Cláudio José Reis de Carvalho

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.

Modeling seasonal and interannual variability in ecosystem carbon cycling for the Brazilian Amazon region

Previous field measurements have implied that undisturbed Amazon forests may represent a substantial terrestrial sink for atmospheric carbon dioxide. We investigated this hypothesis using a regional ecosystem model for net primary production (NPP) and soil biogeochemical cycling. Seasonal and interannual controls on net ecosystem production (NEP) were studied with integration of high-resolution (8-km) multiyear satellite data to characterize Amazon land surface properties over time. Background analysis of temporal and spatial relationships between regional rainfall patterns and satellite observations (for vegetation land cover, fire counts, and smoke aerosol effects) reveals several notable patterns in the model driver data. Autocorrelation analysis for monthly vegetation “greenness” index (normalized difference vegetation index, NDVI) from the advanced very high resolution radiometer (AVHRR) and monthly rainfall indicates a significant lag time correlation of up to 12 months. At lag times approaching 36 months, autocorrelation function (ACF) values did not exceed the 95% confidence interval at locations west of about 47°W, which is near the transition zone of seasonal tropical forest and other (nonforest) vegetation types. Even at lag times of 12 months or less, the location near Manaus (approximately 60°W) represents the farthest western point in the Amazon region where seasonality of rainfall accounts significantly for monthly variations in forest phenology, as observed using NDVI. Comparisons of NDVI seasonal profiles in areas of the eastern Amazon widely affected by fires (as observed from satellite) suggest that our adjusted AVHRR-NDVI captures year-to-year variation in land cover greenness with minimal interference from small fires and smoke aerosols. Ecosystem model results using this newly generated combination of regional forcing data from satellite suggest that undisturbed Amazon forests can be strong net sinks for atmospheric carbon dioxide, particularly during wet (non El Nino) years. However, drought effects during El Nino years can reduce NPP in primary forests of the eastern Amazon by 10–20%, compared to long-term average estimates of regional productivity. Annual NEP for the region is predicted to range from −0.4 Pg C yr−1 (net CO2 source) to 0.5 Pg C yr−1 (net CO2 sink), with large interannual variability over the states of Para, Maranhao, and Amazonas. As in the case of predicted NPP, it appears that periods of relatively high solar surface irradiance combined with several months of adequate rainfall are required to sustain the forest carbon sink for positive yearly NEP estimates.

Respostas de plantas de Schizolobium amazonicum [S. parahyba var. amazonicum] e Schizolobium parahyba [Schizolobium parahybum] à deficiência hídrica

Juvenile plants of S. amazonicum (parica) and S. parahyba (guapuruvu) were subjected to two cycles of water stress in greenhouse conditions. The water deficit induced the increase in total soluble amino acids, proline, total soluble sugars and K + contents in the leaf tissues, lowering osmotic potential values in both species. However, the analysis of the pressure-volume curve results suggests that the species respond differently to water stress, and that S. amazonicum presents greater capacity of osmotic adaptation than S. parahyba.

DROUGHT CONSTRAINTS ON LEAF GAS EXCHANGE BY MICONIA CILIATA (MELASTOMATACEAE) IN THE UNDERSTORY OF AN EASTERN AMAZONIAN REGROWTH FOREST STAND

Analyses of the effects of drought stress on Amazonian regrowth stands are lacking. We measured leaf gas exchange and leaf water potential of Miconia ciliata (Melastomataceae) in a dry-season irrigation experiment in 14-yr-old regrowth. In the dry season, irrigated plants maintained significantly higher leaf water potentials, photosynthetic capacity at light saturation (A(max)), stomatal conductance (g(s)), internal CO(2) concentration (C(i)), and lower A(max)/g(s) than control plants. The degree of dry-season down-regulation of control plant A(max), along with its fast recovery following rain, reveals the importance of occasional dry-season rains to the carbon budget of M. ciliata. During the wet season, we observed higher A(max) for control plants than for plants that had been irrigated during the dry season. We hypothesize that reduced drought constraints on photosynthesis of irrigated plants advanced the flowering and fruiting phenology of irrigated plants into the dry season. Flowers and fruits of control plants developed later, during the wet season, potentially stimulating a compensatory reproductive photosynthesis response in nearby leaves. The relative drought intolerance of M. ciliata may be a deciding factor in its ability to survive through the dynamic successional development of the regrowth stand studied.

