Non-fixing Species Research Articles

Restorative pyric herbivory practices in shrub-encroached grasslands enhance nutrient resource availability and spatial heterogeneity

Pyric herbivory (PH), the combination of prescribed burnings and targeted herbivory, is a promising tool for landscape restoration that emulates historical disturbance regimes. Nitrogen (N) and phosphorus (P) are crucial nutrients for plant growth and although several studies have analysed the single effects of fire or grazers in their availability, the combined effect of both disturbances in the soil nutrient budgets have been rarely considered. This research was planned to analyse the 2-year impact of PH restoration practices on the availability of N and P in two Ulex gallii-encroached grasslands in the Pyrenees. We monitored available forms of N and P for two years using periodic replacements of ion exchange resins to test the hypothesis that mid-term effect of targeted grazing was more relevant than short-term effect of burning. Additionally, we investigated the role of temperature and precipitation on nutrients accumulation and compared its significance to management factors. Burning transformed vegetation and litter into a spatially heterogeneous layer of ash and charred material, which resulted in a variable availability of N and P at the rhizosphere level. After two periods of PH, nutrient availability was higher in soils from grazed plots compared to ungrazed, and the impacts of early burns were scarcely discernible. Nitrate was found to be the most rainfall-dependent nutrient, and grazing also affected its spatial distribution. Our results suggest that the heterogeneous nutrient enrichment enhanced by PH is important for promoting the establishment of a diverse pool of plant species, including both N2-fixing and non-fixing species. In these rainy areas, the use of burnings alone, without grazing, may perpetuate the dynamics of N2-fixing shrub encroachment.

Potential of Bioassays to Assess Consequences of Cultivation of Acacia mangium Trees on Nitrogen Bioavailability to Eucalyptus Trees: Two Case-Studies in Contrasting Tropical Soils.

We hypothesized that the nitrogen-fixing tree Acacia mangium could improve the growth and nitrogen nutrition of non-fixing tree species such as Eucalyptus. We measured the N-mineralization and respiration rates of soils sampled from plots covered with Acacia, Eucalyptus or native vegetation at two tropical sites (Itatinga in Brazil and Kissoko in the Congo) in the laboratory. We used a bioassay to assess N bioavailability to eucalypt seedlings grown with and without chemical fertilization for at least 6 months. At each site, Eucalyptus seedling growth and N bioavailability followed the same trends as the N-mineralization rates in soil samples. However, despite lower soil N-mineralization rates under Acacia in the Congo than in Brazil, Eucalyptus seedling growth and N bioavailability were much greater in the Congo, indicating that bioassays in pots are more accurate than N-mineralization rates when predicting the growth of eucalypt seedlings. Hence, in the Congo, planting Acacia mangium could be an attractive option to maintain the growth and N bioavailability of the non-fixing species Eucalyptus while decreasing chemical fertilization. Plant bioassays could help determine if the introduction of N2-fixing trees will improve the growth and mineral nutrition of non-fixing tree species in tropical planted forests.

A private channel of nitrogen alleviates interspecific competition for an annual legume.

The way resource availability predictably alters interspecific interactions and may favor one resource-acquisition strategy over another is critical for understanding context dependency. The ubiquity of nitrogen (N) limitation across terrestrial environments is a driver of plant competition and the association of some plants with N-fixing bacteria (rhizobia) may alleviate competition with nonfixing plants. Conversely, when available soil N is elevated, competitive advantages imparted by rhizobia are hypothesized to decline because nonfixing species are able to acquire those nutrients readily. We manipulated competition, soil N, and soil microbial inoculation, employing the ground bean Amphicarpaea bracteata, a native annual N-fixing legume, and jewelweed Impatiens capensis, a native co-occurring nonfixing annual. We found that legume performance was negatively impacted by interspecific competition, but less so under lower soil N in both the greenhouse and field. The legume invested a greater proportion of resources in rhizobia when competing, but only under low N. Also consistent with predictions, a competition-by-microbial-inoculation interaction demonstrated that negative effects of competition were alleviated by rhizobia. Finally, we detected an interaction between inoculation and fertilization, whereby N addition resulted in increased performance for uninoculated legumes, but a small decline in performance for inoculated plants, the latter likely representing a cost of mutualism. Thus, several lines of evidence point to the legume-rhizobia mutualism being more beneficial under competition and limited soil N. Competing I. capensis, in contrast, benefited from N addition regardless of the addition of soil microbes. In a survey of natural populations, legume and rhizobia growth were positively correlated at population edges (where interspecific competition is expected to be higher, the mutualism is stronger), whereas at population centers we found no association. Isotopic evidence confirmed a higher degree of rhizobial N-fixation at population edges compared to centers. Taken together, our results demonstrate an important role for the largely private channel of nitrogen in legume competitive performance, but with the benefits imparted by rhizobia being predictably weaker at higher soil fertility. We speculate that alleviation of competitive impacts through resource partitioning is an important and yet largely overlooked aspect of the evolutionary ecology of legume-rhizobia interactions.

