Merrin L Macrae

University of Waterloo | UWaterloo · Department of Geography and Environmental Management (Environment)

Agricultural P losses are a global economic and water quality concern. Much of the current understanding of P dynamics in agricultural systems has been obtained from rainfall‐driven runoff, and less is known about cold‐season processes. An improved understanding of the magnitude, form, and transport flow paths of P losses from agricultural cropland...

Cold agricultural regions are important sites of global food production. This has contributed to widespread water quality degradation influenced by processes and hydrologic pathways that differ from warm region analogues. In cold regions, snowmelt is often a dominant period of nutrient loss. Freeze–thaw processes contribute to nutrient mobilization...

Phosphorus (P) loss in agricultural discharge has typically been associated with surface runoff; however, tile drains have been identified as a key P pathway due to preferential transport. Identifying when and where these pathways are active may establish high‐risk periods and regions that are vulnerable for P loss. A synthesis of high‐frequency, r...

Agricultural phosphorus (P) losses to surface water bodies remain a global eutrophication concern, despite the application of conservation practices on farm fields. Although it is generally agreed upon that the use of multiple conservation practices (“stacking”) will lead to greater improvements to water quality, this may not be cost effective to f...

Soil moisture response to rainfall is a key factor that dictates how well a landscape can support crop growth as well as its susceptibility to water runoff and leaching, however, few studies have investigated how agricultural management impacts this important soil function. This study compares two common agricultural soil treatments (cover crops an...

Systematic tile drainage is used extensively in poorly drained agricultural lands to remove excess water and improve crop growth; however, tiles can also transfer nutrients from farmlands to downstream surface water bodies, leading to water quality problems. Thus, there is a need to simulate the hydrological behaviour of tile drains to understand t...

Phosphorus (P) runoff from agriculture exacerbates eutrophication globally. Despite diverse mitigation options, the issue persists, necessitating the evaluation of slow-release fertilizers for crop production to minimize environmental P losses. Struvite, a P-rich, recycled product with low water solubility, holds promise for minimizing off-site env...

Climate change presents unique challenges for grape growers across the world. In Ontario, three distinct viticultural regions are experiencing climatic shifts towards warmer growing seasons. According to historical records collected from Environment and Climate Change Canada, Lake Erie North Shore has transitioned from an intermediate-to-warm growi...

Phosphorus (P) loss from agricultural systems to surface waters, and ultimately, eutrophication, presents a wicked problem requiring transdisciplinary solutions. The mission of SERA‐17 (Southern Extension and Research Advisory Information Exchange Group‐17) has been to address this problem by developing “Innovative Solutions to Minimize Phosphorus...

Highlights EPIC, SHAW, and DRAINMOD models were evaluated for the simulation of winter hydrology. Energy-based models can better simulate late-winter and early-spring hydrology under winter conditions. Effective simulation of soil temperature and soil hydraulics in winters were identified as potential areas of development in temperature-based model...

The objective of this study is to identify the optimal spatial distribution of Best Management Practices (BMPs) to reduce total phosphorus (TP) runoff from agricultural land in the largest Canadian watershed draining into Lake Erie, the Great Lake most vulnerable to eutrophication. BMP measures include reduced fertilizer application, cover crops, b...

Nutrient and soil loss from agricultural areas impairs surface water quality globally. In the Great Lakes region, increases in the frequency and magnitude of harmful and nuisance algal blooms in freshwater lakes have been linked to elevated phosphorus (P) losses from agricultural fields, some of which are transported via tile drainage. This study e...

Phosphorus (P) budgets can be useful tools for understanding nutrient cycling and quantifying the effectiveness of nutrient management planning and policies; however, uncertainties in agricultural nutrient budgets are not often quantitatively assessed. The objective of this study was to evaluate uncertainty in P fluxes (fertilizer/manure applicatio...

Soil moisture is a known environmental factor influencing carbon dioxide (CO 2) emissions and therefore represents an important variable in predictive models. Establishing relationships between soil CO 2 emissions and soil moisture has long focused on the role of soil organic carbon mineralization by aerobic respiration. This approach, which genera...

