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Understanding phosphorus (P) dynamics in urban landscapes is an emerging research topic as P export from urban landscapes towards aquatic ecosystems causes eutrophication-related challenges in these environments. We investigated P export and forms in four research sites in Ontario, Canada, including three urban catchments and a stormwater pond, all located within the Great Toronto Area in the drainage basin of Lake Ontario. We conducted P speciation laboratory analyses on runoff and suspended sediment samples to measure total P (TP), total dissolved P (TDP), dissolved reactive P (DRP), dissolved unreactive P (TDP–DRP), and PP (TP–TDP). Multiple linear regression (MLR) models were also developed to quantify annual loadings of these P species. Models indicated that P loadings in our sites were close to the lower limit of values reported in the literature, with the simulated range of 0.2—0.46 kg ha-1 yr-1 for TP export, 0.06—0.168 kg ha-1 yr-1 for TDP, 0.011—0.073 kg ha-1 yr-1 for DRP, 0.026—0.095 kg ha-1 yr-1 for DUP, and 0.163—0.288 kg ha-1 yr-1 for PP. In our MLR models, precipitation explained a large fraction of variability in loadings with the median of 58% across all models. Moreover, we realized that as the proportion of residential land within the drainage area increased, larger amounts of P loadings were exported at the catchment scale. Results also implied that pond served as a major P sink, with annual retention of 82, 93, 91, 94, and 77% for TP, TDP, DRP, DUP, and PP, respectively. Mass balance analyses based on sequential P extraction in the sediment core samples revealed that P retention was attributed to sedimentation in the ponds, as well as chemical precipitation of P with calcium mineral phases. In terms of P composition, most of P export in our sites (72—88%) were in particulate form. Besides, the ratio between dissolved forms and TP were the highest in the catchment with the largest amount green spaces. This study demonstrates that, as land-use characteristics impose variations in constituent loadings, urban P management options also have to be varying from a catchment to another. However, sediment removing practices such as the use of ponds will certainly be a reliable P retention approach as most of urban P could be sediment-bound. Furthermore, enhancing the formation of calcium phosphate and other redox-stable mineral phases could be explored as a best management practice in existing and new ponds for improving P retention.

Lakes are key ecosystems within the global biogeosphere. However, the bottom-up controls on the biological productivity of lakes, including surface temperature, ice phenology, nutrient loads and mixing regime, are increasingly altered by climate warming and land-use changes. To better understand the environmental drivers of lake productivity, we assembled a dataset on chlorophyll-a concentrations, as well as associated water quality parameters and surface solar irradiance, for temperate and cold-temperate lakes experiencing seasonal ice cover. We developed a method to identify periods of rapid algal growth from in situ chlorophyll-a time series data and applied it to measurements performed between 1964 and 2019 across 357 lakes, predominantly located north of 40°. Long-term trends show that the algal growth windows have been occurring earlier in the year, thus potentially extending the growing season and increasing the annual productivity of northern lakes. The dataset is also used to analyze the relationship between chlorophyll-a growth rates and solar irradiance. Lakes of higher trophic status exhibit a higher sensitivity to solar radiation, especially at moderate irradiance values during spring. The lower sensitivity of chlorophyll-a growth rates to solar irradiance in oligotrophic lakes likely reflects the dominant role of nutrient limitation. Chlorophyll-a growth rates are significantly influenced by light availability in spring but not in summer and fall, consistent with a switch to top-down control of summer and fall algal communities. The growth window dataset can be used to analyze trends in lake productivity across the northern hemisphere or at smaller, regional scales. We present some general trends in the data and encourage other researchers to use the open dataset for their own research questions.

Canada has extensive peat deposits in northern high latitude wetlands and permafrost ecosystems. Peat accumulation represents a natural long-term carbon sink attributed to the cumulative excess of growing season net ecosystem production over non-growing season net mineralization. However, near-surface peat deposits are vulnerable to climate change and permafrost landscape transition. One specific concern is a potential rapid increase in the non-growing season carbon loss through enhanced organic matter mineralization under a warming climate. Our experimental study explores the response of peat CO2 exchanges to (1) temperature, using the conventional Q10 parameter, and (2) moisture content. The observed responses are expected to reflect, at least in part, differential soil microbial adaptations to varying wetland conditions, across two northern ecoclimatic zones. Laboratory incubations were carried out with shallow peat samples from different depths collected at seven Canadian wetland sites and adjusted to five moisture levels. For each subsample (varying by site, depth and moisture content), CO2 fluxes were measured at 12 sequential temperature settings from -10 to 35˚C. For each subsample, the data were fitted to an exponential equation to derive a Q10 value. In general, boreal peat samples were more temperature sensitive than temperate peat. The optimum moisture level for CO2 release was determined for all the subsamples and related to variations in wetland vegetation and landform types. As a general trend, increasing water saturation reduced the CO2 release rate from a given subsample. We further tested a flexible curve-fitting equation, as recently proposed on a theoretical basis, to recompile the data by ecoclimatic peat type and to account for the non-growing season dynamics. These findings will contribute to Canada’s national carbon budget model by guiding the development and calibration of the peatland module.

