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Published online 1 January 2007
Published in Soil Sci Soc Am J 71:56-63 (2007)
DOI: 10.2136/sssaj2006.0162
© 2007 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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SOIL CHEMISTRY

Effects of Tillage on Phosphorus Release Potential in a Spanish Vertisol

Concepción Saavedra, Juan Velasco, Purificación Pajuelo, Francisco Perea and Antonio Delgado*

Dep. de Ciencias Agroforestales, EUITA, Univ. de Sevilla, Ctra. Utrera Km 1, 41013 Sevilla, Spain

* Corresponding author (adelgado{at}us.es)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
No-till (NT) practices usually increase the P content in soil surfaces. The main purpose of this work was to study the long-term effects of the soil tillage system on the P release potential in the surface layer of a Vertisol from southwest Spain and to correlate it with changes in P content and forms. The three tillage treatments investigated were: (i) conventional tillage (CT) with a moldboard plow and field cultivator; (ii) minimum tillage (MT) with a field cultivator; and (iii) NT, which involved direct sowing into crop residue from the previous year. After 21 yr, NT resulted in enhanced organic matter, Olsen P, organic P, and total P contents relative to CT and MT in the top 5 cm of soil. The ratio of labile inorganic P fractions to P related to sparingly soluble pedogenic Ca phosphates in the surface soil layer was much greater under NT (0.8) than under CT (0.57) or MT (0.55). This was ascribed to a decreased precipitation of sparingly soluble Ca phosphate because of the enrichment in organic matter under NT, and accounted in part for the higher portion of inorganic P related to labile fractions in NT (0.19 vs. 0.09 in CT and 0.10 in MT). The increased P release potential of surface soil as estimated by using P sinks indicated that an increased dissolved P concentration in runoff and a greater amount of potentially releasable P per unit mass of surface soil lost through erosive processes can be expected with NT than with MT or CT.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Many agricultural soils in Europe have an available P content that clearly exceeds the critical values for P fertilizer response (Behrendt and Boekhold, 1993; Barberis et al., 1996). In this situation, soil P has become more of an environmental concern than an agronomic one in areas of intensive cropping and livestock production, where P loads from soil can promote eutrophication (Sharpley et al., 1994; Sharpley, 1995).

Overland flow is believed to be the primary origin of P losses from agricultural soils (Sharpley and Menzel, 1987; Sharpley et al., 1994; Stevens et al., 1999). As readily desorbable P accumulates in topsoil, its concentration decreases with increasing depth (Stamm et al., 1998) and P is transported in solution or associated with eroded particles (Kronvang, 1992). In general, P associated with eroded particles (mainly adsorbed or precipitated P) accounts for most P in overland flow from cultivated agricultural lands (Sharpley et al., 1992; Bundy et al., 2001; Saavedra and Delgado, 2006). Thus, an immediate way of decreasing P movement is by minimizing particulate erosion and transport (Uusitalo et al., 2000; Andraski et al., 2003).

The benefits of long-term NT over CT include higher infiltration rates and reduced soil erosion (Dick et al., 1989; Shipitalo and Edwards, 1998), which help to reduce P loss from soils (Chambers et al., 2000; Kimmell et al., 2001). Cultivation methods using no tillage and leaving an undisturbed soil surface during winter exhibit highly reduced P loads (Puustinen et al., 2005); however, NT increases the stratification of soil organic C and nutrient availability in soil (Franzenluebbers and Hons, 1996; Crozier et al., 1999; Duiker and Beegle, 2006). Specifically, enrichment of the soil surface in Bray P-1 (top 5 cm of soil, Hussain et al., 1999), and bicarbonate- and resin-extractable P (top 8 cm of soil, Zibilske et al., 2002) have been reported. This surface enrichment may affect the pattern of P loss from soil; although total P losses are unrelated to soil test P levels, dissolved P concentrations are highly dependent on soil test P levels (Hooda et al., 2000; Andraski and Bundy, 2003; Andraski et al., 2003). Thus, although NT can reduce soil erosion and loss of P associated with eroded particles, this practice can increase dissolved P in runoff (Puustinen et al., 2005). This is of environmental significance because dissolved P is readily available for algae growth (Dils and Heathwaite, 1998; Ekholm and Krogerus, 2003), and may increase the bioavailable/total P ratio in overland flow under NT.