Light, nitrogen supply, and neighboring plants dictate costs and benefits of nitrogen fixation for seedlings of a tropical nitrogen-fixing tree.

The ability to fix nitrogen may confer a competitive advantage or disadvantage to symbiotic nitrogen-fixing plants depending on the availability of soil nitrogen and energy to fuel fixation. Understanding these costs and benefits of nitrogen fixation is critical to predicting ecosystem dynamics and nutrient cycling. We grew inoculated (with symbiotic bacteria) and uninoculated seedlings of Pentaclethra macroloba (a nitrogen-fixing tree species) both in isolation and with Virola koschnyi (a nonfixing species) under gradients of light and soil nitrogen to assess how the ability to fix nitrogen and fixation activity affect growth, biomass allocation, and responses to neighboring plants. Inoculation itself did not provide a growth advantage to nitrogen fixers, regardless of nitrogen limitation status. Higher nitrogen fixation rates increased biomass growth similarly for nitrogen-limited and nitrogen-saturated fixers. Nodule production was offset by reduced fine-root biomass for inoculated nitrogen fixers, resulting in no change in total belowground allocation associated with nitrogen fixation. Under nitrogen-limited conditions, inoculated nitrogen fixers partially downregulated fixation in the presence of a nonfixing neighbor. These results suggest that nitrogen fixation can provide a growth advantage, even under nitrogen-saturated conditions, and that nitrogen fixers may reduce fixation rates to minimize facilitation of neighbors.

Fertilization and seasonality influence on the photochemical performance of tree legumes in forest plantation for area recovery in the Amazon.

N-fixing leguminous species can reach atmospheric dinitrogen gas (N2), having an advantage under N-limited degraded environments. These N-fixers are constantly used as facilitative species. Chlorophyll a fluorescence (ChF) acknowledges how different species take up and use light energy during photosynthesis. These techniques assess stress and performance responses to photosynthesis and are used for the selection of species with potential for reforestation. Six Fabaceae species were selected for this study: three nonfixing species (Cenostigma tocantinum, Senna reticulata and Dipteryx odorata) and three N-fixing species (Clitoria fairchildiana, Inga edulis and Acacia spp.). Variations in chlorophyll fluorescence under high vs. low water and nutrient conditions were studied. Multivariate analysis was performed to detect the effects of seasonality and fertilization on dark-adapted ChF two years after the experiment was established. The correlation among ChF variables and growth, photosynthesis and foliar nutrient concentrations was evaluated. Under high water- and nutrient-availability conditions, plants exhibited an enhanced performance index on absorption basis values correlated with electron transport fluxes. Under drought and nutrient-poor conditions, most species exhibit increased energy dissipation as photoprotection. High interspecific variation was found; therefore, species-specific responses should be considered in future ChF studies. Corroborating the ability to colonize high-light environments, N-fixers showed an increased performance index correlated with electron transport and Zn and N foliar concentrations. Negative correlations were found between photosynthesis and trapped fluxes. Diameter growth was positively correlated with electron transport fluxes. Given the different responses among species, ChF is an effective technique to screen for seasonality, fertilization and species effects and should be considered for use during forest restoration. Finally, the addition of fertilization treatments may facilitate tropical forest restoration due to the importance of nutrients in physiological processes. N-fixers showed high photochemical performance and tolerance to abiotic stress in degraded areas and therefore should be included to support ecosystem biomass restoration.

