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eDNA refers to DNA extracted from an environmental sample with the goal of identifying the occurrence of past or current biological communities in aquatic and terrestrial environments. However, there is currently a lack of knowledge regarding the soil memory effect and its potential impact on lake sediment eDNA records. To investigate this issue, two contrasted sites located in cultivated environments in France were studied. In the first site, soil samples were collected (n = 30) in plots for which the crop rotation history was documented since 1975. In the second site, samples were collected (n = 40) to compare the abundance of currently observed taxa versus detected taxa in cropland and other land uses. The results showed that the last cultivated crop was detected in 100% of the samples as the most abundant. In addition, weeds were the most abundant taxa identified in both sites. Overall, these results illustrate the potential of eDNA analyses for identifying the recent (< 10 years) land cover history of soils and outline the detection of different taxa in cultivated plots. The capacity of detection of plant species grown on soils delivering sediments to lacustrine systems is promising to improve our understanding of sediment transfer processes over short timescales. [ABSTRACT FROM AUTHOR]

Long‐term records are needed to investigate the impact of extreme events in the current framework of global change. Sedimentary reconstruction with a high resolution remains difficult without conducting expensive, destructive and/or time‐consuming analyses. In this study, high resolution CT‐scan profiles (0·6 mm resolution) were used to investigate their potential for detecting flood deposits induced by heavy rainfall events. This method was applied to a sediment core dated with fallout radionuclides – covering a 120 year period – collected in a pond draining a small forested catchment (French Massif Central – Central France). Between 1960 and 2017, 28 layers were deposited. Seventy‐six percent of these deposits were correlated to the occurrence of heavy rainfall (>50 mm) recorded during one or two consecutive days. The remainder of the deposits detected with the Computer Tomography (CT) scanner (n = 5) were not correlated to weather events. They mainly occurred in response to landscape management operations (for example, afforestation works as a result of the major 1999 storm). This period was indeed characterized by an increase in the delivery of 137Cs‐enriched sediment, demonstrating a greater topsoil contribution to sediment during major forest management operations. The intensity of detrital layers has significantly decreased throughout time after a major land use change that took place in 1948 and land abandonment. The frequency of heavy rainfall and associated detrital deposits has nevertheless increased by 60% and 75%, respectively, between the years 1960 and 2017. These results outline the potential of CT‐scan for reconstructing long‐term flood deposits associated with heavy precipitation. [ABSTRACT FROM AUTHOR]

Abstract: Brazil is a leader in the adoption of conservation agriculture practices and technologies. However, the impact of these practices on sediment sources at the catchment scale has not been quantified yet, particularly in grain growing regions, where a conservationist no‐tillage system is implemented to protect soils. To address this knowledge gap, a sediment fingerprinting study based on elemental geochemistry was carried out in a large agricultural catchment (804 km2) of Southern Brazil where no‐tillage practices dominate. A total of 156 soil samples were taken to characterize the three main potential sediment sources: cropland (n = 79), unpaved roads (n = 41), and channel banks (n = 36). Sediment sampling was performed using a time‐integrated sampler (n = 33) and by collecting fine‐bed material (n = 34) at five locations across the catchment. Sediment was also sampled during flood events at the catchment outlet (n = 20). Sediment source contributions were calculated using an optimal suite of geochemical properties and a mixing model. Results showed that although the catchment is not particularly sensitive to soil erosion (i.e., deep clayey soils with gentle slopes), the amount of sediment supplied by cropland to the river network remains very high (up to 1.63 Mg·ha−1·year−1). Sediment fingerprinting results showed that even when no‐tillage is implemented, cropland remains an important source of sediment, supplying up to 70% of the material transiting the Conceição River. Accordingly, the current conservation farming system in this catchment needs to be improved to further reduce soil erosion and sediment yield. [ABSTRACT FROM AUTHOR]

