THE MEASUREMENT OF BIOAVAILABLE PHOSPHORUS IN AGRICULTURAL RUNOFF

The role of sediment-bound or particulate P in agricultural runoff in accelerating the biological productivity of surface water can be assessed if the biological availability of particulate P (PP) is known. Previous research has indicated amounts of P extracted from deposited river and lake sediments by 0.1 M NaOH to be correlated with P uptake by the alga Selenastrum capricornutum. This study investigates a modification of this extraction to allow routine quantification of potentially bioavailable particulate P (BPP) content of agricultural runoff from the Reddish Prairies and Rolling Red Plains land resource areas. In the proposed method, 20 mL of unfiltered runoff is shaken with 180 mL of 0.11 M NaOH for 17 h and BPP concentration calculated by subtraction of the soluble P (SP) concentration of the runoff sample. Total bioavailable P concentration (TBP) of runoff can be represented by BPP plus SP concentration. Growth of P-straved S. capricornutum, incubated for up to 29 d with runoff sediment from nine watersheds, as the sole P source, was correlated (r2 = 0.76 to 0.95) with potentially BPP content of the added sediment. Sample dilution had no effect on the amount of P extracted from runoff sediment by NaOH across a range in sediment concentration of the extraction medium, equivalent to that observed for 95% of the runoff events. If the sediment concentration of runoff exceeds 20 g L-1, a smaller runoff sample is used in the extraction. The results indicate the applicability of the proposed extraction method to quantify the bioavailability of P transported in agricultural runoff.