| 1. |
- ter Steege, Hans, et al.
(författare)
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Mapping density, diversity and species-richness of the Amazon tree flora
- 2023
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Ingår i: COMMUNICATIONS BIOLOGY. - 2399-3642. ; 6:1
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Tidskriftsartikel (refereegranskat)abstract
- Using 2.046 botanically-inventoried tree plots across the largest tropical forest on Earth, we mapped tree species-diversity and tree species-richness at 0.1-degree resolution, and investigated drivers for diversity and richness. Using only location, stratified by forest type, as predictor, our spatial model, to the best of our knowledge, provides the most accurate map of tree diversity in Amazonia to date, explaining approximately 70% of the tree diversity and species-richness. Large soil-forest combinations determine a significant percentage of the variation in tree species-richness and tree alpha-diversity in Amazonian forest-plots. We suggest that the size and fragmentation of these systems drive their large-scale diversity patterns and hence local diversity. A model not using location but cumulative water deficit, tree density, and temperature seasonality explains 47% of the tree species-richness in the terra-firme forest in Amazonia. Over large areas across Amazonia, residuals of this relationship are small and poorly spatially structured, suggesting that much of the residual variation may be local. The Guyana Shield area has consistently negative residuals, showing that this area has lower tree species-richness than expected by our models. We provide extensive plot meta-data, including tree density, tree alpha-diversity and tree species-richness results and gridded maps at 0.1-degree resolution. A study mapping the tree species richness in Amazonian forests shows that soil type exerts a strong effect on species richness, probably caused by the areas of these forest types. Cumulative water deficit, tree density and temperature seasonality affect species richness at a regional scale.
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| 2. |
- Lundgren, Jens D, et al.
(författare)
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Long-Term Benefits from Early Antiretroviral Therapy Initiation in HIV Infection.
- 2023
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Ingår i: NEJM evidence. - : Massachusetts Medical Society. - 2766-5526. ; 2:3
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Tidskriftsartikel (refereegranskat)abstract
- For people with HIV and CD4+ counts >500 cells/mm3, early initiation of antiretroviral therapy (ART) reduces serious AIDS and serious non-AIDS (SNA) risk compared with deferral of treatment until CD4+ counts are <350 cells/mm3. Whether excess risk of AIDS and SNA persists once ART is initiated for those who defer treatment is uncertain.The Strategic Timing of AntiRetroviral Treatment (START) trial, as previously reported, randomly assigned 4684 ART-naive HIV-positive adults with CD4+ counts .500 cells/mm3 to immediate treatment initiation after random assignment (n = 2325) or deferred treatment (n= 2359). In 2015, a 57% lower risk of the primary end point (AIDS, SNA, or death) for the immediate group was reported, and the deferred group was offered ART. This article reports the follow-up that continued to December 31, 2021. Cox proportional-hazards models were used to compare hazard ratios for the primary end point from randomization through December 31, 2015, versus January 1, 2016, through December 31, 2021.Through December 31, 2015, approximately 7 months after the cutoff date from the previous report, the median CD4+ count was 648 and 460 cells/mm3 in the immediate and deferred groups, respectively, at treatment initiation. The percentage of follow-up time spent taking ART was 95% and 36% for the immediate and deferred groups, respectively, and the time-averaged CD4+ difference was 199 cells/mm3. After January 1, 2016, the percentage of follow-up time on treatment was 97.2% and 94.1% for the immediate and deferred groups, respectively, and the CD4+ count difference was 155 cells/mm3. After January 1, 2016, a total of 89 immediate and 113 deferred group participants experienced a primary end point (hazard ratio of 0.79 [95% confidence interval, 0.60 to 1.04] versus hazard ratio of 0.47 [95% confidence interval, 0.34 to 0.65; P<0.001]) before 2016 (P=0.02 for hazard ratio difference).Among adults with CD4+ counts >500 cells/mm3, excess risk of AIDS and SNA associated with delaying treatment initiation was diminished after ART initiation, but persistent excess risk remained. (Funded by the National Institute of Allergy and Infectious Diseases and others.).
