No abstract

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 (N(2)O). 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.

27B razil has been unique worldwide in terms of land use. Although vast areas of forests and savannahs have been converted into farmland (Fig. 1) -placing the country as a leading global producer of agricultural commodities -it still safeguards the largest tracts of native tropical vegetation on Earth, with extremely high levels of biodiversity. Patterns of land use change, which until recently exhibited the highest worldwide absolute rates of tropical deforestation, largely resulted in low-productivity cattle pastures 2 . Moreover, climate change issues in Brazil are inextricably related to land use and land-use change (LUC) as approximately 80% of the country's total CO 2 -equivalent (CO 2 e) emissions in 2005 were sourced from agriculture and LUC 3 .Demand for farmland is the key immediate driver of LUC in Brazil, and there is little evidence that agricultural expansion is grinding to a halt 4-7 . In fact, Brazil holds the greatest potential for further agricultural expansion in the twenty-first century 8 . Understanding recent LUC patterns (Box 1) and visualizing a sustainable land-use pathway in Brazil have become highly strategic -not only for Brazilians, given that regional and global climate change, food and energy provision, and biodiversity conservation are all at stake. This Review presents an integrated analysis and provides new insights on recent trends in the Brazilian land-use system. In the first two sections we show how Brazil's agriculture is becoming both gradually decoupled from deforestation processes and increasingly intensified and oriented to large-scale farming of trade commodities throughout the country. Next we explain the economic and political factors driving those changes. The fourth section reveals the drawbacks of those changes in aggravating the long history of inequality in land ownership. We then explore repercussions for climate change, namely Agriculture, deforestation, greenhouse gas emissions and local/regional climate change have been closely intertwined in Brazil. Recent studies show that this relationship has been changing since the mid 2000s, with the burgeoning intensification and commoditization of Brazilian agriculture. On one hand, this accrues considerable environmental dividends including a pronounced reduction in deforestation (which is becoming decoupled from agricultural production), resulting in a decrease of ~40% in nationwide greenhouse gas emissions since 2005, and a potential cooling of the climate at the local scale. On the other hand, these changes in the land-use system further reinforce the long-established inequality in land ownership, contributing to rural-urban migration that ultimately fuels haphazard expansion of urban areas. We argue that strong enforcement of sector-oriented policies and solving long-standing land tenure problems, rather than simply waiting for market self-regulation, are key steps to buffer the detrimental effects of agricultural intensification at the forefront of a sustainable pathway for land use in Brazil.for the country's g...

[1] Large Amazonian rivers are known to emit substantial amounts of CO 2 to the atmosphere, while the magnitude of CO 2 degassing from small streams remains a major unknown in regional carbon budgets. We found that 77% of carbon transported by water from the landscape was as terrestrially-respired CO 2 dissolved within soils, over 90% of which evaded to the atmosphere within headwater reaches of streams. Hydrologic transport of dissolved CO 2 was equivalent to nearly half the gaseous CO 2 contributions from deep soil (>2 m) to respiration at the soil surface. Dissolved CO 2 in emergent groundwater was isotopically consistent with soil respiration, and demonstrated strong agreement with deep soil CO 2 concentrations and seasonal dynamics. During wet seasons, deep soil (2 -8 m) CO 2 concentration profiles indicated gaseous diffusion to deeper layers, thereby enhancing CO 2 drainage to streams. Groundwater discharge of CO 2 and its subsequent evasion is a significant conduit for terrestrially-respired carbon in tropical headwater catchments. Citation: Johnson, M. S.,

Following an intense occupation process that was initiated in the 1960s, deforestation rates in the Brazilian Amazon have decreased significantly since 2004, stabilizing around 6000 km(2) yr(-1) in the last 5 years. A convergence of conditions contributed to this, including the creation of protected areas, the use of effective monitoring systems, and credit restriction mechanisms. Nevertheless, other threats remain, including the rapidly expanding global markets for agricultural commodities, large-scale transportation and energy infrastructure projects, and weak institutions. We propose three updated qualitative and quantitative land-use scenarios for the Brazilian Amazon, including a normative 'Sustainability' scenario in which we envision major socio-economic, institutional, and environmental achievements in the region. We developed an innovative spatially explicit modelling approach capable of representing alternative pathways of the clear-cut deforestation, secondary vegetation dynamics, and the old-growth forest degradation. We use the computational models to estimate net deforestation-driven carbon emissions for the different scenarios. The region would become a sink of carbon after 2020 in a scenario of residual deforestation (~1000 km(2) yr(-1)) and a change in the current dynamics of the secondary vegetation - in a forest transition scenario. However, our results also show that the continuation of the current situation of relatively low deforestation rates and short life cycle of the secondary vegetation would maintain the region as a source of CO2 - even if a large portion of the deforested area is covered by secondary vegetation. In relation to the old-growth forest degradation process, we estimated average gross emission corresponding to 47% of the clear-cut deforestation from 2007 to 2013 (using the DEGRAD system data), although the aggregate effects of the postdisturbance regeneration can partially offset these emissions. Both processes (secondary vegetation and forest degradation) need to be better understood as they potentially will play a decisive role in the future regional carbon balance.