Agricultural fields in the Red River Valley of the Northern Great Plains are located on flat clay soils, often drained by shallow, roadside ditches that are not graded and lacking relief. These conditions can result in flow reversals and subsequent flooding of adjacent fields during large runoff events, which can mobilize phosphorus (P). Surface ru...

Systematic tile drainage is used extensively in agricultural lands to remove excess water and improve crop growth; however, tiles can also transfer nutrients from farmlands to downstream surface water bodies, leading to water quality problems. There is a need to simulate the hydrological behaviour of tile drains to understand the impacts of climate...

Losses of phosphorus from soil to surface waters in agricultural areas have been linked to substantial declines in water quality. Riparian wetlands can potentially intercept phosphorus mobilized from upland soils before it reaches connecting waterways, but the capacity of wetlands to buffer against downstream losses of P is poorly understood, espec...

The eutrophication of freshwater systems is a pervasive issue in North America and elsewhere, which has been linked to elevated phosphorus (P) loading from watersheds. Most excess P is thought to originate from non-point agricultural sources, and less attention has been given to small rural point sources, such as bunker silos on livestock farms. So...

Although many studies have examined how improvements in wastewater treatment impact river nutrient concentrations and loads, there has been much less focus on measuring river metabolism to evaluate the wider aquatic ecosystem benefits of reducing nutrient inputs to rivers. The objectives of this study were to evaluate the effects of enhanced wastew...

This study is a meta-analysis of global articles on hydrological nutrient dynamics to determine trends and consensus on: (1) the effects of climate change-induced hydrological and temperature drivers on nutrient dynamics and how these effects vary along the catchment continuum from land to river to lake; (2) the convergence of climate change impact...

Delineating the relative solubility of soil phosphorus (P) in agricultural landscapes is essential to predicting potential P mobilization in the landscape and can improve nutrient management strategies. This study describes spatial patterns of soil extractable P (easily, moderately, and poorly soluble P) in agricultural landscapes of the Red River...

Peatlands store carbon (C), nitrogen (N), and phosphorus (P), and the stoichiometric relationship among them may be modified by ecosystem disturbances, with major implications for boreal peatland ecosystem functions. To understand the potential impact of landscape fragmentation on peatland nutrient stoichiometry, we characterize the stoichiometric...

Improving understanding of how water use efficiency (WUE), evapotranspiration (ET), and gross primary productivity (GPP) (CO2 exchange) vary across agricultural systems can help farmers better prepare for an uncertain future due to climate change by assessing water requirements for a crop as a function of current environmental conditions. This stud...

Quantifying spatial and temporal fluxes of phosphorus (P) within and among agricultural production systems is critical for sustaining agricultural production while minimizing environmental impacts. To better understand P fluxes in agricultural landscapes, P‐FLUX, a detailed and harmonized dataset of P inputs, outputs, and budgets, as well as estima...

Quantifying spatial and temporal fluxes of phosphorus (P) within and among agricul- tural production systems is critical for sustaining agricultural production while min- imizing environmental impacts. To better understand P fluxes in agricultural land- scapes, P-FLUX, a detailed and harmonized dataset of P inputs, outputs, and budgets, as well as...

Algal blooms fueled by phosphorus (P) enrichment are threatening surface water quality around the world. Although P loss from arable land is a critical contributor to P loads in many agricultural watersheds, there has been a lack of understanding of P loss patterns and drivers across regions. Here, we synthesized edge-of-field P and sediment runoff...

Excess nutrients in aquatic ecosystems is a major water quality problem globally. Worsening eutrophication issues are notable in cold temperate areas, with pervasive problems in many agriculturally dominated catchments. Predicting nutrient export to rivers and lakes is particularly difficult in cold agricultural environments because of challenges i...

Phosphorus (P) runoff from agricultural land plays a critical role in downstream water quality. This article summarizes P and sediment runoff data for both snowmelt and rainfall runoff from 30 arable fields in the Canadian provinces of Saskatchewan, Manitoba and Ontario. The data were collected from 216 site-years of field experiments, with climate...

Water use efficiency (WUE) can be calculated using a range of methods differing in carbon uptake and water use variable selection. Consequently, inconsistencies arise between WUE calculations due to complex physical and physiological interactions. The purpose of this study was to quantify and compare WUE estimates (harvest or flux-based) for alfalf...

