Gas movement through soils is important for ecosystems and engineering in many ways such as for microbial and plant respiration, passive methane oxidation in landfill covers and oxidation of mine residues. Diffusion is one of the most important gas movement processes and the determination of the diffusion coefficient is a crucial step in any study. Five laboratory methods used for measuring the relative gas diffusion coefficient (D(s)/D(o)) were compared using a loamy sand, a porous media commonly found in agricultural fields and in several engineered structures, such as in landfill final covers. In the absence of macropores, all methods gave rather similar values of D(s)/D(o). Methods allowing the study of microscale variability indicated that the presence of macropores highly influenced gas movement, thus the value of D(s)/D(o), which, near a macropore may be one order of magnitude higher than in regions without macropores. Repacked columns do not allow the study of heterogeneity in D(s)/D(o). Natural spatial variability in D(s)/D(o) due to water distribution and preferential pathways can only be studied in large systems, but these systems are difficult to handle. Advantages and disadvantages of each method are discussed.

No abstract

Water retention curves are essential for the parameterization of soil water models such as HYDRUS. Although hydraulic parameters are known for a large number of mineral and natural organic soils, our knowledge on the hydraulic behavior of cultivated Histosols is rather limited. The objective of this study was to derive characteristic water retention curves for a large cultivated peatland with lettuce (Lactuca sativa L.) and vegetable farming in southern Quebec, Canada. A comparison showed that the van Genuchten model its better to the water retention data obtained with a Tempe pressure cell experiment than the Groenevelt-Grant model in terms of residual sum of squares; however, the difference in performance was quite small due to the high number of iterations used for itting. Finally, an agglomerative cluster analysis of 85 peat samples allowed us to deine two distinct water retention curves, where the irst water retention curve described samples of relatively shallow (<150 cm) Histosols with an organic content <0.89 and a bulk density >0.3 g cm −3 , and the second curve characterized samples of the deepest (depth 150-230 cm) Histosols with an organic content of up to 0.97 and a bulk density >0.3 g cm −3 , which are the soils that suffered a more dramatic transformation as a result of agriculture. This characterization allows for a multitude of applications, including parameterization of the HYDRUS model for soil water movement, and presents an essential tool for the optimization of water management in cultivated peatlands.Abbreviations: BFGS, Broyden-Fletcher-Goldfarb-Shanno.

Organic soils are an excellent substrate for commercial lettuce (Lactuca sativa L.) farming; however, drainage accelerates oxidation of the surface layer and reduces the water holding capacity, which is often lethal for crops that are sensitive to water stress. In this case study, we analyzed 942 peat samples from a large cultivated peatland complex (18.7 km2) in southern Quebec, Canada, and demonstrated from spatial and temporal patterns that agriculture resulted in a compacted layer below the root zone. We grouped the samples based on the year in which the corresponding fields were created on the previously undisturbed peatland (cutoff years 1970, 1980, 1990, and 2000) and discovered that bulk density has continued to increase, partly due to the overburden pressure, while organic matter has continued to decline since the fields were reclaimed and drained in phases between 1955 and 2006. Saturated hydraulic conductivity (Ks) in the upper 20 cm was remarkably lower on fields older than 10 yr (p = 0.0973 for Wilcoxon rank test), with more samples having a Ks < 2.0 × 10−3 yr. Soil water available capacity (SWAC) was between approximately 5 and 33 cm on fields reclaimed after 2000, while samples from fields reclaimed before 2000 had a lower SWAC between 2 and 23 cm (groups discernable at p = 0.0203). It is possible, however, that the greatest rate of change in Ks and SWAC occurred within even a year of reclamation. The results of this study call for active measures to reduce organic soil degradation such as reducing tillage and on‐field traffic or following a crop rotation scheme.

No abstract