Influence of climate, topography, and soil type on soil extractable phosphorus in croplands of northern glacial-derived landscapes.

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 basin and the southern Great Lakes region. Surface soils (0-30 cm) and select deeper cores (0-90 cm) were collected from 10 cropped fields ranging in terrain (near-level to hummocky), soil texture (clay to loam), composition (calcareous to noncalcareous), and climate across these differing glacial landscapes. Poorly soluble P dominated (up to 91%) total extractable P in the surface soils at eight sites. No differences in the relative solubilities of soil extractable P with microtopography were apparent in landscapes without defined surface depressions. In contrast, in landscapes with pronounced surface depressions, increased easily soluble P (Sol-P), and decreased soil P sorption capacity were found in soil in wetter, low-slope zones relative to drier upslope locations. The Sol-P pool was most important to soil P retention (up to 28%) within the surface depressions of the Red River basin and at sites with low-carbonate soils in the southern Lake Erie watershed (up to 28%), representing areas at elevated risk of soil P remobilization. This study demonstrates interrelationships among soil extractable P pools, soil development, and soil moisture regimes in agricultural glacial landscapes and provides insight into identifying potential areas for soil P remobilization and associated P availability to crops and runoff.

Phosphorus runoff from Canadian agricultural land: A dataset for 30 experimental fields.

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 climates ranging from semi-arid to humid and a wide range of field management practices. In the article, mean annual and seasonal (in terms of snowmelt and rain) precipitation inputs, runoff depths, and P and sediment concentrations and loads are presented, along with ranges of yearly values. In addition, information of field management and soil characteristics (e.g. soil type and soil Olsen P) is also presented for each field. The data have potential to be reused for national and international cross-region comparisons of P and sediment losses, constructing and validating decision-support models and tools for assessing and managing P losses in both snowmelt and rainfall runoff, and informing beneficial management practices to improve agricultural water quality. Interpretation of the data is found in "Phosphorus runoff from Canadian agricultural land: A cross-region synthesis of edge-of-field results" [1].

Concentration-discharge relationships derived from a larger regional dataset as a tool for watershed management.

Concentration-discharge (C-Q) relationships have been widely used to assess the hydrochemical processes that control solute fluxes from streams. Here, using a large regional dataset we assessed long-term C-Q relationships for total phosphorus (TP), soluble reactive phosphorus (SRP), total Kjeldahl nitrogen (TKN), and nitrate (NO3 ) for 63 streams in Ontario, Canada, to better understand seasonal regional behavior of nutrients. We used C-Q plots, Kruskal-Wallis tests, and breakpoint analysis to characterize overall regional nutrient C-Q relationships and assess seasonal effects, anthropogenic impacts, and differences between "rising" and "falling" hydrograph limbs to gain an understanding of the dominant processes controlling overall C-Q relationships. We found that all nutrient concentrations were higher on average in catchments with greater levels of anthropogenic disturbance (agricultural and urban land use). TP, SRP, and TKN showed similar C-Q dynamics, with nearly flat or gently sloping C-Q relationships up to a discharge threshold after which C-Q slopes substantially increased during the rising limb. These thresholds were seasonally variable, with summer and winter thresholds occurring at lower flows compared with autumn and greater variability during snowmelt. These patterns suggest that seasonal strategies to reduce high flows, such as creating riparian wetlands or reservoirs, in conjunction with reducing related nutrient transport during high flows would be the most effective way to mitigate elevated in-stream concentrations and event export. Elevated rising limb concentrations suggest that nutrients accumulate in upland parts of the catchment during drier periods and that these are released during rain events. NO3 C-Q patterns tended to be different from the other nutrients and were further complicated by anthropogenic land use, with greater reductions on the falling limb in more disturbed catchments during certain seasons. There were few significant NO3 hydrograph limb differences, indicating that there was likely to be no dominant hysteretic pattern across our study region due to variability in hysteresis from catchment to catchment. This suggests that this nutrient may be difficult to successfully manage at the regional scale.

Nitrogen dynamics and nitrogen-to-phosphorus stoichiometry in cold region agricultural streams.

