Patterns of soil water repellency change with wetting and drying: the influence of cracks, roots and drainage conditions

The influence of simulated cracks and roots on soil water repellency (SWR) dynamics with and without basal drainage impedance in wetting–drying cycles was investigated in the laboratory experiments. Observations and measurements were taken following water application equivalent to 9.2-mm rainfall and then periodically during 80 h of drying. In total, 180 experiments were carried out using 60 samples of three homogeneous, reconstituted soils with different organic matter contents and textures, but of similar initial severity of SWR [18% molarity of an ethanol droplet (MED)]. Water flowing down the cracks and roots left the soil matrix largely dry and water repellent except for vertical zones adjacent to them and a shallow surface layer. A hydrophilic shallow basal layer was produced in experiments where basal drainage was impeded. During drying, changes in SWR were largely confined to the zones that had been wetted. Soil that had remained dry retained the initial severity of SWR, while wetted soil re-established either the same or slightly lower severity of SWR. In organic-rich soil, the scale of recovery to pre-wetting MED levels was much higher, perhaps associated with temporarily raised levels (up to 36% MED) of SWR recorded during drying of these soils. With all three soils, the re-establishment of the original SWR level was less widespread for surface than subsurface soil and with impeded than unimpeded basal drainage.Key findings are that as follows: (1) with unimpeded basal drainage, the soils remained at pre-wetting repellency levels except for a wettable thin surface layer and zones close to roots and cracks, (2) basal drainage impedance produced hydrophilic basal and surface layers, (3) thorough wetting delayed a return to water-repellent conditions on drying, and (4) temporarily enhanced SWR occurred in organic-rich soils at intermediate moisture levels during drying. Hydrological implications are discussed, and the roles of cracks and roots are placed into context with other influences on preferential flow and SWR under field conditions

A map of the topsoil organic carbon content of Europe generated by a generalized additive model

There is an increasing demand for up-to-date soil organic carbon (OC) data for global environmental and climatic modelling. The aim of this study was to create a map of topsoil OC content at the European scale by applying digital soil mapping techniques to the first European harmonized geo-referenced topsoil (0–20 cm) database, which arises from the Land use/Cover Area frame statistical Survey (LUCAS). A map of the associated uncertainty was also produced to support careful use of the predicted OC contents. A generalized additive model (GAM)was fitted on 85% of the dataset (R2 =0.29), using OC content as dependent variable; a backward stepwise approach selected slope, land cover, temperature, net primary productivity, latitude and longitude as suitable covariates. The validation of the model (performed on 15% of the data-set) gave an overall R2 of 0.27 and an R2 of 0.21 for mineral soils and 0.06 for organic soils. Organic C content in most organic soils was under-predicted, probably because of the imposed unimodal distribution of our model, whose mean is tilted towards the prevalent mineral soils. This was also confirmed by the poor prediction in Scandinavia (where organic soils are more frequent), which gave an R2 of 0.09, whilst the prediction performance (R2) in non-Scandinavian countries was 0.28. Themap of predicted OC content had the smallest values in Mediterranean countries and in croplands across Europe, whereas largest OC contents were predicted in wetlands, woodlands and mountainous areas. The map of the predictions’ standard error had large uncertainty in northern latitudes, wetlands, moors and heathlands, whereas small uncertainty was mostly found in croplands. The map produced gives the most updated general picture of topsoil OC content at the European Union scale

The geochemical transformation of soils by agriculture and its dependence on soil erosion: An application of the geochemical mass balance approach

Agricultural activities alter elemental budgets of soils and thus their long-term geochemical development and suitability for food production. This study examined the utility of a geochemical mass balance approach that has been frequently used for understanding geochemical aspect of soil formation, but has not previously been applied to agricultural settings. Protected forest served as a reference to quantify the cumulative fluxes of Ca, P, K, and Pb at a nearby tilled crop land. This comparison was made at two sites with contrasting erosional environments: relatively flat Coastal Plain in Delaware vs. hilly Piedmont in Pennsylvania. Mass balance calculations suggested that liming not only replenished the Ca lost prior to agricultural practice but also added substantial surplus at both sites. At the relatively slowly eroding Coastal Plain site, the agricultural soil exhibited enrichment of P and less depletion of K, while both elements were depleted in the forest soil. At the rapidly eroding Piedmont site, erosion inhibited P enrichment. In similar, agricultural Pb contamination appeared to have resulted in Pb enrichment in the relatively slowly eroding Coastal Plain agricultural soil, while not in the rapidly eroding Piedmont soils. We conclude that agricultural practices transform soils into a new geochemical state where current levels of Ca, P, and Pb exceed those provided by the local soil minerals, but such impacts are significantly offset by soil erosion.</p

Deciphering human and climatic controls on soil erosion in intensively cultivated landscapes after 1950 (Loire Valley, France)

International audienceIntensification of agricultural practices during the second half of the 20 th century has accelerated of soil erosion around the world. Although this phenomenon has been widely investigated through a combination of monitoring or modelling at short timescales (<10 years), few records are available for reconstructing the trajectory of soil erosion during longer periods (i.e. the 20 th century). Analysis of sediment deposits in reservoirs provides a valuable tool for reconstructing these trends and for identifying the driving factors that may have disturbed the sediment cascade after 1950. Accordingly, sediment cores (n=5) were collected in a reservoir located at the outlet of an intensively cultivated lowland catchment (24.5 km²) representative of those found in central France. Natural (excess lead-210) and artificial radionuclides (caesium-137, americium-241) enable dating of these sedimentary sequences. The corresponding sediment accumulation and erosion rates were calculated for the 1928-2017 period. In addition, daily rainfall records, land use change and agricultural field patterns were reconstructed for the period comprised between 1950 and 2017, based on weather records, digitalized aerial images (n=10) and agricultural census data (n=6). Results showed substantial acceleration of erosion rates after 1928 (+500%). This increase occurred simultaneously with major landscape changes that led to an increase in plot size (+465%) and decrease of the surface occupied by grassland and fallow land (-93%). Both parameters correlated strongly with the erosion rates reconstructed in this catchment (r=0.87 and r=0.95 for the plot size and grassland/fallow land surfaces, respectively). In addition, spectral analyses of daily rainfall records and mass accumulation rates, estimated with high temporal resolution from the sediment core tomography scanner data, showed concomitant short (i.e., 1 year) and long-term (i.e., 16 years) cycles between mass accumulation rates and rainfall. Overall, this study demonstrated the long-term impact of human activities and rainfall dynamics on soil erosion. Between 1928 and 2017, erosion rates increased seven-fold in this lowland catchment, until reaching 31.5 t km-2 yr-1 by 2017. Although landscape modifications likely drove the pluri-decadal trends of erosion, this study has also demonstrated the major role played by rainfall intensity on annual sediment dynamics