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| 3. |
- Sierra, Carlos
(författare)
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How well does ramped thermal oxidation quantify the age distribution of soilcarbon? Assessing thermal stability of physically and chemicallyfractionated soil organic matter
- 2023
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Ingår i: Biogeosciences. - 1726-4170 .- 1726-4189. ; 20, s. 3151-3163
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Tidskriftsartikel (refereegranskat)abstract
- Carbon (C) in soils persists on a range of timescales depending on physical, chemical, and biological processes that interact with soil organic matter (SOM) and affect its rate of decomposition. Together these processes determine the age distribution of soil C. Most attempts to measure this age distribution have relied on operationally defined fractions using properties like density, aggregate stability, solubility, or chemical reactivity. Recently, thermal fractionation, which relies on the activation energy needed to combust SOM, has shown promise for separating young from old C by applying increasing heat to decompose SOM. Here, we investigated radiocarbon (C-14) and C-13 of C released during thermal fractionation to link activation energy to the age distribution of C in bulk soil and components previously separated by density and chemical properties. While physically and chemically isolated fractions had very distinct mean C-14 values, they contributed C across the full temperature range during thermal analysis. Thus, each thermal fraction collected during combustion of bulk soil integrates contributions from younger and older C derived from components having different physical and chemical properties but the same activation energy. Bulk soil and all density and chemical fractions released progressively older and more C-13-enriched C with increasing activation energy, indicating that each operationally defined fraction itself was not homogeneous but contained a mix of C with different ages and degrees of microbial processing. Overall, we found that defining the full age distribution of C in bulk soil is best quantified by first separating particulate C prior to thermal fractionation of mineral-associated SOM. For the Podzol analyzed here, thermal fractions confirmed that similar to 95 % of the mineral-associated organic matter (MOM) had a relatively narrow C-14 distribution, while 5 % was very low in C-14 and likely reflected C from the < 2 mm parent shale material in the soil matrix. After first removing particulate C using density or size separation, thermal fractionation can provide a rapid technique to study the age structure of MOM and how it is influenced by different OM-mineral interactions.
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| 4. |
- Sierra, Carlos
(författare)
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Molecular C-14 evidence for contrasting turnover and temperature sensitivity of soil organic matter components
- 2023
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Ingår i: Ecology Letters. - : Wiley. - 1461-023X .- 1461-0248. ; 26, s. 778-788
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Tidskriftsartikel (refereegranskat)abstract
- Climate projection requires an accurate understanding for soil organic carbon (SOC) decomposition and its response to warming. An emergent view considers that environmental constraints rather than chemical structure alone control SOC turnover and its temperature sensitivity (i.e., Q(10)), but direct long-term evidence is lacking. Here, using compound-specific radiocarbon analysis of soil profiles along a 3300-km grassland transect, we provide direct evidence for the rapid turnover of lignin-derived phenols compared with slower-cycling molecular components of SOC (i.e., long-chain lipids and black carbon). Furthermore, in contrast to the slow-cycling components whose turnover is strongly modulated by mineral association and exhibits low Q(10), lignin turnover is mainly regulated by temperature and has a high Q(10). Such contrasts resemble those between fast-cycling (i.e., light) and mineral-associated slow-cycling fractions from globally distributed soils. Collectively, our results suggest that warming may greatly accelerate the decomposition of lignin, especially in soils with relatively weak mineral associations.
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| 5. |
- Sierra, Carlos, et al.
(författare)
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Recently fixed carbon fuels microbial activity several meters below the soilsurface
- 2023
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 20, s. 827-838
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Tidskriftsartikel (refereegranskat)abstract
- The deep soil, > 1 m, harbors a substantial share of the global microbial biomass. Currently, it is not known whether microbial activity several meters below the surface is fueled by recently fixed carbon or by old carbon that persisted in soil for several hundred years. Understanding the carbon source of microbial activity in deep soil is important to identify the drivers of biotic processes in the critical zone. Therefore, we explored carbon cycling in soils in three climate zones (arid, mediterranean, and humid) of the Coastal Cordillera of Chile down to a depth of 6 m, using carbon isotopes. Specifically, we determined the C-13 :C- 12 ratio (delta C-13) of soil and roots and the C-14 : C-12 ratio (delta C-14) of soil organic carbon and CO2-C respired by microorganisms. We found that the delta C-14 of the respired CO2-C was significantly higher than that of the soil organic carbon in all soils. Further, we found that the delta C-13 of the soil organic carbon changed only in the upper decimeters (by less than 6 parts per thousand ). Our results show that microbial activity several meters below the soil surface is mostly fueled by recently fixed carbon that is on average much younger than the total soil organic carbon present in the respective soil depth increments, in all three climate zones. Further, our results indicate that most decomposition that leads to enrichment of C-13 occurs in the upper decimeters of the soils, which is possibly due to stabilization of organic carbon in the deep soil. In conclusion, our study demonstrates that microbial processes in the deep soil several meters below the surface are closely tied to input of recently fixed carbon.
