Nitrogen input by bamboos in neotropical forest: a new perspective

Background Nitrogen (N) is an important macronutrient that controls the productivity of ecosystems and biological nitrogen fixation (BNF) is a major source of N in terrestrial systems, particularly tropical forests. Bamboo dominates theses forests, but our knowledge regarding the role of bamboo in ecosystem functioning remains in its infancy. We investigated the importance of a native bamboo species to the N cycle of a Neotropical forest. Methods We selected 100 sample units (100 m2 each) in a pristine montane Atlantic Forest, in Brazil. We counted all the clumps and live culms of Merostachys neesii bamboo and calculated the specific and total leaf area, as well as litter production and respective N content. Potential N input was estimated based on available data on BNF rates for the same bamboo species, whose N input was then contextualized using information on N cycling components in the study area. Results With 4,000 live culms ha−1, the native bamboo may contribute up to 11.7 kg N ha−1 during summer (January to March) and 19.6 kg N ha−1 in winter (July to September). When extrapolated for annual values, M. neesii could contribute more than 60 kg N ha−1y−1. Discussion The bamboo species’ contribution to N input may be due to its abundance (habitat availability for microbial colonization) and the composition of the free-living N fixer community on its leaves (demonstrated in previous studies). Although some N is lost during decomposition, this input could mitigate the N deficit in the Atlantic Forest studied by at least 27%. Our findings suggest that M. neesii closely regulates N input and may better explain the high diversity and carbon stocks in the area. This is the first time that a study has investigated BNF using free-living N fixers on the phyllosphere of bamboo

Partitioning of environmental and taxonomic controls on Brazilian foliar content of carbon and nitrogen and stable isotopes

The Neotropics harbor some of the most diversified woody species in the world, and to understand the nutrient dynamics in these ecosystems, it is crucial to understand the role of plant taxonomy. In addition, biological nitrogen (N) fixation (BNF) in the tropics is one of the key processes affecting the global N cycle. Our objective was to (i) investigate the role of taxonomy and sampling site as predictors of foliar carbon (C) and N concentration and its stable isotopes (i.e., δ13C and δ15N); (ii) assess differences in foliar N, C:N ratio, and δ15N among three functional groups: species of N2-fixers and non-fixers of the Fabaceae family, as well as non-Fabaceae species; and (iii) examine the effect of wood density on tree foliar properties. We hypothesized that Fabaceae specimens in symbiosis with N2-fixers would possess a higher foliar N than non-fixing plants, including those of the Fabaceae family, as well as high-density trees would have higher foliar C and C:N ratio relative to low-density trees, where the latter invest in nutrients instead of structural C. We used a data set composed of 3,668 specimens sampled in three main biomes of Brazil: Amazon, Atlantic Forest, and Cerrado. The partitioning of variance had a higher influence of taxonomy on leaf C, N, and C:N ratio. Conversely, foliar δ13C and δ15N were environmentally constrained. While family was the most important taxonomy level for C, N, and C:N ratio, species played a major role for δ13C and δ15N. Foliar N followed the pattern fixers > non-fixers > non-Fabaceae, while C:N ratio had an opposite trend. In addition, foliar C was correlated with wood density, where high-density > medium-density and low-density woods. The large variability of δ15N was observed among Fabaceae species, demonstrates the complexity of using δ15N as an indicator of BNF. The higher foliar N of Fabaceae non-fixers than non-Fabaceae specimens support the hypothesis that an N-demanding lifestyle is an inherent pattern in this family. Lastly, although observed in some studies, the prediction of foliar properties using wood density is challenging, and future research on this topic is needed

Carbon and Beyond:The Biogeochemistry of Climate in a Rapidly Changing Amazon

The Amazon Basin is at the center of an intensifying discourse about deforestation, land-use, and global change. To date, climate research in the Basin has overwhelmingly focused on the cycling and storage of carbon (C) and its implications for global climate. Missing, however, is a more comprehensive consideration of other significant biophysical climate feedbacks [i.e., CH4, N2O, black carbon, biogenic volatile organic compounds (BVOCs), aerosols, evapotranspiration, and albedo] and their dynamic responses to both localized (fire, land-use change, infrastructure development, and storms) and global (warming, drying, and some related to El Niño or to warming in the tropical Atlantic) changes. Here, we synthesize the current understanding of (1) sources and fluxes of all major forcing agents, (2) the demonstrated or expected impact of global and local changes on each agent, and (3) the nature, extent, and drivers of anthropogenic change in the Basin. We highlight the large uncertainty in flux magnitude and responses, and their corresponding direct and indirect effects on the regional and global climate system. Despite uncertainty in their responses to change, we conclude that current warming from non-CO2 agents (especially CH4 and N2O) in the Amazon Basin largely offsets—and most likely exceeds—the climate service provided by atmospheric CO2 uptake. We also find that the majority of anthropogenic impacts act to increase the radiative forcing potential of the Basin. Given the large contribution of less-recognized agents (e.g., Amazonian trees alone emit ~3.5% of all global CH4), a continuing focus on a single metric (i.e., C uptake and storage) is incompatible with genuine efforts to understand and manage the biogeochemistry of climate in a rapidly changing Amazon Basin

