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.

Supply and Transport Limitations on Phosphorus Losses from Agricultural Fields in the Lower Great Lakes Region, Canada.

Phosphorus (P) mobilization in agricultural landscapes is regulated by both hydrologic (transport) and biogeochemical (supply) processes interacting within soils; however, the dominance of these controls can vary spatially and temporally. In this study, we analyzed a 5-yr dataset of stormflow events across nine agricultural fields in the lower Great Lakes region of Ontario, Canada, to determine if edge-of-field surface runoff and tile drainage losses (total and dissolved reactive P) were limited by transport mechanisms or P supply. Field sites ranged from clay loam, silt loam, to sandy loam textures. Findings indicate that biogeochemical processes (P supply) were more important for tile drain P loading patterns (i.e., variable flow-weighted mean concentrations ([]) across a range of flow regimes) relative to surface runoff, which trended toward a more chemostatic or transport-limited response. At two sites with the same soil texture, higher tile [] and greater transport limitations were apparent at the site with higher soil available P (STP); however, STP did not significantly correlate with tile [] or P loading patterns across the nine sites. This may reflect that the fields were all within a narrow STP range and were not elevated in STP concentrations (Olsen-P, ≤25 mg kg). For the study sites where STP was maintained at reasonable concentrations, hydrology was less of a driving factor for tile P loadings, and thus management strategies that limit P supply may be an effective way to reduce P losses from fields (e.g., timing of fertilizer application).

Diversity of integron- and culture-associated antibiotic resistance genes in freshwater floc.

Clinically important antibiotic resistance genes were detected in culturable bacteria and class 1 integron gene cassettes recovered from suspended floc, a significant aquatic repository for microorganisms and trace elements, across freshwater systems variably impacted by anthropogenic activities. Antibiotic resistance gene cassettes in floc total community DNA differed appreciably in number and type from genes detected in bacteria cultured from floc. The number of floc antibiotic resistance gene cassette types detected across sites was positively correlated with total (the sum of Ag, As, Cu, and Pb) trace element concentrations in aqueous solution and in a component of floc readily accessible to bacteria. In particular, concentrations of Cu and Pb in the floc component were positively correlated with floc resistance gene cassette diversity. Collectively, these results identify suspended floc as an important reservoir, distinct from bulk water and bed sediment, for antibiotic resistance in aquatic environments ranging from heavily impacted urban sites to remote areas of nature reserves and indicate that trace elements, particularly Cu and Pb, are geochemical markers of resistance diversity in this environmental reservoir. The increase in contamination of global water supplies suggests that aquatic environments will become an even more important reservoir of clinically important antibiotic resistance in the future.

Comparative floc-bed sediment trace element partitioning across variably contaminated aquatic ecosystems.

Significantly higher concentrations of Ag, As, Cu, Ni and Co are found in floc compared to bed sediments across six variably impacted aquatic ecosystems. In contrast to the observed element and site-specific bed sediment trace element (TE) partitioning patterns, floc TE sequestration is consistently dominated by amorphous oxyhydroxides (FeOOH), which account for 30-79% of floc total TE concentrations, irrespective of system physico-chemistry or elements involved. FeOOH consistently occur in significantly higher concentrations in floc than within bed sediments. Further, comparative concentration factors indicate significantly higher TE reactivity of floc-FeOOH relative to sediment-FeOOH in all systems investigated, indicating that both the greater abundance and higher reactivity of floc-FeOOH contribute to enhanced floc TE uptake. Results indicate that floc-organics (live cells and exopolymeric substances, EPS) directly predict floc-FeOOH concentrations, suggesting an organic structural role in the collection/templating of FeOOH. This, in turn, facilitates the sequestration of TEs associated with floc-FeOOH formation, imparting the conserved FeOOH "signature" on floc TE geochemistry across sites. Results demonstrate that the organic rich nature of floc exerts an important control over TE geochemistry in aquatic environments, ultimately creating a distinct solid with differing controls over TE behavior than bed sediments in close proximity (<0.5 m).

Physical and ecological controls on freshwater floc trace metal dynamics.

Significantly higher concentrations of Ag, As, Cu, Co, Ni, and Pb are found in suspended floc compared to surficial bed sediments for a freshwater beach in Lake Ontario. Contrasting observed element-specific bed sediment metal partitioning patterns, floc sequestration for all elements is dominated by one substrate: amorphous oxyhydroxides. More specifically, floc metal scavenging is controlled by floc biogeochemical architecture. Floc organics, largely living microbial cells and associated exopolymeric substances (EPS), act as scaffolds for the collection and/or templating of amorphous Fe oxyhydroxides. While interactions between floc organics and amorphous Fe oxyhydroxides affected floc sorption behavior, specific element affinities and competition for these limited substrates was important for overall floc partitioning. Further, assessment of metal dynamics during stormy conditions indicated energy-regime driven shifts in floc and bed sediment partitioning that were specifically linked to the exchange of floc and bed sedimentary materials. These novel results demonstrate that the microbial nature of floc formation exerts an important control on floc metal dynamics distinguishable from surficial bed sediments and that hydrologic energy-regime is an important factor to consider in overall floc metal behavior, especially in beach environments.