Phosphorus forms dictate the P release potential of soils in agronomic and environmental terms (Sattell and Morris, 1992; Saavedra and Delgado, 2005a). The use of sequential fractionation methods to study P forms allows the definition of operational P fractions that differ in the ease with which they can be released from soil to water (Ruiz et al., 1997; Delgado and Torrent, 2000; Maguire et al., 2000). Phosphorus fractionation techniques can thus provide useful information about the P release potential of soils (Saavedra and Delgado, 2005b). The increased P concentration in soil surfaces under NT to which P fertilizer is broadcast, and the increase in organic matter content under this tillage system, may significantly affect the relative ratios between different P fractions in soil (Delgado et al., 2002) and may account for changes in P loss patterns in untilled soils.

The objectives of this work were to determine: (i) how the soil tillage system affects P content and forms in the surface of a Vertisol typical of European Mediterranean areas, (ii) how tillage methods affect the P release potential of soil surfaces as estimated by chemical extraction and the use of P sinks; and (iii) how changes in P fractions can explain changes in estimated P release potential.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Design
A long-term tillage experiment was conducted on the experimental farm of the Institute of Agriculture and Fisheries Research of Andalusia (IFAPA) in Carmona, southwestern Spain (37°24'07''N, 5°35'10''W). The mean temperature in the area ranges from 10.0°C (January) to 26°C (August). The average annual precipitation is 500 mm, and the mean potential evapotranspiration is 1600 mm. The soil at the experimental site is a very fine, montomorillonitic, thermic Chromic Haploxerert (Soil Survey Staff, 1998). The most salient properties of the top 5 cm of soil are shown in Table 1, including Fe fractionation data obtained by following the scheme of Ruiz et al. (1997), namely: citrate–ascorbate-extractable Fe, which is an estimate of Fe related to poorly crystalline Fe oxides, and citrate–bicarbonate–dithionite-extractable Fe, which can be assimilated to Fe in crystalline Fe oxides.


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  		Table 1. Selected properties of the studied soil (top 5 cm) at the beginning of the experiment.{dagger}

 
The experiment was started in 1982 and involved three tillage treatments with four replications in a randomized complete block design with 180- by 15-m plots. The treatments applied were as follows: (i) CT, which involved moldboard plowing to a depth of 50 cm in the summer (once every 3 yr), followed by field cultivating to a depth of 15 cm before sowing, crop residues being burnt; (ii) MT, which involved field cultivating to a depth of 15 cm before sowing and partial incorporation of crop residues; and (iii) NT, which involved controlling weeds before sowing by spraying glyphosate [N-(phosphonomethyl)glycine] (36%) at a rate of 2 L ha–1 and sowing directly into the crop residue from the previous year by using a planter with double-disk openers. For all tillage treatments, the crop rotation (annual crops) consisted of wheat (Triticum aestivum L.), sunflower (Helianthus annuus L.), and legumes (pea [Pisum sativum L.], chickpea [Cicer arietinum L.], or faba bean [Vicia faba L.], depending on the year), which were grown under rainfed conditions. Fertilizer was applied only to wheat, by broadcasting 240 kg ha–1 diammonium phosphate (48 kg P ha–1) at preplant and 220 kg ha–1 urea at sidedress. In the CT and MT treatments, the preplant fertilizer was mixed with the soil with the aid of the field cultivator.

In September 2003, a composite soil sample was obtained by mixing 15 subsamples collected from the soil surface (to a depth of 5 cm) in each plot. Samples were air dried and ground to pass a 2-mm sieve before analysis of P forms. Organic matter in the samples was determined by dichromate oxidation (Walkley and Black, 1934).

