Calcium Phosphate Formation along Estuarine Salinity Gradients Printer-friendly Version Interactive Discussion Calcium Phosphate Formation Due to Ph-induced Adsorption/precipitation Switching along Salinity Gradients Bgd Calcium Phosphate Formation along Estuarine Salinity Gradients Printer-friendly

Introduction Conclusions References Tables Figures Back Close Full Screen / Esc This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available. Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract Mechanisms governing phosphorus (P) speciation in coastal sediments remain unknown due to the diversity of coastal environments and poor analytical specificity for P phases. We investigated P speciation along salinity gradients comprising diverse ecosystems in a P-enriched estuary. To determine P load effects on P speciation we 5 compared the high P site with a P-unenriched site. To improve analytical specificity, octacalcium phosphate (OCP), authigenic apatite (carbonate fluorapatite; CFAP) and detrital apatite (fluorapatite) were quantitated in addition to Al/Fe-bound P (Al/Fe-P) and Ca-bound P (CaP). Sediment pH primarily affected P fractions across ecosystems and independent of the P status. Increasing pH caused a pronounced downstream transi-10 tion from adsorbed Al/Fe-P to mineral CaP. Downstream decline in Al/Fe-P was counterbalanced by the precipitation of CaP. This marked upstream-to-downstream switch occurred at near-neutral sediment pH and was enhanced by increased P loads. Accordingly , the site comparison indicated two location-dependent accumulation mechanisms at the P-enriched site, which mainly resulted in elevated Al/Fe-P at pH < 6.6 15 (upstream; adsorption) and elevated CaP at pH > 6.6 (downstream; precipitation). Enhanced CaP precipitation by increased loads was also evident from disproportional accumulation of metastable CaP (CaP meta). The average CaP meta concentration was six-fold, whereas total CaP was only twofold higher at the P-enriched site compared to the P-unenriched site. Species concentrations showed that these largely elevated Ca-20 P meta levels resulted from transformation of fertilizer-derived Al/Fe-P to OCP and CFAP due to decreasing acidity from land to the sea. Formation of OCP and CFAP results in P retention in coastal zones, which may lead to substantial inorganic P accumulation by anthropogenic P input in near-shore sediments. Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc

tion from adsorbed Al/Fe-P to mineral Ca-P. Downstream decline in Al/Fe-P was counterbalanced by the precipitation of Ca-P. This marked upstream-to-downstream switch occurred at near-neutral sediment pH and was enhanced by increased P loads. Accordingly, the site comparison indicated two location-dependent accumulation mechanisms at the P-enriched site, which mainly resulted in elevated Al/Fe-P at pH < 6.6 15 (upstream; adsorption) and elevated Ca-P at pH > 6.6 (downstream; precipitation). Enhanced Ca-P precipitation by increased loads was also evident from disproportional accumulation of metastable Ca-P (Ca-P meta ). The average Ca-P meta concentration was six-fold, whereas total Ca-P was only twofold higher at the P-enriched site compared to the P-unenriched site. Species concentrations showed that these largely elevated Ca-

Introduction
Anthropogenic P inputs more than doubled the global P flux to marine environments (Meybeck et al., 1982;Bennett et al., 2001). In some regions the fluxes have increased to more than ten times their pre-agricultural levels (Howarth et al., 1995). A large proportion of this terrestrial input to marine systems is retained in estuaries and near-5 shore marine environments (Dürr et al., 2010;Howarth et al., 1995;Ruttenberg, 1990Ruttenberg, , 2004Slomp, 2011;Teodoru et al., 2007). The primary anthropogenic P source to this reservoir originates from manure and fertilizer P, which is mainly lost from agricultural systems in the form of erosion-derived particulate P (Kronvang et al., 2007;McDowell et al., 2003;Jordan et al., 2008). Coastal environments may be particularly vulnerable Introduction with adsorbed P at acidic pH to lowest phosphate concentrations in equilibrium with mineral Ca-P under alkaline conditions (e.g. Murrmann and Peech, 1969). Low equilibrium concentrations under alkaline conditions are a result of the decreasing solubility of Ca-P phases, such as carbonate fluorapatite (CFAP) and octacalcium phosphate (OCP), with increasing pH (Hinsinger, 2001;Murrmann and Peech, 1969). Precipita-5 tion of Ca-P may therefore mitigate a desorption-derived P release from sediment (e.g. van Beusekom and de Jonge, 1997). Similarly, Ca-P precipitation is likely to result in the occasionally observed and apparently conflicting decrease of available P by liming to neutral or alkaline pH (Bolan et al., 2003;Haynes, 1982;Naidu et al., 1990). Accordingly, concentrations of Ca-P usually increase downstream as a consequence of 10 enhanced precipitation (Andrieux-Loyer et al., 2008;Coelho et al., 2004;Paludan and Morris, 1999;Sutula et al., 2004). Due to factors that allow for widespread, dispersed formation and persistence of more soluble authigenic Ca-P in marine environments (possibly even in the water column; Faul et al., 2005;Lyons et al., 2011), detrital fluorapatite (FAP) is unlikely to dissolve in these environments (Ruttenberg, 1990;Howarth et al., 1995).
In addition to the dependence on species-specific saturation states (Atlas, 1975;Gunnars et al., 2004), the occurrence of Ca-P minerals depends on their formation kinetics (Atlas and Pytkowicz, 1977;Gulbrandsen et al., 1984;Gunnars et al., 2004;Jahnke et al., 1983;Schenau et al., 2000;Sheldon, 1981) and inhibitors such as Mg 2+ 20 ions (Golubev et al., 1999;Gunnars et al., 2004;Martens and Harriss, 1970). In general, the first solid to form is the one which is thermodynamically least favored (Ostwald step rule; see Morse and Casey, 1988;Nancollas et al., 1989). In view of the slow or inhibited direct nucleation (Golubev et al., 1999;Gunnars et al., 2004;Martens and Harriss, 1970), species of the apatite group may form in sediment by transformation of pre- 25 viously precipitated metastable precursors that are less susceptible to inhibitory effects of Mg 2+ such as OCP (Oxmann, 2014;Oxmann and Schwendenmann, 2014). Hence, rapidly precipitating precursor phases (e.g. days to weeks for OCP; Bell and Black, 1970)  kinetically slow apatite formation occurs (ten to some thousand years; Schenau et al., 2000;Jahnke et al., 1983;Gulbrandsen et al., 1984). Several studies presented field and experimental evidence for this mode of apatite formation in sediment (Gunnars et al., 2004;Jahnke et al., 1983;Krajewski et al., 1994;Oxmann and Schwendenmann, 2014;Schenau et al., 2000;van Cappellen and Berner, 1988). A systematic 5 comparison of P K-edge XANES (X-ray absorption near edge structure spectroscopy) fingerprints from reference materials and marine sediment particles also provided evidence for the occurrence of OCP in sediment (Oxmann, 2014). However, despite significant progress in the determination of different matrix-enclosed Ca-P phases it is not yet clear whether specific conditions at certain locations facilitate successive or direct crystallization of apatite (Slomp, 2011). Provided more soluble Ca-P minerals such as OCP or less stable CFAP form in coastal environments, these minerals might mirror short-term changes of human alterations to the P cycle. Conversely, sparingly soluble apatite minerals may reflect long-term changes due to slow precipitation. Hence, the proportion of more soluble 15 Ca-P should occasionally increase relative to total Ca-P in response to increased P inputs. This human alteration to the solid-phase P speciation due to diagenetic processing of increased P loads may have implications for P fluxes and burial. To better describe P transformations from terrestrial to marine systems and to track the fate of anthropogenic P inputs, we analysed effects on P fractions and species across different 20 ecosystems of a P-enriched site (Firth of Thames, New Zealand). We then compared the findings with data from a P-unenriched coastal site (Saigon River Delta, Vietnam) to identify similarities between temperate and tropical coastal regions and to distinguish speciation differences related to increased P loads. Octacalcium phosphate, authigenic apatite and detrital apatite were determined using a recently validated conversion-Introduction  , 1997). The southern shore of the bay (∼ 7800 ha) is listed as a wetland of international importance under the Ramsar Convention. The Firth of Thames encompasses large tidal flats (up to 4 km wide) and large areas of mangroves (Avicennia marina) at the southern end of the embayment (Brownell, 2004). Mangroves 10 have been expanding seawards leading to a 10-fold increase in area since the mid of the last century (Swales et al., 2007). Mangrove expansion has been related to sediment accumulation and nutrient enrichment but may also coincide with climatic conditions (Lovelock et al., 2010;Swales et al., 2007). The upper coastal intertidal zone is covered by salt marshes. Behind the levee ca. 1.3 million ha is used for pastoral agri-15 culture (∼ half of the total area of the Waikato region; Hill and Borman, 2011;Fig. 1a).

