David A. Lobb

Conventional and conservation tillage: influence on seasonal runoff, sediment, and nutrient losses in the Canadian Prairies.

Conservation tillage has been widely promoted to reduce sediment and nutrient transport from agricultural fields. However, the effect of conservation tillage on sediment and nutrient export in snowmelt-dominated climates is not well known. Therefore, a long-term paired watershed study was used to compare sediment and nutrient losses from a conventional and a conservation tillage watershed in the Northern Great Plains region of western Canada. During the treatment period, dissolved nutrient concentrations were typically greater during spring snowmelt than during summer rainfall events, whereas concentrations of sediment and particulate nutrients were greatest during rainfall events. However, because total runoff was dominated by snowmelt, most sediment and nutrient export occurred during snowmelt. Overall, conservation tillage reduced the export of sediment in runoff water by 65%. Similarly, concentrations and export of nitrogen were reduced by 41 and 68%, respectively, relative to conventional tillage. After conversion to conservation tillage, concentrations and exports of phosphorus (P) increased by 42 and 12%, respectively, with soluble P accounting for the majority of the exported P, especially during snowmelt. Our results suggest that management practices designed to improve water quality by reducing sediment and sediment-bound nutrient export from agricultural fields and watersheds can be less effective in cold, dry regions where nutrient export is primarily snowmelt driven and in the dissolved form. In these situations, it may be more appropriate to implement management practices that reduce the accumulation of nutrients in crop residues and the surface soil.

Tillage translocation and tillage erosion in the complex upland landscapes of southwestern Ontario, Canada

Tillage translocation and tillage erosion were measured throughout the topographically complex landscapes of two fields in the upland region of southwestern Ontario. Translocation of soil by tillage was measured by labelling plots of soil with chloride and measuring the tracers forward displacement in response to single passes by four tillage implements (mouldboard plough, chisel plough, tandem disc and field cultivator). The change in translocation within the landscape was used to measure tillage erosion. All four implements were erosive. A relationship between tillage translocation and slope gradient was observed; however, the variability in translocation could not be explained by slope gradient alone. Slope curvature was responsible for some translocation through the planning action of tillage implements. Tillage depth and speed were subject to considerable discontinuous and inconsistent manipulation by the operator in response to changing topographic and soil conditions. Tillage speed decreased by as much as 60% during upslope tillage and increased by as much as 30% during downslope tillage, relative to that on level ground. Tillage depth decreased by as much as 20% and increased by as much as 30%, relative to that on level ground. This manipulation is typical for tillage in complex landscapes and was presumed largely responsible for the variability in the results. The manipulation of tillage depth and speed are affected by the tractor-implement match and the responsiveness of the tillage operator.

Modelling tillage erosion in the topographically complex landscapes of southwestern Ontario, Canada

Abstract A tillage erosion model was developed for southwestern Ontario based on the relationship between tillage translocation and slope gradient and slope curvature. Two studies of tillage translocation and tillage erosion were used to calibrate this model, one a comparison of upslope and downslope tillage translocation on shoulder slopes, the other an examination of tillage translocation throughout topographically complex landscapes. Two field sites were used for validation of the model. For both sites, past tillage practices were known and past soil erosion was determined using 137 Cs as an indicator of soil redistribution. The model accurately predicted the pattern of soil redistribution that had occurred within the two field sites. Severe soil loss was observed and predicted on convex landscape positions and soil accumulation was observed and predicted on concave landscape positions. The model accounted for almost all of the soil lost from the convex upper slope positions where tillage erosion was expected to be the dominant erosion process. There was considerable soil loss and accumulation elsewhere in the landscapes which could not be accounted for by the model and was presumed to be primarily the result of water erosion. It was concluded that tillage erosion must be incorporated into soil erosion modelling for the purposes of soil conservation.

Pattern of greenhouse gas emission from a Prairie Pothole agricultural landscape in Manitoba, Canada

To obtain accurate N2O and CH4 emission estimates from the Prairie Pothole Region of North America, knowledge of landscape pattern and soil factors is important. A field study was conducted investigating the temporal and spatial variation in N2O and CH4 emissions from spring to fall 2005 and spring-thaw to post-fertilizer application period 2006 using static-vented chambers located at upper, middle and lower landscape elements planted to spring wheat in 2005 and flax in 2006 and riparian areas in an undulating terrain in southern Manitoba. N2O was emitted during spring-thaw and post-fertilizer application periods for cropped positions and CH4 was emitted about 7 wk after soil thaw for lower and riparian elements. While there was no statististical difference in N2O emission from upper, middle and lower landscape elements, there was greater occurrence of N2O emission hotspots at the lower element, associated with its comparatively higher soil moisture and carbon availability. A location of intense CH4 emiss...

