Stable and radiogenic isotopes as tracers for soil degradation

Analyses of soil organic carbon (SOC) content and stable carbon isotope signatures (delta(13)C) of soils were assessed for their suitability to detect early stage soil erosion. We investigated the soils in the alpine Urseren Valley (southern central Switzerland) which are highly impacted by soil erosion. Hill slope transects from uplands (cambisols) to adjacent wetlands (histosols and histic to mollic gleysols) differing in their intensity of visible soil erosion, and reference wetlands without erosion influence were sampled. Carbon isotopic signature and SOC content of soil depth profiles were determined. A close correlation of delta(13)C and carbon content (r > 0.80) is found for upland soils not affected by soil erosion, indicating that depth profiles of delta(13)C of these upland soils mainly reflect decomposition of SOC. Long-term disturbance of an upland soil is indicated by decreasing correlation of delta(13)C and SOC (r </= 0.80) which goes in parallel with increasing (visible) damage at the site. Early stage soil erosion in hill slope transects from uplands to adjacent wetlands is documented as an intermediate delta(13)C value (-27.5 per thousand) for affected wetland soil horizons (0-12 cm) between upland (aerobic metabolism, relatively heavier delta(13)C of -26.6 per thousand) and wetland isotopic signatures (anaerobic metabolism, relatively lighter delta(13)C of -28.6 per thousand). Carbon isotopic signature and SOC content are found to be sensitive indicators of short- and long-term soil erosion processes.

Soil phosphorus flux from emergent marsh wetlands and surrounding grazed pasture uplands

Mercury (Hg) and methylmercury (CH3Hg+) are global pollutants, but little information is available on their distribution and mobility in soils and catchments of Central Europe. The objective of this study was to investigate the pools and mobility of Hg and CH3Hg+ in different forest soils. Upland and wetland forest soils, soil solutions and runoff were sampled. In upland soils the highest contents of total-Hg were found in the Oh layer of the forest floor (>400 ng g-1) and the storage of non geogenic total-Hg (calculated for 60 cm depth) was about 120 mg/m2. The storage of total-Hg was one order of magnitude lower in wetland soils as compared to the upland soils. By far the largest proportion of total-Hg in soils was bound in immobile fractions. The depth gradients of CH3Hg+ did not correspond to those of total-Hg and the highest contents of CH3Hg+ in upland soils were observed in the litter layer of the forest floor and in the Bsv horizon. The CH3Hg+ content of the wetland soils was generally much higher in comparison with upland soils. CH3Hg+ in solution was found in the forest floor percolates of upland soils and in wetland soils, but not in soil solutions from mineral soil horizons. Gaseous losses of Hg as well as methylation of Hg are likely in wetland soils. The latter might be highly relevant for CH3Hg+ levels in runoff.

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Abstract. Soil erosion has been discussed intensively but controversial both as a significant source or a significant sink of atmospheric carbon possibly explaining the gap in the global carbon budget. One of the major points of discussion has been whether or not carbon is degraded and mineralized to CO2 during detachment, transport and deposition of soil material. By combining the caesium-137 (137Cs) approach (quantification of erosion rates) with stable carbon isotope signatures (process indicator of mixing versus degradation of carbon pools) we were able to show that degradation of carbon occurs during soil erosion processes at the investigated mountain grasslands in the central Swiss Alps (Urseren Valley, Canton Uri). Transects from upland (erosion source) to wetland soils (erosion sinks) of sites affected by sheet and land slide erosion were sampled. Analysis of 137Cs yielded an input of 2 and 4.6 tha−1 yr−1 of soil material into the wetlands sites. Assuming no degradation of soil organic carbon during detachment and transport, carbon isotope signature of soil organic carbon in the wetlands could only be explained with an assumed 500–600 and 350–400 years of erosion input into the wetlands Laui and Spissen, respectively. The latter is highly unlikely with alpine peat growth rates indicating that the upper horizons might have an age between 7 and 200 years. While we do not conclude from our data that eroded soil organic carbon is generally degraded during detachment and transport, we propose this method to gain more information on process dynamics during soil erosion from oxic upland to anoxic wetland soils, sediments or water bodies.

Peatland degradation indicated by stable isotope depth profiles and soil carbon loss

15N/ 14N and 18O/ 16O stable isotope ratios of nitrous oxide produced during denitrification in temperate forest soils

