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Agriculture and Agri-Food Canada, Soils and Crops Research and Development Centre, 2560 Hochelaga Blvd., Sainte-Foy, QC, Canada, G1V 2J3
beauchemins{at}em.agr.ca
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Abbreviations: Alox, ammonium oxalate–extractable Al • Feox, ammonium oxalate–extractable Fe • M3P, Mehlich-III extractable P • Po, organic P • Psi, P sorption index • Pox, ammonium oxalate–extractable P • Pt, total soil P • Pw, water-soluble P
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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However, the impact of inorganic fertilizer P on soil P saturation degree has not been extensively studied in agroecosystems in which manure is not the main source of P. This is particularly true for fine-textured soils since P mobility in mineral soils other than those of coarse texture is generally presumed to be largely restricted by P sorption in P-deficient subsoil layers (Pierzynski et al., 1994). Zhang et al. (1995) reported increased P mobility in a context of long-term inorganic fertilization but their observations were limited to the 0- to 40-cm soil layer of a loamy soil from a site receiving an unusually high rate of P fertilizer. Despite this, P transfer from soils to drainage water may become significant in soils with multiple risk factors such as high soil test P, low P sorption capacity and artificial drainage systems that may enhance subsurface transport (Sims et al., 1998). Other factors such as preferential flow through continuous fissures and macropores or through cracks after a storm event on dry clay soils may also favor P transfer to lower horizons or tile drainage systems (Turtola and Jaakkola, 1995; Thomas et al., 1997; Stamm et al., 1998; Gächter et al., 1998).
To assess the risk of downward P movement in such systems, a study was initiated in an area of the St. Lawrence lowlands southeast of Montreal. This agroecosystem is characterized by neutral to slightly alkaline, flat soils generally presenting poor drainage and low to medium P sorption capacities (Beauchemin, 1996). Clayey soils dominate (Nolin et al., 1991) and inorganic fertilizers are the main source of P in most of the area. As these soils are intensively cropped with corn–soybean rotations and were previously reported to be often overfertilized (>500 kg M3P or K ha-1 in the plow layer; Tabi et al., 1990), the flat topography coupled with widespread occurrence of tile-drainage systems led to a particular concern about potential transfer of P from soil to surface waters through tile-drainage systems. In a related study, the concentration and forms of P in tile-drainage waters from a range of soils were first assessed. Although drain water P concentrations were generally low (<0.1 mg P L-1), sandy and clayey sites showed the highest risk for P transfer into drains in concentrations exceeding the surface water quality standard of 0.03 mg total P L-1 (Beauchemin et al., 1998). The present study aimed at characterizing the degree of P saturation of these soils. The objectives were to (i) determine the P status of the A, B, and C horizons of 27 intensively cropped sites showing a range of clay content and (ii) compare the soil P status of this agrosystem in which most sites receive manure infrequently to soils of a very different agrosystem in a watershed of high animal density from the Appalachians.
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Materials and methods |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Soil Characterization
Soil pH was measured in distilled water with a soil/solution ratio of 1:2. Organic C content was determined by wet oxidation (Tiessen and Moir, 1993). Particle-size analysis was performed by the hydrometer method except for the use of the pipette method for soils very rich in clay (Sheldrick and Wang, 1993). Exchangeable Ca was extracted with Mehlich-III solution (Tran and Simard, 1993). Ammonium oxalate–extractable Al and Fe (Alox, Feox) were obtained as described by Ross and Wang (1993).
