Search

This study investigates the rain-initiated mixing and variability in the mixing ratio of selected trace gases in the atmosphere over the central Amazon rain forest. It builds on comprehensive data from the Amazon Tall Tower Observatory (ATTO), spanning from 2013 to 2020 and comprising the greenhouse gases (GHGs) carbon dioxide (CO2) and methane (CH4); the reactive trace gases carbon monoxide (CO), ozone (O3), nitric oxide (NO), and nitrogen dioxide (NO2); and selected volatile organic compounds (VOCs). Based on more than 1000 analyzed rainfall events, the study resolves the trace gas mixing ratio patterns before, during, and after the rain events, along with vertical mixing ratio gradients across the forest canopy. The assessment of the rainfall events was conducted independently for daytime and nighttime periods, which allows us to elucidate the influence of solar radiation. The mixing ratios of CO2 , CO , and CH4 clearly declined during rainfall, which can be attributed to the downdraft-related entrainment of pristine air from higher altitudes into the boundary layer, a reduction of the photosynthetic activity under increased cloud cover, and changes in the surface fluxes. Notably, CO showed a faster reduction than CO2 , and the vertical gradient of CO2 and CO is steeper than for CH4. Conversely, the O3 mixing ratio increased across all measurement heights in the course of the rain-related downdrafts. Following the O3 enhancement by up to a factor of 2, NO , NO2 , and isoprene mixing ratios decreased. The temporal and vertical variability of the trace gases is intricately linked to the diverse sink and source processes, surface fluxes, and free-troposphere transport. Within the canopy, several interactions unfold among soil, atmosphere, and plants, shaping the overall dynamics. Also, the mixing ratio of biogenic VOCs (BVOCs) clearly varied with rainfall, driven by factors such as light, temperature, physical transport, and soil processes. Our results disentangle the patterns in the trace gas mixing ratio in the course of sudden and vigorous atmospheric mixing during rainfall events. By selectively uncovering processes that are not clearly detectable under undisturbed conditions, our results contribute to a better understanding of the trace gas life cycle and its interplay with meteorology, cloud dynamics, and rainfall in the Amazon. [ABSTRACT FROM AUTHOR]

This study investigates the rain-initiated mixing and variability in the concentration of selected trace gases in the atmosphere over the central Amazon rain forest. It builds on comprehensive data from the Amazon Tall Tower Observatory (ATTO), spanning from 2013 to 2020 and comprising the greenhouse gases (GHG) carbon dioxide (CO2) and methane (CH4), the reactive trace gases carbon monoxide (CO), ozone (O3), nitric oxide (NO), and nitrogen dioxide NO2 (NO2) as well as selected volatile organic compounds (VOC). Based on more than 1000 analyzed rainfall incidents, the study resolves the trace gas concentration patterns before, during, and after the rain events, along with its vertical concentration gradients across the forest canopy. The assessment of the rainfall events was conducted independently for daytime and nighttime periods, which allows us to elucidate the influence of solar radiation. The concentrations of CO2, CO, and CH4 clearly declined during rainfall, which can be attributed to the downdraft-related entrainment of pristine air from higher altitudes into the boundary layer, a reduction of the photosynthetic activity under increased cloud cover, as well as changes in the surface fluxes. Notably, CO showed a faster reduction than CO2, and the vertical gradient of CO2 and CO is steeper than for CH4. Conversely, the O3 concentration increased across all measurement heights in the course of the rain-related downdrafts. Following the O3 enhancement by up to a factor of two, NO and VOC concentrations decreased, whereas NO2 increased. The temporal and vertical variability of the trace gases is intricately linked to the diverse sink and source processes, surface fluxes, and free troposphere transport. Within the canopy, several interactions unfold among soil, atmosphere, and plants, shaping the overall dynamics. Also, the concentration of biogenic VOC (BVOC) clearly varied with rainfall, driven by factors such as light, temperature, physical transport, and soil processes. Our results disentangle the patterns in trace gas concentration in the course of the sudden and vigorous atmospheric mixing during rainfall events. By selectively uncovering processes that are not clearly detectable under undisturbed conditions, our results contribute to a better understanding of the trace gas life cycle and its interplay with meteorology, cloud dynamics, and rainfall in the Amazon and beyond. [ABSTRACT FROM AUTHOR]

