Gabriela Bielefeld Nardoto

Global patterns of foliar nitrogen isotopes and their relationships with climate, mycorrhizal fungi, foliar nutrient concentrations, and nitrogen availability.

Ratios of nitrogen (N) isotopes in leaves could elucidate underlying patterns of N cycling across ecological gradients. To better understand global-scale patterns of N cycling, we compiled data on foliar N isotope ratios (delta(15)N), foliar N concentrations, mycorrhizal type and climate for over 11,000 plants worldwide. Arbuscular mycorrhizal, ectomycorrhizal, and ericoid mycorrhizal plants were depleted in foliar delta(15)N by 2 per thousand, 3.2 per thousand, 5.9 per thousand, respectively, relative to nonmycorrhizal plants. Foliar delta(15)N increased with decreasing mean annual precipitation and with increasing mean annual temperature (MAT) across sites with MAT >or= -0.5 degrees C, but was invariant with MAT across sites with MAT < -0.5 degrees C. In independent landscape-level to regional-level studies, foliar delta(15)N increased with increasing N availability; at the global scale, foliar delta(15)N increased with increasing foliar N concentrations and decreasing foliar phosphorus (P) concentrations. Together, these results suggest that warm, dry ecosystems have the highest N availability, while plants with high N concentrations, on average, occupy sites with higher N availability than plants with low N concentrations. Global-scale comparisons of other components of the N cycle are still required for better mechanistic understanding of the determinants of variation in foliar delta(15)N and ultimately global patterns in N cycling.

Recuperation of nitrogen cycling in Amazonian forests following agricultural abandonment.

Phosphorus (P) is generally considered the most common limiting nutrient for productivity of mature tropical lowland forests growing on highly weathered soils. It is often assumed that P limitation also applies to young tropical forests, but nitrogen (N) losses during land-use change may alter the stoichiometric balance of nutrient cycling processes. In the Amazon basin, about 16% of the original forest area has been cleared, and about 30–50% of cleared land is estimated now to be in some stage of secondary forest succession following agricultural abandonment. Here we use forest age chronosequences to demonstrate that young successional forests growing after agricultural abandonment on highly weathered lowland tropical soils exhibit conservative N-cycling properties much like those of N-limited forests on younger soils in temperate latitudes. As secondary succession progresses, N-cycling properties recover and the dominance of a conservative P cycle typical of mature lowland tropical forests re-emerges. These successional shifts in N:P cycling ratios with forest age provide a mechanistic explanation for initially lower and then gradually increasing soil emissions of the greenhouse gas nitrous oxide (N2O). The patterns of N and P cycling during secondary forest succession, demonstrated here over decadal timescales, are similar to N- and P-cycling patterns during primary succession as soils age over thousands and millions of years, thus revealing that N availability in terrestrial ecosystems is ephemeral and can be disrupted by either natural or anthropogenic disturbances at several timescales.

Understanding the Influences of Spatial Patterns on N Availability Within the Brazilian Amazon Forest

Nitrogen variations at different spatial scales and integrated across functional groups were addressed for lowland tropical forests in the Brazilian Amazon as follows: (1) how does N availability vary across the region over different spatial scales (regionalxa0×xa0landscape scale); (2) how are these variations in N availability integrated across plant functional groups (legumexa0×xa0non-legume trees). Leaf N, P, and Ca concentrations as well the leaf N isotope ratios (δ15N) from a large set of legume and non-legume tree species were measured. Legumes had higher foliar N/Ca ratios than non-legumes, consistent with the high energetic costs in plant growth associated with higher foliar P/Ca ratios found in legumes than in non-legumes. At the regional level, foliar δ15N decreased with increasing rainfall. At the landscape level, N availability was higher in the forests on clayey soils on the plateau than in forests on sandier soils. The isotope as well as the non-isotope data relationships here documented, explain to a large extent the variation in δ15N signatures across gradients of rainfall and soil. Although at the regional level, the precipitation regime is a major determinant of differences in N availability, at the landscape level, under the same precipitation regime, soil type seems to be a major factor influencing the availability of N in the Brazilian Amazon forest.

