Efeitos da seca sobre os estoques de carbono no solo e suas frações em um experimento de exclusão de chuva na Amazônia oriental

During the last decades, the understanding of drought impacts became paramount to the understanding of environmental science in the Amazon. Despite of potential large effect of drought on the structure and function of the Amazonian forests, its effects are still poorly understood. Soil carbon stocks are very sensitive to changes in environmental conditions, such as climate alterations, changes in vegetation productivity or land use change. In that context, experiments have been carried out in the Amazon effectively excluding a large fraction of rainfall from forested areas for periods varying from 7 to 10 years. In one of these experiments, I evaluated the influence of extreme drought over the soil carbon stocks up to 2 m deep, using an intensive soil sampling regime for soil C stock determinations, followed by the fractionation of soil organic matter in two contiguous plots in Eastern Amazonia, with one being subjected to the exclusion of ca. 50% of its natural rainfall for the last 10 years and the other being a control plot with natural conditions. For organic matter fractions have been determined. To improve statistical confidence, 40 soil profiles were sampled in each plot and analyzed for soil C content. Profiles were subsequently separated into five groups in each plot, according to the pattern observed in the vertical soil C profile. Similar profiles were used to create bulk samples (per depth) to be analyzed for C fractions and a weighted average was made taking into account the frequencies of each group in each plot. The soil C stock was higher at the drought plot with 106.1 ± 1.8 Mg. C/ha, in comparison to the control plot with 100.6 ± 2.5 Mg C/ha, with the difference being marginally significant at 7% probability (T test: t = 1.82, p=0.07). However, differences were highly significant if compared by soil depth, with the layer from 20 to 100 cm deep corresponding to most of the difference in C stocks between the plots. Approximately 82% of the difference in soil C stocks can be attributed to recently added particulate organic matter (light fraction). The evaluation of changes in the different components of forest carbon cycle suggest that the observed differences in soil C stock between the areas may be associated to changes in root turnover on the drought plot.Durante as últimas décadas, os possíveis impactos da seca se tornaram primordiais para o entendimento sobre a ciência ambiental na Amazônia. Os efeitos da seca sobre as funções e estrutura do ecossistema na Amazônia são potencialmente grandes, mas permanecem mal definidos. Os estoques de C nos solos são muito sensíveis às mudanças em condições ambientais, como alterações de clima, produtividade vegetal ou mudança no uso da terra e experimentos conduzidos na Amazônia tem tentado excluir uma grande quantidade de chuvas em áreas florestadas em períodos de 7-10 anos. Dentro do escopo de um destes experimentos, foi avaliada a influência de seca extrema simulada na Amazônia nas alterações do estoque de carbono no solo até dois metros de profundidade através de uma amostragem intensiva de solo e fracionamento da matéria orgânica em duas parcelas de 1 hectare, sendo uma controle e uma submetida a exclusão de aproximadamente 50% da chuva. Quatro frações da matéria orgânica do solo que compõem os reservatórios ativos e lentos foram separadas. Para melhor representar a área de estudo, 40 perfis de solo foram coletados e analisados para carbono, sendo então organizados em cinco grupos (grupos O, X, I, II e III) de acordo com a variância estatística da concentração do C observada no perfil vertical do solo nas duas áreas de pesquisa. Em seguida, perfiz semelhantes foram agrupados e amostras compostas foram utilizadas para o fracionamento da matéria orgânica, sendo feita uma média ponderada levando em conta a frequência de cada grupo. Em relação ao estoque de carbono no perfil amostrado (0-200 cm), percebeu-se que o estoque médio de C é maior na parcela seca, com um valor de 106,1 ± 1,8 Mg. C/ha, tendo a parcela de controle um estoque 100,6 ± 2,5 Mg C/ha, sendo encontrada diferença marginalmente significativa (probabilidade de 7%) entre elas (T test: t = 1,82, p=0,07). Observa-se, porém, que as diferenças nos estoques foram significativas se observadas por profundidade, com a camada entre 20-100 cm correspondendo a maior parte da diferença. Aproximadamente 82% da diferença entre os estoques pode ser atribuido a adição de novo material particulado ao solo (fração leve livre). A avaliação das mudanças sofridas pelos diferentes componentes do ciclo de carbono florestal sugerem que as alterações observadas nos estoques de C entre as áreas esteja relacionada a uma maior taxa de reposição de raízes no solo da parcela submetida à seca

In situ short-term responses of Amazonian understory plants to elevated CO₂

The response of plants to increasing atmospheric CO2 depends on the ecological context where the plants are found. Several experiments with elevated CO2 (eCO2) have been done worldwide, but the Amazonian forest understory has been neglected. As the central Amazon is limited by light and phosphorus, understanding how understory responds to eCO2 is important for foreseeing how the forest will function in the future. In the understory of a natural forest in the Central Amazon, we installed four open-top chambers as control replicates and another four under eCO2 (+250 ppm above ambient levels). Under eCO2, we observed increases in carbon assimilation rate (67%), maximum electron transport rate (19%), quantum yield (56%), and water use efficiency (78%). We also detected an increase in leaf area (51%) and stem diameter increment (65%). Central Amazon understory responded positively to eCO2 by increasing their ability to capture and use light and the extra primary productivity was allocated to supporting more leaf and conducting tissues. The increment in leaf area while maintaining transpiration rates suggests that the understory will increase its contribution to evapotranspiration. Therefore, this forest might be less resistant in the future to extreme drought, as no reduction in transpiration rates were detected.</p