Summary: Soil nutrient availability can strongly affect root traits. In tropical forests, phosphorus (P) is often considered the main limiting nutrient for plants. However, support for the P paradigm is limited, and N and cations might also control tropical forests functioning.We used a large‐scale experiment to determine how the factorial addition of nitrogen (N), P and cations affected root productivity and traits related to nutrient acquisition strategies (morphological traits, phosphatase activity, arbuscular mycorrhizal colonisation and nutrient contents) in a primary rainforest growing on low‐fertility soils in Central Amazonia after 1 yr of fertilisation.Multiple root traits and productivity were affected. Phosphorus additions increased annual root productivity and root diameter, but decreased root phosphatase activity. Cation additions increased root productivity at certain times of year, also increasing root diameter and mycorrhizal colonisation. P and cation additions increased their element concentrations in root tissues. No responses were detected with N addition.Here we showed that rock‐derived nutrients determined root functioning in low‐fertility Amazonian soils, demonstrating not only the hypothesised importance of P, but also highlighting the role of cations. The changes in fine root traits and productivity indicated that even slow‐growing tropical rainforests can respond rapidly to changes in resource availability. [ABSTRACT FROM AUTHOR]

In the case of tropical forest ecosystems, because overall nutrient stocks are often greater in the aboveground biomass than in soils (Paiva et al. [43]), microbial processes are fundamental to maintain ecosystem processes, and they do so by interacting with soils and plants (Moore et al. [40]; Milton and Kaspari [39]; Camenzind et al. [8]). Plants and soils interact in multiple ways, with nutrient-poor soils often selecting for plant species with conservative strategies, and rich soils selecting for plants with acquisitive strategies (Díaz et al. [17]). Two-way interactions can form feedbacks that influence ecosystem dynamics, for instance, when forest litter transfers nutrients to soils and those soils influence forest growth (e.g., Paiva et al. [43]). Human activities reshaping plant-soil interactions The evolutionary history of plant-soil interactions in the Amazon has contributed to shape how current ecosystems function (de Souza et al. [16]). [Extracted from the article]

Tropical rainforests harbor exceptionally high biodiversity and store large amounts of carbon in vegetation biomass. However, regional variation in plant species richness and vegetation carbon stock can be substantial, and may be related to the heterogeneity of topoedaphic properties. Therefore, aboveground vegetation carbon storage typically differs between geographic forest regions in association with the locally dominant plant functional group. A better understanding of the underlying factors controlling tropical forest diversity and vegetation carbon storage could be critical for predicting tropical carbon sink strength in response to projected climate change. Based on regionally replicated 1-ha forest inventory plots established in a region of high geomorphological heterogeneity we investigated how climatic and edaphic factors affect tropical forest diversity and vegetation carbon storage. Plant species richness (of all living stems >10 cm in diameter) ranged from 69 to 127 ha−1 and vegetation carbon storage ranged from 114 to 200 t ha−1. While plant species richness was controlled by climate and soil water availability, vegetation carbon storage was strongly related to wood density and soil phosphorus availability. Results suggest that local heterogeneity in resource availability and plant functional composition should be considered to improve projections of tropical forest ecosystem functioning under future scenarios. [ABSTRACT FROM AUTHOR]

Background:It has been proposed that the interaction between herbivory and soil nutrient availability drives habitat association of tree species in Peruvian Amazonia. Nevertheless, there is no empirical evidence that this interaction holds across other Amazonian regions. Aims:We address this knowledge gap by testing whether the interaction between herbivory and soil nutrient contributes to habitat association of tree species in white-sand and terra-firme forests in the upper Rio Negro, Brazil. Methods:We conducted a reciprocal transplanting field experiment in which we controlled for the presence of herbivores. We tested for differences in tree-seedling growth and herbivory among seven white-sand and seven terra-firme habitat-specialist species. Additionally, we assessed whether tree seedlings differed in their functional traits. Results:We found no empirical evidence that an interaction between herbivory and soil nutrients shapes habitat association in white-sand and terra-firme forests of the upper Rio Negro. Tree seedlings showed higher mortality when growing in their non-typical habitat. Growth and herbivory were similar regardless of the presence or absence of herbivore protection and type of soil. Conclusions:We suggest that the overall differences in soil nutrient status between white-sand and terra-firme forests in the upper Rio Negro are insufficient to trigger an interaction between herbivory and soil nutrient availability. [ABSTRACT FROM PUBLISHER]