Study of Phosphorus Availability and Forms
The P availability index was determined according to Olsen et al. (1954); the active C treatment to reduce the organic matter interference was avoided and centrifugation (1000 x g) was used instead of filtering. According to Delgado and Torrent (1997), Olsen P is an accurate measure of total plant-available P in this type of soil, which is usually equivalent to 1.5 times the amount of P extractable using the Olsen method. Total P in soil was determined using the molybdenum blue colorimetric method (Murphy and Riley, 1962) following HNO3 digestion in Teflon containers in a microwave oven (Milestone, provided by Gomensoro, Madrid, Spain) according to the manufacturer's instructions.

Sequential soil P fractionation was performed in accordance with two different schemes. One was a modification of that proposed by Ruiz et al. (1997; referred to as mR) involving two additional steps to extract alkali-extractable and residual organic P:

  1. 0.1 M NaOH + 1 M NaCl (NaOH extraction), which releases adsorbed P, P precipitated as Fe and Al phosphates, and P bound by Fe– and Al–organic complexes. In calcareous soils, the amounts extracted in this step can be very small, because most extracted P precipitates or is adsorbed on calcite;
  2. 0.27 M sodium citrate + 0.11 M NaHCO3 (citrate–bicarbonate extraction), which extracts adsorbed P and soluble Ca phosphates, partly precipitated or adsorbed on calcite after NaOH extraction in calcareous soils;
  3. 0.25 M sodium citrate at pH 6 extracted twice, which extracts P ascribed to precipitation as pedogenic Ca phosphates not dissolved in the previous step;
  4. 0.2 M sodium citrate + 0.05 M sodium ascorbate at pH 6 (citrate–ascorbate, mild reductant soluble P), which releases mainly P occluded in poorly crystalline Fe oxides;
  5. 0.27 M sodium citrate + 0.11 M NaHCO3 + 2% sodium dithionite (citrate–bicarbonate–dithionite, reductant-soluble P), which releases P occluded in crystalline Fe oxides;
  6. 1 M NaOAc buffered at pH 4 (acetate), which releases residual pedogenic Ca phosphates previously not dissolved by citrate;
  7. 1 M HCl, which dissolves most lithogenic apatite;
  8. 2 M NaOH, which releases alkali extractable organic P; and
  9. 0.5 M H2SO4 + 0.37 M K2S2O8 for 1 h, which releases residual organic P.

The second fractionation scheme (referred to as mG) involves the use of chelating compounds to determine P related to Fe oxides and Ca phosphates. It is a modification of the sediment P fractionation scheme of Golterman (1996) developed by Díaz-Espejo et al. (1999) and involves the following five fractionation steps:

  1. 0.05 M Ca ethylenediaminetetraacetic acid (EDTA) + 1% sodium dithionite (CaEDTA P) for 1 h to release Fe-related P;
  2. 0.1 M Na2EDTA (NaEDTA P) for 1.5 h to release Ca-bound P;
  3. 0.5 M H2SO4 (H2SO4 P) for 0.5 h;
  4. 2 M NaOH for 2 h to release alkali-extractable organic P; and
  5. 0.5 M H2SO4 + 0.37 M K2S2O8 for 1 h to release residual organic P.

The extraction time in the mR fractionation scheme was 16 h except for the second extraction with sodium citrate (fourth step, 8 h), HCl (1 h), 2 M NaOH (2 h), and the last step (1 h). The soil/extractant ratio was 1:40 in both fractionation methods. All extractions were performed in triplicate, using polyethylene flasks at 25°C with the exception that the extraction with 2 M NaOH was performed at 90°C (hot NaOH-extractable organic P) and the last extraction in both schemes (0.5 M H2SO4 + 0.37 M K2S2O8) was done at 130°C in an autoclave. After each extraction (shaking end over end at 2.5 s–1), the suspension was centrifuged at 1000 x g for 10 min and the supernatant analyzed for molybdate-reactive P (Murphy and Riley, 1962) and total P following sulfuric–persulphate digestion (Diaz-Espejo et al., 1999), except for hot NaOH, which was analyzed for total P only, and in the last step, where only molybdate-reactive P was determined. Differences between molybdate-reactive P and total P in the extracts can be assigned mainly to organic P (Golterman et al., 1998), so a distinction could be made between organic and inorganic (molybdate-reactive) P in each fractionation step.