Convergence of soil nitrogen isotopes across global climate gradients

Quantifying global patterns of terrestrial nitrogen (N) cycling is central to predicting future patterns of primary productivity, carbon sequestration, nutrient fluxes to aquatic systems, and climate forcing. With limited direct measures of soil N cycling at the global scale, syntheses of the (15)N:(14)N ratio of soil organic matter across climate gradients provide key insights into understanding global patterns of N cycling. In synthesizing data from over 6000 soil samples, we show strong global relationships among soil N isotopes, mean annual temperature (MAT), mean annual precipitation (MAP), and the concentrations of organic carbon and clay in soil. In both hot ecosystems and dry ecosystems, soil organic matter was more enriched in (15)N than in corresponding cold ecosystems or wet ecosystems. Below a MAT of 9.8°C, soil δ(15)N was invariant with MAT. At the global scale, soil organic C concentrations also declined with increasing MAT and decreasing MAP. After standardizing for variation among mineral soils in soil C and clay concentrations, soil δ(15)N showed no consistent trends across global climate and latitudinal gradients. Our analyses could place new constraints on interpretations of patterns of ecosystem N cycling and global budgets of gaseous N loss

Consequences of removal of exotic species (eucalyptus) on carbon and nitrogen cycles in the soil-plant system in a secondary tropical Atlantic forest in Brazil with a dual-isotope approach

The impact of exotic species on heterogeneous native tropical forest requires the understanding on which temporal and spatial scales these processes take place. Functional tracers such as carbon (δ13C) and nitrogen (δ15N) isotopic composition in the soil-plant system might help track the alterations induced by the exotic species. Thus, we assess the effects from the removal of the exotic species eucalyptus (Corymbia cytriodora) in an Atlantic forest Reserve, and eucalyptus removal on the alteration of the nutrient dynamics (carbon and nitrogen). The hypotheses were: (1) the eucalyptus permanence time altered δ13C and δ15N in leaves, soils and litter fractions (leaves, wood, flowers + fruits, and rest); and (2) eucalyptus removal furthered decomposition process of the soil organic matter. Hence, we determined the soil granulometry, the δ13C and δ15N in leaves, in the superficial soil layer, and litter in three sites: a secondary forest in the Atlantic forest, and other two sites where eucalyptus had been removed in different times: 12 and 3 months ago (M12 and M3, respectively). Litter samples presented intermediate δ13C and δ15N values in comparison with leaves and soil. In the M3, the greater δ13C values in both litter rest fraction and soil indicate the presence, cycling and soil incorporation of C, coming from the C4 photosynthesis of grassy species (Poaceae). In the secondary forest, the soil δ15N values were twice higher, compared with the eucalyptus removal sites, revealing the negative influence from these exotic species upon the ecosystem N dynamics. In the M12, the leaves presented higher δ13C mean value and lower δ15N values, compared with those from the other sites. The difference of δ13C values in the litter fractions regarding the soil led to a greater fractioning of 13C in all sites, except the flower + fruit fractions in the secondary forest, and the rest fraction in the M3 site. We conclude that the permanence of this exotic species and the eucalyptus removal have altered the C and N isotopic and elemental compositions in the soil-plant system. Our results suggest there was organic matter decomposition in all litter fractions and in all sites. However, a greater organic matter decomposition process was observed in the M3 soil, possibly because of a more intense recent input of vegetal material, as well as the presence of grassy, easily-decomposing herbaceous species, only in this site. Therefore, the dual-isotope approach generated a more integrated picture of the impact on the ecosystem after removing eucalyptus in this secondary Atlantic forest, and could be regarded as an option for future eucalyptus removal studies

Dominance of legume trees alters nutrient relations in mixed species forest restoration plantings within seven years

Failures in reforestation are often attributed to nutrient limitation for tree growth. We compared tree performance and nitrogen and phosphorus relations in adjacent mixed-species plantings of contrasting composition, established for forest restoration on Ultisol soil, originally covered by tropical semi-deciduous Atlantic Forest in Southeast Brazil. Nutrient relations of four tree species occurring in both planting mixtures were compared between a legume-dominated, species-poor direct seeding mixture of early-successional species ("legume mixture"), and a species-diverse, legume-poor mixture of all successional groups ("diverse mixture"). After 7 years, the legume mixture had 6-fold higher abundance of N(2)-fixing trees, 177% higher total tree basal area, 22% lower litter C/N, six-fold higher in situ soil resin-nitrate, and 40% lower in situ soil resin-P, compared to the diverse mixture. In the legume mixture, non-N(2)-fixing legume Schizolobium parahyba (Fabaceae-Caesalpinioideae) had significantly lower proportional N resorption, and both naturally regenerating non-legume trees had significantly higher leaf N concentrations, and higher proportional P resorption, than in the diverse mixture. This demonstrate forms of plastic adjustment in all three non-N(2)-fixing species to diverged nutrient relations between mixtures. By contrast, leaf nutrient relations in N(2)-fixing Enterolobium contortisiliquum (Fabaceae-Mimosoideae) did not respond to planting mixtures. Rapid N accumulation in the legume mixture caused excess soil nitrification over nitrate immobilization and tighter P recycling compared with the diverse mixture. The legume mixture succeeded in accelerating tree growth and canopy closure, but may imply periods of N losses and possibly P limitation. Incorporation of species with efficient nitrate uptake and P mobilization from resistant soil pools offers potential to optimize these tradeoffs