Effects of pioneer N2-fixing plants on the resource status and establishment of neighboring non-N2-fixing plants in a newly formed glacier floodplain, eastern Tibetan Plateau

The ecological roles of dinitrogen-fixing plants (N2-fixers) in the early stage of primary succession remain unclear. We aimed to examine the effects of pioneer N2-fixers on the resource status and establishment of neighboring non-N2-fixing plants (non-fixers) in a newly formed glacier floodplain, eastern Tibetan Plateau. We set up three plots in the floodplain and assessed the abundance, height, and coverage of two N2-fixing and three non-fixing species. We also collected plant and soil samples for the analyses of nutrient status and stable isotope signatures. The two N2-fixing species acquired > 80% of the nitrogen (N) from N2 fixation. The cushion-forming N2-fixer Astragalus mahoshanicus increased the soil nutrient availability compared with the non-fixers and improved the microclimate beneath the cushions. Natural 15N abundance indicated significant transfer of N from A. mahoshanicus to its neighboring non-fixers. Consequently, the neighboring non-fixers were significantly higher in N concentration, above-ground to below-ground biomass ratio and height and density than the non-fixers growing alone, but more deficient in some rock-derived nutrients. The other N2-fixer Hippophae rhamnoides (in seedling stage) generally did not increase the resource availability, and its neighboring non-fixers were similar in height and lower in density than the non-fixers growing alone. The effects of pioneer N2-fixers on the resource status and establishment of neighboring non-fixers in the subalpine floodplain were species dependent and were related to the life history traits of the N2-fixers. A. mahoshanicus has the potential to be used in the restoration of newly exposed land in subalpine regions.

The influence of the invasive alien nitrogen-fixing Robinia pseudoacacia L. on soil nitrogen availability in a mixed Mediterranean riparian forest

Robinia pseudoacacia L. occupies large areas of Mediterranean riparian zones of the northeast of the Iberian Peninsula. This study investigates the influence of the invasive alien nitrogen-fixing R. pseudoacacia on leaf litter nitrogen (N) inputs and soil N availability in a mixed riparian forest in NE Spain. We measured annual leaf litter N inputs, decomposition rates, soil N processes, and soil N concentrations at three sections (near-stream, intermediate, and hillslope) across a riparian forested zone. Moreover, we explored changes in soil N availability associated with the presence of R. pseudoacacia by means of an empirical forest floor model. Leaf litter N content was higher for R. pseudoacacia than for the native non-fixing species. Although the contribution of R. pseudoacacia to annual leaf litter N inputs increased from the near-stream to the hillslope section, soil N mineralization, nitrification, and N availability were similar among sections. Simulations suggest that soil N availability was higher at the near-stream than at the hillslope section without the presence of R. pseudoacacia. However, this pattern smoothed down as R. pseudoacacia spread across the riparian forest. Overall, our results suggest that the spreading of R. pseudoacacia across the riparian zone contributed to homogenize soil N availability over time, and highlight that an integrated spatiotemporal view of the invasive process is needed to assess its impact on soil N biogeochemistry.

Nitrogen-fixing and non-fixing trees differ in leaf chemistry and defence but not herbivory in a lowland Costa Rican rain forest

AbstractNitrogen-fixing plants provide critical nitrogen inputs that support the high productivity of tropical forests, but our understanding of the ecology of nitrogen fixers – and especially their interactions with herbivores – remains incomplete. Herbivores may interact differently with nitrogen fixers vs. non-fixers due to differences in leaf nitrogen content and herbivore defence strategies. To examine these potential differences, our study compared leaf carbon, nitrogen, toughness, chemical defence and herbivory for four nitrogen-fixing tree species (Inga oerstediana, Inga sapindoides, Inga thibaudiana and Pentaclethra macroloba) and three non-fixing species (Anaxagorea crassipetala, Casearia arborea and Dipteryx panamensis) in a lowland tropical rain forest. Leaf chemical defence, not nutritional content, was the primary driver of herbivore damage among our species. Even though nitrogen fixers exhibited 21.1% higher leaf nitrogen content, 20.1% lower C:N ratios and 15.4% lower leaf toughness than non-fixers, we found no differences in herbivory or chemical defence between these two plant groups. Our results do not support the common hypotheses that nitrogen fixers experience preferential herbivory or that they produce more nitrogen-rich defensive compounds than non-fixers. Rather, these findings suggest strong species-specific differences in plant–herbivore relationships among both nitrogen-fixing and non-fixing tropical trees.

