Abstract/Summary

The transport of bioavailable phosphorus (BAP, i.e., algal available) in agricultural runoff can accelerate freshwater eutrophication. As procedural and theoretical limitations have restricted BAP estimation in agricultural soils and runoff, a routine method was developed using iron oxide-impregnated paper strips (Fe-oxide strips) as a sink for BAR Fe-oxide strips were used to investigate the amounts and seasonal dynamics of BAP in 12 Oklahoma soils over 2 years and the effect of agricultural management on BAP loss in runoff from these 12 and 8 additional sites over 4 years. The sites involved native grass, wheat (Triticum aestivum L.), sorghum [Sorghum bicolor (L.) Moench.], and peanut (Arachis hypogaea L.). The strip P content of unfertilized soils was higher in winter (October–March; 10–19 mg kg−1) than spring months (May–June; 3–6 mg kg−1). For the P-fertilized soils, the strip P content increased from 8–17 mg kg−1 prior to P application (20–25 kg ha−1 yr−1 to 33–42 mg kg−1 immediately following application. Seasonal dynamics of strip and organic (OP) and inorganic P (IP) fractions indicated mineralization of moderately labile OP, as a function of phosphatase enzyme activity, and release of moderately labile IP and fertilizer P, as a function of P sorption capacity; these are the major processes controlling the strip P content of unfertilized and fertilized soil, respectively. Dissolved organic P did not reduce the efficiency of IP sorption by the Fe-oxide strips and removal of P from the strips by NaOH rather than H2S04 minimized hydrolysis of OP sorbed on the strip. The loss of BAP in runoff was a function of watershed management. Over the 4-yr study, BAP losses increased in the order reduced till (98 g ha−1 yr−1), native grass (160 g ha−1 yr−1), no till (382 g ha−1 yr−1), and conventional till (678 g ha−1 yr−1). Although the total P loss was 93% lower from no till than conventional till, 73% was bioavailable for no till compared to only 28% for conventional till. Clearly, strip P is a dynamic soil property, which, along with watershed management, can influence the bioavailability of P loss in runoff. Thus, the BAP content of runoff in conjunction with runoff and erosion potential may provide a more reliable index of the trophic response of receiving water bodies than dissolved or total P.