Phosphorus Status
Water-soluble P (Pw) was measured in a 1:27 (weight/volume) ratio of soil/water according to a modification of Sissingh (1971) procedure. Briefly, 2 mL of distilled water were added to 1 g of dry soil for an initial contact time of 22 h. Twenty-five milliliters of distilled water were then added and the mixture shaken for 1 h. The Pw was determined colorimetrically by the molybdenum blue method (Murphy and Riley, 1962) after centrifuging at 27 000 g. Mehlich-III extractable P was also determined (Tran and Simard, 1993). Organic P was determined after extraction with 0.025 M NaOH + 0.05 M Na2EDTA (Bowman and Moir, 1993). Molybdate reactive P (Piox) was measured in the ammonium oxalate extracts by the molybdenum blue method. The oxalate extracts were then digested with H2SO4 + H2SeO3 + H2O2 (Rowland and Grimshaw, 1985) and the total concentration of P in the oxalate digested extracts (Pox) determined with the vanadomolybdophosphoric acid method (Conseil des Productions Végétales du Québec, 1988). Two saturation indices were calculated (%): Piox/(Feox+Alox) and Pox/(Feox+Alox). Total soil P contents (Pt) were obtained after digestion of soil samples with H2SO4 + H2SeO3 + H2O2 (Rowland and Grimshaw, 1985). The Psi was measured by contact of 1.5 g P kg-1 soil (added as KH2PO4) for 18 h at a soil/solution (0.02 M KCl) ratio of 1:20. The Psi was calculated as the quotient X (log C)-1, where X represents the amount of P sorbed (mg kg-1) and C is the P concentration in solution (mg L-1) at the end of the contact period (Bache and Williams, 1971).
Statistical Analysis
Normality of variable distributions was first checked. When necessary, logarithmic transformations were made except for Psi and organic matter content for which square root transformation was the most appropriate to approach normality (Tabachnick and Fidell, 1989). Analysis of variance was conducted according to a nested design (Webster and Oliver, 1990); Pearson's correlations were also calculated. All statistical analyses were made using the SAS software (SAS Inst., 1996, Release 6.12, Cary, NC).
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Results and discussion |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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0.79; P < 0.001) and the logarithm of exchangeable Ca content (r 0.75; P < 0.001). In the B and C horizons, clay content was also positively related to the square root of organic matter content (r 0.67; P < 0.001). This correlation was not significant in the A horizon in which the content of soil organic matter is more likely to be affected by other variables such as soil management. Similarly, the square root of organic matter was correlated with the log (Alox + Feox) content (r 0.65, P < 0.001 for B and C horizons whereas r 0.41, P < 0.05 for A horizon) and with the logarithm of exchangeable Ca (r 0.54, P < 0.01 for the B and C horizons). The positive relationship between organic matter and amorphous Al + Fe contents could be expected to some extent as acid ammonium oxalate solubilizes noncrystalline inorganic forms of Fe and Al as well as organically complexed Fe and Al (Ross and Wang, 1993). Furthermore, the presence of organic ligands has been reported to retard or inhibit crystallization and maintain amorphous forms of Al and Fe hydroxides in soils (Schwertmann, 1985; Lookman et al., 1995). The soil characterization shows that the amorphous Al and Fe content is directly related to the clay content in this agroecosystem where the pedogenic processes are fairly homogeneous and are dominated by hydromorphy. Soils with larger Alox + Feox contents also had higher organic matter contents.
Soil Phosphorus Status Across the Profile
Mehlich-III Extractable Phosphorus
The average M3P contents of the A, B, and C horizons for the 27 sites were 91, 14, and 8 mg kg-1, respectively. According to the M3P contents of the A horizons, nine sites had excessive P levels (112 mg M3P kg-1) and 21 exceeded the level of adequate fertility for corn and soybean (53 mg M3P kg-1; Conseil des Productions Végétales du Québec, 1996; Fig. 1)
. No significant differences in M3P were observed among soil series for the three horizons but significant differences were noted within soil series (Table 3)
. This reflects the impact of soil management (fertilization, tillage practices, and crop type) as the main factor influencing the variability of available P, at least for the A horizon. The logarithm of M3P content of the A horizon was weakly but significantly correlated to log M3P of the
and C horizons
. For a given soil series, differences noted between sites tended to be the same in each horizon; this trend was much more visible for the coarse- to medium-textured soils (MS, JS, AI, and AS) than for the fine-textured soils (KI, DJ, UB, RO, and PV). The MS2 site, in particular, had a very high mean M3P content (295 mg kg-1) in the A horizon and M3P content was also high in the B and C horizons (Fig. 2 and 3)
. This suggests a migration of surface applied P down the profile. Large variability of the M3P content was found in the lower horizons of the MS2 and PV1 sites. In the case of PV1, the field variability seemed mainly related to the variation in organic matter content as one location had an organic matter content in its lower horizons twice as high as those of the other two sampling locations. The higher organic matter content of this subsample was associated with larger M3P, Pw, and Po contents than those of the other two PV1 subsamples (data not shown).