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| 6. |
- Sierra, Carlos
(författare)
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Tree growth periodicity in the ever-wet tropical forest of the Americas
- 2023
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Ingår i: Journal of Ecology. - : Wiley. - 0022-0477 .- 1365-2745. ; 111, s. 889-902
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Tidskriftsartikel (refereegranskat)abstract
- The occurrence of annual growth rings in tropical trees-the result of the seasonal activity of vascular cambium-has been explained by seasonal water deficit or flooding periods. However, little is known about the drivers of annual tree-ring formation under tropical hyper-humid conditions without clear seasonal dry periods or flooding (ever-wet conditions). Shelford's law states that the deficit or the excess of environmental resources limits plant growth. Accordingly, we hypothesize that excess soil moisture, a slight seasonal reduction of precipitation and a reduction in light availability determine rhythmic growth in ever-wet tropical forests.We first assessed the occurrence of rhythmic growth in 14 tree species from the Biogeographic Choco Region (annual rainfall 7200 mm) using three methods: Radiocarbon (C-14) dating (all studied species), tree-ring synchronization (4 species that have replicates) and automatic dendrometers (two species). Then, we assessed the effect of environmental drivers (rainfall, short-wave radiation, temperature and soil moisture) on tree growth based on tree ring and dendrometer observations.We present evidence of annual tree-ring formation in all 14 studied tree species. Depending on the tree species, we observed positive and negative correlations between growth, water availability and light availability. These relationships suggest that both excess or deficit of environmental resources may explain the seasonal pattern of tree growth. Although we cannot differentiate between excess soil water and low light availability by high cloudiness, we suggest that cloudiness frequency could affect tree growth in these forests.Synthesis. We reveal the annual formation of growth rings in the unexplored wetter-end tropical forests, where seasonal growth depends on either high soil moisture and hypoxia or light limitations by cloudiness and photosynthesis constraints.
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| 7. |
- Spohn, Marie, et al.
(författare)
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Continuous decrease in soil organic matter despite increased plant productivity in an 80-years-old phosphorus-addition experiment
- 2023
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Ingår i: Communications earth & environment. - 2662-4435. ; 4
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Tidskriftsartikel (refereegranskat)abstract
- The objective of this study was to understand how eight decades of tillage affected soil element dynamics. We measured soil chemical properties and the C-14:C-12 ratio (& UDelta;C-14) of organic carbon in one of the oldest cropland experiments in Europe with different levels of phosphorus addition. Soil total and organic phosphorus stocks in the uppermost 20 cm did not differ significantly between the control and the phosphorus addition treatments after 80 years, indicating plant phosphorus uptake from the subsoil. Crop yields increased from 220 g dry weight m(-2) in 1936 to more than 500 g dry weight m(-2) in the 2010s. The soil total organic carbon and total organic phosphorus stocks decreased by 13.7% and 11.6%, respectively, in the uppermost 20 cm of the soils during the experiment, irrespective of phosphorus addition. Based on modeling of & UDelta;C-14, we show that the mean transit time of carbon in the soil was below 10 years, indicating that a large share of the carbon inputs to soil is quickly respired. Our results suggest that the current agricultural practice at this long-term experiment is not sustainable because it led to a continuous decrease in soil organic matter over the last decades, despite increases in plant productivity.Several decades of tillage drives continual decreases in soil organic carbon, nitrogen and organic phosphorus with mean carbon transit times in soils estimated at less than 10 years despite increased plant productivity, suggest chemical analyses of European cropland soils.
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