The effects of multiple beneficial management practices on hydrology and nutrient losses in a small watershed in the Canadian prairies.

Most beneficial management practices (BMPs) recommended for reducing nutrient losses from agricultural land have been established and tested in temperate and humid regions. Previous studies on the effects of these BMPs in cold-climate regions, especially at the small watershed scale, are rare. In this study, runoff and water quality were monitored from 1999 to 2008 at the outlets of two subwatersheds in the South Tobacco Creek watershed in Manitoba, Canada. Five BMPs-a holding pond below a beef cattle overwintering feedlot, riparian zone and grassed waterway management, grazing restriction, perennial forage conversion, and nutrient management-were implemented in one of these two subwatersheds beginning in 2005. We determined that >80% of the N and P in runoff at the outlets of the two subwatersheds were lost in dissolved forms, ≈ 50% during snowmelt events and ≈ 33% during rainfall events. When all snowmelt- and rainfall-induced runoff events were considered, the five BMPs collectively decreased total N (TN) and total P (TP) exports in runoff at the treatment subwatershed outlet by 41 and 38%, respectively. The corresponding reductions in flow-weighted mean concentrations (FWMCs) were 43% for TN and 32% for TP. In most cases, similar reductions in exports and FWMCs were measured for both dissolved and particulate forms of N and P, and during both rainfall and snowmelt-induced runoff events. Indirect assessment suggests that retention of nutrients in the holding pond could account for as much as 63 and 57%, respectively, of the BMP-induced reductions in TN and TP exports at the treatment subwatershed outlet. The nutrient management BMP was estimated to have reduced N and P inputs on land by 36 and 59%, respectively, in part due to the lower rates of nutrient application to fields converted from annual crop to perennial forage. Overall, even though the proportional contributions of individual BMPs were not directly measured in this study, the collective reduction of nutrient losses from the five BMPs was substantial.

Patterns of water and tillage erosion on topographically complex landscapes in the North American Great Plains

Two field sites located in the northern region of the North American Great Plains were examined to investigate the contributions of water and tillage erosion towards total soil erosion in topographically complex landscapes (hummocky and undulating landscapes). Results indicated that both water and tillage erosion contributed substantially to total erosion in the undulating landscape while tillage erosion dominated in the hummocky landscape. The patterns of water, tillage and total soil erosion can be predicted using landscape segmentation in such landscapes. Soil properties and crop yield are also related to soil erosion. Landscape segmentation can be used as a simple tool to more easily represent the spatial variability of soil erosion and affected biophysical processes such as crop production, nutrient cycling, greenhouse gas emission and pesticide fate, and to target soil conservation practices toward the most intensive erosion processes on given landform elements.

Modelling tillage translocation using step, linear-plateau and exponential functions

The distance over which soil is displaced and mixed during tillage has important implications for the understanding the dynamics of soil variability within complex soil-landscapes. In two preceding studies of tillage translocation, tillage was observed to displace soil over a length of approximately 1 m following single passes of four tillage implements (chisel plough, mouldboard plough, tandem disc and field cultivator), and over a length of approximately 2 m per sequence of conventional tillage (one pass of mouldboard plough, two passes of tandem disc and one pass of field cultivator). Using data from these studies step, linear-plateau and exponential functions were assessed for their ability to estimate the magnitude of translocation and the redistribution pattern of soil within the till-layer, and to predict the redistribution pattern of soil within the till-layer. On average, step, linear-plateau and exponential models estimated 100.0%, 100.2% and 102.5% of the magnitude of translocation and 76%, 88% and 93% of the soil redistribution pattern, respectively. Based on these results, it was concluded that linear-plateau and exponential functions are suitable models of tillage translocation. The exponential model was superior to the step and linear-plateau models, and an improvement over the existing diffusion model.

The effectiveness of small-scale headwater storage dams and reservoirs on stream water quality and quantity in the Canadian Prairies

