Stratigraphic profiling, slip surface detection, and assessment of remolding in sensitive clay landslides using the CPT

A multi-year cone penetration testing program was initiated at a landslide subject to episodic retrogression in Mud Creek, Ottawa, to assess whether a hand-operated mobile CPT could yield new insights into the current degree of remoulding under progressive failure in metastable areas of a landslide where conventional tracked rigs are unable to gain access. The mobile CPT rig permitted tests to be performed through the entire thickness of the Champlain Sea deposit at a penetration rate of 0.5 cm/s, with similar results to tests performed at the standard 2 cm/s. Measurements of pore pressure varied considerably with cone size, with the magnitude of pore pressure response decreasing with cone size. The elevation of the slip surface was identified in the tip resistance as the point of transition between the remolded soil above the slip surface and the intact soil below the slip surface, whereas a further 0.5 m of penetration was required to elevate pore pressures to values indicative of the intact soil behaviour. In-situ measurements of shear strength of corresponding layers between the intact and remolded profiles to be compared indicating that the soil above the slip surface had remolded to 50% of its fully remolded strength.

Soil Fabric and Transitional Behavior in Completely Decomposed Granite: An Example of Well-Graded Soil

Unlike sedimentary soils, limited studies have dealt with completely decomposed granite (CDG) soils, even though they are plentiful and used extensively in several engineering applications. In this paper, a set of triaxial compression tests have been conducted on well-graded intact and disturbed CDG soils to study the impact of the fabric on soil behavior. The soil behavior was robustly affected by the soil fabric and its mineral composition. The intact soil showed multiple parallel compression lines, while a unique isotropic compression line was present in the case of disturbed soil. Both the intact and disturbed soils showed unique critical state lines (CSL) in both the e-log p′ and q-p′ spaces. The intact soil showed behavior unlike other transitional soils that have both distinct isotropic compression lines ICLs and CSLs. The gradient of the unique ICL of the disturbed soil was much more than that of the parallel compression lines of the intact soil. In the intact soil, the slope of the unique CSL (M) in the q-p′ space was higher than that of the disturbed soil. The isotropic response was present for both the intact and disturbed soils after erasing the inherited anisotropy as the stress increased with irrecoverable volumetric change. Soil fabric is considered the dominant factor in the transitional behavior and such a mode of soil behavior is no longer restricted to gap-graded soil as previously thought.

Soil profile connectivity can impact microbial substrate use, affecting how soil CO₂ effluxes are controlled by temperature

Determining controls on the temperature sensitivity of heterotrophic soil respiration remains critical to incorporating soil–climate feedbacks into climate models. Most information on soil respiratory responses to temperature comes from laboratory incubations of isolated soils and typically subsamples of individual horizons. Inconsistencies between field and laboratory results may be explained by microbial priming supported by cross-horizon exchange of labile C or N. Such exchange is feasible in intact soil profiles but is absent when soils are isolated from surrounding depths. Here we assess the role of soil horizon connectivity, by which we mean the degree to which horizons remain layered and associated with each other as they are in situ, on microbial C and N substrate use and its relationship to the temperature sensitivity of respiration. We accomplished this by exploring changes in C : N, soil organic matter composition (via C : N, amino acid composition and concentration, and nuclear magnetic resonance spectroscopy), and the δ13C of respiratory CO2 during incubations of organic horizons collected across boreal forests in different climate regions where soil C and N compositions differ. The experiments consisted of two treatments: soil incubated (1) with each organic horizon separately and (2) as a whole organic profile, permitting cross-horizon exchange of substrates during the incubation. The soils were incubated at 5 and 15 ∘C for over 430 d. Enhanced microbial use of labile C-rich, but not N-rich, substrates were responsible for enhanced, whole-horizon respiratory responses to temperature relative to individual soil horizons. This impact of a labile C priming mechanism was most emergent in soils from the warmer region, consistent with these soils' lower C bioreactivity relative to soils from the colder region. Specifically, cross-horizon exchange within whole soil profiles prompted increases in mineralization of carbohydrates and more 13C-enriched substrates and increased soil respiratory responses to warming relative to soil horizons incubated in isolation. These findings highlight that soil horizon connectivity can impact microbial substrate use in ways that affect how soil effluxes of CO2 are controlled by temperature. The degree to which this mechanism exerts itself in other soils remains unknown, but these results highlight the importance of understanding mechanisms that operate in intact soil profiles – only rarely studied – in regulating a key soil–climate feedback.