Sampling, field measurements and sample preparation
We established 28 plots along three transects (Fig. 1b,c). Transects extended across the entire tidal inundation gradient and across different ecosystems including bay (n = 4), tidal flat (n = 6), mangrove (n = 9), salt marsh (n = 6) and pasture (n = 3). Tran-20 sects were at least 300 m from rivers to exclude areas affected by sediment aeration. Five additional plots were located at rivers. Sediment cores were taken during low tide using a polycarbonate corer (one core per plot; length: 40 cm; diameter: 9 cm). Immediately after core sampling, sediment pH, redox potential (Eh) and temperature were measured in situ at 0-5, 10-15, 30- on ice and subsequently frozen until further processing. After thawing roots were removed from the sediment samples. Subsamples were then taken for particle size and salinity analysis. The remaining material was dried, ground and sieved (37 • C; < 300 µm mesh; PM 100 ; Retsch, Haan, Germany) for P and nitrogen analyses. Temperature, pH and Eh were measured with a Pt-100 temperature sensor, sul-5 fide resistant SensoLyt SEA/PtA electrodes, and pH/Cond340i and pH 3310 mV-meters (WTW, Weilheim, Germany). The mV-meters were connected to a computer with optoisolators (USB-isolator; Serial: 289554B; Acromag Inc., Wixom, USA) for data visualization and logging (MultiLab pilot; WTW, Weilheim, Germany). Topographic elevation at the plots was measured with a total station (SET530R; Sokkia Co., Atsugi, Japan) relative to a reference point and converted to geo-referenced elevation using a global navigation satellite system (Trimble R8; Trimble Navigation Ltd., Sunnyvale, USA). Inundation duration was calculated from measured elevation above mean sea level and local tide tables (Waikato Regional Council, Hamilton East, New Zealand). 15 Phosphorus fractions and total P were analysed using three different methods. (i) The relative proportion of more soluble Ca-P was determined by semi-selective extraction analysis using the Morgan test method (Morgan, 1941); (ii) Al/Fe-P and Ca-P fractions were quantitated by sequential extraction of P after Kurmies (Kurmies, 1972); and (iii) total P (TP) was analysed after Andersen (1976) as modified by Ostrofsky (2012 defined as the sum of inorganic P fractions (Ca-P, Al/Fe-P). Organic P was calculated by subtracting TIP from TP. Octacalcium phosphate, CFAP (authigenic apatite) and FAP (detrital apatite) were quantitated using the CONVEX method (Oxmann and Schwendenmann, 2014). This method employs a conversion procedure by parallel incubation of sediment subsam-5 ples at varying pH in 0.01 M CaCl 2 for differential dissolution of OCP, CFAP and FAP (Fig. S2c). The concentration of OCP, CFAP and FAP is determined by the difference of Ca-P concentrations before and after differential dissolution of OCP, CFAP and FAP, respectively. These Ca-P concentrations are quantitated by the method of Kurmies. Differential dissolution was verified by standard addition experiments. Reference compounds included OCP, hydroxylapatite (HAP), various CFAP specimens, FAP and biogenic apatite. Methodology, instrumentation and the suite of reference minerals are described in Oxmann and Schwendenmann (2014). CONVEX analysis was conducted for seven sediment samples (differential dissolution shown in Fig. S2a, b), which covered the observed pH gradient and included sediments from each ecosystem. The sum 15 of OCP and CFAP represents more soluble Ca-P (similar to Ca-P meta ) and was termed "Ca-P OCP+CFAP ". Phosphate concentrations in chemical extracts were determined after Murphy and Riley (1962) using a UV-Vis spectrophotometer (Cintra 2020; GBC Scientific Equipment, Dandenong, Australia).