Effects of seasonality, litter removal and dry-season irrigation on litterfall quantity and quality in eastern Amazonian forest regrowth, Brazil

Litterfall quantity and quality may respond to alterations in resource availability expected with ongoing land-use and climate changes. Here, we quantify the effects of altered resource availability on non-woody litterfall quantityandquality(nitrogenandphosphorusconcentrations)ineasternAmazonianforestregrowth(Brazil)through two multi-year experimental manipulations: (1) daily irrigation (5mm d −1 ) during the dry season; and (2) fortnightly litterremoval.Consistentwithothertropicalforestdatalitterfallexhibitedseasonalpatterns,increasingwiththeonset of the dry season and declining with the onset of the rainy season. Irrigation did not affect litterfall mass and had little impactonnitrogen(N)orphosphorus(P)concentrationsandreturn,exceptfordecreasinglitterPconcentrationatthe end of two irrigation periods. Litter removal did not alter litterfall mass or P concentration, but progressively reduced litterfall N during the course of the experiment. Overall, these results suggest significant resistance to altered resource availabilitywithintheboundsofourexperimentaltreatments;ourfindingsmayhelptoconstraincarbonandnutrient cycling predictions for tropical forests in response to land-use and climate changes.

Carbon content in Amazonian Oxisols after forest conversion to pasture.

Soil plays an important role in the C cycle, and substitution of tropical forest by cultivated land affects C dynamic and stock. This study was developed in an area of expansion of human settlement in the Eastern Amazon, in Itupiranga, State of Para, to evaluate the effects of native forest conversion to Brachiaria brizantha pasture on C contents of a dystrophic Oxisol. Soil samples were collected in areas of native forest (NF), of 8 to 10 year old secondary forest (SF), 1 to 2 year old SF (P1-2), 5 to 7 year old SF (P5-7), and of 10 to 12 year old SF (P10-12), and from under pastures, in the layers 0-2, 2-5 and 5-10 cm, to evaluate C levels and stocks and carry out separation of OM based on particle size. After deforestation, soil density increased to a depth of 5 cm, with greater increase in older pastures. Variation in C levels was greatest in the top soil layer; C contents increased with increasing pasture age. In the layers 2-5 and 5-10 cm, C content proved to be stable for the types of plant cover evaluated. Highest C concentrations were found in the silt fraction; however, C contents were highest in the clay fraction, independent of the plant cover. An increase in C associated with the sand fraction in the form of little decomposed organic residues was observed in pastures, confirming greater sensitivity of this fraction to change in soil use.

Respostas de pupunheiras (Bactris gasipaes Kunth) jovens ao alagamento

The objective of this work was to study the effects of waterlogged soil on the stomatal conductance, the relative water content, the chlorophyll content and on the N, P, K and soluble sugar concentrations of leaf, bulb and root tissues of young pijuayo palms ( Bactris gasipaes Kunth). Six month old age plants were submitted to flooding by continuous periods of seven, 14 and 21 days. Flooding induced the closure of the stomata, although the leaf tissues have maintained high relative water contents (about 90%). Root anoxia also induced reduction of the contents of total chlorophyll, organic N, P and mainly K in leaf tissues and significant reduction of the root biomass. In the flooded plants, the soluble sugar contents of the leaves, bulb and roots were higher than in the same tissues of the control plants. Although no death of plants have been detected to the end of the experimental period, these metabolic alterations allow to affirm that this species ( Bactris gasipaes Kunth) is sensi- tive to the root flooding.

