RESUMO

Riverine nitrogen distribution is increasingly controlled by anthropogenic activities in their watersheds, regardless of spatial scale, climate, and geographical zone. Consequently, modelling efforts to predict the export of nitrogen from rivers worldwide have used attributes such as population density, land use, urbanization and sanitation. These models have greatly enhanced our understanding of the sources and fate of nitrogen added to terrestrial systems and transported to rivers and streams, especially for developed countries of the North temperate zone. However, much of the world's population lives in developing countries of the tropics, where the effects of human activities on riverine N exports are still poorly understood. In an effort to close this gap, we compare riverine nitrogen data from 32 Brazilian rivers draining two contrasting regions in this tropical country in terms of economic development - the State of São Paulo and the Amazon. Our data include nitrogen in different dissolved forms, such as Dissolved Inorganic Nitrogen (DIN) and Dissolved Organic Nitrogen (DON). The results show that nitrogen concentrations decreased as river runoff increased in both study areas, and that concentrations were significantly higher in rivers draining the most economically developed region. The relationships between nitrogen concentrations and fluxes with demographic parameters such as population density were also determined and compared to those in temperate systems. In contrast to temperate watersheds, we found that nitrogen fluxes increased only after population densities were higher than 10 individuals per km².

Assuntos

RESUMO

High levels of wet N and acidic deposition were measured in southeast Brazil. In this study we addressed the sensitivity of water bodies and soils to acidification and N deposition in the Piracicaba River basin (12,400 km2). Average acid neutralization capacity (ANC) at 23 river sampling sites varied from 350 to 1800 microeq l(-1). Therefore, rivers and streams in the Piracicaba basin are well buffered, if the lower limit of 200 microeq l(-1) is assumed as an indication of poorly buffered waters. ANC is increased by untreated wastewaters discarded into rivers and streams of the region. Average NO3 concentrations varied from 20 to 70 microeq l(-1). At the most polluted river sites, NO3 concentration is not highest, however, probably due to NO3 reduction and denitrification. Most of the nitrogen in streams is also provided by wastewaters and not by wet deposition. The majority of the soils in the basin, however, are acidic with a low base cation content and high aluminum concentration. Therefore, soils in this basin are poorly buffered and, in areas of forest over sandy soils, acidification may be a problem.

Assuntos

RESUMO

The chemical composition of ground waters and stream waters is thought to be determined primarily by weathering of parent rock. In relatively young soils such as those occurring in most temperate ecosystems, dissolution of primary minerals by carbonic acid is the predominant weathering pathway that liberates Ca2+, Mg2+ and K+ and generates alkalinity in the hydrosphere. But control of water chemistry in old and highly weathered soils that have lost reservoirs of primary minerals (a common feature of many tropical soils) is less well understood. Here we present soil and water chemistry data from a 10,000-hectare watershed on highly weathered soil in the Brazilian Amazon. Streamwater cation concentrations and alkalinity are positively correlated to each other and to streamwater discharge, suggesting that cations and bicarbonate are mainly flushed from surface soil layers by rainfall rather than being the products of deep soil weathering carried by groundwater flow. These patterns contrast with the seasonal patterns widely recognized in temperate ecosystems with less strongly weathered soils. In this particular watershed, partial forest clearing and burning 30 years previously enriched the soils in cations and so may have increased the observed wet season leaching of cations. Nevertheless, annual inputs and outputs of cations from the watershed are low and nearly balanced, and thus soil cations from forest burning will remain available for forest regrowth over the next few decades. Our observations suggest that increased root and microbial respiration during the wet season generates CO2 that drives cation-bicarbonate leaching, resulting in a biologically mediated process of surface soil exchange controlling the streamwater inputs of cations and alkalinity from these highly weathered soils.

Assuntos

RESUMO

C4 plant species were proposed to generally represent inferior food sources compared to C3 plants thus are avoided by herbivores, particularly insects. This was tested in semi-aquatic and terrestrial arthropods from Amazonian river-floodplains by carbon isotope discrimination (delta13C). Two semi-aquatic grasshopper species (Stenacris f. fissicauda, Tucavaca gracilis-Acrididae) obtain their carbon during development from specific C4 macrophytes and two semi-aquatic species (Cornops aquaticum-Acrididae, Paulinia acuminata-Pauliniidae) from specific C3 macrophytes. The terrestrial millipede Mestosoma hylaeicum (Paradoxosomatidae) obtains about 45% of its carbon from roots of one C4 macrophyte during the development of immatures whereas adults use other food sources, including C3 trees. Results suggest, that (1) both C4 and C3 plants represent distinct hosts for terrestrial arthropods in Amazonia; (2) immatures may use plant species with a different photosynthetic pathway than adults.