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Data of the Firth of Thames site were compared with those of a contrasting Punenriched site of the Saigon River Delta (Oxmann et al., 2008;. The site was located ca. 50 km from Ho Chi Minh City and was not significantly influenced by anthropogenic P input (remote location; no agriculture). It comprised an area exhibiting physical-chemical sediment characteristics very similar to those of the Firth of Thames site and an area of acid sulphate sediments (analysed separately; Sect. 3.4).

Particle size, pH and Eh
Inundation duration ranged from 365 days yr −1 in the bay to 0 days yr −1 in the pas-5 ture (Table S1). Clay, silt and sand fractions varied between 0-20 %, 60-80 % and 0-30 %, respectively (not shown). Particle size distribution differed only slightly among transects. In contrast, considerable differences in particle size distribution were found along transects with higher silt and lower sand contents in mangrove plots. Salinity decreased from the bay (32 ‰) to tidal flats (21 ‰). Highest values were measured in the 10 mangroves (35 ‰), which declined to 25 ‰ in the salt marshes and 0 ‰ in the pastures (Table S1). Sediment humidity ranged between 60 % and 70 % in the bay, tidal flat and mangrove fringe plots and decreased to 20-30 % in the pastures (not shown). Sediment pH ranged from 5.18 in the pasture to 7.4 in the bay (Fig. 2a, b; Table S1). Redox potential varied between 621 mV in the pasture and −141 mV in the  Table S1). This pronounced and relatively constant pH increase and Eh decrease towards the bay was closely correlated with inundation duration (Fig. 2a, b; p < 0.0001). While systematic differences among depth intervals were less apparent for pH, Eh typically decreased with increasing sediment depth (Fig. 2b). 20 Ecosystem-averaged Ca-P concentrations varied considerably and ranged from 3.37 µmol g −1 (pasture, 0-5 cm) to 11.37 µmol g −1 (bay, 10-15 cm) (Fig. 2c, d; Table S1). In contrast, Al/Fe-P was highest in the pasture (17.9 µmol g −1 ; 0-5 cm) and lowest (3.94 µmol g −1 ; 30-35 cm) in the bay. On average, the lowest Ca-P and highest Al/Fe-P concentrations were measured in 0-5 cm depth (Fig. 2c, d). Averaged percentages of Ca-P (% of TIP) steadily increased and averaged percentages Al/Fe-P (% of TIP) steadily decreased from pasture to bay (Fig. 2c, d). Along the marked downstream transition from Al/Fe-P (2.7-fold decrease) to Ca-P (2.6-fold increase) the average drop in Al/Fe-P from pasture to bay approximately matched the average Ca-P 5 increase (−8.27 vs. +6.73 µmol g −1 ; averages across all plots and depth intervals of each ecosystem; Table S1). Furthermore, mean Al/Fe-P concentrations in the different systems were negatively correlated with those of Ca-P (r = −0.66, p < 0.001; Table S1).

Phosphorus fractions: transformations
In addition, the decline in Al/Fe-P with depth was counterbalanced by the Ca-P increase with depth (−1.16 vs. +1.18 µmol g −1 ; Table S1). The pasture was excluded from estimating these changes with depth because here the large loss of Al/Fe-P with depth was not counterbalanced by Ca-P (apparent surface runoff: −6.00 vs. +0.69 µmol g −1 ; Table S1).

Phosphorus fractions: P load, pH and Eh effects
Mean Ca-P concentration in sediments of the P-enriched site was approximately twice 15 the level of Ca-P in the P-unenriched site (Fig. 3a). Mean Al/Fe-P concentration was approximately 30 % higher in sediments of the P-enriched site compared to the Punenriched site. However, at both sites Ca-P concentrations increased exponentially with pH ( Fig. 3a). Al/Fe-P concentrations showed a peak at ∼ pH 6.6 ( Fig. 3b). In summary, despite large differences in P fraction concentrations between the two sites pH 20 dependencies of both fractions were similar between the sites, except for Al/Fe-P concentrations in 0-5 cm depth. Concentrations of Al/Fe-P in this depth range showed a linear decrease with pH at the P-enriched site due to high Al/Fe-P levels in acidic surface sediment of the pasture (linear regression in Fig. 3b; Table 1; r = −0.81, p < 0.0001).
In sediments with pH < 6.6 the average concentration of Al/Fe-P was 70 % higher 25 at the P-enriched site than at the P-unenriched site (Table 2; Fig. 3b). Despite these largely elevated levels of Al/Fe-P at topographically higher areas, Al/Fe-P was only slightly increased (13 %) in the lower intertidal zone and the bay (pH > 6.6;  Fig. 3b). Calcium phosphate in contrast showed the opposite pattern of enrichment at the P-enriched site. In comparison to the P-unenriched site the average concentration of Ca-P was only 49 % higher in upstream environments (pH < 6.6) but increased by 88 % in the lower intertidal zone and the bay (pH > 6.6; Table 2; Fig. 3a). Although Ca-P and Al/Fe-P clearly showed opposite trends along the three transects 5 of the P-enriched site (Fig. 2c, d), both fractions increased exponentially with pH below pH 6.6 ( Fig. 3a, b). Both fractions were positively correlated at pH < 6.6 (shown in Fig. 3c for lower depth of both sites). At pH > 6.6, however, Ca-P increased further, whereas Al/Fe-P abruptly decreased (cf. Fig. 3a and b). This suggests a pH-induced switch towards Ca-P predominance (see arrows in Fig. 3c). Because this switch occurred in the downstream direction, it is in agreement with the observed Ca-P increase and Al/Fe-P decrease towards the bay (Fig. 2c,d). In contrast to the substantial pH effects, a potential reduction-mediated P desorption was restricted to pH > 6.6. The Al/Fe-P fraction declined with decreasing Eh only in that pH range (cf. Fig. 3b, d; Table 1).

Metastable calcium phosphate
Metastable Ca-P (Ca-P meta ) increased exponentially with pH ( Fig. 4a), similar to Ca-P ( Fig. 3a), and correlated with Ca-P at both sites ( Fig. 4b). Yet the concentration of Ca-P meta in sediments of the P-enriched site was six-fold higher compared to the Punenriched site (Fig. 4c). In contrast, total Ca-P was only twofold higher compared to 20 the P-unenriched site (Fig. 3a). On average, Ca-P meta comprised ca. 35 % of total Ca-P at the P-enriched site and only 10 % at the P-unenriched site (Fig. 4d). To verify that the higher Ca-P meta concentrations were not generated by differences in pH and Eh values we restricted the comparison to mangrove plots with similar pH and Eh values at both sites (Table 3). By doing this, previous figures did not change. 25 The difference in Ca-P meta concentrations between mangrove plots of the two sites was just as disproportionate when compared to the difference in total Ca-P concentrations between those plots (six-fold vs. twofold). The portion of Ca-P meta was still ca.

