Examining sources and pathways of phosphorus transfer in a ditch-drained field.

Understanding the processes that mobilize and transport dissolved phosphorus (P) during storms is critical to managing P in flat landscapes with open ditch drainage and legacy soil P. In this study, we used routine baseflow monitoring and intensive storm sampling at a ditch-drained site on Maryland's Lower Eastern Shore (July 2017-September 2018) to assess whether concentration-discharge (C-Q) relationships and chemical and isotopic hydrograph separation could provide insight into the processes that mobilize and transport dissolved P in ditch drainage. Using a segmented regression model, we determined that long-term C-Q relationships for dissolved P differed above and below a discharge threshold of 6.4 L s-1 . Intensive storm sampling revealed that small storms (n = 3) occurring at or below the discharge threshold generally exhibited complex hysteresis and dissolved P dilution patterns that were consistent with deeper (>122 cm) groundwater inputs with low dissolved P concentrations (0.04 mg L-1 ). In contrast, large storms occurring well above the discharge threshold (n = 4) induced rising water tables and preferential flow pathways that most likely tapped dissolved P-enriched shallow (<20 cm) soil waters (0.89 mg L-1 ), producing consistent clockwise hysteresis and dissolved P flushing patterns. Notably, chemical and isotope hydrograph separation during two of the largest storms revealed significant event water fractions (59-68%) that strongly suggested a role for the rapid delivery of dissolved P via preferential flow pathways. Findings highlight the need to mitigate vertical P stratification as a means for reducing dissolved P flushing from ditch-drained landscapes with legacy P.

Perovskite Carrier Transport: Disentangling the Impacts of Effective Mass and Scattering Time Through Microscopic Optical Detection.

While carrier mobility is a practical and commonly cited measure of transport, it conflates the effects of two more fundamental material properties: the effective mass and mean scattering time of charge carriers. This Letter describes the correlation of two ultrafast imaging techniques to disentangle the effect of each on carrier transport in lead halide perovskites. Two materials are compared: methylammonium lead tri-iodide (MAPbI3) and cesium lead bromide diiodide (CsPbBrI2). By correlating photoinduced changes to the refractive index with a direct measure of carrier diffusion, both the carrier optical mass and mean scattering time are uniquely determined on microscopic length scales. These results show that the factor of 4 lower mobility of CsPbBrI2 is due not to differing optical masses of charge carriers, which are measured to be similar in CsPbBrI2 and MAPbI3, but rather to a difference in mean carrier scattering time. The scope and limitations of the approach are discussed.

Screened Charge Carrier Transport in Methylammonium Lead Iodide Perovskite Thin Films.

While organometal halide perovskites are promising for a variety of optoelectronic applications, the morphological and compositional defects introduced by solution processing techniques have hindered efforts at understanding their fundamental properties. To provide a detailed picture of the intrinsic carrier transport properties of methylammonium lead iodide without contributions from defects such as grain boundaries, we utilized pump-probe microscopy to measure diffusion in individual crystalline domains of a thin film. Direct imaging of carrier transport in 25 individual domains yields diffusivities between 0.74 and 1.77 cm2 s-1, demonstrating single-crystal-like, long-range transport characteristics in a thin film architecture. We also examine the effects of excitation density on carrier diffusivity, finding that transport is nearly independent of photoexcited carrier density between 6 × 1017 cm-3 and 4 × 1019 cm-3. Transport modeling of the observed density independence suggests that strong carrier-phonon scattering coupled with a large static relative permittivity is responsible for the unusual transport characteristics of methylammonium perovskite.

Managing Surface Water Inputs to Reduce Phosphorus Loss from Cranberry Farms.

Cranberry ( Ait.) production in Massachusetts represents one-fourth of the US cranberry supply, but water quality concerns, water use, and wetland protection laws challenge the future viability of the state's cranberry industry. Pond water used for harvest and winter flooding accounts for up to two-thirds of phosphorus (P) losses in drainage waters. Consequently, use of P sorbing salts to treat pond water holds promise in the mitigation of P losses from cranberry farms. Laboratory evaluation of aluminum (Al)-, iron (Fe)-, and calcium (Ca)-based salts was conducted to determine the application rate required for reducing P in shallow (0.4 m) and deep (3.2 m) water ponds used for cranberry production. Limited P removal (<22%) with calcium carbonate and calcium sulfate was consistent with their relatively low solubility in water. Calcium hydroxide reduced total P up to 49%, but increases in pond water pH (>8) could be detrimental to cranberry production. Ferric sulfate and aluminum sulfate applications of 15 mg L (ppm) resulted in near-complete removal of total P, which decreased from 49 ± 3 to <10 µg P L (ppb). However, ferric sulfate application lowered pH below the recommend range for cranberry soils. Field testing of aluminum sulfate demonstrated that at a dose of 15 mg L (∼1.4 Al mg L), total P in pond water was reduced by 78 to 94%. Laboratory and field experiments support the recommendation of aluminum sulfate as a cost-effective remedial strategy for reducing elevated P in surface water used for cranberry production.

Improving Children's Knowledge of Fraction Magnitudes.