Globally, maize (Zea mays, a C4-plant) and alfalfa (Medicago sativa, a C3-plant) are common and economically important crops. Predicting the response of their water use efficiency, WUE, to changing hydrologic and climatic conditions is vital in helping farmers adapt to a changing climate. In this study, we assessed the effective leaf area index (eL...

Nutrient losses from agricultural operations are a major contributor to the eutrophication of freshwaters. Although many studies have quantified diffuse nutrient losses, less is known about agricultural point-source contributions, such as bunker silos, to watershed phosphorus (P) loads. This study examined the contributions of a dairy farm bunker s...

Agricultural tile drainage is expanding in the northern Great Plains of North America. Given ongoing environmental and political concerns related to the eutrophication of Lake Winnipeg in Canada and the potential for tile drains to transport significant quantities of nutrients from agricultural fields, an improved understanding of nutrient dynamics...

Excess nutrients in aquatic ecosystems is a major water quality problem globally. Worsening eutrophication issues are notable in cold temperate areas, with pervasive problems in many agriculturally dominated catchments. Predicting nutrient export to rivers and lakes is particularly difficult in cold agricultural environments because of challenges i...

Peatlands contain a globally significant store (30%) of soil carbon (C). Within the Canadian Western Boreal Plains, where peatlands are a dominant feature, the climate is becoming warmer and drier, coupled with an increase in forest fire incidence. The response of peatlands to forest fire is likely to be a key determinant in the future of C storage...

Peatlands dominate the landscape (∼50% of total area) in the Athabasca Oil Sands Region (AOSR) in northern Alberta, and as such, are susceptible to hydrological disturbance from oil sands mining and exploration. In this study, we explore the hydrochemical response of a disturbed fen (Firebag Fen) in the AOSR following the removal of a temporary acc...

The detrimental impacts of agricultural subsurface tile flows and their associated pollutants on water quality is a major environmental issue in the Great Lakes region and many other places globally. A strong understanding of water quality indicators along with the contribution of tile-drained agriculture to water contamination is necessary to asse...

Soil erosion from agricultural lands continues to be a global societal problem. The movement of soils is often accompanied by nitrogen and phosphorus that are crucial to crop growth, but their redistribution from farm fields to waterways can reduce crop yields and degrade water quality. While within-field sediment and nutrient movement has been qua...

The dynamics and processes of nutrient cycling and release were examined for a lowland wetland‐pond system, draining woodland in southern England. Hydrochemical and meteorological data were analyzed from 1997 to 2017, along with high‐resolution in situ sensor measurements from 2016 to 2017. The results showed that even a relatively pristine wetland...

Tile drainage of agricultural fields is a conduit for nutrient losses. Preferential flow in the soil can more directly connect surface runoff with tile drainage compared with matrix flow, which may also increase P losses. In this study, water temperature was monitored in surface runoff and tile drainage and electrical conductivity (EC) was measured...

Warming conditions across Canada's subarctic and arctic regions are causing permafrost landforms to thaw, resulting in rapid land cover change, including conversion of peat plateaus to wetland and thermokarst. These changes have important implications for northern ecosystems, including shifting controls on carbon uptake and release functions, as we...

Water quality issues, including harmful and nuisance algal blooms (HNABs), related to nitrogen (N) and phosphorus (P) exported from agricultural lands persist in the Great Lakes region. Previous work examining N and P loss from agricultural fields in portions of the United States (US) and Canada (CA) that drain into Lake Erie, consistently indicate...

Understanding the controls on carbon dioxide (CO2) exchange within saline boreal fens, appropriate natural analogs for landscape reclamation in areas affected by salinization in the Athabasca Oil Sands Region, can provide insights into the biogeochemical and carbon sink functioning of constructed wetlands under saline conditions. Hence, this study...

Algal blooms in the Great Lakes are a concern due to excess nutrient loading from non-point sources; however, there is uncertainty over the relative contributions of various non-point sources under different types of land use in rural watersheds, particularly over annual time scales. Four nested subwatersheds in Southern Ontario, Canada (one natura...

Northern peatlands are experiencing increased wildfire disturbance, threatening peatland biogeochemical function and ability to remain major stores of carbon (C) and macronutrients (nitrogen—N, and phosphorus—P). The impacts of climate change-driven drying on peatland nutrient dynamics have been explored previously; however, the impacts of wildfire...

