Sébastien F. Lange

Water content as a function of apparent dielectric permittivity in a Fibric Limnic Humisol

Volumetric water content (θv) was estimated from time domain reflectometry (TDR) measurementsof apparent dielectric permittivity (Ka) in an organic soil (Humisol). The goals of this study were: (i) to test the accuracy of existing θv-Ka relationships in this soil and if found insufficient (ii) to develop alternative θv-Ka relationships for this organic soil. The Ka values were measured over a wide range of θv in intact soil cores taken from three horizons (Ohp, Of, Oco). Empirical θv-Ka relationships found in the literature for organic porous media could not accurately describe the θv-Ka relationships of any horizon of this Humisol, probably because of the its very large organic matter content (> 75%) of this soil. New θv-Ka relationships for each horizon were consequently developed. Key words: Organic soil, TDR, coprogenic soil, volumetric water content, apparent dielectric permittivity

Potential Efficiency of Riparian Vegetated Buffer Strips in Intercepting Soluble Compounds in the Presence of Subsurface Preferential Flows.

Buffer strips have been widely recognized as to promote infiltration, deposition and sorption of contaminants for protecting surface water against agricultural contamination. However, such strips do not intercept all contaminants, particularly soluble ones. Although preferential flow (PF) has been suggested as one factor among several decreasing the efficiency of buffer strips, the mechanisms involved are not well understood. This project examines buffer strip efficiency at intercepting solutes when subsurface PF occurs. Two soluble sorbed tracers, FD&C Blue #1 and rhodamine WT, were applied on an agricultural sandy loam soil to evaluate the ability of a naturally vegetated buffer strip to intercept soluble contaminants. Rhodamine was applied about 15 m from the creek, while the Blue was applied 15 m to 165 m from the creek. Tracer concentration was measured over a two-year period in both the creek and the buffer strip through soil and water samples. Although the tracers traveled via different pathways, they both quickly moved toward the creek, passing beneath the buffer strip through the soil matrix. Our results demonstrate that the risk of water contamination by soluble contaminants is high in such systems, even when a well-vegetated buffer strip is used. The design of buffer strips should be modified to account for underground bypass, either by using plants that have deep, fine roots that do not favour PF or by adding a filter extending deep underground that can be regularly changed.

N2O and CO2 dynamics in a pasture soil across the frozen period

Abstract: Since the process of gas dynamics in agricultural soils is mainly studied during plant growth, only a few studies have focused on these dynamics in frozen soils covered with snow. Nevertheless, gas dynamics during the cold season is important to quantify the yearly mass balance of gas emitted to the atmosphere. Spatiotemporal concentrations of CO2 and N2O have been measured from the prefreezing to the thawing period in a pasture soil during two cold seasons along with soil temperature and other soil properties. The spatial dynamics of these gases differed from each other and depended on the spatial and temporal variability of soil temperature as long as the soil surface temperature was above 0 °C. Two main occurrences of gas release occurred during thawing, one related to trapped gases, similar for both gases, and the other to reactivation of microorganisms, different between both gases. Once the soil was frozen, both gas concentrations increased throughout the frozen period, even during very cold conditions, indicated a gases production faster than the loss. Under frozen condition, their spatial variability was independent of soil temperature during which their correlation was up to 90%. Three periods related to gas dynamics were observed during both cold seasons: freezing with spatiotemporal trends different between both gases, completely frozen with similar trends, and partial to complete thawing with trends different between both gases.

Carbon sequestration vs. agricultural yields in tree-based intercropping systems as affected by tree management1

Abstract: Tree-based intercropping (TBI) may increase carbon (C) sequestration in agroecosystems, but may reduce crop yields. In this study of TBI, we used ecosys, a comprehensive mathematical model of terrestrial ecosystems, which represents interspecific competition for light, nutrients, and water, to evaluate the concurrent effects of TBI on C sequestration and crop yields in TBI experiments conducted at St. Paulin and St. Edouard in southern Quebec. Total gains in C sequestration vs. total losses in crop yields over 11 yr relative to monocropping were 682 vs. 396 g C m-2 at St. Paulin and 841 vs. 168 g C m-2 at St. Edouard. These gains and losses were generally consistent with the measurements at the two TBI sites and with those at TBI experiments under similar environmental conditions elsewhere. Gains and losses depended on competition for light by trees and crops, and so were affected by different fractions of tree foliage removal used to manage this competition in the model. The modelling protocol developed for this study provides a robust, process-based methodology to evaluate economic and environmental benefits of TBI under diverse climates, soils, and tree and crop management practices. Some of the key assumptions used to model TBI are also discussed.

Tree-based intercropping may reduce, while fertilizer nitrate may increase, soil methane emissions1

Abstract: Tree-based intercropping (TBI) systems have shown some promise in mitigating greenhouse gas emissions, such as by sequestering carbon and decreasing soil nitrous oxide emissions. However, the effects of TBI on soil methane fluxes remain unknown. In a field study, we failed to show differences in soil CH4 production between TBI and conventional monocropping (CM) systems. Within TBI plots, however, we found significantly lower CH4 concentrations near the middle of the alleys than closer to tree rows. Soil CH4 concentrations also decreased with soil depth, even dipping below mean global atmospheric concentrations. Laboratory assays revealed a higher CH4 oxidation potential in soils collected from TBI plots compared with CM plots. These assays also revealed a decrease in CH4 oxidation potential after soils were amended with nitrate. We conclude that TBI could potentially reduce soil CH4 emissions, whereas fertilizer nitrate may increase them.

Vertical and lateral subsurface preferential flow along an agricultural catena

Abstract: Despite all efforts, agricultural contaminants remain at alarming concentrations in Quebec surface waters. Preferential flow (PF) of soluble contaminants has been suggested as a contributing factor but has not been specifically studied in the humid climate of eastern Canadian soils. Three tracers were surface applied on plots along a catena on a loamy sand under intensive agricultural production in Beauce, Quebec. Tracer distribution in soil profiles was monitored three times over a 12-month period. At the summit, finger flow rapidly transported tracers into the subsurface. The subsurface preferential lateral flow rapidly brought the tracers downslope. Narrow points of preferential seepage and discharge and underground lateral PF were observed at the footslope. The summit and the backslope of the catena showed strong vertical and lateral subsurface PF, which made their contribution to subsurface tracer movement toward surface water equal to or greater than that of the footslope, in part because of the hydrological connectivity between summit, backslope, and surface water. PF and matrix flow were both significant in all parts of the catena. Therefore, all parts of a catena, even those far from surface water, should be considered when evaluating potential belowground contaminant transport toward surface water.