Assuntos

RESUMO

Leaves of 208 trees were collected for isotopic analysis together with wood from 36 tree boles and 18 samples of fine litter from a terra-firme forest located at Samuel Ecological Reserve, Rondônia State, in the southwestern Amazon region. The range of δ13C values in leaves was from -28 to -36‰, with an average (±1 SD) of -32.1 ± 1.5‰, which was more negative than the δ13C values of bole samples (-28.4 ± 2.0‰) and fine litter (-28.7 ± 2.0‰). These values are within the range found for tropical and subtropical forests. Pooling the δ13C values for leaf samples from trees of the same height gave averages which were positively correlated with plant height at a highly significant level, with a slope of 0.06 and an intercept of -33.3‰ and a correlation coefficient r 2=0.70 (P<0.001).

RESUMO

The 18O content of rain and river waters was used as an isotopic tracer in order to carry out the hydrograph separation of the Amazon river, during the 1973-1974 hydrological years, and to estimate the contributions of the surface runoff (event water) and baseflow (pre-event water) components to the total river flow. The average surface runoff and baseflow contributions were 30.3 and 69.7% respectively. At peak discharge, the mean contribution of the baseflow was about 57%. The results of the isotopic separation model were compared with the filter-separation autoregressive method, showing similar behavior and magnitude.

Os teores de 18O em águas de chuva e amostras de água de rio foram utilizados como traçadores num modelo isotópico de separação de hidrógrafas do rio Amazonas durante o período 1973-1974, com o intuito de se estimar as contribuições do escoamento superficial rápido e escoamento de base para o escoamento total da bacia de drenagem. As contribuições médias desses reservatórios foram de 30,3 e 69,7% respectivamente. Durante o período de pico de cheia, o fluxo de base contribuiu com cerca de 57% do volume total de água escoada. Os resultados do modelo de separação isotópico foram comparados com o método estatístico de filtros auto-recursivos, mostrando comportamentos similares.

RESUMO

The 18O content of rain and river waters was used as an isotopic tracer in order to carry out the hydrograph separation of the Amazon river, during the 1973-1974 hydrological years, and to estimate the contributions of the surface runoff (event water) and baseflow (pre-event water) components to the total river flow. The average surface runoff and baseflow contributions were 30.3 and 69.7% respectively. At peak discharge, the mean contribution of the baseflow was about 57%. The results of the isotopic separation model were compared with the filter-separation autoregressive method, showing similar behavior and magnitude.

Os teores de 18O em águas de chuva e amostras de água de rio foram utilizados como traçadores num modelo isotópico de separação de hidrógrafas do rio Amazonas durante o período 1973-1974, com o intuito de se estimar as contribuições do escoamento superficial rápido e escoamento de base para o escoamento total da bacia de drenagem. As contribuições médias desses reservatórios foram de 30,3 e 69,7% respectivamente. Durante o período de pico de cheia, o fluxo de base contribuiu com cerca de 57% do volume total de água escoada. Os resultados do modelo de separação isotópico foram comparados com o método estatístico de filtros auto-recursivos, mostrando comportamentos similares.

RESUMO

Paleoecological and geomorphological studies indicate that, during the middle Holocene, there was a predominance of drier conditions with grassy savannahs replacing forests across the South American continent. Modern savannahs are composed mainly of C4 plants and soils developed under this type of vegetation show enrichment in 13C compared to soils under C3 vegetation cover. If soils contain stabilized organic matter formed in the middle Holocene, we hypothesize that former C4 vegetation would be evidenced by a large enrichment of 13C in soil organic matter (SOM). We investigate this possibility examining the depth variation of carbon isotopic composition in 21 soil profiles collected by different researchers at 14 different sites in Brazil. Of these, profiles from only three sites showed a marked increase of 13C with depth (9-10‰ enrichment in δ13C difference between the surface soil and deepest depth); two sites showed intermediate enrichment (4-5‰), and nine sites showed a small enrichment of approximatelly 2.5‰. The majority of sites showing all-C3 derived SOM were in the Amazon region. Possible causes for the absence of a large 13C enrichment with depth are: (1) dominance of C3 rather than C4 grasses in mid-Holocene savannahas, (2) soil profiles did not preserve organic matter derived from mid-Holocene plants, (3) the retreat of forest areas did not occur on a regional scale, but was a much more localized phenomenon.

