«A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Crop, Soil, and Environmental Sciences ...»
We are unsure why pH declined after this value, but a reduction in pH does help explain responses observed in the soil fraction study discussed later. The Hoberg SiL responsed similar to the Captina SiL (Fig. 3.3) with the pH plateauing at STP concentrations of 1070 mg P kg"1 soil (Fig. 3.3), similar to the peak where DRP began to decrease (1084 mg P ka"1) (Fig. 3.1). A linear response was observed with pH in Frederick SiL (Fig. 3.2), possibly due to tillage of the soil profile or absence of mid-range Mehlich-3 STP values.
Mehlich-3 extractable soil Ca was influenced by long term PL additions due to high Ca concentrations in PL (Tabler and Berry, 2003; Sharpley et al, 2004) (Fig. 3.3).
Statistically, Captina and Hoberg SiL had curvilinear relationships between Mehlich-3 STP and extractable Ca that plateaued at 508 and 2997 mg P kg"1, respectively. Frederick SiL had a linear relationship that was likely due to topsoil dilution by less concentrated
formation of water insoluble P compounds (Siddique and Robinson, 2003). A power relationship (r2 = 0.63) between exchangeable soil Ca versus water-extractable P as a
that as Mehlich-3 P increased Ca became a key controller of water extractable P.
Asymptotic values from Mehlich-3 STP versus DRP relationships (Fig. 3.1) were solved in regression equations established between Mehlich-3 STP and Mehlich-3 Ca (Fig. 3.3) to find exchangeable Ca values when DRP plateaued or was reduced in runoff (Fig. 3.1).
For example, Captina SiL had reduction of DRP concentrations in runoff at Mehlich-3 STP concentrations of 495 mgPkg 1 (Fig.3.1) even though STP continued to increase.
At this point, exchangeable Ca concentrations were 1791 mg Ca kg"1 and fell within the range of the power function where slope drastically changes. When the power function's slope changes, Ca becomes more responsible for P solubility than other cations and P is rendered less water soluble (Fig. 3.4). In Fig. 3.4, soils with low exchangeable Ca values had greater percentages of water-soluble P than soils with higher exchangeable Ca values. For example using the power function in Fig. 3.4 (y = 47500x"103, r2 = 0.63), a soil with 800 mg Ca kg"1 had a ratio of 49 kg water extractable P per 100 kg Mehlich-3 STP while soils with 4000 mg Ca kg"1 had a ratio of 9 kg water extractable P per 100 kg Mehlich-3 STP. Lower concentrations of water extractable P equate to less runoff DRP as reported in studies on soils from New York, Oklahoma, Pennsylvania, and Texas (Torbert et al., 2002; Sharpley et al., 2004).
Total recoverable Ca, Fe, Al, and P varied significantly with STP ranges as a result of long-term PL applications (Table 3.2). In all three soils, Ca increased as Pt increased due to high concentrations of Ca in PL (Tabler and Berry, 2003). Although significant differences were observed, no consistent pattern was seen with either Fe or Al in the various STP ranges. Differences in management, such as using chemically amended PL and inherent soil mineralogical differences likely caused this effect. Total P and Pj increased as Mehlich-3 STP increased, while P0 generally decreased. Higher concentrations of Pj traditionally mean more environmental concerns as Pj may be readily available to aquatic systems (Sharpley et al, 1994).
All soil-P fractions increased or decreased as PL applications increased Mehlich-3 STP (Table 3.3). More Pt was present as SLB-P in the high STP range for Captina and Frederick SiL (19.5 and 9.8%, respectively); while Hoberg SiL soils had highest percentage of SLB-P in the medium and high STP ranges (11.5 and 12.0%, respectively).
In all soils, percentage of SLB-P decreased at very high STP ranges (9.6, 3.9, and 8.9% for Captina, Frederick and Hoberg SiL, respectively). Total P consistently increases as STP increases (Table 3.2); therefore, additional P and portions of SLB-P formed more stable secondary minerals less detrimental to the environment. Decreasing the SLB-P soil fraction was likely responsible for the quadratic response of decreasing runoff DRP at very high STP ranges in rainfall simulations (Fig. 3.1).