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0.66; P < 0.001). The Pw content of the B and C horizons was particularly high in the MS and PV soils. The sites within a given soil that showed high M3P were also high in Pw in the B horizon (Fig. 2). In contrast to M3P, the highest Pw values in the A horizon were associated with the clayey sites RO2 (24 mg kg-1) and PV1 (25 mg kg-1). The Mehlich-III solution has been shown to underestimate plant-available P in some Quebec soils rich in clay (Tran and Giroux, 1985; Simard et al., 1991) and this may explain the discrepancy. Also, the soil/water ratio of 1:27 used for the extraction may cause clay dispersion and release P bound forms. Soils with elevated clay contents are more likely to be affected by dispersion and to release P. Overall, the logarithm of Pw of the A horizon was very weakly but significantly correlated to log Pw of the
and C horizons
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In a very different Quebec agroecosystem, Simard et al. (1995) determined the P status of acidic, sandy to loamy Appalachian soils in the Beaurivage river watershed characterized by high livestock density and forage-based cropping systems (Table 4)
. They reported comparable levels of labile P in surface horizons
but higher levels in
and C horizons
. The increase of labile P in lower horizons of agricultural soils compared to forested soils of the Beaurivage watershed showed that long-term manure inputs had resulted in the transfer of P from the A to the C horizon in spite of the large P sorption capacities of the B horizons in these soils. In the case of the lowland soils, the lower levels of labile P in the B and C horizons compared to the soils of the Beaurivage watershed suggest a less intensive transfer of P, in spite of the generally lower P sorption capacities of the B horizon. Eghball et al. (1996) showed that P inputs from manure were more mobile in the soil than P from inorganic fertilizers. Crop management which is dominated by forages in the Beaurivage watershed may have also resulted in more P migrating in the soil profiles as compared to corn–soybean rotations (Sims et al., 1998).
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0.57, P < 0.01) and with the logarithm of Pt in A and B horizons (r 0.57; P < 0.01). Several studies have also reported positive correlation between total organic P and clay, organic matter, or total P contents in soils (Harrison, 1987).
The DJ3 site had particularly high Po content in the lower horizons (221 and 134 mg kg-1 for the B and C horizon, respectively; Fig. 2 and 3). This was the only site in ridge tillage. Vanasse et al. (1997) found higher drainage water P concentrations under ridge tillage than under moldboard plowing in clayey soils of the same region. The large standard deviation of the DJ3 site expresses, again, the effect of soil management on the field variability in P content. In this specific case, this may be due to the adopted pattern of sampling. The three samples were collected from different locations in regard to the ridge: on the ridge, between ridges and near the ridge. Even though the standard deviation was large, the smallest Po value for the B horizon of this site was 99 mg kg-1, which is still twice the amount found in most other sites. In the case of the PV1 and PV2 sites, the large Po and organic matter contents of the subsoil may be the result of manure inputs in the past considering their proximity to an old abandoned farm building, although organic matter may also have accumulated due to their localization in a depression. Across the 27 sites, the logarithm of Po content of the A horizon was significantly correlated with log Po of the
and the C horizons
. The coefficients of correlation between P content of A and those of B or C horizons were slightly higher with Po than with M3P or Pw. This suggests that Po may be a better indicator of P mobility in soils from the lowlands than the two other soil P measurements.
The Po contents of these lowland soils were at least two to three times lower than those reported for the Beaurivage watershed in which soils received long-term manure additions (Table 4). In this latter watershed, the mean NaHCO3–Po + NaOH–Po contents were 388, 136, and 84 mg kg-1 for the A, B, and C horizons, respectively, compared to mean Po contents of 152, 45, and 25 mg kg-1 in the A, B, and C horizons from the lowlands. Given that the NaHCO3–Po + NaOH–Po fractions represent only the labile to moderately labile Po, even higher total Po contents were probably present in the Beaurivage soils. The proportion of Pt as Po was also two to three times higher in the Beaurivage than in the lowland soils. On average, 34, 19, and 12% of Pt was in organic form in the A, B, and C horizons, respectively, of the Beaurivage watershed compared to 16, 6, and 3% for the A, B, and C horizons from the 27 sites studied. In the Beaurivage watershed, long-term organic fertilization and crop rotations dominated by grasslands in which tillage is infrequent favor the maintenance of larger Po pools compared to the intensively cropped soils of the lowlands receiving mainly inorganic P fertilizers (Hedley et al., 1982; Brossard and Laurent, 1988; Tran and N'dayegamiye, 1995).