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 35 % at the P-enriched site and 10 % at the P-unenriched site (Table 3). Moreover, the proportion of Ca-P meta to total Ca-P was equal for two areas of the P-unenriched site (10 %) despite their very different average pH and Eh values (Table 3). In summary, comparatively large amounts of metastable Ca-P accumulated at the P-enriched site.

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Concentrations of OCP, authigenic apatite and detrital apatite were largely affected by the pH in sediments of both sites. Strongly acidic sediments (∼ pH < 4) contained just detrital apatite (FAP), whereas slightly acidic sediments (∼ pH 4-7) contained also authigenic apatite (CFAP). Octacalcium phosphate was additionally present in alkaline mangrove, river, bay and tidal flat sediments. Hence, the concentration of more sol-  Table 4). However, the portion of Ca-P OCP+CFAP as a percentage of total Ca-P was significantly larger (70.5 ± 17.5 %; numbers above columns in Fig. 5) for sediments of the P-enriched site compared to the P-unenriched site (29.5 ± 26.0 %), t(11) = 3.346, p = 0.0065. This larger portion of Ca-P OCP+CFAP provided supporting 15 evidence for the larger portion of Ca-P meta in sediments of the P-enriched site (cf. Sect. 3.4). Overall, more soluble Ca-P determined by the two independent methods (CONVEX method: Ca-P OCP+CFAP; Morgan test: Ca-P meta ) yielded comparable results. Accordingly, corresponding values obtained by the two methods were significantly correlated (r = 0.74, p < 0.05; Table 4).

Phosphorus status, Firth of Thames
The Firth of Thames sediments were high in P compared to the Saigon River Delta site and sediments from other coastal areas ( Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | P g −1 ) and "very enriched" (> 32 µmol P g −1 ) according to the New Zealand classification system (Robertson and Stevens, 2009;Sorensen and Milne, 2009). This is due largely to high P fertilizer application rates, which provide the main P source (∼ 90 %) for the high P inputs to the watershed (Waikato region; Total input: 41 Gg P yr −1 ; Fertilizer: 37 Gg P yr −1 ; Rate: 28 kg P ha −1 yr −1 ; Atmosphere and Weathering: 4 Gg P yr −1 ; 5 Parfitt et al., 2008). A significant increase in TP is correlated with intensification of pastoral farming and contributes to the deterioration of the river water quality by surface runoff (Vant and Smith, 2004). Further, elevated levels of P and nitrogen in ground water at coastal farmlands agreed with specific fertilizer application rates (Brownell, 2004). The application rates and previous findings strongly suggest that the P accumulation 10 measured in this study was largely related to P fertilization practices. Hence, the Firth of Thames site is characterized by anthropogenic P enrichment.

Phosphorus fractions
Phosphorus fractions showed a strong and continuous increase of Ca-P and decrease of Al/Fe-P with increasing inundation duration from pasture to bay (Fig. 2). These 15 changes were driven by pH and Eh gradients (Fig. 2). Salinity had no detectable effect (Table 1), similar to findings of Maher and DeVries (1994). Both an increase in Ca-P (decreasing solubility of calcium phosphates with increasing pH; Lindsay et al., 1989;Hinsinger, 2001) and a decline in Al/Fe-P (desorption of P from metal (hydr)oxides with increasing pH or decreasing Eh; e.g. Mortimer, 1971) commonly occur across estuar-20 ine inundation gradients. Similar upstream-to-downstream changes in sediment P fractions have been observed, for example, at marsh systems in Portugal (Coelho et al., 2004) and South Carolina (Paludan and Morris, 1999), estuarine zones of a French river (Andrieux-Loyer et al., 2008) and an estuarine transect from the Mississippi River to the Gulf of Mexico (Sutula et al., 2004). In the latter case, the opposing upstream- 25 to-downstream changes in sediments (Sutula et al., 2004)  the same trends were observed along a continuum from agricultural soils across hardwater stream sediments to lake sediments (Noll et al., 2009). The pH dependence of the solubility of Ca-P phases was mirrored by the increase in Ca-P with decreasing acidity (Fig. 3a). In contrast, concentrations of Al/Fe-P showed a maximum amount of adsorbed P at pH 6.6 ( Fig. 3b) and therefore agreed with the 5 commonly occurring maximum P availability at pH 6.5 in agricultural soils (e.g. Chapin III et al., 2011). This maximum P availability is caused by the highest P solubility in equilibrium with various P minerals at pH 6.5 (Lindsay et al., 1989). Considering that low amounts of P are precipitated with Ca, Fe and Al at pH 6.5 (e.g. Chapin III et al., 2011), large amounts of soluble reactive P could be available for adsorption on 10 metal (hydr)oxides. Further, Ca-P phases predominated in alkaline downstream environments and could undergo dissolution and subsequent adsorption after upstream transport by tides (cf. Fig. 2a , c, d, Sect. 4.5, De Jonge and Villerius, 1989). If these phases are transported to close locations of ∼ pH 6.5 and subsequently dissolve and adsorb, they contribute to the elevated Al/Fe-P concentrations at this pH. The maximum 15 amount of adsorbed P at pH 6.6 also indicated that Eh, which showed the maximum decline at ∼ pH 6.6 ( Fig. 3d), did not cause significant desorption of P below this pH. Yet, both Eh and Al/Fe-P decreased at pH > 6.6 (cf. Fig. 3b and d) and were correlated in this range (Table 1). Because the drop in Al/Fe-P correlated also with an increase in pH in this range (Table 1, Fig. 3b), effects of pH and Eh on P desorption could not 20 be distinguished above pH 6.6. The decreasing amount of adsorbed P above pH 6.6 may be due to (i) charge changes of metal (hydr)oxides with pH (Oh et al., 1999;Spiteri et al., 2008;Barrow et al., 1980;Sundareshwar and Morris, 1999); (ii) less efficient sorption by iron in the Fe(II) state compared to the Fe(III) state (e.g. Sundareshwar and Morris, 1999); or (iii) a combination of charge changes and Fe reduction. 25 However, pH apparently affected both Ca-P and Al/Fe-P across the pH range, whereas a possible reduction-mediated decline in Al/Fe-P was restricted to pH > 6.6. Thus, pH exerted strong control on inorganic P fractions and therefore largely accounted for the downstream transition from Al/Fe-P to Ca-P. Interestingly, these pH- driven P redistributions largely agreed with those from the P-unenriched site despite considerable differences of P fraction concentrations between both sites (Fig. 3a, b). In summary, the dominating effect of the pH was therefore independent of the P status of the diverse coastal sites.