The Al-P soil fraction served as a sink for P added as STP and SLB-P increased in Captina and Hoberg SiL soils (Table 3.3). We expected the fraction of soil Al-P to decrease as soil pH increased in relation to traditional partition diagrams (Lindsay, 1979);
acidic and basic soils in several soil series from Illinois, Michigan, Wisconsin, and New York receiving varying amounts of long-term P fertilization (Pierzynski et al., 1990).
Pierzynski and coworkers (1990) found that Al was the predominant or secondary cation associated with P in 57 categories out of 65 regardless of soil pH. At STP concentrations of 425 and 1070 mg P kg"1 for Captina and Hoberg SiL soils respectively, the pH peaked at 7.3 and began to decrease (Fig. 3.2). A decrease in pH would shift soil reaction towards formation of Al-P compounds (Lindsay, 1979). Frederick SiL soils had higher concentrations of Al-P formation in the medium and very high STP ranges than the low STP range (Table 3.3).
As expected, percentage of Fe-P minerals generally decreased (Table 3.3) as pH increased (Fig. 3.2) based on previous equilibrium experiments (Lindsay, 1979). Captina SiL had the lowest percentage of Fe-P in the high STP range (8.6%) and increased to 11.3% as pH began to decline at very high STP (Fig. 3.2). In Captina SiL, Fe-P may be partially responsible for reducing DRP in runoff water at very high Mehlich-3 STP concentrations. Iron-P consistently decreased from 16.8% to 0.8% in Hoberg SiL (Table 3.3) as STP increased and pH increased and plateaued (Fig. 3.2). Frederick SiL had the highest percentage of Fe-P at low STP and the percentage decreased at higher STP ranges (Table 3.3).
The RS-P fraction of Pt increased from 4.1 to 11.3% as STP increased for Captina SiL soils (Table 3.3). As our soils were aerobic, increases in this fraction should not increase DRP in runoff water since particulates were immediately filtered from water samples. However, the RS-P fraction will be easily dissolved when washed into
STP range, the percentage of RS-P was lower than at the STP high range for Frederick and Hoberg SiL soils. Reductant-soluble P trends for Frederick and Hoberg SiL correlate with reduced DRP concentrations in runoff at very high STP ranges (Fig. 3.1).
Calcium bound P reacted in similar manners to Al-P and RS-P in all three soils (Table 3.3). Highest Ca-P percentage of Pt was in the high and very high STP range for the Captina SiL. Calcium soil concentration plateaued at a Mehlich-3 extractable soil concentration of 508 mg P kg"1 and may have not been available for additional Ca-P formation as STP increased (Fig. 3.3). Frederick SiL had the highest Ca-P percentage at the STP high range (37.9%) and decreased with higher STP concentrations (23.4%) (Table 3.3). We are unsure why Ca-P decreased at high STP concentrations since pH and available Ca was still increasing (Fig. 3.3). Hoberg SiL Ca-P increased from 9.3 to 31.7% for low to very high STP ranges, respectively, as STP increased (Table 3.3). Hoberg SiL may be able to buffer more P from being released in runoff as insoluble Ca-P compounds since available Ca does not peak until STP concentrations of 2997 mg P kg'1 (Fig. 3.3).
Generally, an increase of Ca-P formation across all of the soils was seen as STP increased and likely inhibited additional runoff DRP as Mehlich-3 STP increased. At values above Sharpley and coworker's (2004) 412 mg Mehlich-3 P kg"1 critical value, the acidic Mehlich-3 extract dissolved Ca-P compounds and artificially inflated predictions for runoff DRP.
Silt loam soils with long histories of PL applications may have quadratic relationships with DRP in runoff and Mehlich-3 extractable STP concentrations. As soil
cations increased or decreased. Iron-P was not a significant source for reducing DRP in runoff for Hoberg or Frederick SiL, but did sequester some additional P in Captina SiL.
Generally, Al-P, RS-P, and Ca-P formations were the main sinks for P and proportionally reduced P loss in runoff at very high STP concentrations even though Mehlich-3 and water extractable STP continuously increased. Mehlich-3 extract relationships were generally better models for predicting DRP than water extractable P; however, Mehlich-3 overestimates environmentally available P in soils with high pH and high exchangeable Ca concentrations by extracting Ca-P that are less soluble in water.
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