Total Phosphorus
Significant differences of total P content were observed among soil series in the A and C horizons (Table 3). In the A, B, and C horizons, the mean total P contents were 932, 703, and 696 mg kg-1, respectively. The highest levels of Pt in the A horizon were found in soils with the highest clay content such as UB, RO, and PV (>800 mg kg-1) whereas other soil series tended to have Pt contents of around 800 mg kg-1 (Fig. 1). In the B and C horizons, differences in Pt contents between textural groups were visible mainly for the PV soil series which had higher Pt content than other soils (Fig. 2 and 3). The fact that differences in Pt levels among textural groups were more evident in the A than in the lower horizons may indicate the specific impact of fertilization according to soil texture. Tabi et al. (1990) reported that loamy to clayey soils from this region were more prone to show overfertilization in Mehlich-III extractable K or P than sandy soils. This may be due to their higher sorption capacity which favors greater P accumulation over time.
Differences in Pt level within soil series were only significant in the surface horizon. This suggests that the impact of agricultural practices on the soil Pt level is mostly restricted to the surface horizon. In the Beaurivage watershed, Simard et al. (1995) reported an increase in total soil P level with animal density, even in the C horizon. Sandy to loamy soils from the Beaurivage watershed had much higher Pt contents (up to 2019 mg kg-1; Table 4) in the Ap horizons than soils with comparable texture from the present study (Beauchemin, 1996). This indicates higher P accumulation in the Beaurivage watershed soils than in the lowlands. It is difficult to assess the real extent of Pt accumulation in soils from the lowlands as no corresponding, unfertilized or forested soils were investigated as a reference. However, the study of Tabi et al. (1990) on the main soil series of this region showed that overfertilization was more frequent in corn and cereals than in forage-based systems. Also, more than 74% of the row crop area in continuous corn, which was a common cropping system up to 1990, was overfertilized in P or K. This indicates that fertilization has contributed to P enrichment of these soils.
Phosphorus Sorption Index
The P sorption index is a single-point method to characterize the relative P sorbing properties of soils (Bache and Williams, 1971). This index allows one to rank soils according to their sorption capacities. In the A, B, and C horizons, the mean Psi values were 223, 268, and 304, respectively. In all horizons, differences among soil series were significant (Table 3). The Psi was higher in clayey (>200) than in sandy to loamy soil series (<200; Fig. 1–3). Similarly, Mozaffari and Sims (1994) and Lookman et al. (1995) also observed that higher clay contents were associated with higher P sorption capacities. This observation remains valid as long as compared soils are relatively homogeneous in regard to the pedogenic processes. Clayey soil series (DJ, UB, RO, PV) had comparable Psi values except for KI which had a lower clay content (Table 1).
The square root of Psi was better correlated with the logarithm of Alox + Feox content (r 0.89; P < 0.001) and with clay content (r 0.74; P < 0.001) in each horizon than with the square root of organic matter content (r 0.54, P < 0.01 for B and C horizons) or with the logarithm of exchangeable
. The correlations clay vs. square root of Psi were stronger in the subsoil than in the A horizon, suggesting that clay content has a greater effect on soil P sorption capacity in lower horizons. Tran and Giroux (1987) also observed that the P sorption capacity of similar neutral to calcareous topsoils was strongly correlated with the Alox and Feox contents, and to a lesser extent, with the clay or organic matter contents. As noted by Lookman et al. (1996), clay particles may present pH-dependent positive charges at their edges but their relationship with P sorption capacity is more likely to be an indirect one as a source of amorphous Al and Fe hydroxides resulting from their weathering.
