Risk, N., Snider, D. and Wagner-Riddle, C. 2013. Mechanisms leading to enhanced soil nitrous oxide fluxes induced by freeze-thaw cycles. Can. J. Soil Sci. 93: 401-414. The freezing and thawing of soil in cold climates often produces large emissions of nitrous oxide (N₂O) that may contribute significantly to a soil's annual greenhouse gas emission budget. This review summarizes the state of knowledge of the physical and biological mechanisms that drive heightened N₂O emissions at spring melt. Most studies of freeze-thaw N₂O emissions have concluded that denitrification is the dominant process responsible for the large thaw fluxes. Soil moisture, availability of carbon and nitrogen substrates, and freeze temperature and duration are the major factors identified as controlling freeze-thaw cycle (FTC) N₂O emissions. Two mechanisms are proposed to lead to enhanced N₂O emissions at thaw: (1) the physical release of N₂O that is produced throughout the winter and trapped under frozen surface layers and/or within nutrient-rich water films in the frozen layers, and (2) the emission of newly produced (de novo) N₂O at the onset of thaw, which is stimulated by increased biological activity and changes in physical and chemical soil conditions. Early studies implicated the physical release of N₂O from subsurface soil layers as the main mechanism contributing to spring thaw emissions, but most current studies do not support this hypothesis. Mounting evidence suggests that most of the emitted N₂O is produced de novo. This may be fueled by newly available denitrification substrates that are liberated from dead microbes, fine roots, and/or the disintegration of soil aggregates. The release of N₂O trapped in shallow surface layers may represent a small, but important contribution of the total emissions. Application of new techniques to study microbial communities in their natural environments, such as metagenomics and stable isotope studies, have the potential to enhance our understanding of the soil N cycle and its linkages to FTC N₂O emissions. Future field studies of N₂O emissions ought to quantify both overwinter accumulation/release and the de novo production of N₂O so that the contribution of each mechanism to the annual emission budget is known.

Pelster, D. E., Chantigny, M. H., Rochette, P., Angers, D. A., Laganière, J., Zebarth, B. and Goyer, C. 2013. Crop residue incorporation alters soil nitrous oxide emissions during freeze-thaw cycles. Can. J. Soil Sci. 93: 415-425. Freeze-thaw (FT) cycles stimulate soil nitrogen (N) and carbon (C) mineralization, which may induce nitrous oxide (N₂O) emissions. We examined how soybean (Glycine max L.) and corn (Zea mays L.) residue incorporation affect N₂O emissions from high C content (35 g kg^-1) silty clay and low C content (19 g kg^-1) sandy loam soils over eight 10-d FT cycles, as a function of three temperature treatments [constant at 1°C (unfrozen control), 1 to -3°C (moderate FT), or 1 to -7°C (extreme FT)]. In unamended soils, N₂O emissions were stimulated by FT, and were the highest with extreme FT. This was attributed to the increased NO₃ availability measured under FT. Application of mature crop residues (C:N ratios of 75 for soybean and 130 for corn) caused rapid N immobilization, attenuating FT-induced N₂O emissions in the silty clay. In the sandy loam, residue addition also induced immobilization of soil mineral N. However, N₂O emissions under moderate FT were higher with than without crop residues, likely because N₂O production in this low-C sandy loam was stimulated by C addition in the early phase of incubation. We conclude that FT-induced N₂O emissions could be reduced through incorporation of mature crop residues and the subsequent immobilization of mineral N, especially in C-rich soils.