In situ short-term responses of Amazonian understory plants to elevated CO₂

The response of plants to increasing atmospheric CO2 depends on the ecological context where the plants are found. Several experiments with elevated CO2 (eCO2) have been done worldwide, but the Amazonian forest understory has been neglected. As the central Amazon is limited by light and phosphorus, understanding how understory responds to eCO2 is important for foreseeing how the forest will function in the future. In the understory of a natural forest in the Central Amazon, we installed four open-top chambers as control replicates and another four under eCO2 (+250 ppm above ambient levels). Under eCO2, we observed increases in carbon assimilation rate (67%), maximum electron transport rate (19%), quantum yield (56%), and water use efficiency (78%). We also detected an increase in leaf area (51%) and stem diameter increment (65%). Central Amazon understory responded positively to eCO2 by increasing their ability to capture and use light and the extra primary productivity was allocated to supporting more leaf and conducting tissues. The increment in leaf area while maintaining transpiration rates suggests that the understory will increase its contribution to evapotranspiration. Therefore, this forest might be less resistant in the future to extreme drought, as no reduction in transpiration rates were detected.</p

Direct evidence for phosphorus limitation on Amazon forest productivity

The productivity of rainforests growing on highly weathered tropical soils is expected to be limited by phosphorus availability1. Yet, controlled fertilization experiments have been unable to demonstrate a dominant role for phosphorus in controlling tropical forest net primary productivity. Recent syntheses have demonstrated that responses to nitrogen addition are as large as to phosphorus2, and adaptations to low phosphorus availability appear to enable net primary productivity to be maintained across major soil phosphorus gradients3. Thus, the extent to which phosphorus availability limits tropical forest productivity is highly uncertain. The majority of the Amazonia, however, is characterized by soils that are more depleted in phosphorus than those in which most tropical fertilization experiments have taken place2. Thus, we established a phosphorus, nitrogen and base cation addition experiment in an old growth Amazon rainforest, with a low soil phosphorus content that is representative of approximately 60% of the Amazon basin. Here we show that net primary productivity increased exclusively with phosphorus addition. After 2 years, strong responses were observed in fine root (+29%) and canopy productivity (+19%), but not stem growth. The direct evidence of phosphorus limitation of net primary productivity suggests that phosphorus availability may restrict Amazon forest responses to CO2 fertilization4, with major implications for future carbon sequestration and forest resilience to climate change.The authors acknowledge funding from the UK Natural Environment Research Council (NERC), grant number NE/L007223/1. This is publication 850 in the technical series of the BDFFP. C.A.Q. acknowledges the grants from Brazilian National Council for Scientific and Technological Development (CNPq) CNPq/LBA 68/2013, CNPq/MCTI/FNDCT no. 18/2021 and his productivity grant. C.A.Q., H.F.V.C., F.D.S., I.A., L.F.L., E.O.M. and S.G. acknowledge the AmazonFACE programme for financial support in cooperation with Coordination for the Improvement of Higher Education Personnel (CAPES) and the National Institute of Amazonian Research as part of the grants CAPES-INPA/88887.154643/2017-00 and 88881.154644/2017-01. T.F.D. acknowledges funds from FundacAo de Amparo a Pesquisa do Estado de SAo Paulo (FAPESP), grant 2015/50488-5, and the Partnership for Enhanced Engagement in Research (PEER) programme grant AID-OAA-A-11-00012. L.E.O.C.A. thanks CNPq (314416/2020-0)

Direct evidence for phosphorus limitation on Amazon forest productivity

The productivity of rainforests growing on highly weathered tropical soils is expected to be limited by phosphorus availability1. Yet, controlled fertilization experiments have been unable to demonstrate a dominant role for phosphorus in controlling tropical forest net primary productivity. Recent syntheses have demonstrated that responses to nitrogen addition are as large as to phosphorus2, and adaptations to low phosphorus availability appear to enable net primary productivity to be maintained across major soil phosphorus gradients3. Thus, the extent to which phosphorus availability limits tropical forest productivity is highly uncertain. The majority of the Amazonia, however, is characterized by soils that are more depleted in phosphorus than those in which most tropical fertilization experiments have taken place2. Thus, we established a phosphorus, nitrogen and base cation addition experiment in an old growth Amazon rainforest, with a low soil phosphorus content that is representative of approximately 60% of the Amazon basin. Here we show that net primary productivity increased exclusively with phosphorus addition. After 2 years, strong responses were observed in fine root (+29%) and canopy productivity (+19%), but not stem growth. The direct evidence of phosphorus limitation of net primary productivity suggests that phosphorus availability may restrict Amazon forest responses to CO2 fertilization4, with major implications for future carbon sequestration and forest resilience to climate change

