4R Management of Phosphorus Fertilizer in the Northern Great Plains.

Phosphorus (P) fertilizer has played a vital role in increasing the productivity of crop production in the northern Great Plains for approximately 100 years. Throughout this period, agricultural production practices have changed dramatically, while our knowledge of P behavior and beneficial management practices has improved. Some of the more recent and substantial changes in farming practices on the northern Great Plains include widespread adoption of reduced tillage systems, introduction of new crops and high-yielding cultivars, intensification and extension of crop rotations, development of new fertilizer products, increased appreciation of the role of microbial interactions in P dynamics, and growing concern for the effects of P on water quality. As cropping systems, technology, and societal demands evolve over time, nutrient management practices must also evolve to address concerns and take advantage of emerging opportunities. Classic principles and new P fertilizer technologies and management practices must be integrated into packages of 4R practices that optimize crop yield and agronomic efficiency while minimizing negative environmental impact and conserving P resources. Although a wide range of products and practices can be combined for this approach, placing ammonium phosphate fertilizer in a band, in or near the seed-row, at the time of seeding and at a rate that matches P removal by the crop generally provides the greatest P efficiency, long-term sustainability, and environmental protection for small grain, oilseed, and pulse crop production in the northern Great Plains.

Effectiveness of Vegetated Buffer Strips in Controlling Legacy Phosphorus Exports from Agricultural Land.

The continued phosphorus (P) impairment of freshwaters and the associated risk of eutrophication raise questions regarding the efficiency of current beneficial management practices (BMPs) for improving water quality. Vegetated buffer strips (VBSs) are widely encouraged BMPs for reducing P export from agricultural land. However, there is a lack of evidence regarding the long-term efficiency of VBSs for reducing legacy P losses. This research used soil analyses to investigate the P removal efficiency of an unmanaged VBS for controlling P loss from agricultural land in Manitoba, Canada, between 1954 and 2011. The results showed statistically significant retention of total P, Olsen extractable P, and 0.01 M CaCl extractable P by a 5-m wide VBS compared with field soils. We found that surface soils at 5-m into the VBS had a significantly greater P sorption capacity and a smaller degree of P saturation (DPS) than adjacent field soils. The elevated DPS in field soils is likely associated with gradual P enrichment as a result of manure or fertilizer application over time and the strong affinity of P compounds for soil. Although P stratification in the VBS over 57 yr resulted in a significant increase (∼11%) in DPS of VBS topsoil compared with VBS subsoil, our findings do not support the saturation of VBS soils with P. However, cutting and removal of vegetation from VBS could be a useful strategy to remove P from VBS and minimize possible P remobilization associated with vegetation senescence, especially where the climate is cold and runoff is dominated by snowmelt.

Uncertainties in vegetated buffer strip function in controlling phosphorus export from agricultural land in the Canadian prairies.

Vegetated buffer strips (VBSs) are widely encouraged as a cost-effective strategy to address phosphorus (P) pollution associated with agricultural production. However, there is a lack of evidence in the effectiveness of these measures for tackling diffuse P pollution in cold-climate regions under concentrated runoff flow conditions. This research aimed to investigate the effects of VBSs on reducing P concentrations in surface runoff at three different watersheds in Manitoba, Canada. Surface runoff samples were collected in four sub-catchments from each watershed by installing paired weirs at 0.5-m and at 5-m into the VBSs along the expected runoff flow path. In addition, P concentrations were measured in soil samples collected within and outside of the runoff flow path to gain further insight into P dynamics within VBSs at each study site. The results indicate that VBSs had little or no significant effect on reducing the concentration of P forms in surface runoff in the majority of situations, resulting in reduced runoff losses of total, dissolved and particulate P concentrations in only 23, 12 and 12% of the situations, respectively. In addition, Olsen extractable P concentrations in VBS soils were not significantly different from field soils both within and outside of the flow path. The ineffective P retention by VBSs in this region is likely associated with the fact that the majority of the runoff flow is concentrated through small portions of VBSs and occurs during snowmelt when biogeochemical processes responsible for P retention in VBSs are limited. Further research is needed to develop alternative management practices that enhance P retention during concentrated snowmelt runoff events in such cold-climate regions.

Nutrient and sediment losses in snowmelt runoff from perennial forage and annual cropland in the canadian prairies.

An 8-yr field-scale study, 2005 to 2012, investigated effects of agricultural land use on nutrient and sediment losses during snowmelt runoff from four treatment fields in southern Manitoba. In 2005, two fields with a long-term history of annual crop (AC) production were planted to perennial forage (PF), while two other fields were left in AC production. In 2009, the AC fields were converted to PF, while the PF fields were returned to AC. Runoff flow rates were monitored at the lower edge of the fields, and nutrient concentrations of runoff water were determined. The effects of AC and PF on selected variables were similar for the spatial (between-fields) and temporal (within-field) comparisons. The flow-weighted mean concentrations (FWMCs) and loads of particulate N, P, and sediment were not affected by treatment. Soil test N and the FWMC and load of NO (NO + NO) were significantly greater in the AC treatment, but the FWMC and load of NH were greater in the PF treatment. Loads of total dissolved N (TDN) and total N (TN) were not affected by treatment, although the concentrations of TDN and TN were greater in the AC treatment. The PF treatment significantly increased FWMCs and loads of total dissolved P (TDP) and total P (TP). On an annual snowmelt runoff basis, the PF treatment increased the FWMC of TDP by 53% and TP by 52% and increased the load of TDP by 221% and TP by 160% compared with the AC treatment. The greater P and NH losses in the PF treatment were attributed mainly to nutrient release from forage residue due to freezing.