Correlation but no causation between leaf nitrogen and maximum assimilation: The role of drought and reproduction in gas exchange in an understory tropical plant Miconia ciliata (Melastomataceae)

Alternative hypotheses were tested to explain a previously reported anomaly in the response of leaf photosynthetic capacity at light saturation (A(max)) in Miconia ciliata to dry-season irrigation. The anomaly is characterized by an abrupt increase in leaf A(max) for nonirrigated plants at the onset of the rainy season to values that significantly exceeded corresponding measurements for plants that were irrigated during the previous dry season. Hypothesis 1 posits that a pulse in leaf nitrogen increases CO(2) assimilation in nonirrigated plants at the onset of the wet season and is dampened for irrigated plants; this hypothesis was rejected because, although a wet-season nitrogen pulse did occur, it was identical for both irrigated and nonirrigated plants and was preceded by the increase in assimilation by nonirrigated plants. Hypothesis 2 posits that a reproduction-related, compensatory photosynthetic response occurs in nonirrigated plants following the onset of the wet season and is dampened in irrigated plants; consistent with hypothesis 2, high maximum assimilation rates for control plants in the wet season were significantly correlated with fruiting and flowering, whereas irrigation caused flowering and fruiting in the dry season, spreading M. ciliata reproductive activity in irrigated plants across the entire year.

Avaliação de indicadores de qualidade do solo sob alternativas de recuperação do solo no Nordeste Paraense

Os indicadores convencionais de fertilidade de solo, mais comumente utilizados, enfatizam o balanco acido-base e o conteudo de elementos quimicos, mas nao consideram adequadamente as propriedades biologicas do solo, responsaveis por grande parte do seu funcionamento. Os indicadores biologicos, ainda pouco difundidos, levam em consideracao a atividade microbiana do solo que e mais sensivel a dinâmica do solo. Em Marapanim-PA, e utilizando um desenho experimental inteiramente casualizado, com quatro repeticoes, avaliou-se o comportamento dos indicadores do solo carbono e nitrogenio da biomassa microbiana, fosfatase acida, materia orgânica, carbono orgânico e fosforo disponivel, em um solo com cultivo sequencial de mandioca e milho, preparo com corte-e-trituracao da capoeira original. Os fatores corte-e-trituracao, fosfato natural e adubacao verde foram arranjados nos seguintes tratamentos: capoeira; testemunha; fosfato natural (FN); feijao-de-porco [Canavalia ensiformis (L.) DC] (FP); FN + FP; FN + guandu [Cajanus cajan (L.) Millspaugh]. (G); FN + titonia [Tithonia diversifolia (Hemsley) A. Gray] (T); FP + G; FP + T. Em quatro epocas de amostragens, foram tiradas amostras compostas do solo (0 - 10 cm) de cada parcela para analise dos indicadores. Os indicadores estudados foram influenciados pela epoca de amostragem do solo. Os indicadores fosfatase acida, nitrogenio da biomassa microbiana e carbono da biomassa microbiana foram capazes de detectar os efeitos de maior numero de tratamentos de recuperacao do solo testados, enquanto a materia orgânica do solo foi o menos capaz. Ao longo do tempo, a resposta dos indicadores variou em funcao dos fatores estudados.

Técnicas avançadas de sensoriamento remoto aplicadas ao estudo de mudanças climáticas e ao funcionamento dos ecossistemas amazônicos

This paper aims to assess the contribution of remote sensing technology in addressing key questions raised by the Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA). The answers to these questions foster the knowledge on the climatic, biogechemical and hydrologic functioning of the Amazon, as well as on the impact of human activities at regional and global scales. Remote sensing methods allow integrating information on several processes at different temporal and spatial scales. By doing so, it is possible to perceive hidden relations among processes and structures, enhancing their teleconnections. Key advances in the remote sensing science are summarized in this article, which is particularly focused on information that would not be possible to be retrieved without the concurrence of this technology.