Efeitos da seca sobre os estoques de carbono no solo e suas frações em um experimento de exclusão de chuva na Amazônia oriental

During the last decades, the understanding of drought impacts became paramount to the understanding of environmental science in the Amazon. Despite of potential large effect of drought on the structure and function of the Amazonian forests, its effects are still poorly understood. Soil carbon stocks are very sensitive to changes in environmental conditions, such as climate alterations, changes in vegetation productivity or land use change. In that context, experiments have been carried out in the Amazon effectively excluding a large fraction of rainfall from forested areas for periods varying from 7 to 10 years. In one of these experiments, I evaluated the influence of extreme drought over the soil carbon stocks up to 2 m deep, using an intensive soil sampling regime for soil C stock determinations, followed by the fractionation of soil organic matter in two contiguous plots in Eastern Amazonia, with one being subjected to the exclusion of ca. 50% of its natural rainfall for the last 10 years and the other being a control plot with natural conditions. For organic matter fractions have been determined. To improve statistical confidence, 40 soil profiles were sampled in each plot and analyzed for soil C content. Profiles were subsequently separated into five groups in each plot, according to the pattern observed in the vertical soil C profile. Similar profiles were used to create bulk samples (per depth) to be analyzed for C fractions and a weighted average was made taking into account the frequencies of each group in each plot. The soil C stock was higher at the drought plot with 106.1 ± 1.8 Mg. C/ha, in comparison to the control plot with 100.6 ± 2.5 Mg C/ha, with the difference being marginally significant at 7% probability (T test: t = 1.82, p=0.07). However, differences were highly significant if compared by soil depth, with the layer from 20 to 100 cm deep corresponding to most of the difference in C stocks between the plots. Approximately 82% of the difference in soil C stocks can be attributed to recently added particulate organic matter (light fraction). The evaluation of changes in the different components of forest carbon cycle suggest that the observed differences in soil C stock between the areas may be associated to changes in root turnover on the drought plot.Durante as últimas décadas, os possíveis impactos da seca se tornaram primordiais para o entendimento sobre a ciência ambiental na Amazônia. Os efeitos da seca sobre as funções e estrutura do ecossistema na Amazônia são potencialmente grandes, mas permanecem mal definidos. Os estoques de C nos solos são muito sensíveis às mudanças em condições ambientais, como alterações de clima, produtividade vegetal ou mudança no uso da terra e experimentos conduzidos na Amazônia tem tentado excluir uma grande quantidade de chuvas em áreas florestadas em períodos de 7-10 anos. Dentro do escopo de um destes experimentos, foi avaliada a influência de seca extrema simulada na Amazônia nas alterações do estoque de carbono no solo até dois metros de profundidade através de uma amostragem intensiva de solo e fracionamento da matéria orgânica em duas parcelas de 1 hectare, sendo uma controle e uma submetida a exclusão de aproximadamente 50% da chuva. Quatro frações da matéria orgânica do solo que compõem os reservatórios ativos e lentos foram separadas. Para melhor representar a área de estudo, 40 perfis de solo foram coletados e analisados para carbono, sendo então organizados em cinco grupos (grupos O, X, I, II e III) de acordo com a variância estatística da concentração do C observada no perfil vertical do solo nas duas áreas de pesquisa. Em seguida, perfiz semelhantes foram agrupados e amostras compostas foram utilizadas para o fracionamento da matéria orgânica, sendo feita uma média ponderada levando em conta a frequência de cada grupo. Em relação ao estoque de carbono no perfil amostrado (0-200 cm), percebeu-se que o estoque médio de C é maior na parcela seca, com um valor de 106,1 ± 1,8 Mg. C/ha, tendo a parcela de controle um estoque 100,6 ± 2,5 Mg C/ha, sendo encontrada diferença marginalmente significativa (probabilidade de 7%) entre elas (T test: t = 1,82, p=0,07). Observa-se, porém, que as diferenças nos estoques foram significativas se observadas por profundidade, com a camada entre 20-100 cm correspondendo a maior parte da diferença. Aproximadamente 82% da diferença entre os estoques pode ser atribuido a adição de novo material particulado ao solo (fração leve livre). A avaliação das mudanças sofridas pelos diferentes componentes do ciclo de carbono florestal sugerem que as alterações observadas nos estoques de C entre as áreas esteja relacionada a uma maior taxa de reposição de raízes no solo da parcela submetida à seca

The green ocean amazon experiment (GOAMAZON2014/5) observes pollution affecting gases, aerosols, clouds, and rainfall over the rain forest

The susceptibility of air quality, weather, terrestrial ecosystems, and climate to human activities was investigated in a tropical environment. © 2017 American Meteorological Society