In response to flooding and soil erosion impacting the South Tobacco Creek watershed in southcentral Manitoba, local landowners constructed a network of small dams and reservoirs in the headwaters. Between 1999 and 2007, two of the small dams/reservoirs (Steppler multipurpose dam and Madill dry dam) were intensively monitored for their effectiveness in reducing peak flows and downstream sediment and nutrient loading during spring snowmelt (typically mid-March to mid-April) and summer rainfall (typically May to November) periods. These small-scale headwater storage dams were effective in reducing peak flows from agricultural land. The two dams/reservoirs monitored also reduced annual concentrations of sediment and total nitrogen (TN) to downstream receiving waters. However, annual concentrations of total phosphorus (TP) were only significantly reduced at the Madill dry dam, and the average concentrations of nitrogen (N) and phosphorus (P) within outflow water samples still exceeded guidelines for freshwater in the Canadian Prairies. Both dams/reservoirs significantly reduced annual loads of sediment, TN, and TP (Steppler dam, average of 77%, 15%, and 12%, respectively; Madill dam, average of 66%, 20%, and 9%, respectively). This corresponded to an average annual retention of 25 Mg y−1 (28 tn yr−1) of sediment, 166 kg N y−1 (366 lb N yr−1) and 17 kg P y−1 (37 lb P yr−1) by the Steppler dam, while 6 Mg y−1 (7 tn yr−1) of sediment, 181 kg N y−1 (399 lb N yr−1) and 10 kg P y−1 (22 lb P yr−1) were retained by the Madill dam. Both reservoirs reduced annual loads of dissolved N and P to downstream water bodies (Steppler, average of 14% and 10%, respectively; Madill, average of 23% and 15%, respectively), and were generally effective in removing dissolved N and P during both snowmelt and rainfall-generated runoff. The percent retention of dissolved nutrients was consistently higher during the summer than the spring. While the reservoirs removed particulates during snowmelt-generated runoff, they were often sources of suspended nutrients during rainfall-generated events. However, since dissolved nutrients were the dominant form of both N and P (>70% for both snowmelt and rainfall events), the two dams/reservoirs successfully reduced overall nutrient loads to downstream water bodies, annually and seasonally. In combination with improving flood and erosion control for the region, small headwater storage dams and reservoirs deserve consideration when developing watershed nutrient management plans, especially for undulating and hummocky regions on the Great Plains.

Herbicide Sorption Coefficients in Relation to Soil Properties and Terrain Attributes on a Cultivated Prairie

The sorption of 2,4-D and glyphosate herbicides in soil was quantified for 287 surface soils (0-15 cm) collected in a 10 x 10 m grid across a heavily eroded, undulating, calcareous prairie landscape. Other variables that were determined included soil carbonate content, soil pH, soil organic carbon content (SOC), soil texture, soil loss or gain by tillage and water erosion, and selected terrain attributes and landform segments. The 2,4-D sorption coefficient (Kd) was significantly associated with soil carbonate content (-0.66; P < 0.001), soil pH (-0.63; P < 0.001), and SOC (0.47; P < 0.001). Upper slopes were strongly eroded and thus had a significantly greater soil carbonate content and less SOC compared with lower slopes that were in soil accumulation zones. The 2,4-D Kd was almost twice as small in upper slopes than in lower slopes. The 2,4-D Kd was also significantly associated with nine terrain attributes, particularly with compounded topographic index (0.59; P < 0.001), gradient (-0.48; P < 0.001), mean curvature (-0.43; P < 0.001), and plan curvature (-0.42 P < 0.001). Regression equations were generated to estimate herbicide sorption in soils. The predicted power of these equations increased for 2,4-D when selected terrain attributes were combined with soil properties. In contrast, the variation of glyphosate sorption across the field was much less dependent on our measured soil properties and calculated terrain attributes. We conclude that the integration of terrain attributes or landform segments in pesticide fate modeling is more advantageous for herbicides such as 2,4-D, whose sorption to soil is weak and influenced by subtle changes in soil properties, than for herbicides such as glyphosate that are strongly bound to soil regardless of soil properties.

Critical Factors Affecting Field-Scale Losses of Nitrogen and Phosphorus in Spring Snowmelt Runoff in the Canadian Prairies

A long-term, field-scale study in southern Manitoba, Canada, was used to identify the critical factors controlling yearly transport of nitrogen (N) and phosphorus (P) by snowmelt runoff. Flow monitoring and water sampling for total and dissolved N and P were performed at the edge of field. The flow-weighted mean concentrations and loads of N and P for the early (the first half of yearly total volume of snowmelt runoff), late (the second half of yearly total volume of snowmelt runoff), and yearly snowmelt runoff were calculated as response variables. A data set of management practices, weather variables, and hydrologic variables was generated and used as predictor variables. Partial least squares regression analysis indicated that critical factors affecting the water chemistry of snowmelt runoff depended on the water quality variable and stage of runoff. Management practices within each year, such as nitrogen application rate, number of tillage passes, and residue burial ratio, were critical factors for flow-weighted mean concentration of N, but not for P concentration or nutrient loads. However, the most important factors controlling nutrient concentrations and loads were those related to the volume of runoff, including snow water equivalent, flow rate, and runoff duration. The critical factors identified for field-scale yearly snowmelt losses provide the basis for modeling of nutrient losses in southern Manitoba and potentially throughout areas with similar climate in the northern Great Plains region, and will aid in the design of effective practices to reduce agricultural nonpoint nutrient pollution in downstream waters.