Fijación biológica del nitrógeno en tres especies silvestres del género Lupinus (Leguminosae, Papilionoideae) en México

Antecedentes y Objetivos: La fijación biológica del nitrógeno (FBN) en leguminosas nativas de México ha sido poco estudiada; particularmente en especies del género Lupinus no se tiene información al respecto. En este sentido, el objetivo de esta investigación fue cuantificar la acumulación de biomasa y nitrógeno (N) en tres lupinos silvestres del estado de Jalisco, México (Lupinus exaltatus, L. rotundiflorus y L. mexicanus), así como evaluar la FBN en términos de cantidad de N fijado (kg ha-1) y proporción de N derivado de la atmósfera (% Ndda).Métodos: Las especies en estudio al igual que la cebada, una especie conocida como no fijadora de N y usada de referencia, fueron cultivadas en suelos agrícolas de Zapopan, Jalisco, en el periodo otoño-invierno de 2015-2016. Durante el crecimiento de las especies en estudio se realizaron muestreos destructivos a los 93, 108 y 122 días después de la siembra (Dds). En cada muestreo se cuantificó el número de nódulos por planta, contenido de materia seca (Ms) y % de N. Con estos valores se estimó mediante el método de la diferencia de N, la proporción de N derivado de la atmósfera (% Ndda) y cantidad de N fijado (kg ha-1).Resultados clave: A los 93, 108 y 122 Dds, L. exaltatus fijó 87.4, 86.9 y 187 kg de N ha-1, lo cual fue equivalente a 71.0, 70.9 y 83.8% de Ndda, respectivamente. Las especies L. rotundiflorus y L. mexicanus solamente mostraron valores significativos de N fijado hasta los 122 Dds.Conclusiones: Bajo las condiciones de este estudio se puede concluir que L. exaltatus se caracterizó por ser la especie más eficiente en la fijación de N atmosférico, debido a una favorable simbiosis entre las raíces y las bacterias nativas del suelo fijadoras de N atmosférico.

Concentración foliar de macro- y micro-nutrimentos en cuatro leguminosas maderables del trópico estacionalmente seco de Costa Rica

The information about nutritional aspects of trees in the tropical dry forest is scarce. This investigation aims to establish a normal range of foliar nutrient concentration to four forestry species as related to their growth rates and biological nitrogen fixation capacity in the seasonally dry ecosystem at Guanacaste, Costa Rica. Foliar samples for four species, Dalbergia retusa (cocobolo), Enterolobium cyclocarpum (guanacaste), Hymenaea courbaril (guapinol or jatobá) and Schizolobium parahyba (gallinazo or pachaco), all belonging to the Fabaceae family, were analyzed for N, P, Ca, Mg, K, S, Fe, Mn, Cu, Zn, B and Na. Nutrient concentration varied with species according to their ability to fix nitrogen and their rate of growth. The overall concentration of macronutrients was higher in nitrogen-fixing species compared with non-fixing species, and within each one of those groups, the species with higher growth rate presented more macronutrients than species with low growth rate. According to the concentration of foliar nutrients accumulated in the foliage, we recommend that H. courbaril and D. retusa could be grown in the region soils with medium fertility and E. cyclocarpum and S. parahyba in the more fertile soils. Values ranges of foliar concentrations considered as satisfactory presented in this work are new for D. retusa and H. courbaril and improve the scarce information available for E. cyclocarpum and S. parahyba.

Rising CO2 accelerates phosphorus and molybdenum limitation of N2-fixation in young tropical trees

Nitrogen fixation may be critical for supplying the nitrogen (N) needed to maintain the tropical carbon sink in a world of rising atmospheric CO2. However, we do not know whether increased CO2 acts to exacerbate nutrient limitation on the fixation process itself. We experimentally test this idea by growing N2-fixing plants in pre-Industrial (280 ppm), present-day (400 ppm), and doubled (800 ppm) atmospheric CO2. In a greenhouse experiment, we grew tree seedlings from N2-fixing species and a non-fixing species at three CO2 concentrations with control, +P (phosphorus), +Mo (molybdenum), and + P +Mo nutrient treatments. We found nutrient limitation to be minimal at pre-Industrial CO2, but with increasing CO2 fixer growth and fixation became increasingly limited by P and by a P-by-Mo interaction. At 400 ppm, plants with +P grew ~50% faster and fixed 10-15× more N2 based on nodule mass and nitrogenase activity. At 800 ppm, plants with +P +Mo grew 200% more, and fixed 25× more N2, suggesting Mo-P co-limitation at elevated CO2. Our findings imply that complex patterns of nutrient limitation can develop as CO2 rises, potentially suppressing tropical N2-fixation and the new inputs of N needed to sustain the tropical carbon sink.