Conversion of Conservation Tillage to Rotational Tillage to Reduce Phosphorus Losses during Snowmelt Runoff in the Canadian Prairies.

In a preceding study, converting conventional tillage (ConvT) to conservation tillage (ConsT) was reported to decrease nitrogen (N) but to increase phosphorus (P) losses during snowmelt runoff. A field-scale study was conducted from 2004 to 2012 to determine if conversion of ConsT to rotational tillage (RotaT), where conservation tillage was interrupted by a fall tillage pass every other year, could effectively reduce P losses compared with ConsT. The RotaT study was conducted on long-term paired watersheds established in 1993. The ConvT field in the pair has remained under ConvT practice since 1993, whereas tillage was minimized on the ConsT field from 1997 until 2007. In fall 2007, RotaT was introduced to the ConsT field, and heavy-duty cultivator passes were conducted in the late fall of years 2007, 2009, and 2011. Runoff volume and nutrient content were monitored at the edge of the two fields, and soil and crop residue samples were taken in each field. Greater soil Olsen P and more P released from crop residue are likely the reasons for the increased P losses in the ConsT treatment (2004-2007) relative to the ConvT treatment (2004-2007). Analysis of covariance indicated that, compared with ConsT (2004-2007), RotaT (2008-2012) increased the concentrations of dissolved organic carbon (DOC) by 62%, total dissolved N (TDN) by 190%, and total N (TN) by 272% and increased the loads of DOC by 34%, TDN by 34%, and TN by 60%. However, RotaT (2008-2012) decreased soil test P in surface soil, P released from crop residue, and duration of runoff compared with ConsT (2004-2007) and thus decreased the concentrations of total dissolved P (TDP) by 46% and total P (TP) by 38% and decreased the loads of TDP by 56% and TP by 42%. In the Canadian Prairies, where P is a major environmental concern compared with N, RotaT was demonstrated to be an effective practice to reduce P losses compared with ConsT.

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.

Critical source area management of agricultural phosphorus: experiences, challenges and opportunities.

The concept of critical source areas of phosphorus (P) loss produced by coinciding source and transport factors has been studied since the mid 1990s. It is widely recognized that identification of such areas has led to targeting of management strategies and conservation practices that more effectively mitigate P transfers from agricultural landscapes to surface waters. Such was the purpose of P Indices and more complex nonpoint source models. Despite their widespread adoption across the U.S., a lack of water quality improvement in certain areas (e.g. Chesapeake Bay Watershed and some of its tributaries) has challenged critical source area management to be more restrictive. While the role of soil and applied P has been easy to define and quantify, representation of transport processes still remains more elusive. Even so, the release of P from land management and in-stream buffering contribute to a legacy effect that can overwhelm the benefits of critical source area management, particularly as scale increases (e.g. the Chesapeake Bay). Also, conservation tillage that reduces erosion can lead to vertical stratification of soil P and ultimately increased dissolved P loss. Clearly, complexities imparted by spatially variable landscapes, climate, and system response will require iterative monitoring and adaptation, to develop locally relevant solutions. To overcome the challenges we have outlined, critical source area management must involve development of a 'toolbox' that contains several approaches to address the underlying problem of localized excesses of P and provide both spatial and temporal management options. To a large extent, this may be facilitated with the use of GIS and digital elevation models. Irrespective of the tool used, however, there must be a two-way dialogue between science and policy to limit the softening of technically rigorous and politically difficult approaches to truly reducing P losses.

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.

Are current phosphorus risk indicators useful to predict the quality of surface waters in southern manitoba, Canada?

Many phosphorus (P) risk indicators have been developed to assess the risk of P loss from agricultural land to surface water. Most of these indicators are designed for land and climates where rainfall-induced erosion of particulate P from sloping landscapes is the main process of P transport. No indicators have been validated in the Canadian Prairies, where P losses are driven by snowmelt-driven runoff over nearly level landscapes and frozen soils. The objective of this project was to evaluate the relationship between water quality data for P from 14 watersheds and three existing P risk indicators for their potential use in the southern Manitoba prairie region of Canada. None of the indicators, including Birr and Mulla's P Index, a preliminary P risk indicator for Manitoba, and a preliminary version of Canada's National Indicator of Risk of Water Contamination by Phosphorus, was significantly correlated with mean concentrations of total P in water or P export per hectare. Although erosion risk was a significant factor influencing the value of these indexes, erosion risk was not correlated with either measure of P loss in these watersheds. Several other watershed characteristics, including average soil test P concentrations, livestock density, proportion of land in annual crops, and the land's inherent capability for agricultural production, were strongly correlated with P concentrations in water (r = 0.80***, r = 0.63**, 0.76***, and -0.70**, respectively). Therefore, these types of P risk indicators require modifications to estimate the risk of P loss under the soil, landscape, and climatic conditions of southern Manitoba.