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The increase in Ca-P along transects, which correlated with an equivalent decrease in Al/Fe-P, strongly suggests that Ca-P formed at the expense of adsorbed P along the salinity gradient. By plotting Al/Fe-P and Ca-P concentrations on a pH scale (Fig. 3a, b) it became evident that the downstream transition from Al/Fe-P to Ca-P was related to a switch from P adsorption to Ca-P precipitation at ∼ pH 6.6 (see also Fig. 3c). Above 10 this pH, reduction processes are less important for P desorption (Reddy and DeLaune, 2008), P adsorption is usually less pronounced (Murrmann and Peech, 1969) and thermodynamically less stable Ca-P phases such as octacalcium phosphate (OCP) may form (Bell and Black, 1970;Oxmann and Schwendenmann, 2014). A similar switch has been suggested for observed fraction changes with increasing sediment depth 15 in non-upwelling continental margin environments, but from organic P to authigenic Ca-P (Ruttenberg and Berner, 1993). This switch was partly explained by the redox state, which could be the controlling parameter for diagenetic redistributions and related downcore changes of P fractions in marine environments. The main difference between P redistributions in this study and results of Ruttenberg and Berner (1993) re-20 lates to the P source to the formation of authigenic Ca-P. Our data indicate strong interactions between inorganic P forms, with Al/Fe-P being a significant P source for Ca-P. In contrast to these strong interactions between Al/Fe-P and Ca-P all correlations with organic P were not significant. However, P redistributions along marine sediment cores may strongly differ from those across intertidal zones. The marked fraction changes 25 suggest that the pH regulates an alternative switch between Al/Fe-P and Ca-P at the investigated coastal sites. This pH-driven P redistribution could be a common mechanism at coastal pH gradients because it took place along different transects, comprising 10244 Introduction

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | diverse ecosystems, and it was independent of the P status (Fig. 3a, b). Hence, this mechanism could also be important for processes of P accumulation by increased P loads, as discussed next.

Phosphorus accumulation processes
The anthropogenic P input at the P-enriched site caused two different location-5 dependent accumulation mechanisms, which mainly resulted in elevated Al/Fe-P at pH < 6.6 (upstream) and elevated Ca-P at pH > 6.6 (downstream). This site comparison therefore implies that fertilizer-derived P was largely included in the Al/Fe-P fraction (adsorbed P) of acidic upstream sediments. Phosphorus inputs by runoff or erosion to downstream environments apparently led to enhanced precipitation of Ca-P by in-10 creasing pH. The accumulation pattern in this site comparison therefore corresponds to location-dependent transformations between Al/Fe-P and Ca-P, which are to be expected from P redistributions at individual sites (Sect. 4.3). We hypothesize that more soluble Ca-P minerals accumulate relative to total Ca-P by anthropogenic P enrichment because the formation of sparingly soluble Ca-P min-15 erals is too slow for balancing increased formation rates of thermodynamically less stable Ca-P minerals. This hypothesis is consistent with comparatively large amounts of metastable Ca-P, which apparently accumulated at the P-enriched site due to external factors (Sect. 3.4, Table 3; Fig. 4c, d). In agreement with several studies (Sect. 4.6) our findings showed that Morgan's weakly acidic acetate-acetic acid solution extracts 20 mainly metastable Ca-P phases. Because sparingly soluble Ca-P minerals, such as detrital apatite, are unlikely to dissolve in Morgan's solution (pH 4.8; cf. Ruttenberg, 1992), the correlations between Morgan P (Ca-P meta ) and Ca-P (p < 0.0001 at both sites; Fig. 4b) are attributable to more soluble Ca-P minerals. Our hypothesis is also consistent with concentrations of OCP, CFAP and FAP, which were separately deter-25 mined for sediments of both sites (Fig. 5). Sediments of the P-enriched site showed a significantly larger portion of Ca-P OCP+CFAP compared to the P-unenriched site. Results of both independent methods, which were significantly correlated ( fore provide strong evidence for the proposed accumulation of thermodynamically less stable Ca-P by anthropogenic P inputs.

