| 1. |
- Hao, Qian, et al.
(författare)
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Holocene carbon accumulation in lakes of the current east Asian monsoonal margin: Implications under a changing climate
- 2020
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Ingår i: Science of the Total Environment. - : Elsevier. - 0048-9697 .- 1879-1026. ; 737, s. 1-13
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Tidskriftsartikel (refereegranskat)abstract
- Carbon (C) present in lake sediments is an important global sink for CO2; however, an in-depth understanding of the impact of climate variability and the associated changes in vegetation on sediment C dynamics is still lacking. A total of 13 lakes were studied to quantify the influence of climate and vegetation on the reconstructed Holocene C accumulation rate (CAR) in lake sediments of the modern East Asian monsoonal margin. The corresponding paleoclimate information was assessed, including the temperature (30–90°N in the Northern Hemisphere) and precipitation (indicated by the δ18O of the Sanbao, Dongge, and Hulu caves). The Holocene vegetation conditions were inferred by pollen records, including arboreal pollen/non-arboreal pollen and pollen percentages. The results showed that the peak CAR occurred during the mid-Holocene, coinciding with the strongest period of the East Asian summer monsoon and expansion of forests. Lakes in the temperate steppe (TS) regions had a mean CAR of 13.41 ± 0.88 g C m−2 yr−1, which was significantly greater than the CARs of temperate desert (TD) and highland meadow/steppe (HMS; 6.76 ± 0.29 and 7.39 ± 0.73 g C m−2 yr−1, respectively). The major influencing factor for the TS sub-region was vegetation dynamics, especially the proportion of arboreal vegetation, while temperature and vegetation coverage were more important for the HMS. These findings indicate that C accumulation in lake sediments is linked with climate and vegetation changes over long timescales; however, there was notable spatial heterogeneity in the CARs, such as opposing temporal changes and different major influencing factors among the three sub-regions during the mid-Holocene. Aridification and forest loss would decrease C storage. However, prediction of C accumulation remains difficult because of the spatial heterogeneity in CARs and the interaction between the CAR and various factors under future climate change conditions.
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| 2. |
- Lu, Haibo, et al.
(författare)
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Comparing machine learning-derived global estimates of soil respiration and its components with those from terrestrial ecosystem models
- 2021
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Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9318 .- 1748-9326. ; 16:5
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Tidskriftsartikel (refereegranskat)abstract
- The CO2 efflux from soil (soil respiration (SR)) is one of the largest fluxes in the global carbon (C) cycle and its response to climate change could strongly influence future atmospheric CO2 concentrations. Still, a large divergence of global SR estimates and its autotrophic (AR) and heterotrophic (HR) components exists among process based terrestrial ecosystem models. Therefore, alternatively derived global benchmark values are warranted for constraining the various ecosystem model output. In this study, we developed models based on the global soil respiration database (version 5.0), using the random forest (RF) method to generate the global benchmark distribution of total SR and its components. Benchmark values were then compared with the output of ten different global terrestrial ecosystem models. Our observationally derived global mean annual benchmark rates were 85.5 ± 40.4 (SD) Pg C yr-1 for SR, 50.3 ± 25.0 (SD) Pg C yr-1 for HR and 35.2 Pg C yr-1 for AR during 1982-2012, respectively. Evaluating against the observations, the RF models showed better performance in both of SR and HR simulations than all investigated terrestrial ecosystem models. Large divergences in simulating SR and its components were observed among the terrestrial ecosystem models. The estimated global SR and HR by the ecosystem models ranged from 61.4 to 91.7 Pg C yr-1 and 39.8 to 61.7 Pg C yr-1, respectively. The most discrepancy lays in the estimation of AR, the difference (12.0-42.3 Pg C yr-1) of estimates among the ecosystem models was up to 3.5 times. The contribution of AR to SR highly varied among the ecosystem models ranging from 18% to 48%, which differed with the estimate by RF (41%). This study generated global SR and its components (HR and AR) fluxes, which are useful benchmarks to constrain the performance of terrestrial ecosystem models.
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| 3. |
- Snöälv, Jo, et al.
(författare)
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Flocculation boundaries in the landscape: transformation processes of dissolved organic matter in coastal moorland streams, UK
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- From headwaters to estuaries, organic carbon is transported and transformed. Some carbon may be mineralised and outgassed to the atmosphere, while another fraction is buried in sediments. Organic carbon can end up in the sediment through flocculation. As the dissolved organic matter (DOM) is transported through the landscape, it passes through several flocculation boundaries. Examples of such boundaries are the exposure to suspended minerogenic material derived from soil erosion or weathering processes that causes adsorption or coagulation, and estuarine mixing where riverine DOM encounters a substantial increase in salinity. In addition, both of these processes may occur as river flow causes sediment resuspension in the saline estuary. This study aims to simulate these flocculation boundaries by experimental additions of clay particles, saline mixing, and a combined clay and salt treatment. We performed experiments on water from one peatland pond and eight second and third order streams in a coastal moorland that served as a setting for these processes. Analysis of DOM composition through fluorescence spectroscopy and mass spectrometry showed that flocculation has the potential to modify the composition of DOM, as compounds with a terrestrial signature are preferentially removed by flocculation. This effect could be observed for clay and combined clay and salt mixing, while saline mixing alone caused limited changes in the DOM composition. Contrary to previous studies, combined clay and saline mixing did not lead to more flocculation compared to clay mixing alone. Instead, the added salt desorbed a small amount of organic matter already associated with the clay particles, which confounded the overall flocculation effect. In addition to organic carbon removal by flocculation, we expect that the altered composition of the DOM remaining in solution will also affect its reactivity to other DOM transformation processes.
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