Global warming is expected to change the thermal and hydrological soil regime in permafrost ecosystems which might impact soil erosion processes. Erosion assessment using radionuclides can provide information on past and ongoing, i.e. time-split, processes. The focus of this work was to find out if permafrost soils in the Swiss Alps differ in their medium- and long-term erosion rates from non-permafrost soils and if rates have accelerated during the last few decades. Using cosmogenic (meteoric 10Be) and anthropogenic radionuclides (137Cs, 239 + 240Pu), a time-split approach was achieved by determining erosion activities on the long (millennia; 10Be) and medium term (decades; 137Cs, 239 + 240Pu). Additionally, the stable isotope δ 13C signature in soil organic matter was used as a qualitative indicator for soil disturbance patterns. We compared soil erosion processes in permafrost soils and nearby unfrozen soils in the alpine (sites at 2,700 m asl, alpine tundra) and the subalpine (sites 1,800 m asl, natural forest) range of the Swiss Alps (Upper Engadine). 137Cs, 239 + 240Pu and δ 13C measurements were performed at the alpine sites only. Depending on the calculation procedure (profile distribution model or inventory method), the 137Cs measurements revealed soil accumulation rates of 1–3 t/km2/year in permafrost soils and 34–52 t/km2/year in non-permafrost soils. However, due to snow cover and subsequent melt-water runoff during 137Cs deposition after the Chernobyl accident, caesium does not seem to be an appropriate soil erosion tracer on the investigated alpine sites. With 239 + 240Pu, more reliable results were achieved. 239 + 240Pu measurements provided erosion rates of 31–186 t/km2/year in permafrost soils and accumulation rates of 87–218 t/km2/year in non-permafrost soils. Erosion and accumulation were relatively low and related to the vegetation community. The long-term (10Be) soil redistribution rates (erosion rates up to 49 t/km2/year and accumulation rates up to 4 t/km2/year) were low with no significant differences between permafrost and non-permafrost sites. The δ 13C signature indicated soil disturbances in permafrost and non-permafrost soils compared to the reference site. Our results highlight that soil redistribution rates have increased during the last few decades. However, whether the higher medium-term erosion rates obtained for the last decades are the result of the ongoing climate warming and related accelerated soil erosion or if other factors (e.g. measurement uncertainties) have been responsible for such an increase could not fully be clarified.

The Kasai basin (DRCongo), southwest part of the Congo Basin, displays a unique array of climatic conditions, mineralogical compositions and land use trajectories. It is currently experiencing an explosive demographic expansion, which leads to drastic land use and land cover changes (LULCC) such as deforestation and cropland expansion. The net terrestrial C exchange from both vegetation and soils accompanying land use change is relatively well constrained. In contrast, C exchange associated with accelerated soil erosion following the degradation of natural habitats is an uncertain component of the carbon budgeta,b,c, particularly in tropical settingsd,e,f.To remedy this lack of knowledge, we used remote sensing techniques in conjunction with field observations and preexisting datasets describing potential drivers such as Worldpopg, global Human Modificationh, etc. We analyzed the temporal evolution of LULCC and its drivers within the Kasai Basin at high spatial resolutions. Preliminary results show a discernable surge in deforestation rates, varying across the different sub-basins, alongside expansions in cropland cultivation and artisanal mining activities, all recognized as contributors to heightened soil erosion. Using estimated time series of suspended sediment yield derived from in-situ measurements and remote sensing products as an indicator for upland soil erosion, we compared observed changes in sediment yield to LULCC trajectories. We also conducted a robust sampling campaign and collected over 5000 soil samples from 15 hillslopes transects alongside the Kasai River. Multiple soil organic carbon (SOC) analyses were realized to investigate C dynamics in tropical eroding uplands: carbon and nitrogen content and stable isotopes, soil texture, clay mineralogy and fallout radionuclide inventory of a selection of samples, and MIR spectroscopy to have high depth resolution results.The results of our study will provide insights into the environmental drivers responsible for the heightened soil erosion and lateral carbon fluxes observed within the Kasai Basin. The forthcoming study of these fluxes will improve the knowledge of the tropical C budget in eroding upland soils. &amp;#160; REFERENCESa Stallard, R. F. (1998). Global Biogeochemical Cycles, 12(2), 231&amp;#8211;257. https://doi.org/10.1029/98GB00741 b Berhe, A. A. et al. (2007). Bioscience, 57, 337&amp;#8211;346, https://doi.org/10.1641/B570408 cVan Oost, K. et al. (2007). Science, 318(5850), 626&amp;#8211;629. https://doi.org/10.1126/science.1145724 d Don, A. et al. (2011). Global Change Biology, 17(4), 1658&amp;#8211;1670. https://doi.org/10.1111/j.1365-2486.2010.02336.x e Reichenbach, M. et al. (2023). Global Change Biology, 00, 1&amp;#8211;17. https://doi.org/10.1111/gcb.16622 f Wilken, F. et al. (2020). Soil, 1&amp;#8211;22. https://doi.org/10.5194/SOIL-7-399-2021 g Lloyd, C. T. et al. (2017). Scientific Data, 4(1), 170001. https://doi.org/10.1038/sdata.2017.1 h Kennedy, C.M. et al. (2019). Global Change Biology, 25(3), 811&amp;#8211;826. doi:10.1111/gcb.14549