Water use efficiency in perennial forage species: Interactions between nitrogen nutrition and water deficit

Interactions between nitrogen and water may vary between N2 fixing species and species that rely only mineral soil nitrogen. Here we compared above ground biomass accumulation (W), nitrogen uptake (N), evapotranspiration (ET) of alfalfa (Medicago sativa L.) and tall fescue (Festuca arundinacea Schreb.) during regrowth periods under contrasting supply of water (irrigated vs non-irrigated) and N (for non-fixing species). Using previously published data, we estimated the dynamics of the two components of ET, evaporation from soil (E) and transpiration (T), in order to analyse the impact of E/T, of the transpiration efficiency (TE = W/T) and the role of crop N nutrition on water use efficiency (WUE= W/ET). In tall fescue, limiting N supply reduced WUE by both increasing E/T ratio and decreasing TE. Water limitation in both alfalfa and tall fescue led to crop nitrogen deficit. This drought-induced N deficiency resulted in a proportional reduction in TE irrespective of the source of N for the plant. We propose that the ratio N/T, representing the apparent N concentration of water transpired by the crop, is relevant for analysing N-water interactions. Comparisons of dynamics of N/T ratio must be done at similar biomass or similar transpiration because N and T are related allometrically.

Effects of including forbs on N2-fixation and N yield in red clover-ryegrass mixtures

Legume biological nitrogen (N2)-fixation is stimulated by neighbouring non-fixing species, but studies of legume N2-fixation in temporary grasslands including non-leguminous forage herb (forb) species are rare. We investigated N2-fixation, N yield, and total herbage production in a range of species mixtures consisting of three forb species—chicory (Cichorium intybus L.), ribwort plantain (Plantago lanceolata L.), and caraway (Carum carvi L.)—mixed into a traditional red clover (Trifolium pratense L.) and perennial ryegrass (Lolium perenne L.) mixture at two fertilisation levels. The percentage of red clover N derived from the atmosphere (%Ndfa) was higher in mixtures containing non-legumes than in pure stand but, did not increase with inclusion of forbs. On a whole-seasonal basis, red clover in mixtures derived 90% or more of its N from fixation even when fertilised with 216 kg total N ha−1. Forbs, in particular chicory, reduced the amount of N2-fixation and total N yields by affecting the red clover proportion in the harvested biomass. Generally, inclusion of forbs in red clover-ryegrass mixtures had no negative effect on total herbage production and percentage of legume N2-fixation. However, to maintain a high total N and N2-fixation yields, mixtures should not include a high seeding proportion of chicory.

Effects of mixing forage species contrasting in vertical and temporal nutrient capture on nutrient yields and fertilizer recovery in productive grasslands

Mixing grasses with N2-fixing legumes benefits grassland yields. We examined if grass/forb/legume mixtures combining species with complementary vertical and temporal nutrient capture further promote nutrient yields and improve fertilizer N recovery. Four species differing in N2-fixing ability and in vertical and temporal nutrient capture were grown in monoculture and two- and four-species mixtures. 15N was used to examine vertical complementarity in nutrient capture and fertilizer N recovery. Asynchrony was quantified from the N yield distribution over the year. The positive mixture effects on N and P yields (P < 0.001) between species contrasting in vertical and temporal nutrient capture (with same N2-fixing ability) were of a similar magnitude (+45 kg N) as those between N2-fixing and non-fixing species with shallow roots (+52 kg N). Mixture benefits were maximized up to +99 kg N when constituent species differed in N2-fixing ability and vertical and temporal nutrient capture. Species modified their vertical N capture when grown in mixture (niche shift). Remarkably, mixing Trifolium pratense and Cichorium intybus, which were similar in both vertical and temporal nutrient capture, generated negative mixture effects. The 4-species mixture captured as much fertilizer N as the grass monoculture, despite significant abundance of legumes. Combining species differing in multiple characteristics for nutrient capture optimizes nutrient capture of non-legume/legume-mixtures.