Effects of fire on soil nitrogen dynamics and microbial biomass in savannas of Central Brazil

The objective of this work was to study the effects of fire on net N mineralization and soil microbial biomass in burned and unburned cerrado stricto sensu sites. The study was carried out from April 1998 to April 2000. The pH values were significantly higher in the burned site while soil moisture content was significantly higher in the unburned site (P<0.05). The soil C/N ratio was 22/1 and the available NO3-N ranged between 1.5 and 2.8xa0mgxa0kg-1 dry weight. However, the NH4-N concentration ranged between 3 and 34xa0mgxa0kg-1 dry weight in the burned site and between 3 and 22xa0mgxa0kg-1 dry weight in the unburned site. The NH4-N increased after fire, but no significant changes were observed for NO3-N (P<0.05). The NO3-N accumulation occurred in short periods during the rainy season. The rates of net N mineralization increased during the rainy season while reductions in soil microbial biomass were observed at both sites. This suggested that the peak in microbial activities occurred with the first rain events, with an initial net immobilization followed by net mineralization. Both sites presented the same pattern for mineralization/immobilization, however, the amount of inorganic-N cycled annually in unburned site was 14.7xa0kgxa0ha-1 per year while the burned site presented only 3.8xa0kgxa0ha-1 of inorganic-N, one year after the burning.

Frozen chicken for wild fish: nutritional transition in the Brazilian Amazon region determined by carbon and nitrogen stable isotope ratios in fingernails.

Amazonian populations are experiencing dietary changes characteristic of the nutrition transition. However, the degree of change appears to vary between urban and rural settings. To investigate this process, we determined carbon and nitrogen stable isotope ratios in fingernails and dietary intake of Amazonian populations living along a rural to urban continuum along the Solimões River in Brazil.

Stable carbon and nitrogen isotopic fractionation between diet and swine tissues

Naturally occurring stable isotope ratios can be a powerful tool in studies of animal nutrition, provided that the assumptions required for dietary reconstruction are validated by studies such as the one presented here. The objective of this study was to document the magnitude of isotopic fractionation between swine diet and their different tissues. For this, the isotopic ratios of carbon and nitrogen of the diet and selected tissues (hair, nail, liver, muscle, fat and cartilage) were determined. The d13C and d15N of the diet were -15.9x89 and 1.3x89, respectively, and all d15N of swine tissues were 2.2 to 3.0x89 enriched in 15N in relation to the diet. Little variation in d15N occurred among tissues, with exception to liver that was less enriched in 15N than the nail. Nail and hair presented no 13C enrichment relative to diet. Cartilage was ~1.0x89 enriched in 13C as compared to diet. Liver and muscle were on average 2.1x89 more depleted in 13C in relation to diet as well as fat tissues. Some of the C and N isotope ratios of swine tissues differed in organs, but the isotopic fractionation trends among tissues appears to be similar to other mammals. Therefore our data provide a good baseline to interpret stable isotope patterns in domestic mammals (such as swine) in controlled or semi-controlled experiments.

Nitrogen availability patterns in white-sand vegetations of Central Brazilian Amazon

Addressing spatial variability in nitrogen (N) availability in the Central Brazilian Amazon, we hypothesized that N availability varies among white-sand vegetation types (campina and campinarana) and lowland tropical forests (dense terra-firme forests) in the Central Brazilian Amazon, under the same climate conditions. Accordingly, we measured soil and foliar N concentration and N isotope ratios (δ15N) throughout the campina-campinarana transect and compared to published dense terra-firme forest results. There were no differences between white-sand vegetation types in regard to soil N concentration, C:N ratio and δ15N across the transect. Both white-sand vegetation types showed very low foliar N concentrations and elevated foliar C:N ratios, and no significant difference between site types was observed. Foliar δ15N was depleted, varying from −9.6 to 1.6‰ in the white-sand vegetations. The legume Aldina heterophylla had the highest average δ15N values (−1.5‰) as well as the highest foliar N concentration (2.1%) while the non-legume species had more depleted δ15N values and the average foliar N concentrations varied from 0.9 to 1.5% among them. Despite the high variation in foliar δ15N among plants, a significant and gradual 15N-enrichment in foliar isotopic signatures throughout the campina–campinarana transect was observed. Individual plants growing in the campinarana were significantly enriched in 15N compared to those in campina. In the white-sand N-limited ecosystems, the differentiation of N use seems to be a major cause of variations observed in foliar δ15N values throughout the campina–campinarana transect.