The stable oxygen isotope signature (delta(18)O) of soil is expected to be the result of a mixture of components within the soil with varying delta(18)O signatures. Thus, the delta(18)O of soils should provide information about the soil's substrate, especially about the relative contribution of organic matter versus minerals. As there is no standard method available for measuring soil delta(18)O, the method for the measurement of single components using a high-temperature conversion elemental analyser (TC/EA) was adapted. We measured delta(18)O in standard materials (IAEA 601, IAEA 602, Merck cellulose) and soils (organic and mineral soils) in order to determine a suitable pyrolysis temperature for soil analysis. We consider a pyrolysis temperature suitable when the yield of signal intensity (intensity of mass 28 per 100 microg) is at a maximum and the acquired raw delta(18)O signature is constant for the standard materials used and when the quartz signal from the soil is still negligible. After testing several substances within the temperature range of 1075 to 1375 degrees C we decided to use a pyrolysis temperature of 1325 degrees C for further measurements. For the Urseren Valley we have found a sequence of increasing delta(18)O signatures from phyllosilicates to upland soils, wetland soils and vegetation. Our measurements show that the delta(18)O values of upland soil samples differ significantly from wetland soil samples. The latter can be related to the changing mixing ratio of the mineral and organic constituents of the soil. For wetlands affected by soil erosion, we have found intermediate delta(18)O signatures which lie between typical signatures for upland and wetland sites and give evidence for the input of upland soil material through erosion.

Effect of soil and water conservation measures on hydrological processes and sediment yield in the highlands of North-Western Ethiopia

Dealing with flooding, upland soil and streambank erosion, sedimentation, and contamination of water from agricultural, rural, and urban watersheds, and understanding the underlying natural processes are continued challenges to the environmental hydraulics field and in the management of sustainable water and environmental resources around the world. Watershed simulation models are useful tools to understand and analyze the processes and the problems and help mitigate those through evaluating the effects of land-use changes and best management practices. Developing adequate watershed simulation models and verifying those on real world watersheds with measured and monitored data are challenging. A Dynamic Watershed Simulation Model (DWSM) is being developed at the Illinois State Water Survey to simulate surface and subsurface storm water runoff, propagation of flood waves, soil erosion, and transport of sediment and agricultural chemicals in agricultural and rural watersheds. Different components of the DWSM have been applied to watersheds in Illinois for testing these components and assisting local watershed planning groups in planning restoration projects. Some of the recent progresses made in this ongoing modeling study are presented here. The soil erosion and sediment transport component was tested (calibrated and verified) on the Big Ditch watershed in Illinois, a 100-square-kilometer tributary subwatershed of the Upper Sangamon River basin draining into Lake Decatur. Two different divisions of the watershed, one with coarse subdivisions and the other with fine subdivisions were used in the simulations to investigate scaling effects on the parameter values and the model results.

Land degradation resulting from soil erosion is a global concern, with the greatest risk in developing countries where food and land resources can be limited. The use of fallout radionuclides (FRNs) is a proven method for determining short and medium-term rates of soil erosion, to help improve our understanding of soil erosion processes. There has been limited use of these methods in tropical Africa due to the analytical challenges associated with 137Cs, where inventories are an order of magnitude lower than in the Europe. This research aimed to demonstrate the usability of 239+240Pu as a soil erosion tracer in western Kenya compared to conventional isotopes 210Pbex and 137Cs through the determination of FRN depth profiles at reference sites. Across six reference sites 239+240Pu showed the greatest potential, with the lowest coefficient of variation and the greatest peak-to-detection limit ratio of 640 compared to 5 and 1 for 210Pbex and 137Cs respectively. Additionally, 239+240Pu was the only radionuclide to meet the ‘allowable error’ threshold, demonstrating applicability to large scale studies in Western Kenya where the selection of suitable reference sites presents a significant challenge. The depth profile of 239+240Pu followed a polynomial function, with the maximum areal activities found between depths 3 and 12 cm, where thereafter areal activities decreased exponentially. As a result, 239+240Pu is presented as a robust tracer to evaluate soil erosion patterns and amounts in western Kenya, providing a powerful tool to inform and validate mitigation strategies with improved understanding of land degradation.

Reconciling petrological and isotopic mixing mechanisms in the Pitcairn mantle plume using stable Fe isotopes

The objective of this research is to trace the sources of stream sediments in a small watershed influenced by anthropogenic and lithogenic origins identified by the spatial distributions and temporal variations of stream sediments using geochemical interpretation of the stable and radiogenic isotopes, major components, and heavy metals data and principal component analysis. To know the effects of both present and past mining, the stream sediments were sampled at the stream tributaries and sediment coring work. The spatial distributions of heavy metals clearly showed the effects of Cu and Pb–Zn mineralization zones at the site. Anthropogenic Pb was elevated at the downstream area by the stream sediments due to an active quarry. The results of principal components analysis also represent the effects of the stream sediments origins, including anthropogenic wastes and the active quarry and lithogenic sediment. Anomalous Cu, indicating the effect of past Guryong mining, was identified at the deep core sediments of 1.80–5.05 m depth. The influence of active quarry was shown in the recently deposited sediments of <1.50 m depth, which was proved by the profiles of radioactive 210Pb and stable Pb and Sr isotopes. This study suggests that the chemical studies using radiogenic and stable isotopes and heavy metals and multivariate statistical method are useful tools to discriminate the sources of stream sediments with different origins.