Octacalcium phosphate and authigenic apatite
A dominant proportion of more soluble Ca-P was contributed by OCP in alkaline sediments (Fig. 5). These OCP concentrations therefore suggest authigenic apatite forma-5 tion by initial precipitation of OCP in alkaline mangrove, river, bay and tidal flat sediments. Octacalcium phosphate has been suggested to occur in surface marine sediment using XANES (Oxmann, 2014; µXANES spectra of Brandes et al., 2007), which requires minimal sample preparation and is minimally affected by common sample matrices. Results of the CONVEX method, which was validated by the matrix effect-free 10 method of standard addition (Oxmann and Schwendenmann, 2014), therefore agree with field and experimental evidence for the occurrence of OCP in sediment (see also Gunnars et al., 2004;Jahnke et al., 1983;Krajewski et al., 1994;Morse and Casey, 1988;Nancollas et al., 1989;Schenau et al., 2000;van Cappellen and Berner, 1988). We conclude that this precursor plays a crucial role in P redistributions (see arrows 15 in Fig. 5), including the pH-dependent switch from adsorbed P to Ca-P in the downstream direction; the potential reverse transformation after upstream transport; and the pH-dependent accumulation processes. Further, apatite formation by successive crystallization is possibly mainly restricted to alkaline sediments.  Grossl and Inskeep, 1992), lake sediment (e.g. Avnimelech, 1983 and references therein) and marine sediment (see above references). In agreement with the high OCP concentrations found in this study (Fig. 5), solid-state NMR and XANES spectroscopy based studies implied that OCP does not only belong to 25 the most commonly reported but also to the most prevalent inorganic P forms in alkaline environments (Beauchemin et al., 2003;Kizewski et al., 2011;Oxmann, 2014 Howarth et al. (1995) proposed that the suggested rapid CFAP formation in surface layers of non-upwelling sediments (Ruttenberg and Berner, 1993), which was derived from authigenic P fractions (extracted with pH 4 acetate buffer; SEDEX method; Ruttenberg, 1992), may be 5 biased by biogenic apatite. Slomp (2011) similarly suggested that the occurrence of more amorphous, not operationally defined authigenic Ca-P phases causes overestimates of operationally defined Ca-P phases and that, on the other hand, long-term diagenesis of authigenic Ca-P can cause overestimates of detrital Ca-P. We found large solubility differences between FAP and CFAP in sediment (Oxmann 10 and Schwendenmann, 2014), which agree with notable differences between solubilities of laboratory synthesized FAP and CFAP detected by Jahnke (1984). The pH at which differential dissolution of FAP and CFAP occurred during addition experiments for validation of the CONVEX method agrees with a pH of 4, at which best separation of FAP and CFAP is attained in the SEDEX acetate buffer (Ruttenberg, 1992). It 15 is important to note that several CFAP specimens with different degrees of carbonate substitution as well as biogenic apatite were distinguishable from FAP in these addition experiments. We therefore believe that the SEDEX method gives a reliable estimate of continental-derived FAP and that the method reliably distinguishes this detrital FAP from more soluble Ca-P. More soluble Ca-P, however, may include CFAP and OCP, both 20 of which can be expected to be largely generated where they are observed. However, the assumption that less stable Ca-P phases, which dissolve in acetate buffer of pH 4, did only form at sampling sites may be misleading. Given the possibility that some CFAP or other less stable Ca-P phases do not readily dissolve in alkaline seawater (Faul et al., 2005;Lyons et al., 2011;Sheldon, 1981; see also Gulbrandsen et al., 1984 25 as cited in Slomp, 2011), yet in the pH 4 acetate buffer of the SEDEX method (∼ 4 pH units difference), some of this more soluble Ca-P may be of allochthonous origin.
To define the entire "non-detrital" Ca-P fraction (i.e. Ca-P phases other than FAP; separated by e.g. pH 4 acetate buffer) in marine sediment as being an oceanic sink for BGD Introduction

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | SRP (i.e. authigenically precipitated from seawater) assumes that, contrary to detrital FAP, "non-detrital" Ca-P of terrestrial sources completely dissolves before or after entering the marine environment. From the downstream formation of OCP that occurred despite higher solubility of OCP compared to CFAP and on the basis of several recent findings, we speculate that this assumption is likely inaccurate and needs further eval-5 uation, at least for specific near-shore marine environments. Pertinent recent findings include, for example, (i) large amounts of less stable Ca-P in soil and parent rock (determined by the SEDEX method or comparable sequential extractions with acetate buffer; Alt et al., 2013;Hansen et al., 2004) that could enter the marine realm, e.g. through erosion processes; (ii) apparent occurrence or even formation of less stable Ca-P in 10 terrestrial and marine waters (also confirmed by the SEDEX method; Faul et al., 2005;Lyons et al., 2011;Slomp, 2011;Sutula et al., 2004); and (iii) common Ca-P formation in intertidal sediments (confirmed by several independent methods; see Sect. 4.2). In general, the established Ca-P precipitation in sediments along salinity gradients provides some insight into the relevance of factors influencing this precipitation such as 15 changes in salinity, dissolved phosphate and pH. In fact, the apparent Ca-P solubility increases strongly with increasing salinity for a given phosphate concentration and pH (Atlas, 1975). Yet, increasing Ca-P concentrations imply that the salt effect may be more than offset by P desorption, the rise in pH and other potential factors in interstitial waters along salinity gradients. 20 Our results imply that when P enters the marine environment, enhanced Ca-P formation takes place in the intertidal zone. Some non-detrital Ca-P at sites further offshore could be derived from Ca-P-generating areas of the lower intertidal zone or even from freshwater environments (see e.g. Raimonet et al., 2013). Although P redistributions may strongly differ between sediments and soils (e.g. effects of the redox state and 25 ionic strength), the fundamental processes operating on P speciation are comparable. This fact is reflected, for example, in the extensive direct utilization or adaptation of soil P methods for sedimentological studies. It is therefore appropriate to identify potential similarities between sediments and soils in terms of P speciation as described below, especially when analysing transformations from freshwater to seawater environments.

Adsorption/precipitation switching in terrestrial soil
It is well-known that P availability and, hence, P composition differs substantially between acidic (mainly Al/Fe-(hydr)oxide-bound P) and calcareous soils (mainly Ca-P 5 of varying solubility; e.g. Batjes, 2011). Thus soil pH, in particular the distinction between acidic and calcareous soils, is empirically the best predictor of appropriate soil tests for available P (e.g. Batjes, 2011). Correlations between two soil tests should diverge at a specific soil pH provided that (i) pH-driven adsorption/precipitation processes strongly influenced P speciation of soils analysed; and (ii) one P test extracts more soluble Ca-P (Morgan test) and the other mainly adsorbed P (most available P tests) (cf. Ahmad et al., 1968;Curran, 1984;Curran and Ballard, 1984;Dabin, 1980 and references therein; Herlihy and McCarthy, 2006). Figure 6 shows that the Morgan test is indeed fundamentally different from other methods used to determine available P. Available P tests were significantly correlated with each other except Morgan P (Ca-15 P meta ; all correlations non-significant; Fig. 6a). However, by separating those data into acidic and neutral to alkaline soils we found that all correlations with Morgan P were statistically significant (Fig. 6b). Hence, the divergence between correlations of P tests with pH equals the pH of the proposed switch from adsorption to Ca-P meta precipitation. While the pH-related divergence can certainly not be explained by a similar pH of spe-20 cific extraction solutions (Morgan: 4.8; Olsen: 8.5; Bray: 1.6; M3P: 2.5) it agrees with decreasing amounts of adsorbed P (Figs. 5, 3b) and the onset of OCP precipitation at near-neutral pH (Figs. 5;S2a,b). These P redistributions in soils and the described redistributions in particulate matter (Sutula et al., 2004) imply that the proposed switch is not necessarily dependent on changes of the ambient redox conditions. Further-25 more, the comparable switch for sediments (this study) and soils with quite different characteristics ( Fig. 6; Bair and Davenport, 2012;Curran, 1984)  The Morgan test is evidently an unreliable indicator for the determination of P availability in sample sets with both acidic and alkaline substrates (Fig. 6a). Permanently alkaline environments are usually characterized by lower P availability than slightly acidic environments due to decreasing Ca-P solubility with pH (Figs. 2a,c,3a,5,S2a,b) and lower amounts of Al/Fe-P (Figs. 3b; 5), whereas Morgan P continuously increases with 5 pH across the pH range (Fig. 4a, c). This conclusion is consistent with known overestimates of plant-available P by the Morgan test at high soil pH (Aura, 1978;Foy et al., 1997). Hence, despite highest Morgan test values for bay sediments (Ca-P meta ; Table S1) the availability of P is likely lower in permanently alkaline environments thereby mitigating coastal eutrophication. Because limited P availability is generally negatively 10 correlated with P mobility, this positive effect comes at the expense of an increased P accumulation at the coastal zone.

