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Measuring Phosphorus Release in Laboratory Microcosms for Water Quality Assessment
Chapters
Summary July 22nd, 2019
Accurate quantification of phosphorus (P) desorption potential in saturated soils and sediments is important for P modeling and transport mitigation efforts. To better account for in situ soil-water redox dynamics and P mobilization under prolonged saturation, a simple approach was developed based on repeated sampling of laboratory microcosms.
Transcript
Our method helps answer the question of how likely a given soil or sediment is to release soluble phosphorous under prolonged saturation. Quantifying phosphorous release potential in variably saturated environments is important for determining transport risk to streams and designing mitigation practices. The main advantage of the method is its ability to simulate important biogeochemical processes affecting soluble phosphorous release in mobility under field conditions.
A scientist trying this method for the first time should not struggle, as its simplicity is one of its main advantages. To begin, collect approximately four liters of soil from desired sites. Limit collection areas to approximately 10 square meters to reduce the amount of spacial variability represented by the sample.
Sieve samples through a coarse 20 millimeter screen, followed by a two millimeter screen. Thoroughly hand-mix the samples after sieving. Weigh out 100 grams of the field-moist soil, dry in an oven at 105 degrees Celsius for 24 hours.
Weigh the dry soil and calculate percent gravimetric water content. Then, measure out a 500 milliliter subsample using an empty beaker and reserve for chemical analysis. Use the remaining sieved soil for microcosm studies or store in polypropylene bags at five degrees Celsius for later use.
Use one liter graduated polypropylene or other nonreactive plastic beakers as individual experimental microcosm units. Wash beakers in 10%hydrochloric acid and triple rinse with distilled water. Measure two centimeters up from the bottom and place a mark next to beaker graduations.
Drill a 1.25 centimeter diameter hole at the mark for drainage ports. Place a small bead of silicone around the inside edge of hose barb. Carefully insert the drainage port into the hole.
Allow air-drying for 24 hours before proceeding. Trace the outside circumference of hose barbs onto nylon mesh filter screen. Cut out with scissors, apply a thin bead of silicone around the outside edge of the filter screen, and gently press onto the hose inlet.
Allow at least 24 hours of drying time before using. Next, fit a short piece of latex hose on hose barbs and clamp with 3.3-centimeter-wide paper binder clips. Fill beakers with approximately 500 milliliters of distilled water, to test for possible leaks.
Load 500 milliliters of sample into duplicate microcosms and gently apply distilled water along beaker walls until floodwater reaches the one liter mark. Remove the paraffin film to induce porewater flow through drainage port at desired initial sampling time point. Collect samples by placing clean 20 milliliter beakers directly beneath porewater drainage ports.
Allow several milliliters of porewater to drain, discard, and use the next 10 milliliters as a representative sample volume. Filter porewater samples through 0.45 micron membrane filters and immediately analyze for soluble reactive phosphorus on a spectrophotometer. Record absorbance values and time of measurements.
Take initial floodwater sample by inserting a 10 milliliter bulb syringe pipette halfway down the water column, and withdraw a sample using a circular motion. Dispense into beakers, filter through 0.45 micron membrane filters, and analyze immediately for soluble reactive phosphorus. Refill beakers to the one liter level with distilled water to consistently maintain a total volume of flooded soil and water column at one liter in all microcosms.
Repeat the analysis of soluble reactive phosphorus at desired time points. In this protocol, a riparian site with low soil pH had nearly continuous soluble reactive phosphorus sorption from porewater. Soil that was sampled from an adjoining maize production field with elevated labile inorganic phosphorus demonstrated nearly a sevenfold increase in porewater-soluble reactive phosphorus over the first month of inundation.
Porewater ferrous iron concentration, as a proxy for redox status, increased substantially after approximately three weeks, indicating reducing conditions. In contrast, floodwater-soluble reactive phosphorus tended to decrease over time. Flooding dry soil substantially increased inorganic phosphorus desorption to porewater and subsequent mobilization to overlying water compared to flooding the same soil in a field-moist state.
The reliability of soil phosphorus tests to predict average soluble reactive phosphorus concentrations, was evaluated. Distilled water and modified Morgan extractable phosphorus were among the best predictors of average porewater and floodwater-soluble reactive phosphorus concentrations. Modified Morgan extractable phosphorus measured by inductively coupled plasma optical emission spectroscopy was not as good of a predictor compared to modified Morgan extractable phosphorus or distilled water measured by molybdate colorimetry.
The ratio of porewater-soluble reactive phosphorus over floodwater-soluble reactive phosphorus increased linearly as a function of soil pH. Other experiments characterizing phosphorus dynamics are also possible, for example, phosphorus removal capacity of wetland soils is an important process and can be simulated by spiking floodwater with phosphorus and measuring its rate of disappearance over time. The analytical procedure used for measuring phosphorus involves the use of hydrochloric acid.
Proper safety equipment and laboratory facilities are therefore required.
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