Kreyling, J., Haei, M. and Laudon, H. 2013. Snow removal reduces annual cellulose decomposition in a riparian boreal forest. Can. J. Soil Sci. 93: 427-433. Decomposition is a key process in carbon and nutrient cycling. However, little is known about its response to altered winter soil temperature regimes in boreal forests. Here, the impact of soil frost on cellulose decomposition over 1 yr and soil biotic activity (bait-lamina sticks) over winter, in spring, and in summer was investigated using a long-term (9-yr) snow-cover manipulation experiment in a boreal Picea abies forest. The experiment consisted of the treatments: snow removal, increased insulation, and ambient control. The snow removal treatment caused longer and deeper soil frost (minimum temperature -8.6°C versus -1.4°C) at 10 cm soil depth in comparison with control, while the increased insulation treatment resulted in nearly no soil frost during winter. Annual cellulose decomposition rates were reduced by 46% in the snow removal manipulation in comparison with control conditions. Increased insulation had no significant effect on decomposition. The decomposition was mainly driven by microorganisms, as no significant difference was observed for containers enclosed with a 44-µm and a 1-mm mesh. Soil biotic activity was slightly increased by both the snow removal and the increased insulation treatment in comparison with control conditions over winter. However, this effect disappeared over spring and summer. We conclude that soil frost can have strong effects on decomposition in boreal ecosystems. Further studies should investigate to which degree the observed reduction in decomposition due to reduced snow cover in winter slows or even offsets the expected increase in decomposition rates with global warming.

Elliott, J. 2013. Evaluating the potential contribution of vegetation as a nutrient source in snowmelt runoff. Can. J. Soil Sci. 93: 435-443. On the Canadian prairies, most nutrient transport to surface waters takes place during snowmelt. The potential for a range of 11 residue types to release nitrogen (N), phosphorus (P) and carbon (C) was assessed by snowmelt simulation. Interactions between soils and residues were measured for two contrasting residues. Samples (taken in late fall) were frozen prior to snowmelt simulations that consisted of three diurnal temperature cycles from -5°C to 9°C followed by a final melt at 5°C. Releases of total and total dissolved P (TP and TDP), total dissolved N (TDN), and dissolved organic C (DOC) during simulated snowmelt were greater from actively growing residues than from crop stubble and were significantly related to plant moisture and nutrient contents. Nutrient release from wheat stubble (WS) was statistically similar to that from the underlying surface soil but releases of P and ammonia (NH₃) from winter wheat (WW) were at least four times greater than for the corresponding soil. When combined samples of residue and soil were tested, releases of most nutrients were less than when the residue and soil were tested separately. Potential release of nutrients from vegetation is a factor for consideration in the design of practices to reduce nutrient transport.

Cade-Menun, B. J., Bell, G., Baker-Ismail, S., Fouli, Y., Hodder, K., McMartin, D. W., Perez-Valdivia, C. and Wu, K. 2013. Nutrient loss from Saskatchewan cropland and pasture in spring snowmelt runoff. Can. J. Soil Sci. 93: 445-458. To develop appropriate beneficial management practices (BMPs) for a watershed, it is essential to quantify the nutrients lost from agricultural fields and to identify the mechanisms of nutrient transport. To determine appropriate BMPs for a watershed in southeastern Saskatchewan, nutrient concentrations were measured in spring 2010 in snowmelt runoff from fertilized annual cropland (zero till) and perennial tame pastures. The majority of nutrient loss was in dissolved form, rather than as particulates. Significantly more nitrogen (N) was lost from pastures as dissolved ammonium than from cropland, while significantly more dissolved organic N was lost from croplands. Although there were no significant differences in total phosphorus (P) loss, there were significantly higher concentrations of dissolved reactive P in runoff from cropland, and significantly higher particulate P in runoff from pastures. Total carbon (C) in runoff was higher from cropland, due mainly to significantly higher dissolved organic C concentrations. Runoff from cropland contained significantly higher concentrations of dissolved potassium and sulfur, reflecting the fertilization of cropland fields with these nutrients. These preliminary results demonstrate that nutrients may be transported from agricultural lands by different mechanisms (e.g., in dissolved versus particulate forms) as a function of cropping system, requiring the development of specific types of BMPs to best control nutrient losses.