According to Saavedra and Delgado (2005b), the two sequential fractionation schemes provided an accurate distinction of inorganic P fractions including: (i) adsorbed P and soluble Ca phosphates, which can be supposed to be the more labile P forms in soil (extracted by NaOH and citrate–bicarbonate in mR), (ii) nonlithogenic Ca phosphates (NaEDTA in mG), (iii) sparingly soluble pedogenic Ca phosphates (citrate-extractable P in mR), (iv) lithogenic Ca phosphates (HCl in mR), (v) P occluded in poorly crystalline Fe oxides (citrate–ascorbate in mR), and (vi) P occluded in crystalline Fe oxides (citrate–bicarbonate–dithionite in mR). Also, the combined amounts of organic P in each fraction can be used as accurate estimates of total organic P in soil; similarly, the combined amounts of molybdate-reactive P in each fraction provide an estimate of total inorganic P in the soil (Saavedra and Delgado, 2005b).

Phosphorus Release Potential
Water-extractable P (WP), which can be used to estimate the P release potential (Torrent and Delgado, 2001), was quantified by the extraction of P using a soil/water ratio of 1:10 for 1 h. Phosphorus desorbed to a near-infinite sink can be taken to be an estimate of the amount of P that can be released to a solution with a near-zero P concentration (Delgado and Torrent, 2001). A paper strip impregnated with Fe oxide (FeO strip), prepared according to Chardon et al. (1996), was used as a near-infinite sink. The extraction using the FeO strip was performed by placing 1 g of soil, 40 mL of deionized water, and a paper strip in a 50-mL polyethylene flask at room temperature in an orbital shaker at 0.5 s–1 for 16 h. After extraction, the strip was carefully washed and dried at 25°C, and adsorbed P in the strip was extracted by 40 mL of 0.1 M H2SO4 (1-h extraction with shaking in the same manner as for soil extraction).

The kinetics of P release were studied by using a Cl––saturated anion-exchange resin as a P sink. Nylon bags holding 2.2 g of Dowex 1 x 4 resin (diameter ranging from 0.05 to 1.3 mm) were placed in 60-mL polyethylene flasks containing 40 mL of 0.002 M CaCl2 and 1 g of soil. The flasks were then placed in a reciprocating shaker oscillating at 1.2 s–1. At 1, 2, 4, and 8 h, and at 1, 2, 4, 8, 16, 30, 60, and 90 d, the resin bags were removed and replaced with fresh resin. The resin was eluted twice with 0.5 M HCl to remove adsorbed P.

All extractions were performed in duplicate at 25°C, and molybdate-reactive P determined in all extracts according to Murphy and Riley (1962).

Statistical Analyses
The P-release data obtained by using the anion-exchange resins were plotted as a function of time via a logarithmic equation. Regressions, analysis of variance according to the experimental design, and mean comparisons according to Student's t-test were done by using Statgraphics Plus 5.1 (StatPoint, 2000).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Organic Matter and Phosphorus Contents in Surface Soil
Soil tillage had a significant effect on the organic matter and P concentrations of the soil surface (0- to 5-cm layer): after 21 yr, NT exhibited the highest organic matter, total P (determined by soil digestion or as the combination of P fractions separated according to the proposed schemes), FeO strip extractable P, Olsen P (Olsen et al., 1954), and organic P concentrations (Table 2).


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  		Table 2. Effect of soil tillage on the contents in organic matter and different P forms.{dagger}

 
The organic matter concentration in the surface layer (0–5 cm) was significantly different among tillage systems and decreased in the following sequence: NT > MT > CT (Table 2). Similar results were previously reported by Hussain et al. (1999). Increased organic matter contents in the soil surface layers under NT are a common finding (Blevins et al., 1983; Dick et al., 1998; Dao, 1998) and result from an increased accumulation through decreased soil mixing, and from a decreased decomposition rate as a result of less soil residue contact with soil, lower aeration, and a lower soil temperature (Hussain et al., 1999). The differences between MT and CT can be attributed at least partly to the small losses of soil organic matter promoted by noninversion tillage relative to moldboard plowing (Follett and Peterson, 1988; Reicosky and Lindstrom, 1993).