We examined whether playing a computerized fraction game, based on the integrated theory of numerical development and on the Common Core State Standards' suggestions for teaching fractions, would improve children's fraction magnitude understanding. Fourth and fifth-graders were given brief instruction about unit fractions and played Catch the Monster with Fractions, a game in which they estimated fraction locations on a number line and received feedback on the accuracy of their estimates. The intervention lasted less than 15 minutes. In our initial study, children showed large gains from pretest to posttest in their fraction number line estimates, magnitude comparisons, and recall accuracy. In a more rigorous second study, the experimental group showed similarly large improvements, whereas a control group showed no improvement from practicing fraction number line estimates without feedback. The results provide evidence for the effectiveness of interventions emphasizing fraction magnitudes and indicate how psychological theories and research can be used to evaluate specific recommendations of the Common Core State Standards.

Imaging theory of structured pump-probe microscopy.

With sub-micron spatial resolution and femtosecond temporal resolution, pump probe microscopy provides a powerful spectroscopic probe of complex electronic environments in bulk and nanoscale materials. However, the electronic structure of many materials systems are governed by compositional and morphological heterogeneities on length scales that lie below the diffraction limit. We have recently demonstrated Structured Pump Probe Microscopy (SPPM), which employs a patterned pump excitation field to provide spectroscopic interrogation of sub-diffraction limited sample volumes. Herein, we develop the imaging theory of SPPM in two dimensions to accompany the previously published experimental methodology. We show that regardless of pump and probe wavelengths, a nearly two-fold reduction in spectroscopic probe volume can be achieved. We also examine the limitations of the approach, with a detailed discussion of ringing in the point spread function that can reduce imaging performance.

Spatial Scale and Field Management Affect Patterns of Phosphorus Loss in Cranberry Floodwaters.

Although cranberries ( Ait.) are indigenous to the northeastern United States, phosphorus (P) fertilizer additions and periodic flooding make commercial cranberry a potential source of P to the region's lakes and streams. In this study, we report values of P export in cranberry floodwaters that range from <0.8 to 4.7 kg P ha, generally reflecting differences in the hydrological, edaphic, and management factors underlying soil P transfer to floodwater. The relatively high P loading rate (4.7 P kg P ha) was associated with harvest flooding of organic-rich soils. Periods of winter flooding and the discharge of harvest floodwater from mineral soils resulted in relatively low P loss (<0.8 kg P ha). Increases in concentrations of total dissolved P (DP) and total particulate P (PP) in floodwater as stage decreased below the surface of the cranberry bed were consistent with the transport of dissolved P in soil porewater and mobilization of particulate P in ditches. Variations in floodwater DP, as well as conservative and reactive tracer concentrations, suggested that the processes by which soil P is released to porewater included desorption of near-surface soil P and anaerobic dissolution of iron-P compounds deeper in the soil profile. At the farm scale, concentrations of DP and PP steadily increased over time, presumably because drainage waters from beds farther upgradient had longer contact times with P-rich sources, such as soil porewater and ditch sediments. Overall, the study illustrates the role that scale-dependent processes impart on patterns of P loss in agricultural production systems.

Chemical and Isotopic Tracers Illustrate Pathways of Nitrogen Loss in Cranberry Floodwaters.

Limited research exists on the sources of nitrogen (N) in cranberry floodwaters, which have been identified as a prominent cause of concern to watershed N loading in the cranberry-producing region of southeastern Massachusetts. In this study, we used naturally occurring chemical and isotopic tracers to infer the sources of N transported by harvest floodwaters. In 2012, the cranberry bed was a net source of total N (TN), exporting 0.8 kg N ha (primarily as organic N) to a nearby lake. Systematic increases in TN concentration were associated with increasing fractions of pre-event soil water and groundwater ("porewater") in discharge. Results showed that N concentrations in porewater generally derive from the natural mixing of soil water and perched groundwater within the cranberry bed but locally display a connection to deep groundwater where the underlying peat is absent. These findings illustrate the environmental significance of stored pools of porewater in cranberry beds and the ability to focus on moments of disproportionate N transfer to most efficiently curtail floodwater N losses (i.e., 58% of N export occurred in only 22% of floodwater discharge).

Temporal variation of oxygen isotope ratios (δ18O) in drinking water: implications for specifying location of origin with human scalp hair.

Previous work suggests that δ(18)O values of human hair can be used to constrain the region-of-origin of unknown individuals, but robust assessments of uncertainties in this method are lacking. Here we assess one source of uncertainty - temporal variation in the δ(18)O value of drinking water - using a monthly tap water survey of δ(18)O to develop geospatial models (i.e., maps) of the intra-annual variation (seasonality) in tap water δ(18)O for the contiguous USA. Temporal variation in tap water δ(18)O was correlated with water-supply type, and was related to geographic patterns of precipitation δ(18)O seasonality and water residence time. The maps were applied in a Bayesian framework to identify the geographic origin of an unidentified woman found in Utah, based on measured δ(18)O of scalp hair. The results are robust in specifying parts of the western USA as the most likely region-of-origin. Incorporation of tap water δ(18)O seasonality in the analysis reduces the precision of geographic assignments, but other sources of uncertainty (e.g., spatial interpolation uncertainty) have an equal or larger effect.