Phosphorus (P) plays a crucial role in agriculture as a primary fertilizer nutrient—and as a cause of the eutrophication of surface waters. Despite decades of efforts to keep P on agricultural fields and reduce losses to waterways, frequent algal blooms persist, triggering not only ecological disruption but also economic, social, and political cons...

In permafrost landscapes, ponds and lakes are ubiquitous and important features that support long-term carbon storage and diverse diatom communities. With climate warming, thermokarst lake expansion may modify the ability of these aquatic systems to maintain these functions, as a consequence of changes to nutrient inputs and cycling. A recent (2006...

Cover crops (CC) have both agronomic and environmental benefits but also have the potential to increase losses of dissolved reactive P after freeze–thaw cycles (FTC). This field study, conducted over one nongrowing season (NGS) in Ontario, Canada, characterized water‐extractable P (WEP) content in different CC species and compared observed changes...

This study quantified the contributions of overland and tile flow to total runoff (sum of overland and tile flow) and nutrient losses in a Vertisolic soil in the Red River valley (Manitoba, Canada), a region with a cold climate where tile drainage is rapidly expanding. Most annual runoff occurred as overland flow (72–89%), during spring snowmelt an...

The use of cover crops and crop residues is a common strategy to mitigate sediment and nutrient losses from land to water. In cold climates, elevated dissolved P losses can occur associated with freeze–thaw of plant materials. Here, we review the impacts of cover crops and crop residues on dissolved P and total P loss in cold climates across ∼41 st...

In northern regions, a high proportion of annual runoff and phosphorus (P) export from cropland occurs with snowmelt. In this study, we analyze 57 site‐years of field‐scale snowmelt runoff data from 16 small watersheds draining fine‐textured soils (clay or clay loam) in Manitoba, Canada. These fields were selected across gradients of soil P (2.4 to...

Hydrologic models partition flows into surface and subsurface pathways, but their calibration is typically conducted only against streamflow. Here we argue that unless model outcomes are constrained using flow pathway data, multiple partitioning schemes can lead to the same streamflow. This point becomes critical for biogeochemical modeling as indi...

Preferential flowpaths transport phosphorus (P) to agricultural tile drains. However, if and to what extent this may vary with soil texture, moisture conditions, and P placement is poorly understood. This study investigated (1) interactions between soil texture, antecedent moisture conditions, and the relative contributions of matrix and preferenti...

Agricultural runoff containing P and N from drainage tiles contributes to nutrient loading in waterways, leading to downstream eutrophication. Recent studies suggest that nutrient losses through tile drains can be reduced if nutrients are applied in the subsurface. This study explored interactions between nutrient supply and infiltrating water duri...

Core Ideas Preferential flow is prevalent in clay soil under both frozen and thawed conditions. Preferential flow dominates the infiltration regime under frozen soil conditions in silt loam. Subsurface placement of fertilizer can limit subsurface nutrient leaching. Subsurface placement is particularly effective in soil with abundant preferential fl...

Water quality problems are frequently influenced by hydrological processes, particularly in landscapes in which land drainage has been modified. The expansion of agricultural tile drainage in the Northern Great Plains of North America is occurring, yet is controversial due to persistent water quality problems such as eutrophication. Runoff‐generati...

Studies on phosphorus (P) dynamics in prairie pothole wetlands (PWs) are scarce. • 13 prairie PWs with differing soil, hydrology and human disturbance were surveyed. • Intact wetlands appear to be more effective at storing P than drained wetlands. • Wetland volume, soil texture and short-term stage fluctuations control P dynamics. • The relative im...

Across the circumpolar north, the degradation of permafrost has resulted in an increase in the extent of thermokarst landforms. Within thermally disturbed ecosystems, climatically driven changes to hydrology and temperature regimes have the potential to modify nutrient cycling processes. To assess the impacts of changing moisture and thermal condit...

Phosphorus (P) losses from agricultural soils are a growing economic and water-quality concern in the Lake Erie watershed. While recent studies have explored edge-of-field and watershed P losses related to land-use and agricultural management, the potential for soils developed from contrasting parent materials to retain or release P to runoff has n...