RESUMO

Seedpieces with one node (one bud) plus half of the internodes at each side, were cut from the middle third of cane stalks, variety NA 56-79, grown with 1S N. The dry weight, N content and 15N abundance of cuttings were estimated from samples of each original stalk. The cuttings were planted in pots filled with 6kg of a Dark Red Latosol (RHODIC KANHAPLUDALF) sample. The experiment was conducted under greenhouse conditions, using a random design with 16 treatments (harvest times) and three replicates. The harvested plants were divided into shoots, roots and remaining seedpiece. The dry weight, N content and 15N abundance were determined in each sample. The results showed that the reduction of the dry weight of the seedpiece was compensated by new matter yield (shoot and root) only after three months of growth. During this period the emergence of fixing roots and of the primary shoot depended solely on the seedpiece reservoir. The later is also essential as a nitrogen source for the cane plant during 50-60 days after planting. After 6 months the dry matter of the original cuttings is reduced by 50% and 50% of its nitrogen is translocated to the cane plant. It may be estimated that, under field conditions, 6-12kg.ha-1 can be translocated from the original seedpieces to the cane plant (shoots and roots). These values can represent 5-10% of total N required by the cane plant.

Toletes (colmo-semente) com um nó (uma gana) mais metade do entre-nó inferior e metade do superior foram obtidos do terço médio de colmos de cana-de-açúcar, marcados com 15N, da variedade NA 56-79. A massa seca, o conteúdo de nitrogênio e a abundância isotópica de 15N dos toletes foram estimados tomando-se por base os valores obtidos em amostra de cada colmo. Os toletes foram plantados em vasos com 6kg de terra colhida de uma Terra Roxa Estruturada Latossólica (RHODIC KANHAPLUDALF). O experimento foi conduzido em casa-de-vegetação, em delineamento inteiramente casualizado, com 16 tratamentos correspondentes à colheitas em diferentes tempos após o plantio. Nas plantas, subdivididas em amostras de parte aérea, raízes e tolete remanescente, foram determinados a massa seca, N-total e abundância de 15N. Dos resultados obtidos verificou-se que a redução da matéria seca do tolete somente foi compensada pela produção de novos tecidos por volta dos 3 meses do plantio. Nesse período, a emissão das raízes de fixação e a brotação do perfilho primário dependeu da reserva orgânica do tolete. A reserva de nitrogênio do tolete foi fundamental como fonte do nutriente à cana-planta nos 50-60 dias após o plantio. A degradação de 50% da reserva orgânica do tolete ocorreu até os 6 meses e possibilitou a liberação de 50% da reserva de nitrogênio que foi translocada aos novos tecidos da cana-planta. Pode-se estimar que de 6-12 kg.ha-1 do N-tolete são translocados aos novos tecidos (50% do N-tolete), valores estes que podem representar 5-10% das necessidades de N da cultura.

RESUMO

Seedpieces with one node (one bud) plus half of the internodes at each side, were cut from the middle third of cane stalks, variety NA 56-79, grown with 1S N. The dry weight, N content and 15N abundance of cuttings were estimated from samples of each original stalk. The cuttings were planted in pots filled with 6kg of a Dark Red Latosol (RHODIC KANHAPLUDALF) sample. The experiment was conducted under greenhouse conditions, using a random design with 16 treatments (harvest times) and three replicates. The harvested plants were divided into shoots, roots and remaining seedpiece. The dry weight, N content and 15N abundance were determined in each sample. The results showed that the reduction of the dry weight of the seedpiece was compensated by new matter yield (shoot and root) only after three months of growth. During this period the emergence of fixing roots and of the primary shoot depended solely on the seedpiece reservoir. The later is also essential as a nitrogen source for the cane plant during 50-60 days after planting. After 6 months the dry matter of the original cuttings is reduced by 50% and 50% of its nitrogen is translocated to the cane plant. It may be estimated that, under field conditions, 6-12kg.ha-1 can be translocated from the original seedpieces to the cane plant (shoots and roots). These values can represent 5-10% of total N required by the cane plant.