Compared to soils from the Beaurivage watershed, soils from the lowlands had, on average, smaller Psi values in A and B horizons (Table 4). However, Psi values of the clayey A horizons were comparable to those reported for sandy to loamy soils from the Beaurivage watershed. Those latter soils are usually considered to have large P sorption capacities. In clayey soils, Psi values were also larger in subsurface than in surface horizons. This suggests reduced P sorption capacities of fertilized A horizons although corresponding unfertilized A horizons should be characterized before making this conclusion. Among sandy soils, the JS series had the lowest P sorption index in the B and C horizons which was associated with low Alox + Feox contents. Differences within soil series were significant only for the surface horizon (Table 3), suggesting a limited impact of the P inputs on the P sorption capacities of the subsoil.
Phosphorus Saturation Index
The Pox/(Alox + Feox) saturation degrees ranged from 7 to 33% in the A horizon
and from 5 to 17%
in the B and C horizons (Fig. 1–3). In the case of Piox/(Alox + Feox), these values were 5 to 25% for the A horizon and 4 to 15% for the B and C horizons (data not shown). Although no significant differences among soil series were found in the A and B horizons (Table 3), the saturation level of the A horizon tended to be higher in sandy to loamy (>15%) than in clayey soils (<15%). This is in agreement with the higher Psi values noted in clayey than in sandy to loamy soils. The logarithm of Pox/(Alox + Feox) was negatively related to the square root of
in the A horizon. Significant differences within soil series were noted only in the A horizon. No significant correlation
existed between the P saturation indices of the A horizon and those of the lower horizons. In the surface horizon, log (Pox/(Alox + Feox)) was weakly but significantly correlated with log M3P
but not with
. No such correlation was observed in lower horizons. Slightly better coefficients of correlation were obtained when considering the proportion of labile P (Pw/Pt or M3P/Pt) rather than the labile P content alone (Fig. 4B)
. Other studies on acidic soils have found linear correlation between extractable P and P saturation indices but the closeness of the relationship seems to depend on the extractant used for the soil P content and the P component of the saturation index and on the range of studied soils (Beauchemin and Simard, 1999).
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In a regional inventory in Belgium, a critical saturation degree of Pox/0.5(Alox + Feox) of 24% (Lookman et al., 1995) or 30% (De Smeth et al., 1996) was arbitrarily retained to define noncalcareous soils potentially saturated in P. Considering our Pox/(Alox + Feox) saturation index, critical limits of 12 to 15% would be equivalent. With 15% as an arbitrary P saturation degree, 10 out of the 12 investigated sandy to loamy sites (MS, JS, AI, and AS) would be P saturated in their A horizon whereas only 3 out of the 15 clayey sites would exceed that threshold. Only one site, PV1, would be P saturated on a whole profile basis. If a more stringent value of 12% was considered instead of 15%, the KI1 and PV2 sites would also be P saturated on a whole profile basis. According to Breeuwsma and Reijerink (1992), this suggests that these three sites would be at greater risk than the others to leach ortho-P in concentrations >0.1 mg L-1 at the bottom of their profile where drainage systems are usually located. Tile-drainage waters from the PV1 and PV2 sites had very high total P concentrations (>0.5 mg L-1) while the KI1 site had tile-drain water total P concentrations <0.05 mg L-1 (Beauchemin et al., 1998). In spite of low P saturation degrees in the B and C horizons, the JS1 site also presented relatively high total P concentrations in drain water (> 0.06 mg L-1) for both samplings.
These observations indicate that additional factors should be considered to fully assess the risk of P transfer from soils to tile-drainage water. For instance, previous results on these clayey soils suggested that preferential flow might be a pathway for P loss to drainage water (Beauchemin et al., 1998). Thus, clayey soils with low P saturation degree might also present high P concentrations in drainage water under specific conditions. It is also clear that these critical limits of degree of P saturation, chosen here only arbitrarily, may not be adequate for neutral to slightly alkaline soils from the lowlands as the saturation index was first defined for non-calcareous soils. The weak correlation or lack of correlation between the P saturation index and M3P or Pw, respectively, does not necessarily indicate that the index is a poor environmental indicator for the neutral to slightly alkaline soils. It rather suggests the need to form homogeneous soil groups to better assess the risk of P transfer into drainage water as discussed in more details by Beauchemin and Simard (1999). Beauchemin (1996) reported that the saturation index along with the Pw content could significantly contribute to explain the variation of tile-drainage water P concentration in the subgroup of soils with calcareous substratum.
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Conclusions |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Received for publication March 8, 1999.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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= 0.05, 0.01 and 0.001, respectively)