Nitrogen fixation by Alnus species boosts soil nitrous oxide emissions

The aim of this study was to assess how higher soil temperatures and increased nitrogen (N) availability, mediated by the N2 fixation of different Alnus species, affect soil nitrous oxide (N2O) emissions. We measured rates of N2O emission in situ under non‐N2‐fixing tree species or grassland (controls) and under Alnus species by the closed chamber technique along a temperature gradient of 5.7 K from the lowlands to the upper montane belt in Switzerland. Similar soil conditions and representative tree and bush taxa at a given elevation were a key prerequisite for the site selection. Across the 12 test plots, and irrespective of the elevation, N2O emissions were on average 12‐fold larger in Alnus stands than in non‐N2‐fixing vegetation. The mean emission was 1.3 ± 0.2 kg N2O‐N ha−1 under Alnus and 0.1 ± 0.1 kg N2O‐N ha−1 under non‐N2‐fixing vegetation from mid‐June to mid‐October. The largest rates of emission were observed in Alnus viridis (Chaix.) D.C. shrubland at 1680 m a.s.l. (mid‐elevation), with a seasonal emission of 4.2 ± 0.5 kg N2O‐N ha−1. Differences in soil temperature along the elevation gradient did not affect the release of N2O in non‐N2‐fixing vegetation or across the different Alnus species. We conclude that larger N availability through N2 fixation increases N2O emissions in ecosystems even without intense agricultural management. The fast expansion of Alnus viridis across the Alps has been documented mainly in the context of ‘ecological consequences’. Here, we provide evidence that Alnus viridis also adds substantially to greenhouse gas emissions.Highlights We evaluated soil N2O emissions under Alnus and non‐fixing species along a temperature gradient of 5.7 K. Alnus stands emitted on average 12‐fold more N2O than non‐N2‐fixing vegetation. Neither short‐ nor long‐term variation in temperature affected N2O emissions in Alnus stands. During the growing season, soil under Alnus viridis emitted 1.5% of the annual Swiss N2O emission.

Effect of initial soil properties on six-year growth of 15 tree species in tropical restoration plantings.

In restoration plantings in degraded pastures, initial soil nutrient status may lead to differential growth of tropical tree species with diverse life history attributes and capacity for N2 fixation. In 2006, we planted 1,440 seedlings of 15 native tree species in 16 fenced plots (30 × 30 m) in a 60‐year‐old pasture in Los Tuxtlas, Veracruz, Mexico, in two planting combinations. In the first year, we evaluated bulk density, pH, the concentration of organic carbon (C), total nitrogen (N), ammonia (NO3−), nitrate (NH4+), and total phosphorus (P) in the upper soil profile (0–20 cm in depth) of all plots. The first two axes of two principal component analyses explained more than 60% of the variation in soil variables: The axes were related to increasing bulk density, NO3−, NH4+, total N concentration, and pH. Average relative growth rates in diameter at the stem base of the juvenile trees after 6 years were higher for pioneer (45.7%) and N2‐fixing species (47.6%) than for nonpioneer (34.7%) and nonfixing species (36.2%). Most N2‐fixing species and those with the slowest growth rates did not respond to soil attributes. Tree species benefited from higher pH levels and existing litter biomass. The pioneers Ficus yoponensis, Cecropia obtusifolia, and Heliocarpus appendiculatus, and the N2‐fixing nonpioneers Cojoba arborea, Inga sinacae, and Platymiscium dimorphandrum were promising for forest restoration on our site, given their high growth rates.

Functional Plant Types Drive Plant Interactions in a Mediterranean Mountain Range.