Toletes (colmo-semente) com um nó (uma gana) mais metade do entre-nó inferior e metade do superior foram obtidos do terço médio de colmos de cana-de-açúcar, marcados com 15N, da variedade NA 56-79. A massa seca, o conteúdo de nitrogênio e a abundância isotópica de 15N dos toletes foram estimados tomando-se por base os valores obtidos em amostra de cada colmo. Os toletes foram plantados em vasos com 6kg de terra colhida de uma Terra Roxa Estruturada Latossólica (RHODIC KANHAPLUDALF). O experimento foi conduzido em casa-de-vegetação, em delineamento inteiramente casualizado, com 16 tratamentos correspondentes à colheitas em diferentes tempos após o plantio. Nas plantas, subdivididas em amostras de parte aérea, raízes e tolete remanescente, foram determinados a massa seca, N-total e abundância de 15N. Dos resultados obtidos verificou-se que a redução da matéria seca do tolete somente foi compensada pela produção de novos tecidos por volta dos 3 meses do plantio. Nesse período, a emissão das raízes de fixação e a brotação do perfilho primário dependeu da reserva orgânica do tolete. A reserva de nitrogênio do tolete foi fundamental como fonte do nutriente à cana-planta nos 50-60 dias após o plantio. A degradação de 50% da reserva orgânica do tolete ocorreu até os 6 meses e possibilitou a liberação de 50% da reserva de nitrogênio que foi translocada aos novos tecidos da cana-planta. Pode-se estimar que de 6-12 kg.ha-1 do N-tolete são translocados aos novos tecidos (50% do N-tolete), valores estes que podem representar 5-10% das necessidades de N da cultura.

RESUMO

Two experiments were conducted on commercial sugar cane fields cropped with the variety SP70-1143, with the objective of evaluating a single row microplot design to determine plant recovery of 15N fertilizer nitrogen. One of them used 15N-aqua ammonia and 15N-urea applied to two linear meter microplots of a ratoon crop (four replicates). The second used one linear meter microplots (three replicates) which received 15N-aqua ammonia only. The fertilizers were applied on 15cm deep furrows, located 25cm from both sides of the cane row. One linear meter of ratoon cane, inside and outside of the microplot, and on the same and adjacent rows were harvested twelve months after fertilization. The results indicate the feasibility of using single row segments of ratoon cane with 15N-fertilizer. The main advantage of this microplot design, when compared to the classical 3 contiguous row segments, is that only one third of the labeled fertilizer is needed. In a single row, in order to separate the nitrogen taken up by plants from the fertilizer applied to the row (Nr dff r), from that applied to adjacent rows (Nr dff r+1, and Nr dff r-1), the following should be considered: (a) a border segment of 0.5 to 1.0m inside the plot, so that Ndff results from plants harvested in the center of the microplot represent the actual value of fertilizer nitrogen taken up from that applied to the same row, and (b) harvest of plants from adjacent rows at equivalent positions to those sampled inside the microplot, to quantify the 15N-fertilizer uptake by outside plants (Nr+1dff r and Nr-1dff r), which is assumed to be the same as non labeled fertilizer applied to adjacent rows (Nr dff r+l and Nr dff r-1) taken up by inside plants. The Ndff total values should be calculated by the equation: Ndff total = Nr dff r + Nr+1dff r + Nr-1dff r.

Com o objetivo de avaliar um modelo de parcela, na determinação do aproveitamento pela cana-de-açúcar de fertilizantes nitrogenados-15N, foram conduzidos dois experimentos em áreas comerciais plantadas com a variedade SP 70-1143. Num experimento foram aplicados os adubos aquamônia-15N e uréia-15N, em parcelas constituídas de segmentos simples de linha de soqueira de cana, com 2 metros lineares de comprimento (4 repetições). No segundo, parcelas de l metro linear (3 repetições) receberam aquamônia-15N. Os fertilizantes foram aplicados em sulcos com 15cm de profundidade, abertos a 25cm de distância nos dois lados da linha de cana. Foram realizadas colheitas de l metro linear de cana no interior e fora das parcelas, na mesma linha e nas adjacentes à elas. Os resultados isotópicos evidenciaram ser perfeitamente possível a utilização dessas parcelas com fertilizante-15N. A principal vantagem do modelo, comparado às parcelas convencionais com 3 segmentos contíguos, é a redução a um terço, da quantidade de fertilizantes-15N necessária. Na determinação do nitrogênio absorvido pelas plantas do fertilizante aplicado na linha com adubo-15N (Nr dff r), e daquele absorvido do fertilizante não marcado com o isótopo, aplicado nas linhas adjacentes (Nr dff r+1 e Nr dff r-1), deve-se considerar para as parcelas em questão: (a) bordadura interna de 0,5 a 1m de comprimento, para que os valores de Ndff de plantas colhidas no centro delas, representem o valor máximo do nitrogênio absorvido do fertilizante aplicado na linha; e (b) colheita de plantas nas linhas imediatamente adjacentes, em posições equivalentes à efetuada no interior das parcelas, a fim de quantificar o N absorvido do fertilizante-15N (Nr+1dff r e Nr-1dff r), que representa o valor do N absorvido do adubo aplicado nas linhas adjacentes, por plantas da parcela (Nr dff r+I e Nr dff r.,). O valor de Ndff

