Introduction:
Phosphorus is an essential macronutrient that is often limiting primary production in rivers and lakes. In terms of the Redfield ratio (106C:16N:1P), even a small addition of P under P-limited conditions could lead to a significant increase in the primary production of aquatic ecosystems. This may lead to eutrophication and hypoxia, a water dead zone, with significant loss of biodiversity and changes in the food web (Orihel et al., 2017; Dijkstra et al., 2018). As algae may bind nitrogen, an increase in P-availability may trigger algal outbreaks. This has been observed in many cases to occur due to excessive fertilizer and manure applications
For many aquatic systems, soil and particulate-P erosion together with runoff water are main sources of inputs of P to surface water (Brendel et al., 2019). Sediment pore water and desorption from its solid phase replenish P in overlying waters, and become available for consumption by aquatic organisms. It has long been recognized that soil and sediment bound P is only partly biologically available to plants and aquatic organisms as algae (Van Rotterdam et al., 2012). The bioavailable fraction of P can be evaluated by algal bioassays (Liu et al., 2016). Although algal assays are difficult to conduct and time-consuming, the bioavailable P estimated by assays is considered well to quantify the potential of sediment P to cause eutrophication (Young et al., 1985; Ekholm and Krogerus, 2003). Indeed, the quantification of bioavailable phosphorus for algal growth was shown to be essential to estimate eutrophication risks (Ellison and Brett, 2006; Okubo et al., 2012). However, it is attractive to predict bioavailable P using a chemical measure that is simple, routinely applicable, and cost-efficient. A single chemical extraction that allows to screen a large number of samples and monitor the health of aquatic ecosystems requires that the relationship between bioavailability of P from algal bioassay experiments and chemically extracted P is clear and unambiguous.
Several chemical extraction methods have been developed in the field of soil fertility to estimate bioavailable P in soil and sediments such as those developed by Morgan (1954), Mehlich (1983), Bray-Kurtz (1961), Colwell (1952) and Olsen et al. (1954). Chemical extractants are based on different mechanisms and abilities to extract P and may differ regarding the chemical used for extraction as well as e.g. sediment: water ratio, equilibration (shaking) time, and targeted P pools. For instance Bray-P may be useful for acid, but not for calcareous materials. Also Mehlich-P is based on extraction by acid, but performs better for high pH soils. Olsen-P extracts with NaHCO3and is more commonly used for pH-values exceeding slightly acid values, as it was developed for alkaline soils. Phosphorus extracted by 0.1 M NaOH is one of first methods developed to quantify algal available P (Young, 1982). Anderson and Magdoff (2005) reported that Olsen-P had the highest correlation with Selenastrum capricornutum growth in comparison with other extractants. Huettl et al, (1979) equilibrated sediment suspensions with hydoxy-Al saturated cation exchange resin and found that the P retained by resin correlated with Selenastrum capricornutum (r2 = 0.98; n = 5). The well-known 0.01M CaCl2-extraction (Boekhold et al., 1993) provided excellent results for agricultural soils that are regularly limed. Sharpley (1993) used a selective extraction that was based on establishing contact between a soil suspension and Fe-oxide coated paper strips that were developed by Van Der Zee et al. (1987). The so-called Fe-paper extracted P content of runoff sediment was closely correlated (r2=0.96) to the growth of P-starved Selenastrum capricornutum incubated for 29d with runoff as the sole source of P (Sharpley, 1993).
To improve phosphorus management and deal effectively with adverse legacy effects of sedimentary phosphorus, we investigate bioavailability of P for the aquatic ecosystem. The scope of this research was to 1) study the bioavailability P in sediments according to several single chemical extractions and assessment of their relationship with P-inhibited algal growth, and 2) recognize influencing factors in P extraction in sustaining algal growth.