Conclusions
Our findings indicate a pH-induced switch from P adsorption to Ca-P precipitation at near-neutral pH, which apparently leads to inorganic P accumulation in near-shore 15 sediments. Further, this P redistribution is apparently driven by OCP formation and enhanced by anthropogenic P inputs. Hence, a significant proportion of authigenic Ca-P may be derived from anthropogenic sources in some coastal regions.
The proposed mechanism, including relatively rapid formation of an apatite precursor, explains several independent observations: the downstream transition from Al/Fe-20 P to Ca-P at ∼ pH 6.6; the Ca-P formation at the expense of adsorbed P; the large increase of Ca-P meta with increasing pH; the dominant proportion of OCP in alkaline sediments; the pH-dependent accumulation mechanisms of Al/Fe-P and Ca-P, and the accumulation of Ca-P meta and Ca-P OCP+CFAP at the P-enriched site; the comparable pH-driven switch in various soils. The suggested switch appears to be a very common 25 mechanism because it was observed across different ecosystems and it was independent of the P status. Further evidence that this mechanism operates in different envi- ronments comes from the analogue pH dependence in very different soils and similar downstream transitions reported by several studies. Less stable Ca-P is mainly formed and buried during sedimentation rather than being allochthonous material. Hence, CFAP and OCP act as diagenetic sinks for P at the investigated sites and are mainly responsible for the accumulation of inorganic P in 5 the lower intertidal zone and bay. Some authigenic Ca-P, however, could be dissolved when physical-chemical conditions change or after upstream transport by tides. Some of it could also be resuspended and transported further offshore, similar to detrital FAP. In general, OCP formation may mitigate a desorption-derived P release from sediment and seems to occur when P adsorption is usually less pronounced -that is, under Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Alt, F., Oelmann, Y., Schöning, I., and Wilcke, W.: Phosphate release kinetics in calcareous grassland and forest soils in response to H + addition, Soil Sci. Soc. Am. J., 77, 2060Am. J., 77, -2070Am. J., 77, , 2013. Andersen, J. M.: An ignition method for determination of total phosphorus in lake sediments, Water Res., 10,[329][330][331]1976. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | DeLaune, R. D., Reddy, C. N., and Patrick Jr., W. H.: Effect of pH and redox potential on concentration of dissolved nutrients in an estuarine sediment, J. Environ. Qual., 10,[276][277][278][279]1981. Dürr, H. H., Laruelle, G. G., van Kempen, C. M., Slomp, C. P., Meybeck, M., and Middelkoop, H.: Worldwide typology of nearshore coastal systems: defining the estuarine filter of river inputs  Heggie, D. T., Skyring, G. W., O'Brien, G. W., Reimers, C., Herczeg, A., Moriarty, D. J. W., Burnett, W. C., and Milnes, A. R.: Organic carbon cycling and modern phosphorite formation on the East Australian continental margin: an overview, in: Phosphorite Research and Development, Special Publ., 52, edited by: Notholt, A. J. G. and Jarvis, I., Geological Society, London, 87-117, 1990. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Spiteri, C., Cappellen, P. V., and Regnier, P.: Surface complexation effects on phosphate adsorption to ferric iron oxyhydroxides along pH and salinity gradients in estuaries and coastal aquifers, Geochim. Cosmochim. Ac., 72, 3431-3445, 2008. Sundareshwar, P. V. and Morris, J. T.: Phosphorus sorption characteristics of intertidal marsh sediments along an estuarine salinity gradient, Limnol. Oceanogr., 44, 1693-1701. Al/Fe-P: Al/Fe-bound P; Ca-P: calcium-bound P; Ca-P meta : metastable Ca-P a Blank rows indicate complete sample set analysed b All correlations with organic P non-significant c Surface layer showed a stronger correlation between Al/Fe-P and pH than the complete data set because other depth intervals showed a peak at pH 6.6 (see Fig. 3c). d Note that r and p values of Ca-P vs. Al/Fe-P strongly depend on selected pH intervals (cf.  Table 2. Phosphorus fractions at different pH intervals in P-enriched sediments of the Firth of Thames and P-unenriched sediments of the Saigon River Delta. a n pH Ca-P Al/Fe-P Site µmol g −1 µmol g −1 < 6.6 > 6.6 < 6.6 > 6.6 < 6.6 > 6.6 < 6.6 > 6.6   Plots (n = 28) along transects were located in the following ecosystems: bay (dark blue), tidal flat (blue), mangrove (light green), salt marsh (green) and pasture (dark green). Tidal flat plots close to mangrove forests included mangrove seedlings. Five additional plots were located at rivers (grey). Isolines indicate elevations in meters below mean sea level. Google Earth images for areas from pasture to tidal flat in (c). BGD 11,2014 Calcium phosphate formation along estuarine salinity gradients   Table S1 for the number of averaged data and their values). Mean values of all depth 4 intervals (112 samples) are also shown for each parameter. Mean fraction concentrations in % 5 of total inorganic P (TIP) are given in c and d. X-axis labels include mean distance (Dist, 6 meters, downstream fringing mangroves are set as zero) and mean inundation durations (ID,7 days/year). r and p values for correlations among these parameters are given in Table 1. 8 Figure 2. Physical-chemical sediment characteristics (a: pH; b: Eh) and sediment phosphorus fractions (c: Ca-P; d: Al/Fe-P) across ecosystems at the Firth of Thames. Each depth interval includes average values of several plots of each ecosystem across the entire site (all transects; see Table S1 for the number of averaged data and their values). Mean values of all depth intervals (112 samples) are also shown for each parameter. Mean fraction concentrations in % of total inorganic P (TIP) are given in (c) and (d). X-axis labels include mean distance (Dist, meters, downstream fringing mangroves are set as zero) and mean inundation durations (ID, days yr −1 ). r and p values for correlations among these parameters are given in Table 1  Ca-P increase with pH (exponential; both sites, all samples). Mean Ca-P concentration of 3 Firth of Thames samples c. twice that of Saigon River Delta samples. (b) Al/Fe-P peak at ~ 4 pH 6.6 due to increase below and decrease above that value (exponential; both sites, all 5 samples). (c) Linear regressions between Al/Fe-P and Ca-P at deeper depths (30-35, 35-40 6 cm) for different pH intervals (cf. Fig. 3a, b). Arrows indicate switch from Al/Fe-P to Ca-P 7 with increasing pH (downstream direction). (d) Eh vs. pH (NZ; all samples). Different 8 symbols denote surface (0-5 cm), intermediate (10-15 cm) and deeper depth intervals (30-35, 9 35-40 cm)   Ca-P increase with pH (exponential; both sites, all samples). Mean Ca-P concentration of Firth of Thames samples ca. twice that of Saigon River Delta samples. (b) Al/Fe-P peak at ∼ pH 6.6 due to increase below and decrease above that value (exponential; both sites, all samples). (c) Linear regressions between Al/Fe-P and Ca-P at deeper depths (30-35, 35-40 cm) for different pH intervals (cf. Fig. 3a, b). Arrows indicate switch from Al/Fe-P to Ca-P with increasing pH (downstream direction). Firth of Thames (NZ) and Saigon River Delta (VN) site ordered by pH (all concentrations in 2 µmol P g -1 ). The sampling site (N: NZ, New Zealand; V: VN, Vietnam) and depth interval 3 (cm) is given at the top axis. The portion of more soluble Ca-POCP+CFAP (hatched area) is given 4 as a percentage of total Ca-P above columns. A typical decrease of adsorbed P and increase of 5 OCP with increasing pH from 6.7 to 7.3 suggests a reversible transformation in that range 6 (arrows; adsorption/precipitation switch). Strong Al/Fe-P predominance in acidic surface 7 sediment of the pasture is denoted (star). Org. P: total organic P; Inorg. P: total inorganic P; 8 OCP: octacalcium phosphate; CFAP: carbonate fluorapatite; FAP: fluorapatite; Res. P: 9 residual P; Man*: strongly acidic mangrove; Man: mangrove; Past: pasture; TiFl: tidal flat; 10 Rive: river. Residual P correlated with Al/Fe-P at a significance level of 0.0001 (asterisks). 11