RESUMO

Two experiments were conducted on commercial sugar cane fields cropped with the variety SP70-1143, with the objective of evaluating a single row microplot design to determine plant recovery of 15N fertilizer nitrogen. One of them used 15N-aqua ammonia and 15N-urea applied to two linear meter microplots of a ratoon crop (four replicates). The second used one linear meter microplots (three replicates) which received 15N-aqua ammonia only. The fertilizers were applied on 15cm deep furrows, located 25cm from both sides of the cane row. One linear meter of ratoon cane, inside and outside of the microplot, and on the same and adjacent rows were harvested twelve months after fertilization. The results indicate the feasibility of using single row segments of ratoon cane with 15N-fertilizer. The main advantage of this microplot design, when compared to the classical 3 contiguous row segments, is that only one third of the labeled fertilizer is needed. In a single row, in order to separate the nitrogen taken up by plants from the fertilizer applied to the row (Nr dff r), from that applied to adjacent rows (Nr dff r+1, and Nr dff r-1), the following should be considered: (a) a border segment of 0.5 to 1.0m inside the plot, so that Ndff results from plants harvested in the center of the microplot represent the actual value of fertilizer nitrogen taken up from that applied to the same row, and (b) harvest of plants from adjacent rows at equivalent positions to those sampled inside the microplot, to quantify the 15N-fertilizer uptake by outside plants (Nr+1dff r and Nr-1dff r), which is assumed to be the same as non labeled fertilizer applied to adjacent rows (Nr dff r+l and Nr dff r-1) taken up by inside plants. The Ndff total values should be calculated by the equation: Ndff total = Nr dff r + Nr+1dff r + Nr-1dff r.

Com o objetivo de avaliar um modelo de parcela, na determinação do aproveitamento pela cana-de-açúcar de fertilizantes nitrogenados-15N, foram conduzidos dois experimentos em áreas comerciais plantadas com a variedade SP 70-1143. Num experimento foram aplicados os adubos aquamônia-15N e uréia-15N, em parcelas constituídas de segmentos simples de linha de soqueira de cana, com 2 metros lineares de comprimento (4 repetições). No segundo, parcelas de l metro linear (3 repetições) receberam aquamônia-15N. Os fertilizantes foram aplicados em sulcos com 15cm de profundidade, abertos a 25cm de distância nos dois lados da linha de cana. Foram realizadas colheitas de l metro linear de cana no interior e fora das parcelas, na mesma linha e nas adjacentes à elas. Os resultados isotópicos evidenciaram ser perfeitamente possível a utilização dessas parcelas com fertilizante-15N. A principal vantagem do modelo, comparado às parcelas convencionais com 3 segmentos contíguos, é a redução a um terço, da quantidade de fertilizantes-15N necessária. Na determinação do nitrogênio absorvido pelas plantas do fertilizante aplicado na linha com adubo-15N (Nr dff r), e daquele absorvido do fertilizante não marcado com o isótopo, aplicado nas linhas adjacentes (Nr dff r+1 e Nr dff r-1), deve-se considerar para as parcelas em questão: (a) bordadura interna de 0,5 a 1m de comprimento, para que os valores de Ndff de plantas colhidas no centro delas, representem o valor máximo do nitrogênio absorvido do fertilizante aplicado na linha; e (b) colheita de plantas nas linhas imediatamente adjacentes, em posições equivalentes à efetuada no interior das parcelas, a fim de quantificar o N absorvido do fertilizante-15N (Nr+1dff r e Nr-1dff r), que representa o valor do N absorvido do adubo aplicado nas linhas adjacentes, por plantas da parcela (Nr dff r+I e Nr dff r.,). O valor de Ndff