Cold agricultural regions are getting warmer and experiencing shifts in precipitation patterns, which affect hydrological transport of nutrients through reduced snowpack and higher annual proportions of summer rainfall. Previous work has demonstrated that the timing of phosphorus (P) concentrations is regionally coherent in streams of the northern Great Plains, suggesting a common climatic driver. There has been less investigation into patterns of stream nitrogen (N), despite its importance for water quality. Using high-frequency water quality data collected over 6 yr from three southern Manitoba agricultural streams, the goal of this research was to investigate seasonal patterns in N and P concentrations and the resultant impacts of these patterns on N/P stoichiometry. In the spring, high concentrations of inorganic N were associated with snowmelt runoff, while summer N was dominated by organic forms; inorganic N concentrations remained consistently low in the summer, suggesting increased biological N transformation and N removal. Relationships between N concentration and discharge showed generally weak model fits (r2 values for significant relationships ranging from .33 to .48), and the strength and direction of model fits differed among streams, seasons, and forms of N. Dissolved organic N concentrations were strongly associated with dissolved organic carbon. Nitrogen-to-phosphorus ratios varied among streams but were significantly lower during summer storm events (p < .0001). These results suggest that climate-driven shifts in temperature and precipitation may negatively affect downstream water quality in this region.

Hydrological and Seasonal Controls of Phosphorus in Northern Great Plains Agricultural Streams.

Controls on nutrient transport in cold, low-relief agricultural regions vary dramatically among seasons. The spring snowmelt is often the dominant runoff and nutrient loading event of the year. However, climate change may increase the proportion of runoff occurring with rainfall, and there is an urgent need to understand seasonal controls on nutrient transport to understand how patterns may change in the future. In this study, we assess patterns and drivers of total P (TP) dynamics in eight streams draining agriculturally dominated watersheds, located in southern Manitoba, Canada. Data from three years of monitoring revealed highly coherent patterns of TP concentrations in streams, with pronounced peaks in the spring and midsummer across the region. This coherent pattern was in spite of considerable interannual variability in the magnitude and timing of discharge; in particular, a major storm event occurred in summer 2014, which resulted in more discharge than the preceding spring melt. Concentration-discharge model fits were generally poor or not significant, suggesting that runoff generation is not the primary driver of TP dynamics in the majority of streams. Seasonal patterns of conductivity and stream temperature suggest that mechanisms controlling TP vary by season; a spring TP concentration maximum may be related to surface runoff over frozen soils, whereas the summer TP maximum may be related to temperature-driven biogeochemical processes, which are not well represented in current conceptual or predictive models. These findings suggest that controls on stream TP concentrations are dynamic through the year, and responses to increases in dormant and nondormant season temperatures may depend on seasonally variable processes.

Landscape Controls on Nutrient Export during Snowmelt and an Extreme Rainfall Runoff Event in Northern Agricultural Watersheds.

In the northern Great Plains, most runoff transport of N, and P to surface waters has historically occurred with snowmelt. In recent years, significant rainfall runoff events have become more frequent and intense in the region. Here, we examine the influence of landscape characteristics on hydrology and nutrient export in nine tributary watersheds of the Assiniboine River in Manitoba, Canada, during snowmelt runoff and with an early summer extreme rainfall runoff event (ERRE). All watersheds included in the study have land use that is primarily agricultural, but with differing proportions of land remaining as wetlands, grassland, and that has been artificially drained. Those watersheds with greater capacity for storage of water in surface depressions (noneffective contributing areas) exhibited lower rates of runoff and nutrient export with snowmelt. During the ERRE, higher export of total P (TP), but not total N, was observed from those watersheds with larger amounts of contributing area that had been added through artificial surface drainage, and this was associated primarily with higher TP concentrations. Increasing or restoring the storage of water on the landscape is likely to reduce nutrient export; however, the importance of antecedent conditions was evident during the ERRE, when small surface depressions were at or near capacity from snowmelt. Total P concentrations observed during the summer ERRE were as high as those observed with snowmelt, and N/P ratios were significantly lower. If the frequency of summer ERREs increases with climate change, this is likely to result in negative water quality outcomes.

Agricultural Water Quality in Cold Climates: Processes, Drivers, Management Options, and Research Needs.