The total P content in the soil surface under NT was consistent with the accumulation of P fertilizer applied to the upper 5 cm of soil for 20 yr (an increase of about 1 g P kg–1), taking into account the low P transport resulting from crop residue accumulation on the soil surface. The available P index (Olsen P) was three times higher under NT than under CT or MT; on the other hand, total soil P under NT was only twice CT or MT (Table 2). Therefore, the large relative increase in Olsen P relative to total P under NT compared with other soil tillage systems could have been the result not only of accumulation of applied P under NT, but also of other factors affecting P dynamics in the surface layer.

Inorganic Phosphorus Forms in Surface Soil
Precipitated Ca phosphates may account for the most relevant P form in the studied soil according to the relative contribution of the combination of inorganic P extracted by citrate–bicarbonate, citrate, acetate, and HCl to total P in the mR scheme, and to the relative contribution of inorganic P extracted by NaEDTA in the mG scheme (Tables 3 and 4; Ruiz et al., 1997; Saavedra and Delgado, 2005b). The combined inorganic P fractions related to Ca phosphates in the mR scheme (citrate–bicarbonate, citrate, acetate, and HCl) and NaEDTA in mG were not significantly different (P < 0.001) when all tillage treatments were considered; this suggests that, in both cases, similar P forms in soil were extracted. Citrate–ascorbate-extractable inorganic P in mR and H2SO4–extractable inorganic P in mG were not significantly different (P < 0.001) either. A significant portion of acid-extractable inorganic P in mG might correspond to P occluded in Fe oxides because nonsignificant Fe oxide reduction has been observed in the first step of this scheme (Saavedra and Delgado, 2005b). Our results support the hypothesis that most of this fraction corresponds to P occluded in poorly crystalline Fe oxides only. In fact, this P fraction accounted for a significant proportion of inorganic P in the soil samples (about 30%), which was higher than the usual values for similar soils in the area (Ruiz et al., 1997). However, when Fe related to poorly crystalline Fe oxides (citrate–ascorbate soluble, Table 1) accounts for such a high proportion of Fe in oxides (around 17%), a high contribution of occluded P in poorly crystalline Fe oxides to total P in soil can be expected (Delgado et al., 2000).


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  		Table 3. Effect of soil tillage on different P fractions (inorganic and organic) separated by following the modified Ruiz et al. (1997) scheme.

 

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  		Table 4. Effect of soil tillage on different P fractions (inorganic and organic) separated by following the Golterman scheme as modified by Díaz-Espejo et al. (1999).

 
Plant-available P is assumed to consist mainly of inorganic P included in NaOH and citrate–bicarbonate fractions in mR (Delgado et al., 2000; Domínguez et al., 2001) and to be closely related to CaEDTA-extractable inorganic P in the mG scheme (Saavedra and Delgado, 2005b). Olsen P accounted for 18% of the combination of NaOH and citrate–bicarbonate inorganic P (Y = 0.18X, R2 = 0.98, P < 0.001, n = 12) and was slightly higher than CaEDTA-extractable inorganic P (Y = 1.27X, R2 = 0.96, P < 0.001, n = 12), which consists largely of adsorbed P (Saavedra and Delgado, 2005b). According to Delgado and Torrent (1997), total plant-available P must be equivalent to twice the inorganic P extracted with CaEDTA; thus, not only adsorbed P but also soluble Ca phosphates must contribute to the available P pool in soil.

Overall, no significant differences in inorganic or organic P fractions were observed between CT and MT (Tables 3 and 4); however, extracted P in most of the P fractions was significantly greater under NT than under CT or MT (Tables 3 and 4).