The allocation and cycling of carbon (C) within forests is an important component of the biospheric C cycle, but is particularly understudied within tropical forests. We synthesise reported and unpublished results from three lowland rainforest sites in Amazonia (in the regions of Manaus, Tapajós and Caxiuanã), all major sites of the Large-Scale Biosphere–Atmosphere Programme (LBA). We attempt a comprehensive synthesis of the C stocks, nutrient status and, particularly, the allocation and internal C dynamics of all three sites. The calculated net primary productivities (NPP) are 10.1±1.4 Mg C ha−1 yr−1 (Manaus), 14.4±1.3 Mg C ha−1 yr−1 (Tapajós) and 10.0±1.2 Mg C ha−1 yr−1 (Caxiuanã). All errors bars report standard errors. Soil and leaf nutrient analyses indicate that Tapajós has significantly more plant-available phosphorus and calcium. Autotrophic respiration at all three sites (14.9–21.4 Mg C ha yr−1) is more challenging to measure, with the largest component and greatest source of uncertainty being leaf dark respiration. Comparison of measured soil respiration with that predicted from C cycling measurements provides an independent constraint. It shows general good agreement at all three sites, with perhaps some evidence for measured soil respiration being less than expected. Twenty to thirty percent of fixed C is allocated belowground. Comparison of gross primary productivity (GPP), derived from ecosystem flux measurements with that derived from component studies (NPP plus autotrophic respiration) provides an additional crosscheck. The two approaches are in good agreement, giving increased confidence in both approaches to estimating GPP. The ecosystem carbon-use efficiency (CUEs), the ratio of NPP to GPP, is similar at Manaus (0.34±0.10) and Caxiuanã (0.32±0.07), but may be higher at Tapajós (0.49±0.16), although the difference is not significant. Old growth or infertile tropical forests may have low CUE compared with recently disturbed and/or fertile forests. [ABSTRACT FROM AUTHOR]

The biomass of tropical forests plays an important role in the global carbon cycle, both as a dynamic reservoir of carbon, and as a source of carbon dioxide to the atmosphere in areas undergoing deforestation. However, the absolute magnitude and environmental determinants of tropical forest biomass are still poorly understood. Here, we present a new synthesis and interpolation of the basal area and aboveground live biomass of old-growth lowland tropical forests across South America, based on data from 227 forest plots, many previously unpublished. Forest biomass was analyzed in terms of two uncorrelated factors: basal area and mean wood density. Basal area is strongly affected by local landscape factors, but is relatively invariant at regional scale in moist tropical forests, and declines significantly at the dry periphery of the forest zone. Mean wood density is inversely correlated with forest dynamics, being lower in the dynamic forests of western Amazonia and high in the slow-growing forests of eastern Amazonia. The combination of these two factors results in biomass being highest in the moderately seasonal, slow growing forests of central Amazonia and the Guyanas (up to 350 Mg dry weight ha−1) and declining to 200–250 Mg dry weight ha−1 at the western, southern and eastern margins. Overall, we estimate the total aboveground live biomass of intact Amazonian rainforests (area 5.76 × 106 km2 in 2000) to be 93±23 Pg C, taking into account lianas and small trees. Including dead biomass and belowground biomass would increase this value by approximately 10% and 21%, respectively, but the spatial variation of these additional terms still needs to be quantified. [ABSTRACT FROM AUTHOR]

Low nutrient availability in large parts of the Amazon rainforest may prevent forest growth in response to elevated atmospheric carbon dioxide. Greater allocation of carbon belowground could however stimulate plant nutrient acquisition and alleviate this constraint. The ground litter layer is the main source of nutrients and fine roots are heavily exploring this layer. Therefore, we hypothesized that increased root growth in the litter layer could stimulate leaf litter decomposition and nutrient release, both by physical effect on litter fragmentation and by biochemical interactions, such as extracellular enzyme activity and labile carbon exudation. We carried out a leaf litter bag experiment (188 days) in a nutrient impoverished humid tropical rainforest in Central Amazonia to investigate: 1) the effect of roots, and 2) the effect of simulated root labile carbon inputs, on decomposition and nutrient release, and their interaction with microorganisms. We combined a root exclusion treatment and a labile carbon fertilization treatment with two different low molecular weight carbon compounds (citric acid; glucose) and analyzed the litter mass for carbon and nutrients (nitrogen, phosphorus, and cations), microbial carbon:nitrogen:phosphorus, and extracellular enzyme activity. Contrary to our expectation, root exclusion and labile carbon inputs had no significant effect on leaf litter mass loss. Although mass loss was not affected, the presence of roots resulted in significant direct release of phosphorus and cations from the litter. On the other hand, labile carbon addition only influenced the release of potassium under citric acid inputs, which inhibited potassium release. We found no significant effect of roots and labile carbon addition on microbial carbon:nitrogen:phosphorus. Phosphatase activity was significantly reduced due to root exclusion by 24%. We show that root growth in the litter layer is an effective mechanism for direct nutrient release by plants, which occurred without affecting the litter mass loss rate. Our results further indicate that phosphatase exudation is likely a very effective plant-based mechanism for phosphorus release from litter. We suggest that labile carbon exudation is an ineffective way of acquiring nutrients since litter microbial activity was not limited by carbon. We conclude that higher allocation of carbon to root production and phosphatase exudation in the litter layer could liberate more phosphorus and cations for plant uptake under increasing atmospheric carbon dioxide. [ABSTRACT FROM AUTHOR]

