Algal growth and sediment test phosphorus
A linear relationship between algal growth and 7 of the sediment P tests is shown in Figure 4, which shows that algal growth was different in the growth medium incubated with different sediments. Moreover, this figure reveals that there is a significant relationship between the P tests andSelenastrum capricornutum growth (Figure 4). Of the seven sediment P test measurements, Colwell-P correlated best with algal growth (r2=0.92,P <0.001). Part of the explanation for the high correlation between Colwell-P and Olsen-P with algal growth may be that the pH of algal growth solution, buffered at pH 7.5 with NaHCO3, was much closer chemically to the Olsen and Colwell extracts of 0.5 M NaHCO3 at pH 8.5 than the other extractants. The solid : solution ratio (1:50) and the long extraction time of Colwell-P are more similar to the S. capricornutum growth conditions (1:100), which is the plausible reason that it outperformed the Olsen-P (1:20) extraction.
Similarly, 0.1 M NaOH had a significant relationship with algal growth. Fe/Al-P represents phosphorus bound to Fe and Al oxides and is exchangeable with OH¯ and other inorganic phosphorus compounds, which are soluble in bases (Wu et al., 2011). Dorich et al. (1984) also found that 0.1 M NaOH-extractable phosphorus was significantly correlated with 2-day and 14-day available phosphorus for an alga (S. capricornutum ). The high correlation between 0.1 M NaOH and algal growth indicated the sensitivity of P bioavailability to redox conditions in river system.
Morgan-P (NaActate pH = 4.8) was expected to extract P from Ca-P pools bound to the authigenic carbonate fluorapatite (CFAP) + biogenic apatite (Ruttenberg, 1992). The correlation between Morgan-P and algal growth was therefore significant. Actually, in calcareous soils, it is the high activity of Ca2+ cations, rather than that of carbonate anions, that affects P mobility, due to the precipitation of Ca-phosphates with low solubility (Jalali and Jalali., 2107). Torbert et al. (2002) studied relationship of soil test P and runoff in calcareous and non-calcareous soils. They indicated that in calcareous soils losses of P in runoff was reduced due to the reaction of Ca with soluble P to form insoluble Ca-P minerals. William et al. (1980) reported that uptake of P by the cells alga (Scenedesmus quadricauda ) varied from 8 to 50% of total P and from 38 to 83% of nonapatite inorganic P when measured directly. Xiao-Fei et al. (2015) reported that part of Ca-P can be uptaken by Microcystis aeruginosa .
All sediment P measurements were consistently high in sediments with high P concentrations, however, in some sediments, the response was very high during the two-week growth period and even continued for a long period of time. One possible explanation is the presence of internal conversion of P from recalcitrant P pool to bioavailable P is high in this sediment (S20). The Colwell extractant separated sediments well against the algal population. It means that sediments with high bioavailable P had high algal growth. Some sediment P can be easily desorbed and released, particularly when the concentration of phosphorus in the water column is depleted. It is expected that Colwell- extracted P would separate from potentially available P to immediately available to algae. So, 14 days cultural time was well correlated with Colwell P. The proportion of Colwell-P to Total-P in the sediments of rivers was the most consistent of all the extractions used (Table 4).