Extraction of nutrients from sediment
Water-extractable nutrients in sediment were analysed following a 2:5 w/v aqueous extraction. A 10 g sub-sample of homogenised wet sediment was mixed with 25 mL of deionised water (resistivity greater than or equal to 18 M?-1, Milli-Q, Millipore) in a polypropylene centrifuge tube (50 mL, Sarstedt AG and Co.) and tumbled for 1 h at room temperature. The mixture was centrifuged for 15 minutes at 12 000 rpm and the supernatant filtered through a 0.45 micron membrane cartridge filter (Minisart, Sartorius AG) into a clean tube for storage. Filtrates were stored at 4 degrees C until analysed.
Methods of nutrient analysis
Analysis of anions in water extracts of soil was carried out by ion chromatography using a Metrohm 761 Compact IC connected to a 766 IC Sample Processor. Additionally, a Bischoff Lambda 1010 UV-vis detector was employed to confirm the identification of NO2- and NO3- in samples. Separation of anions (F-, Cl-, Br-, NO2-, NO3-, PO42-, and SO42-) was achieved within 10 minutes by injecting a 20 micro litre sample onto a Metrosep A Supp 5 column flushed with 3.2 mM Na2CO3 / 1.0 mM NaHCO3 eluent solution at 0.7 mL min-1. Calibration of the conductivity detector was achieved using standard solutions in the range 0-20 mg L-1; where necessary, more-concentrated samples were diluted to allow quantification within this range.
Ammonium by colorimetry
Ammonium (NH4+) in sediment extract (water) was determined in the range 0-10 mg L-1 Sample extracts (2.00 mL) were pipetted into a 10 mL plastic vial mixed with 2.00 mL of buffer / complexing agent (120 mM sodium potassium tartrate / 90 mM sodium citrate, pH 5.2), 1.2 mL of a fresh 2:1 v/v mixture of alkaline 0.5 M sodium salicylate (0.63 M in NaOH) and 0.1% w/v sodium nitroprusside catalyst followed by 800 micro litres of 16 mM sodium dichloroisocyanurate. The sample was agitated by inversion and the colour developed for 60 minutes before measurement of the absorbance of the blue indophenol dye at 660 nm. A linear calibration was constructed for standard solutions to calculate the concentration of NH4+ in the extracts.
Phosphate by colorimetery
Phosphate (PO43-) in sediment extract (water) was determined in the range 0-10 mg L-1. Sample extracts (5 mL) were transferred to a 10 mL plastic vial. To the sample 1 mL of the prepared colour reagent, preparation outlined below, was added. The sample was agitated by inversion and the colour developed for 30 minutes before measurement of the absorbance of the reduced antimony phosphomolybdate complex at 880 nm. A linear calibration was constructed for standard solutions to calculate the concentration of PO43- in the extracts.
Colour reagent Preparation
Molybdate antimonyl tartrate sulphuric acid reagent was prepared by 2:2:5:1 (v/v/v/v) of the following reagents 0.02 M ammonium molybdite / 0.001 M potassium antimonyl tartrate / 9.19 M H2SO4 / Milli Q. The colour reagent was prepared by 3:2 (v/v) 95 mM abscorbic acid / molybdate antimonyl tartrate sulphuric acid reagent.