BGD
See Table 4 for further correlations among these species distributions. 12 Figure 5. Phosphorus species distributions in sediments across different ecosystems at the Firth of Thames (NZ) and Saigon River Delta (VN) site ordered by pH (all concentrations in µmol P g −1 ). The sampling site (N: NZ, New Zealand; V: VN, Vietnam) and depth interval (cm) is given at the top axis. The portion of more soluble Ca-P OCP+CFAP (hatched area) is given as a percentage of total Ca-P above columns. A typical decrease of adsorbed P and increase of OCP with increasing pH from 6.7 to 7.3 suggests a reversible transformation in that range (arrows; adsorption/precipitation switch). Strong Al/Fe-P predominance in acidic surface sediment of the pasture is denoted (star). Org. P: total organic P; Inorg. P: total inorganic P; OCP: octacalcium phosphate; CFAP: carbonate fluorapatite; FAP: fluorapatite; Res. P: residual P; Man*: strongly acidic mangrove; Man: mangrove; Past: pasture; TiFl: tidal flat; Rive: river. Residual P correlated with Al/Fe-P at a significance level of 0.0001 (asterisks). See Table 4  Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Figure 6. Influence of soil pH on Al/Fe-(hydr)oxide-bound P (Al/Fe-P) and calcium-bound P 1 (Ca-P) as derived from correlations between Morgan P and other available P tests (Olsen P, 2 Bray P and Mehlich M3P). All correlations were non-significant (p > 0.05) for the entire pH 3 range (a) due to extraction of different P fractions (Morgan P: mainly metastable Ca-P; other 4 available P tests: mainly Al/Fe-P). By separating data into acidic and neutral-alkaline soils all 5 correlations were significant (b; p < 0.05). Data from (i) Bair and Davenport (2012), 6 excluding a heavily fertilized sample (Morgan P ten times higher than the second highest 7 Morgan P value) and a sample with high carbonate content despite low pH; and (ii) from 8 Curran (1984), excluding an OM sample (forest floor). Values for Mehlich P only from Bair 9 and Davenport (2012) due to different Mehlich tests (Bair and Davenport: M3P; Curran: 10 Figure 6. Influence of soil pH on Al/Fe-(hydr)oxide-bound P (Al/Fe-P) and calcium-bound P (Ca-P) as derived from correlations between Morgan P and other available P tests (Olsen P, Bray P and Mehlich M3P). All correlations were non-significant (p > 0.05) for the entire pH range (a) due to extraction of different P fractions (Morgan P: mainly metastable Ca-P; other available P tests: mainly Al/Fe-P). By separating data into acidic and neutral-alkaline soils all correlations were significant (b, p < 0.05). Data from (i) Bair and Davenport (2012), excluding a heavily fertilized sample (Morgan P ten times higher than the second highest Morgan P value) and a sample with high carbonate content despite low pH; and (ii) from Curran (1984), excluding an OM sample (forest floor). Values for Mehlich P only from Bair and Davenport (2012) due to different Mehlich tests (Bair and Davenport: M3P; Curran: M2P). Note the significant correlations in 5b despite very different soils: developed in marine clay, native, garden, manured and unfertilized agriculture, pasture, P-fertilized and unfertilized forest; different horizons; varying pH.