Consumption of water polluted by faecal contaminants is responsible for 2 million deaths annually, most of which occur in developing countries without adequate sanitation. In tropical aquatic systems, streambeds can be reservoirs of persistent pathogenic bacteria and high rainfall can lead to contaminated soils entering streams and to the resuspension of sediment-bound microbes in the streambed. Here, we present a novel method using fallout radionuclides (Be and Pb) to estimate the proportions of Escherichia coli, an indicator of faecal contamination, associated with recently eroded soil particles and with the resuspension of streambed sediments. We show that using these radionuclides and hydrograph separations we are able to characterize the proportion of particles originating from highly contaminated soils and that from the resuspension of particle-attached bacteria within the streambed. We also found that although overland flow represented just over one tenth of the total flood volume, it was responsible for more than two thirds of the downstream transfer of E. coli. We propose that data obtained using this method can be used to understand the dynamics of faecal indicator bacteria (FIB) in streams thereby providing information for adapted management plans that reduce the health risks to local populations.[Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Abstract: Soil erosion leads to important environmental problems (e.g. muddy floods, reservoir sedimentation) in cultivated areas of the European loess belt. This study aimed to determine the impact of rainfall seasonality and land use change on soil erosion over the last 40 years in a 94-ha cultivated catchment of Normandy (France). To this end, scenarios representative of the different land use conditions were simulated using the STREAM expert-based erosion model. A 13 years long sequence of rainfall events was run with this model. Results showed that erosion increased dramatically after land consolidation (+168% on average). Interannual variability of erosion is important. After land consolidation, 79% of erosion was observed in summer and autumn, even though these seasons only accounted for 58% of annual rainfall kinetic energy. The bulk of erosion was hence produced by a few intense thunderstorms during this period. Thunderstorms correspond to 5% of rainfall events and to 15% of the total rainfall depth, but they generate 51% of total annual erosion after land consolidation (and up to 57% of erosion before land consolidation). Confrontation of the STREAM model outputs with the erosion rates modelled based on Cs-137 measurements suggested that soil redistribution within the catchment was very high but that sediment exports from the catchment remained limited (sediment delivery ratio between 1% and 10%). Local erosion rates derived from Cs-137 measurements showed an important and organised spatial variability, but erosion rates integrated over larger areas remained in the same order of magnitude as those simulated by the STREAM model or were slightly higher. Water erosion would hence not be the only process generating erosion within this catchment. In this context, our results show that tillage erosion cannot be neglected to calculate the sediment budget over several decades. [Copyright &y& Elsevier]

The definition of the Anthropocene boundary is being discussed across various disciplines. In this study, we have focused our research on the sedimentary responses of landscape to human activity, as a response to agricultural changes over the 20th century in Europe.In the absence of long-term records of sediment transported by river systems, sediment deposited in lakes may represent a powerful archive for reconstructing the evolution of sediment yields in response to global changes. Accordingly, sedimentary fluxes were reconstructed in 10 ponds located at the outlet of small agricultural headwater catchments distributed across the Loire Basin, in Western France. These study sites were selected in order to represent a large variety of land cover/land use changes, landscape morphologies and agriculture dynamics. The objective was to investigate whether a common evolution of sediment yields was observed.The results obtained show the occurrence of a general transition in the sediment production during the second half of the 20th century (between 1945 and 1960), with two contrasting trajectories. On the one hand, sediment delivery decreased in catchments witnessing a decline of agriculture, with a 70% decrease of sediment delivery. On the other hand, in lowland areas affected by intensification of land management after WWII (land consolidation, stream redesign, edge removal or agricultural drainage implementation), sediment deposits record a sharp acceleration, by one order of magnitude. The recent sediment dynamics therefore emphasizes a significant change associated to the post-1950 period, with a sharp positive or negative trend. These local observations are in agreement with data compiled from other countries across the Northern Hemisphere (Europe, USA, Japan). In contrast, there is a lack of similar data from catchments located in the Southern Hemisphere (e.g. in South America, developing countries). [ABSTRACT FROM AUTHOR]

Cropland cultivated under no-tillage has dramatically increased in Brazil during the last three decades. However, soil degradation by water erosion remains excessive in the country. Furthermore, the impact of inappropriate agricultural practices on soil erosion is often neglected by farmers. Therefore, the contribution of potential sources of sediment should be quantified in order to raise awareness among rural communities on the need to improve land management for protecting soil and water resources. This study quantified the contribution of potential sources to sediment transiting the Guaporé River, draining a 2032-km 2 cultivated catchment, in Southern Brazil. Potential sediment source types were surface of cropland soils ( n = 159), unpaved roads ( n = 58), and stream channel banks ( n = 46). A total of 175 suspended sediment samples were collected following different sampling strategies (including sampling of river water during floods, installation of time-integrated suspended sediment samplers, and collection of bed sediment) in 10 sub-catchments, from January 2011 to March 2014. Discriminant properties were selected among the concentrations in 22 geochemical elements and total organic carbon. Results showed that sediment source contributions were similar for the different sediment sampling strategies. Although, the contributions of sediment sources varied in space across Guaporé catchment, they were dominated by cropland (91 ± 15%), while stream channels (5 ± 2%) and unpaved roads (4 ± 10%) were sources of minor importance. Cropland contribution increased with the drained cropland surface area. The unambiguous dominance of cropland as the main source supplying sediment to the Guaporé River justifies the urgent need to better plan land use and to promote the adoption of appropriate conservation farming practices in similar areas of Southern Brazil. Soil losses in cropland could be reduced by implementing crop rotation, mechanical runoff control strategies, and by avoiding the construction of unpaved roads in thalwegs. [ABSTRACT FROM AUTHOR]