The error in stream sediment phosphorus fractionation and sorption properties effected by drying pretreatments

Abstract

Purpose: Stream sediment can control phosphorus (P) in the water column at baseflow. Two common laboratory analyses of sediment P are the equilibrium phosphate concentration at net zero sorption (EPC₀ ) and P fractionation. Good sample handling ensures representative results, but oftentimes, studies rely on air-dried or freeze-dried samples, which alters sediment biogeochemistry. How and to what extent this influences EPC₀ and P fractionation remains unclear. We therefore examine pretreatment effects on sediment EPC₀ and P fractionation.

Materials and methods: We collected fine sediments (< 2 mm) from streams in the Tukituki River and Reporoa Basins in New Zealand (n = 31 sediments). Subsamples were then either kept fresh, frozen then lyophilized (freeze-dried), or dried at 40 °C for 2 weeks (air-dried). Measurements of EPC₀ and P fractionation were made in triplicate. The sequential P fractionation scheme determined five different P pools: NH₄ Cl (labile P), NaOH reactive P (RP; metal oxide-bound P) and unreactive P (URP; organic P), HCl (Ca-mineral P), and residual P. Along with statistical comparisons between fresh results and the two pre-treatments, we explored correlations between pre-treatment effects and sediment physicochemical characteristics. Results and discussion: The sediments had generally low EPC₀ (majority < 0.020 mg P L⁻¹), and uncertainty in EPC₀ increased with concentration magnitude. While there were sediment-specific changes in EPC₀ with pre-treatment, there was no consistent bias caused by pre-treatment. However, the differences between the fresh and air-dried sediment EPC₀ were larger and more variable than between fresh and freeze-dried sediment. For P fractionation, the Tukituki sediments were enriched in HCl-P, while Reporoa sediments had more NaOH-RP and NaOH-URP. Despite large sediment-specific changes, the overall effects of freeze- and air-drying sediment were increased NH₄ Cl-P (estimated average effect, θ̂ = + 0.63 and + 3.7 mg P kg⁻¹ ), no significant changes for NaOH-RP, contrasting changes in NaOH-URP (− 3.4 and + 3.3 mg P kg⁻¹ ), and decreased HCl-P (− 40 and − 33 mg P kg −1 ). Conclusions: We found that drying sediment significantly influenced EPC₀ and P fractions (especially the NH₄Cl-P fraction). Air-drying was particularly error-prone and should be avoided. The use of freeze-drying to preserve samples for later analyses and improve ease of handling may be used with appropriate consideration of the research objectives and the error introduced by freeze-drying. However, we recommend using fresh sediments for analyses whenever possible, as they best represent natural conditions.