exóticas invasoras: propuesta metodológica

Tanto en España como en Europa existen demasiadas plantas exóticas invasoras para poder afrontar la gestión de todas ellas, por lo que es necesario priorizar las más nocivas. Sin embargo, ello requiere disponer de medidas cuantitativas, sistemáticas y comparables de su impacto. La información disponible es desigual en cuanto a los criterios y variables para medir impactos y por tanto difícil de integrar. Proponemos el siguiente método para integrar medidas de impactos procedentes de distintos estudios: 1) Búsqueda de casos de estudio, 2) cálculo de tamaños del efecto; 3) clasificación de los casos por nivel de organización, 4) integración de los tamaños del efecto para cada especie y nivel de impacto con técnicas de meta-análisis, y 5) estima de un índice de fiabilidad (basado en el número de casos) y otro de consistencia (basado en la heterogeneidad entre casos). Aplicamos este método para estimar los impactos en España de tres árboles invasores (Ailanthus altissima, Robinia pseudoacacia y Ulmus pumila). Encontramos 50 casos para A. altissima, 39 para R. pseudoacacia y 15 para U. pumila. Los impactos en el nivel de ecosistema (fertilidad) fueron los más estudiados, mientras que los de comunidad e individuo están menos documentados. Robinia pseudoacacia tiende a incrementar la fertilidad, mientras que A. altissima no altera esta propiedad. La metodología propuesta tiene la ventaja de permitir estimar el impacto con datos de estudios diversos, pero su aplicación está limitada por la disponibilidad de casos de estudio