High-latitude cold regions are warming more than twice as fast as the rest of the planet, with the greatest warming occurring during the winter. Warmer winters are associated with shorter periods of snow cover, resulting in more frequent and extensive soil freezing and thawing. Freeze-thaw cycles influence soil chemical, biological, and physical properties and any changes to winter soil processes may impact carbon and nutrients export from affected soils, possibly altering soil health and nearby water quality. These impacts are relevant for agricultural soils and practices in cold regions as they are critical in governing water flows and quality within agroecosystems. In this study, a soil column experiment was conducted to assess the leaching of nutrients from fertilized agricultural soil during the non-growing season. Four soil columns were exposed to a non-growing season temperature and precipitation model and fertilizer amendments were made to two of the columns to determine the efficacy of fall-applied fertilizers and compared to other two unfertilized control columns. Leachates from the soil columns were collected and analyzed for cations and anions. The experiment results showed that a transition from a freeze period to a thaw period resulted in significant loss of chloride (Cl-), sulfate (SO42-) and nitrate (NO3-). Even with low NO3- concentrations in the applied artificial rainwater and fertilizer, high NO3- concentrations (~150 mg l-1) were observed in fertilized column leachates. Simple plug flow reactor model results indicate the high NO3- leachates are found to be due to active nitrification occurring in the upper oxidized portion of the soil columns mimicking overwinter NO3- losses via nitrification in agricultural fields. The low NO3- leachates in unfertilized columns suggest that freeze-thaw cycling had little effect on N mineralization in soil. Findings from this study will ultimately be used to bolster winter soil biogeochemical models by elucidating nutrient fluxes over changing winter conditions to refine best management practices for fertilizer application.

Soil heterotrophic respiration has been considered as a key source of CO2 flux into the atmosphere and thus plays an important role in global warming. Although the relationship between soil heterotrophic respiration and soil water content has been frequently studied both theoretically and experimentally, model development has thus far been empirically based. Empirical models are often limited to the specific condition of their case studies and cannot be used as a general platform for modeling. Moreover, it is difficult to extend the empirical models by theoretically defined affinities to any desired degree of accuracy. As a result, it is of high priority to develop process-based models that are able to describe the mechanisms behind this phenomenon with more deterministic terms.Here we present a mechanistic, mathematically-driven model that is based on the common geometry of a pore in porous media. Assuming that the aerobic respiration of bacteria requires oxygen as an electron acceptor and dissolved organic carbon (DOC) as a substrate, the CO2 fluxes are considered a function of the bioavailable fraction of both DOC and oxygen. In this modeling approach, the availability of oxygen is controlled by its penetration into the aquatic phase through the interface between air and water. DOC on the other hand is only available to a section of the soil that is in contact with water. As the water saturation in the pore changes, it dynamically and kinematically impacts these interfaces through which the mass transfer of nutrients occurs, and therefore the CO2 fluxes are directly controlled by water content. We showcased the model applicability on several case studies and illustrated the model capability in simulating the observed microbial respiration rates versus the soil water contents. Furthermore, we showed the model potential to accept additional physically-motivated parameters in order to explain respiration rates in frozen soils or at different temperatures.

Stringent environmental policies in many countries have played an extensive role in reducing external phosphorus (P) loading to lakes from agriculture and urban sources. Nonetheless, such reductions in external P loading to many eutrophic lakes have not resulted in the expected concurrent restitution of water quality. Such a delayed recovery of many lakes is blamed both on internal loading of legacy P from lake sediments (i.e., benthic recycling) and the amplification of such internal P loading processes due to the reduction in external P concentrations. Hence, a detailed process understanding of P cycling at the sediment-water interface (SWI) is critical to understand nutrient loading, water quality and associated effects on lake water quality. Much of the work on sedimentary P cycling has traditionally focused on inorganic processes of soluble phosphate, particularly sorption to metals (Fe, Mn, Al) oxyhydroxides and clays. However, there is increasing recognition that organic forms of P, along with interactions between phosphate and humic substances, also play a decisive role in controlling P fluxes between sediments and the overlying water column.This study focused on gaining further understanding of the such processes through the collection of sediment cores from the oxygenated epilimnion and the mostly anoxic hypolimnion of Lake 227 of the Experimental Lakes Area (ELA) in Ontario, Canada. Since 1969, this unique experimental lake has been fertilized with phosphorus (P), which triggered a relatively rapid trophic transition from oligotrophic to eutrophic conditions. The cores contain a chronological record of changes in sediment burial rates and sediment P speciation across this trophic transition.Interpretation of such changes was undertaken by coupling results of chemical extractions with 210Pb sediment dating, 31P NMR, XANES and Mössbauer spectroscopy. The major sedimentary P fraction prior to lake enrichment starting in 1969 was organic P (POrg). Fertilization of the lake in 1969 coincided with significant increases in the accumulation rate of sediment, total organic carbon (TOC) and total P (TP), in addition to a marked relative contribution of NaHCO3 extractable P. The combined proportion of PHum and POrg desposited since artificial fertilization in 1969 account for ≥70% of total P burial in the sediments. The anticipated composition of such PHum fractions was hypothesized to be ternary phosphate (PO4) complexes with humic substances. In support of this, the strong linear correlation between P and iron (Fe) extracted by NaHCO3 implies a close association of the two elements in the humic fraction. Furthermore, XANES and Mössbauer spectra indicate that most Fe in the post-1969 sediments is conserved in the +3 oxidation state, which may be ascribed to the stabilization of reducible Fe by organic matter, partially due to the formation of ternary PO4-Fe(III)-humic complexes. Our findings suggest the artificial eutrophication of Lake 227 resulted in the accelerated accumulation of a large sedimentary reservoir of reactive sediment P that may drive continued internal P loading to the water column following the cessation of artificial fertilization.