Shrubs have positive (facilitation) and negative (competition) effects on understory plants, the net interaction effect being modulated by abiotic conditions. Overall shrubs influence to great extent the structure of plant communities where they have significant presence. Interactions in a plant community are quite diverse but little is known about their variability and effects at community level. Here we checked the effects of co-occurring shrub species from different functional types on a focal understory species, determining mechanisms driving interaction outcome, and tested whether effects measured on the focal species were a proxy for effects measured at the community level. Growth, physiological, and reproductive traits of Euphorbia nicaeensis, our focal species, were recorded on individuals growing in association with four dominant shrub species and in adjacent open areas. We also recorded community composition and environmental conditions in each microhabitat. Shrubs provided environmental conditions for plant growth, which contrasted with open areas, including moister soil, greater N content, higher air temperatures, and lower radiation. Shrub-associated individuals showed lower reproductive effort and greater allocation to growth, while most physiological traits remained unaffected. Euphorbia individuals were bigger and had more leaf N under N-fixing than under non-fixing species. Soil moisture was also higher under N-fixing shrubs; therefore soil conditions in the understory may counter reduced light conditions. There was a significant effect of species identity and functional types in the outcome of plant interactions with consistent effects at individual and community levels. The contrasting allocation strategies to reproduction and growth in Euphorbia plants, either associated or not with shrubs, showed high phenotypic plasticity and evidence its ability to cope with contrasting environmental conditions.

A global meta-analysis of changes in soil carbon, nitrogen, phosphorus and sulfur, and stoichiometric shifts after forestation

Planted forests, established on non-forest lands, play an important role in enhancing terrestrial carbon (C) sequestration. Understanding the changes in soil C, nutrients and stoichiometry in planted forests is important for forest management. We conducted a global meta-analysis of changes in C, nitrogen (N), phosphorus (P) and sulfur (S) and their stoichiometry in mineral soils of planted forest across broad climatic zones from 139 papers. Soil C and N are slightly decreased after forestation on grassland, moderately increased after forestation on cropland, and substantially increased after forestation on barren land. Forestation does not affect total soil P, but the available P is significantly depleted after the forestation of grassland and cropland with N-fixers. Changes in soil nutrients (N, P and S) and shifts in stoichiometry (ratios of C:N, C:P and N:P) are significantly related to soil C dynamics (p < 0.05). Soil C sequestration is the lowest in the boreal zone, and greater under plantation with N-fixing species than under non-fixing species. Changes in soil C and nutrients after forestation mainly differ to prior land use. Compared with forestation of grassland, forestation of barren land is a more effective approach to enhancing C sequestration.

Nitrogen isotopic patterns in tropical forests along a rainfall gradient in Northeast Brazil

The interpretation of ecosystem nitrogen isotopic patterns is subject to controversies since there are few surveys of signals along climate gradients within the same region. We determined the effects of a rainfall gradient, under high temperatures, on the soil and plant δ15N signals in Northeast Brazil. Nitrogen, carbon and δ15N signals were determined from coastal perennial to subhumid and semiarid deciduous forests to savanna – deciduous forest transition. Soil C and N concentrations were linearly related and both related quadratically with mean annual rainfall while δ15N concentrations decreased with higher rainfall (16 to 4‰). Plant δ15N was 1–3‰ lower than soil δ15N (Δδ15N) in all areas. Only in the deciduous forests, leaves of target species had significantly lower 15 N signals than those of non-fixing species, allowing estimation of fixation. In the perennial forests the low signals indicated that the 15 N natural abundance method was not suitable to estimate fixation, while in the savanna – deciduous forest transition, high signals indicated that the legume species were not fixing. The smaller Δδ15N than those reported in cooler areas and the absence of rainfall effect indicates that, when high, temperature overrides the rainfall effect.

Foliar resorption in nitrogen-fixing and non-fixing species in a swamp forest in northern Turkey

Foliar resorption of nutrients in plants is a key factor to conserve nutrients especially nitrogen (N) and phosphorus (P) and makes plant species less-dependent to soil nutrient status. There is much debate whether N-fixing and non-fixing species differ or not with respect to N and P usage strategies. Two N-fixing (one actinorhizal and one legume) and four non-fixing species were sampled in a swamp forest in northern Turkey to compare nitrogen-fixing and non-fixing species with respect to resorption efficiency (RE) and resorption proficiency (RP). Actinorhizal and legume species were Alnus glutinosa (L.) Gaertner subsp. glutinosa and Robinia pseudoacacia L., respectively. Non-fixing species were Quercus hartwissiana Stev., Acer campestre L. subsp. campestre, Euonymus europaeus L. and Fraxinus excelsior L. It has been found that N-fixing species had lower NRE than non-fixing species in the present study, while N-fixing species were more P-proficient than non-fixing species (except for F. excelsior). Additionally, N/ P ratios of N-fixing species were higher than those of non-fixing species. Foliar resorption was not highly proficient in both N-fixing and non-fixing species in the present study.