Agricultural Practices Modulate the Beneficial Activity of Bacterial-Feeding Nematodes for Plant Growth and Nutrition: Evidence from an Original Intact Soil Core Technique

Free-living nematodes have beneficial effects on plant growth and nutrition. Exploring how agricultural practices modulate these beneficial effects is still challenging. A study was conducted in Ferralsols from Madagascar from one unmanaged grassland and 16 upland rainfed rice fields, representative of different agricultural practices: rotation, agroforestry and monoculture. Intact soil cores in plastic cylinders were sampled in the field to assess the effects of agricultural practices on changes in plant growth and nutrition induced by the presence of bacterial-feeding nematodes. The soil cores were fumigated to kill the nematodes and moistened with a filtered fresh soil suspension containing only microbial cells. A rice seed was introduced in the core, which was then incubated under natural climatic conditions for 40 days with or without inoculation of the bacterial-feeding nematode Acrobeloides sp. The inoculation of the nematodes induced lower, similar or higher plant biomass and nutrient content in comparison to the control according to the agricultural practices. Positive effects of Acrobeloides sp. on plant functions were frequent in soil cores sampled from fields with high plant diversity, especially from agroforestry systems. The intact soil core technique appears to be a robust means of mimicking field conditions and constitutes a promising tool to assess effects on soil processes of the ecological intensification of agricultural practices.

Ivermectin transfer through percolation and surface runoff from intact soil mesocosms under rainfall simulation

<p>Ivermectin (IVM) is one of the few pharmaceutics that are still used in a preventive, systematic manner in extensive cattle breeding in our study region in the Ardèche region (France), amongst others. It is an efficient antiparasitic agent with an extreme acute toxicity for most invertebrates, especially aquatic organisms like daphnia (ng/l), and is also highly toxic to different fish species (µg/l). Due to its strong sorption to soil and sediment and quick photodegradation, early environmental risk assessments (ERA) conclude a low risk for aquatic organisms. More recent studies conclude an inacceptable risk for daphnia and dung organisms. One of the critical parameters between these contradictory conclusions is IVM export from cow dung and transfer towards the streams.</p><p>The study region is characterized by a Mediterranean climate with a dry summer and intense convective storm events leading to regular flash flood events that coincide with the cattle treatment seasons in spring and autumn. The study region encompasses the Claduègne catchment which is part of the OHMCV observatory and the OZCAR and eLTER research infrastructures.</p><p>The key question concerning the risk for aquatic organisms is to what extent and in which conditions IVM is mobilized and transferred from cow dung to soil and river via surface runoff and percolation in this environment prone to rapid flow processes. We approach this question on the scale of 60*30*22 (L*W*D) cm3 intact soil mesocosms, for which we developed an adapted field sampling and laboratory experimentation case. Soil mescosms are collected in the Claduègne catchment. IVM is applied in form of spiked cow dung at realistic environmental concentrations before simulating several rainfall events, representative of this Mediterranean region. Runoff and drainage water are sampled for major anions (including Br- tracer), non-particulate organic carbon and IVM concentrations on a high temporal frequency in order to gain an insight on the intra- and inter-event dynamics of water and IVM transfer. Tested parameters include dung ageing, soils types, initial soil humidity and consecutive rainfall events.</p><p>The first results highlight the importance of runoff for the overall export of IVM on the event scale. Concerning the water flux, initial humidity is found to determine the runoff / drainage partitioning as well as the rapidity of percolation through the occurrence of preferential flow. In this context, hydrophobicity seems to play an important role.</p>