Tropical forests harbor a significant portion of global biodiversity and are a critical component of the climate system. Reducing deforestation and forest degradation contributes to global climate-change mitigation efforts, yet emissions and removals from forest dynamics are still poorly quantified. We reviewed the main challenges to estimate changes in carbon stocks and biodiversity due to degradation and recovery of tropical forests, focusing on three main areas: (1) the combination of field surveys and remote sensing; (2) evaluation of biodiversity and carbon values under a unified strategy; and (3) research efforts needed to understand and quantify forest degradation and recovery. The improvement of models and estimates of changes of forest carbon can foster process-oriented monitoring of forest dynamics, including different variables and using spatially explicit algorithms that account for regional and local differences, such as variation in climate, soil, nutrient content, topography, biodiversity, disturbance history, recovery pathways, and socioeconomic factors. Generating the data for these models requires affordable large-scale remote-sensing tools associated with a robust network of field plots that can generate spatially explicit information on a range of variables through time. By combining ecosystem models, multiscale remote sensing, and networks of field plots, we will be able to evaluate forest degradation and recovery and their interactions with biodiversity and carbon cycling. Improving monitoring strategies will allow a better understanding of the role of forest dynamics in climate-change mitigation, adaptation, and carbon cycle feedbacks, thereby reducing uncertainties in models of the key processes in the carbon cycle, including their impacts on biodiversity, which are fundamental to support forest governance policies, such as Reducing Emissions from Deforestation and Forest Degradation.

Carbon and nitrogen isotope ratios of human fingernails were measured in 490 individuals in the western US and 273 individuals in southeastern Brazil living in urban areas, and 53 individuals living in a moderately isolated area in the central Amazon region of Brazil and consuming mostly locally grown foods. In addition, we measured the carbon and nitrogen isotope ratios of common food items to assess the extent to which these isotopic signatures remain distinct for people eating both omnivorous and vegetarian diets and living in different parts of the world, and the extent to which dietary information can be interpreted from these analyses. Fingernail delta13C values (mean +/- standard deviation) were -15.4 +/- 1.0 and -18.8 +/- 0.8 per thousand and delta15N values were 10.4 +/- 0.7 and 9.4 +/- 0.6 per thousand for southeastern Brazil and western US populations, respectively. Despite opportunities for a "global supermarket" effect to swamp out carbon and nitrogen isotope ratios in these two urbanized regions of the world, differences in the fingernail isotope ratios between southeastern Brazil and western US populations persisted, and appeared to be more associated with regional agricultural and animal production practices. Omnivores and vegetarians from Brazil and the US were isotopically distinct, both within and between regions. In a comparison of fingernails of individuals from an urban city and isolated communities in the Amazonian region, the urban region was similar to southeastern Brazil, whereas individuals from isolated nonurban communities showed distinctive isotopic values consistent with their diets and with the isotopic values of local foods. Although there is a tendency for a "global supermarket" diet, carbon and nitrogen isotopes of human fingernails hold dietary information directly related to both food sources and dietary practices in a region.

Here we present the within-site, seasonal, and interannual variations of the carbon (d 13 C) and nitrogen (d 15 N) isotope ratios of leaves, wood, bark and litter from four sites in the Amazon region, Brazil. Samples were collected in Manaus (3°06¢07¢¢ S; 60°01¢30¢¢ W), Ji-Parana( 10°53¢07¢¢ S; 61°57¢06¢¢ W), and Santare´m (2°26¢35¢¢ S; 54°42¢30¢¢ W) with mean annual precipitation of 2207, 2040 and 1909 mm respectively. The overall average for all leaf samples wasÀ32:3 AE 2:5& for d 13 C andþ5:8 AE 1:6& for d 15 N (n=756). The leaf d values at these sites were often but not always statistically distinct from each other. The d 13 C values varied fromÀ37:8& toÀ25:9&. Pronounced differences in d 13 C values occurred with height associated with differences in forest structure. The d 13 C of leaf dry matter showed seasonal variations associated with the length of the dry season, despite the fact that total annual precipitation was similar among the studied sites. Leaf d 15 N values ranged fromþ0:9& to a maximum value ofþ10:9&, and the Santare´m sites showed more enriched values than Manaus and Ji-Parana´sites. No seasonal variation was detected in the d 15 N of leaves, but significant differences were observed among sites and with changes in canopy height. The isotope ratio data are consistent with our current understanding of the roles of light, water availability, and recycling of soil-respired CO 2 influences on d 13 C and consistent with our understanding that an open nitrogen cycle can lead to high d 15 N values despite a significant number of legumes in the vegetation.