Amazon forests are a key but poorly understood component of the global carbon cycle. If, as anticipated, they dry this century, they might accelerate climate change through carbon losses and changed surface energy balances. We used records from multiple long-term monitoring plots across Amazonia to assess forest responses to the intense 2005 drought, a possible analog of future events. Affected forest lost biomass, reversing a large long-term carbon sink, with the greatest impacts observed where the dry season was unusually intense. Relative to pre-2005 conditions, forest subjected to a 100-millimeter increase in water deficit lost 5.3 megagrams of aboveground biomass of carbon per hectare. The drought had a total biomass carbon impact of 1.2 to 1.6 petagrams (1.2 x 1015 to 1.6 x 1015 grams). Amazon forests therefore appear vulnerable to increasing moisture stress, with the potential for large carbon losses to exert feedback on climate change. [ABSTRACT FROM AUTHOR]

The majority of forests across the Amazon basin grow in low fertility soils and plants might use a wide range of above and belowground mechanisms to overcome nutrient limitation. Phosphorus is hypothesised to be the main nutrient limiting tropical forest productivity, but more recent evidence suggests that multiple nutrients could regulate forest functioning. Belowground, root functional traits usually represent a balance between maximising the acquisition of limiting resources and minimising root tissue construction and maintenance. On low-fertility sites, plant economic theory predicts that if the supply of the limiting nutrient in soils is increased, plant investment in root biomass and nutrient uptake strategies should decrease. To test this, we investigated the response of fine root properties to the addition of nutrients in slow-growing primary rainforest established on low fertility soils in central Amazon. Using the ingrowth core method, we sampled young fine roots (<2 mm diameter) and measured root morphological traits (root diameter, specific root length, specific root area and root tissue density), root biomass production, root phosphatase enzyme activity and root mycorrhizal colonisation in 32 plots after seven months of phosphorus, nitrogen (N) and cations addition in a fully factorial design. We hypothesised that the addition of P would reduce root productivity, phosphatase activity and mycorrhizal colonisation, with root morphology shifting as a sign of alleviation of P limitation by increasing root diameter and tissue density and decreasing specific length and area. We also hypothesised that the addition of N and/or cations would exacerbate P limitation and therefore root productivity and phosphatase exudation would increase, with morphological traits changing towards finer and longer roots. Contrary to expectations, root productivity in the first seven months post-fertilisation was >50% higher in plots where cations were added, with no effects of P or N addition observed. The addition of cations and P increased root diameter, mainly for the 0-10 cm soil layer, with no significant effects on other root morphological traits. As predicted, root phosphatase activity strongly decreased with P addition, indicating alleviation of P limitation. Mycorrhizal colonisation, on the other hand, increased with P addition, suggesting a potential shift in P-uptake strategies, from mining (e.g. release of phosphatases) to foraging root traits (e.g. association with mycorrhizas). These results support the hypothesis that P limits some aspects of plant functioning in this central Amazon forest, but also suggest that cations could play an important role in controlling fine root production and the expression of root traits. Multiple nutrients may limit belowground processes in central Amazon forests and even slow-growing tropical rainforest can respond rapidly to changes in soil nutrient availability. These results therefore shed new light on the belowground mechanisms by which tropical forests thrive under low soil fertility, and contribute to the aim of better understanding Amazon forest functioning under current and future climate. [ABSTRACT FROM AUTHOR]