Edwards, L. M. 2013. The effects of soil freeze-thaw on soil aggregate breakdown and concomitant sediment flow in Prince Edward Island: A review. Can. J. Soil Sci. 93: 459-472. The importance of aggregate size and integrity in soil productivity and crop production is paramount, and aggregate size reduction or increase invariably becomes a primary concern in such soil management practices as tillage and organic matter manipulation. In this regard, therefore, the present review looks particularly at the consequence of freeze-thaw cycling (FTC) on agricultural lands in Prince Edward Island (PEI) where an annual average of 40 cycles induce measurable aggregate breakdown with mixed consequences. On the one extreme, the consequences are manifest in increased soil erosion. On the other extreme, reduced (or reversed) soil compaction and improved seedbed conditions are welcomed consequences where temperature alternation breaks up hard pans or soil clods, or where the predominance of smaller aggregates can be an asset in seedbed environments, favouring improved crop emergence and early-spring establishment. In the PEI soils studied, the greatest changes in aggregate size distribution with FTC occurred in the largest and smallest size fractions wherein fractions <0.5 mm showed a 33% average increase while, simultaneously, the 4.75-9.5 mm fractions showed a 28% average decrease. This breakdown is reflected most contrastingly where FTCs to maximum (asymptotic) breakdown averaged up to 3.5 times for a loam as it did for a sandy loam or a fine sandy loam soil. This review also examines FTC in a broader agricultural and environmental context where it can potentially impact agro-sustainability. Where FTC effects on a fine sandy loam were measured in terms of erosion, there was a sediment mass increase of about 90% in interrill flow and about 25% in rill flow. Further, this review emphasizes methodology that has proven to be workable under the circumstances of PEI's dominant agricultural soils and the FTC research objectives that they helped to shape. It was considered important in this review, also, to highlight the need for expanded research (commencing with regional cooperation), particularly on frost depth, to feed into moisture-availability modelling towards improved clarity for end-user benefit.

Dagesse, D. F. 2013. Freezing cycle effects on water stability of soil aggregates. Can. J. Soil Sci. 93: 473-483. The freezing process is commonly implicated as a key factor in defining the state of soil structural stability following the winter months. Controversy exists, however, regarding the efficacy, and even the net effect, of this process. The objective of the study was to establish the separate effects of the freezing, freeze-thaw and freeze-drying processes in defining soil structural stability following the over-winter period. Aggregates from soils of varying clay content (0.11, 0.33, 0.44 kg kg^-1) and initial water content (0.10, 0.20 or 0.30 kg kg^-1) were subjected to freeze-only (F), freeze-thaw (FT) and freeze-dry (FD) treatments. Post-treatment aggregate stability determination was via wet aggregate stability (WAS) and dispersible clay (DC). Freezing alone and freeze-dry treatments generally resulted in greater aggregate stability, while the freeze-thaw generally resulted in lower aggregate stability as compared with a control, not frozen treatment (T). These data suggest the freezing-induced desiccation process improves aggregate stability, while the addition of a thaw component following freezing, with the attendant liquid water, is responsible for degradation of aggregate stability. Clay content and initial water content are important factors governing the magnitude of this process.

Fouli, Y., Cade-Menun, B. J. and Cutforth, H. W. 2013. Freeze-thaw cycles and soil water content effects on infiltration rate of three Saskatchewan soils. Can. J. Soil Sci. 93: 485-496. Many soils at high latitudes or elevations freeze and thaw seasonally. More frequent freeze-thaw cycles (FTCs) may affect ecosystem diversity and productivity because freeze-thaw cycles cause changes in soil physical properties and affect water movement in the landscape. This study examined the effects of FTCs (0, 1, 5, and 10) and antecedent soil water content [at soil water potentials (SWP) -1.5, -0.033 and -0.02 MPa] on the infiltration rate of three Saskatchewan soils (a clay, a loam, and a loamy sand). A tension infiltrometer was used at tensions [water potentials of the tension infiltrometer (WPT)] -5, -10 and -15 cm. Infiltration rates increased with increasing SWPs for the loam and clay soils due to higher infiltrability into drier soils. Infiltration rates decreased with increasing SWPs for the loamy sand, probably the result of less surface tension, unimodal porosity, and increased gravitational potential. Infiltration rates either decreased or did not change with increasing FTCs, and this may be due to increased water viscosity as temperatures approach freezing. Also, ice may have formed in soil pores after frequent FTCs, causing lower infiltration rates. Infiltration rates for clay at -1.5 MPa were higher than for loam or loamy sand, probably the result of clay mineralogy and potential shrinking and cracking. Soil texture and initial water content had a significant effect on infiltration rates, and FTCs either maintained or lowered infiltration rates.

