Phosphorus stocks and pools in eroding streambank soils

Phosphorus (P) in streambank soils can directly enter waterbodies via streambank erosion. How much and what form of P is entailed by this terrestrial-aquatic transfer has implications for the degree and timespans of downstream water quality impacts. Quantifying the bioavailability and spatial distribution of soil P species in streambanks is needed to comprehensively assess the fate of P eroded into streams and their potential risk to downstream water quality. This study quantitatively evaluated the abundance and speciation of P in streambank profiles of Polecat Creek, IL, USA using sequential (Hedley) P fractionation. The HCl extractable fraction, interpreted as Ca-P and insoluble in water, was the dominant form of P accounting for 12–73% of total P (14–344 g m−2) with higher stocks in C horizons (131 ± 26 g m−2) than the overlying B (36 ± 7.6 g m−2) and A (41 ± 12 g m−2) horizons. Ca-P was strongly positively correlated with soil pH and inorganic carbon (IC) content, demonstrating the influence of apatite-rich loess parent material of this region in driving P speciation. Organic P (Org-P) accounted for 1–33% of total P (0.1–104 g m−2) with significantly higher stocks in A horizon (39 ± 5 g m−2) than B (11 ± 7 g m−2) and C (22 ± 7 g m−2) horizons, concomitant with the pattern of organic carbon (OC) and reflective of strong biological (riparian vegetation) controls on this pool of potentially mineralizable P. Subsurface increases in Org-P occurred for buried A horizons in banks created by alluvial depositions indicated the strong influence of fluvial processes on streambank P distribution and speciation. Fe/Al-P accounted for 1–21% of total P stocks and was generally enriched in the surface A (50 ± 30 mg kg−1) and B (35 ± 39 mg kg−1) horizons. The most immediately bioavailable pool, exchangeable P (Exc-P), was the smallest across streambanks, constituting < 10% of total P and largely relegated to A horizon (1.2–20.4 g m−2). This proof of concept offers valuable information about the variation of P stocks and speciation by genetic horizons in streambanks.