ISOTOPIC VIEW OF VEGETATION AND CARBON AND NITROGEN CYCLES IN A CERRADO ECOSYSTEM, SOUTHEASTERN BRAZIL

Os ciclos biogeoquimicos do carbono e do nitrogenio em savanas sao fortemente regulados pela distribuicao sazonal de precipitacao e pulsos de nutrientes liberados apos eventos de chuva, que sao criticos para o comportamento dos microrganismos e da vegetacao. Investigou-se a variabilidade espacial e sazonal dos isotopos estaveis de C e N como indicadores da ciclagem destes elementos em uma area de cerrado sensu stricto de uma area protegida em um Parque Estadual no estado de Sao Paulo, Brasil. Os valores de δ13C e δ15N foliar variaram de -33,6 a -24,4 ‰ e -2,5 a 4,5 ‰, respectivamente. Os valores de δ13C apresentaram uma relacao consistente com a altura do dossel, mostrando a importância da estrutura da vegetacao na assinatura isotopica do C da vegetacao. A variacao isotopica do Carbono associada com a duracao da estacao seca indica a importância do C recentemente fixado para integrar a assinatura isotopica do C orgânico da folha. Com relacao ao Nitrogenio, as especies de Cerrado estudadas apresentaram uma grande variacao no δ15N foliar. Nao houve relacao entre o δ15N foliar com a altura do dossel. No entanto, os valores de δ15N foliar apresentaram uma variabilidade sazonal, com maiores valores de δ15N na transicao entre as estacoes seca e chuvosa. A variacao encontrada na composicao isotopica do C e N foliares foi consistente com o fato de a vegetacao ser altamente diversa, tendo que lidar com uma alta disponibilidade de energia, mas baixa disponibilidade de agua e N.

Variation in nitrogen use strategies and photosynthetic pathways among vascular epiphytes in the Brazilian Central Amazon

A variacao nas estrategias de uso do nitrogenio e das vias fotossinteticas de familias de epifitas vasculares foi investigada em uma vegetacao de areia branca na Amazonia Central. Foram medidas as concentracoes e composicoes isotopicas de nitrogenio e carbono (δ15N e δ13C, respectivamente) de folhas de epifitas (Araceae, Bromeliaceae e Orchidaceae), assim como de suas arvores hospedeiras. As folhas da arvore hospedeira Aldine heterophylla tiveram a maior concentracao de nitrogenio foliar e menor razao C:N (2,1 ± 0,06% e 23,6 ± 0,8) que de suas hospedes. O valor de δ15N foliar da arvore somente diferiu do valor das orquideas. Ao comparar as familias de epifitas, a maior concentracao de nitrogenio foliar e menor razao C:N foi observada nas araceas (1,4 ± 0,1% e 34,9 ± 4,2, respectivamente). As orquideas tiveram valores mais negativos de δ15N foliar (-3,5 ± 0,2‰) que araceas (-1,9 ± 0,7‰) e bromelias (-1,1 ± 0,6‰). Ao comparar os taxons de cada familia, observou-se que tanto os taxons de araceas como os de orquideas diferiram em relacao ao nitrogenio foliar e razao C:N, enquanto que nao foi detectada variacao entre os taxons de bromelias. As diferencas nos valores de δ15N foliar aqui observadas podem ser relacionadas a variacao na dependencia das fontes de nitrogenio disponiveis para as epifitas, assim como na variacao da qualidade do microhabitat no dossel. Em relacao aos valores de δ13C foliar das epifitas analisadas, verificou-se que a maioria usa a via fotossintetica CAM (valores em torno de -17‰), comumente associada com plantas que vivem em condicoes de suprimento de agua limitado ou intermitente. Apenas as araceas e um taxon de orquidea mostraram usar a via C3 (valores em torno de -30‰).

The Use of Carbon and Nitrogen Stable Isotopes to Track Effects of Land‐Use Changes in the Brazilian Amazon Region

Publisher Summary On an average, 18,000 km2 of Brazilian Amazon terra firme forest vegetation is burned every year. Most of this area is replaced by pastures cultivated to support cattle ranching. The replacement of tropical terra firme forests by cultivated pastures, has introduced different species of African C4 tropical grasses, mainly of the genus Brachiaria. This change opens up a unique opportunity for the use of carbon stable isotope composition to track the fate of introduced C4 grass species since the main matrix of plant species in the Amazon terra firme forests is composed by C3 plants. It has also been documented that tropical terra firme forests generally have an open nitrogen cycle, where losses of N are significantly higher in relation to the inputs of N. As a consequence of these high losses, soils and plants of tropical terra firme forests are characterized by distinctly high values of δ15N. As N availability increases, losses from the ecosystem, such as gaseous N losses and leaching of NO3– as a result of incomplete nitrification, should increase and lead to 15N-enriched plants. Such a fact opens up a second window of opportunity for using the N stable isotopic composition as a tool to investigate changes in the N cycle due to land-use changes that are occurring in the Amazon basin. The stable C and N isotopic composition is used to evaluate changes in ecosystem functioning due to land-use changes that have been occurring in the Amazon region. The chapter discusses isotopic shifts from individual components of the ecosystems, like plants, and shows how the signals of the C4 vegetation have already been incorporated in other reservoirs of C, such as soils, rivers, and atmosphere.