Christensen, A. F., He, H., Dyck, M. F., Turner, L., Chanasyk, D. S., Naeth, M. A. and Nichol, C. 2013. In situ measurement of snowmelt infiltration under various topsoil cap thicknesses on a reclaimed site. Can. J. Soil Sci. 93: 497-510. Understanding the soil and climatic conditions affecting the partitioning of snowmelt to runoff and infiltration during spring snow ablation is a requisite for water resources management and environmental risk assessment in cold semi-arid regions. Soil freezing and thawing processes, snowmelt runoff or infiltration into seasonally frozen soils have been documented for natural, agricultural or forested systems but rarely studied in severely disturbed systems such as reclaimed lands. The objective of this study was to quantify the snowmelt infiltration/runoff on phosphogypsum (PG) tailings piles capped with varying thicknesses of topsoil (0.15, 0.3, and 0.46 m) at a phosphate fertilizer production facility in Alberta. There are currently no environmental regulations specifying topsoil capping thickness or characteristics for these types of tailings piles. Generally, the function of the topsoil cap is to facilitate plant growth and minimize the amount of drainage into the underlying PG. Experimental plots were established in 2006 to better understand the vegetation and water dynamics in this reconstructed soil. In 2011, time domain reflectometry (TDR) probes and temperature sensors were installed at various depths for continuous, simultaneous, and automated measurement of composite dielectric permittivity (ε_eff) and soil temperature, respectively. An on-site meteorological station was used to record routine weather data. Liquid water and ice content were calculated with TDR-measured effective permittivity (ε_eff) and a composite dielectric mixing model. Spatial and temporal change of total water content (ice and liquid) revealed that snowmelt infiltration into the topsoil cap increased with increasing topsoil depth and net soil water flux from the topsoil cap into the PG material was positive during the snowmelt period in the spring of 2011. Given the objective of the capping soil is to reduce drainage of water into the PG material it is recognized that a capping soil with a higher water-holding capacity could reduce the amount of meteoric water entering the tailings.

Mohammed, G. A., Hayashi, M., Farrow, C. R. and Takano, Y. 2013. Improved characterization of frozen soil processes in the Versatile Soil Moisture Budget model. Can. J. Soil Sci. 93: 511-531. Soil freezing and thawing influence the infiltration of rain and snow melt water and subsequent redistribution, runoff generation, and a host of other processes. Accurate characterization of frozen soil processes in hydrological models is important for their use in managing agricultural activities and water resources. The Versatile Soil Moisture Budget (VSMB) is a relatively simple soil water balance model, which has been widely used in Canada for several decades, but its application has primarily been for crop-growing seasons. We have modified the VSMB to include new algorithms for snow accumulation and melt, soil freezing and thawing, and snowmelt infiltration and runoff; and evaluated its performance using field data from a grassland site in Alberta. The new VSMB model simulates snow processes with reasonable accuracy and predicts the day of thawing within several days of observation. It also estimates the amount of runoff and its inter-annual variability reasonably well, although the model still has limitations in accurately predicting the vertical distribution of water content. Despite these limitations, the model will be useful for estimating the amount of snowmelt runoff that provides the critical water inputs to wetlands and dugouts, and for understanding the effects of landuse variability on these processes.

Taina, I. A., Heck, R. J., Deen, W. and Ma, E. Y. T. 2013. Quantification of freeze-thaw related structure in cultivated topsoils using X-ray computed tomography. Can. J. Soil Sci. 93: 533-553. The aim of this work was to test the utility of various CT procedures for the investigation of topsoil microstructural features formed under the influence of annual freeze-thaw processes in an agricultural Orthic Gray Brown Luvisol. Assuming that various tillage systems have different propensity to develop and preserve these features, soil cores were collected, prior to spring tillage, from plots characterized by no-till, moldboard plow and disc tillage systems. Micromorphological characterization of CT imagery indicated that soils from no-till treatments exhibited clearly developed platy structures. However, for two of these treatments, quantitative determinations of porosity in binary imagery did not reveal the presence of fine planar mesovoids associated with platy structures. Semivariograms of attenuation, in the three orthogonal axes of the CT imagery, revealed a higher dissimilarity in the vertical direction than in either horizontal direction in all the treatments that were most affected by freeze-thaw processes. The variability of the grayscale values along the soil vertical and horizontal directions were also employed to establish morphometric indices for the platy structure.