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. Frozen ground can limit infiltration and interaction with soils, and minimal nutrient uptake during the nongrowing season may govern nutrient export from agricultural catchments. This paper reviews agronomic, biogeochemical, and hydrological characteristics of cold agricultural regions and synthesizes findings of 23 studies that are published in this special section, which provide new insights into nutrient cycling and hydrochemical processes, model developments, and the efficacy of different potentially beneficial management practices (BMPs) across varied cold regions. Growing evidence suggests the need to redefine optimum soil phosphorus levels and input regimes in cold regions to allow achievement of water quality targets while still supporting strong agricultural productivity. Practices should be considered through a regional and site-specific lens, due to potential interactions between climate, hydrology, vegetation, and soils, which influence the efficacy of nutrient, crop, water, and riparian buffer management. This leads to differing suitability of BMPs across varied cold agricultural regions. We propose a systematic approach (""), to achieve water quality objectives in variable and changing climates, which combines nutrient transport process onceptualization, nderstanding BMP functions, redicting effects of variability and change, onsideration of producer input and agronomic and environmental tradeoffs, practice daptation, nowledge mobilization, and valuation of water quality improvement.

Impacts of Soil Phosphorus Drawdown on Snowmelt and Rainfall Runoff Water Quality.

Managing P export from agricultural land is critical to address freshwater eutrophication. However, soil P management, and options to draw down soil P have received little attention in snowmelt-dominated regions because of limited interaction between soil and snowmelt. Here, we assessed the impacts of soil P drawdown (reducing fertilizer P inputs combined with harvest removal) on soil Olsen P dynamics, runoff P concentrations, and crop yields from 1997 to 2014 in paired fields in Manitoba, Canada. We observed that Olsen P concentrations in the 0- to 5-cm soil layer were negatively correlated with the cumulative P depletion and declined rapidly at the onset of the drawdown practice (3.1 to 5.4 mg kg yr during 2007-2010). In both snowmelt runoff and rainfall runoff, concentrations of total dissolved P (TDP) were positively correlated with the concentrations of soil Olsen P. Soil P drawdown to low to moderate fertility levels significantly decreased mean annual flow-weighted TDP concentrations in snowmelt runoff from 0.60 to 0.30 mg L in the field with high initial soil P and from 1.17 to 0.42 mg L in the field with very high initial soil P. Declines in TDP concentration in rainfall runoff were greater. Critically, yields of wheat ( spp.) and canola ( L.) were not affected by soil P depletion. In conclusion, we demonstrate that relatively rapid reductions in P loads are achievable at the field scale via managing P inputs and soil P pools, highlighting a management opportunity that can maintain food security while improving water security in cold regions.

Impacts of Cover Crops and Crop Residues on Phosphorus Losses in Cold Climates: A Review.

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 studies, exploring linkages between water-extractable P (WEP) in plant materials and P loss in surface runoff and subsurface drainage. Water-extractable P concentrations are influenced by plant type and freezing regimes. For example, WEP was greater in brassica cover crops than in non-brassicas, and increased with repeated freeze-thaw cycles. However, total P losses in surface runoff and subsurface drainage from cropped fields under cold climates were much lower than plant WEP, owing to retention of 45 to >99% of released P by soil. In cold climatic regions, cover crops and crop residues generally prevented soil erosion and loss of particle-bound P during nongrowing seasons in erodible landscapes but tended to elevate dissolved P loss in nonerodible soils. Their impact on total P loss was inconsistent across studies and complicated by soil, climate, and management factors. More research is needed to understand interactions between these factors and plant type that influence P loss, and to improve the assessment of crop contributions to P loss in field settings in cold climates. Further, tradeoffs between P loss and the control of sediment loss and N leaching by plants should be acknowledged.

Options for Improved Phosphorus Cycling and Use in Agriculture at the Field and Regional Scales.

Soil phosphorus (P) cycling in agroecosystems is highly complex, with many chemical, physical, and biological processes affecting the availability of P to plants. Traditionally, P fertilizer recommendations have been made using an insurance-based approach, which has resulted in the accumulation of P in many intensively managed agricultural soils worldwide and contributed to the widespread water quality issue of eutrophication. To mitigate further environmental degradation and because future P fertilizer supplies are threatened due to finite phosphate rock resources and associated geopolitical and quality issues, there is an immediate need to increase P use efficiency (PUE) in agroecosystems. Through cultivar selection and improved cropping system design, contemporary research suggests that sufficient crop yields could be maintained at reduced soil test P (STP) concentrations. In addition, more efficient P cycling at the field scale can be achieved through agroecosystem management that increases soil organic matter and organic P mineralization and optimizes arbuscular mycorrhizal fungi (AMF) symbioses. This review paper provides a perspective on how agriculture has the potential to utilize plant and microbial traits to improve PUE at the field scale and accordingly, maintain crop yields at lower STP concentrations. It also links with the need to tighten the P cycle at the regional scale, including a discussion of P recovery and recycling technologies, with a particular focus on the use of struvite as a recycled P fertilizer. Guidance on directions for future research is provided.

