Availability of phosphorus to algae from eroded soil fractions

A study was conducted to determine the concentrations of algal-available P (P_aa) in unfractionated samples and size-classified separates of soil eroded from wheat stubble plots during a simulated rainfall experiment conducted in Warren County, Indiana, U.S.A. Run-off samples were collected, concentrated, dispersed by sonification, separated according to size and incubated with algal cells (Selanastrum capricornutum) in P-free nutrient medium for 2 weeks. Algal-available P in soil separates was determined by decreases in sediment inorganic P (P_i) over the incubation period. The initial concentrations of dissolved molybdate-reactive P_i (DMRP), P_i extractable with NaOH (NaOH-P_i), P_i extractable with HCl (HCl-P_i), organic P (P_o, and total P increased with decreasing particle size. A higher proportion of P_i was HCl-extractable in fractions > 2 μm than in clay-size separates. The level of P_aa in samples increased with decreasing particle size. Available P concentrations in separates < 0.2, 0.2–2, 2–50, > 50 μm and unfractionated samples averaged 447, 282, 113, 26 and 261 μg g^?1, respectively. From 42 to 69% of the P_i and 20–37% of the total P in unfractioned samples was P_aa. A lower proportion of P_i in the > 50 μm fraction was P_aa than in the other size fractions (34 vs. 50%). There was no relationship between particle size and the proportion of total P that was available. The majority (> 84%) of P_aa originated in the P_i fraction extractable with NaOH, whereas HCl-P_i only contributed small amounts of P_aa. The proportion of initial NaOH-P_i assimilated by algae in 2 weeks ranged between 43 and 77%, whereas an average of 16% of HCl-P_i in separates was P_aa. On average, a greater proportion of NaOH-P_i was assimilated by algae in samples from no-till plots as compared to those from chisel-plowed plots.