The prevalence of nonlinearity and detection of ecological breakpoints across a land use gradient in streams.

Human activities can alter aquatic ecosystems through the input of nutrients and carbon, but there is increasing evidence that these pressures induce nonlinear ecological responses. Nonlinear relationships can contain breakpoints where there is an unexpected change in an ecological response to an environmental driver, which may result in ecological regime shifts. We investigated the occurrence of nonlinearity and breakpoints in relationships between total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), and total dissolved carbon (DOC) concentrations and ecological responses in streams with varying land uses. We calculated breakpoints using piecewise regression, two dimensional Kolmogorov-Smirnov (2DKS), and significant zero crossings (SiZer) methods. We found nonlinearity was common, occurring in half of all analyses, with some evidence of multiple breakpoints. Linearity, by contrast, occurred in less than 14% of cases, on average. Breakpoints were related to land use gradients, with 34-43% agricultural cover associated with DOC and TDN breakpoints, and 15% wetland and 9.5% urban land associated with DOC and nutrient breakpoints, respectively. While these breakpoints are likely specific to our study area, our study contributes to the growing literature of the prevalence and location of ecological breakpoints in streams, providing watershed managers potential criteria for catchment land use thresholds.

Riverine Export of Aged Carbon Driven by Flow Path Depth and Residence Time.

The flux of terrestrial C to rivers has increased relative to preindustrial levels, a fraction of which is aged dissolved organic C (DOC). In rivers, C is stored in sediments, exported to the ocean, or (bio)chemically processed and released as CO2. Disturbance changes land cover and hydrology, shifting potential sources and processing of DOC. To investigate the likely sources of aged DOC, we analyzed radiocarbon ages, chemical, and spectral properties of DOC and major ions from 19 rivers draining the coterminous U.S. and Arctic. DOC optics indicated that the majority is exported as aromatic, high molecular weight, modern molecules while aged DOC tended to consist of smaller, microbial degradation products. Aged DOC exports, observed regularly in arid basins and during base flow in arctic rivers, are associated with higher proportion of mineral weathering products, suggesting deeper flows paths. These patterns also indicate potential for production of microbial byproducts as DOC ages in soil and water with longer periods of time between production and transport. Thus, changes in hydrology associated with landscape alteration (e.g., tilling or shifting climates) that can result in deeper flow paths or longer residence times will likely lead to a greater proportion of aged carbon in riverine exports.

Effects of crop rotation and management system on water-extractable organic matter concentration, structure, and bioavailability in a chernozemic agricultural soil.

Water-extractable organic matter (WEOM) in soil affects contaminant mobility and toxicity, heterotrophic production, and nutrient cycling in terrestrial and aquatic ecosystems. This study focuses on the influences of land use history and agricultural management practices on the water extractability of organic matter and nutrients from soils. Water-extractable organic matter was extracted from soils under different crop rotations (an annual rotation of wheat-pea/bean-wheat-flax or a perennial-based rotation of wheat-alfalfa-alfalfa-flax) and management systems (organic or conventional) and examined for its concentration, composition, and biodegradability. The results show that crop rotations including perennial legumes increased the concentration of water-extractable organic carbon (WEOC) and water-extractable organic nitrogen (WEON) and the biodegradability of WEOC in soil but depleted the quantity of water-extractable organic phosphorus (WEOP) and water-extractable reactive phosphorus. The 30-d incubation experiments showed that bioavailable WEOC varied from 12.5% in annual systems to 22% for perennial systems. The value of bioavailable WEOC was found to positively correlate with WEON concentrations and to negatively correlate with C:N ratio and the specific ultraviolet absorbance of WEOM. No significant treatment effect was present with the conventional and organic management practices, which suggested that WEOM, as the relatively labile pool in soil organic matter, is more responsive to the change in crop rotation than to mineral fertilizer application. Our results indicated that agricultural landscapes with contrasting crop rotations are likely to differentially affect rates of microbial cycling of organic matter leached to soil waters.