Under NT, the combination of inorganic P extracted by NaOH and citrate–bicarbonate in mR (adsorbed P and soluble Ca phosphates) and the inorganic P extracted by CaEDTA in mG (adsorbed P) were three times greater than under CT or MT; these were thus the fractions exhibiting the largest relative increase (Tables 3 and 4). This increase in the fractions including the more labile P forms under NT was not solely the result of the P enrichment of the surface layer, as total P increased twofold; it also accounts for the fact that the ratio of such fractions to the combined inorganic P fractions was also significantly higher in NT (Table 5).


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  		Table 5. Effect of soil tillage on different inorganic P form ratios.

 
Pedogenic Ca phosphates were also significantly increased under NT relative to CT and MT (Tables 3 and 4) as a result of the significant contribution of precipitation of Ca phosphates to P sorption in calcareous soils (Castro and Torrent, 1998; Tunesi et al., 1999). As expected, no significant differences in P related to lithogenic Ca phosphates were observed (inorganic P extracted by HCl, Table 3).

The ratio of labile inorganic P fractions in mR (NaOH + citrate–bicarbonate) to P related to sparingly soluble pedogenic Ca phosphates (citrate extractable) in the surface soil layer was much higher under NT (0.8) than under CT (0.57) or MT (0.55). This revealed a decreased precipitation of sparingly soluble Ca phosphates under NT that can be ascribed to the enrichment of the surface layer with organic matter (Table 2) and accounted partly for the significantly higher ratios of Olsen P and labile P fractions to total inorganic P under NT relative to MT and CT (Table 5). Organic matter (particularly humic compounds) decreases the precipitation of sparingly soluble Ca phosphates by favoring the formation of soluble Ca phosphates over sparingly soluble forms (Inskeep and Silvertooth, 1988; Grossl and Inskeep, 1991; Delgado et al., 2002). This inhibitory effect can also increase the rate of P adsorption to the soil. Also, the increase in available P and soluble Ca phosphates has been shown to be higher in P-rich soils than in low-P soils with identical applied fertilizer rates (Fixen et al., 1983). Because the surface layer under NT exhibited more marked P enrichment than those under other treatments, this might also account partly for its increased ratio of labile P forms to the combination of inorganic P fractions.

The amount of P occluded in Fe oxides was also significantly greater with NT than with CT or MT, the difference being largely due to P occluded in poorly crystalline Fe oxides (citrate–ascorbate, Table 3, or H2SO4, Table 4). Occlusion of P fertilizer applied to soil has been encountered in soils of the study area—even in crystalline Fe oxides (Smeck et al., 1994). Poorly crystalline Fe oxides contain more occluded P per unit mass Fe than do crystalline oxides, however, which suggests that fertilizer P can be preferentially occluded in poorly crystalline Fe oxides (Ruiz et al., 1997; Domínguez et al., 2001). In fact, most P occluded in crystalline oxides is assumed to be related to the amount of these soil components formed through weathering, which is a long-term process (Saavedra and Delgado, 2005b). Thus, one can expect a large increase in P occluded in poorly crystalline Fe oxides relative to crystalline ones when the soil is enriched with P. The increase in the ratio of inorganic P extracted by citrate–ascorbate to the combined inorganic P fractions was not significant (Table 5), so P occluded in poorly crystalline Fe oxides was essentially related to the degree of P enrichment of the soil. On the other hand, the ratio of inorganic P extracted by citrate–bicarbonate–dithionite to the combined inorganic P fractions was significantly lower in NT, which indicated that this fraction was not so strongly affected as citrate–ascorbate by the degree of P enrichment of the soil.

Organic Phosphorus Fractions
The environmental and agronomic significance of organic P is determined by its hydrolysis potential, which dictates whether these forms become bioavailable in soils or water (Shand and Smith, 1997; Tarafdar et al., 2002). Organic P, estimated as the combination of organic P fractions in both fractionation schemes, was also significantly greater with NT than with the other treatments (Table 2). However, a nonsignificant increase in the ratio of organic P fractions to the combined P fractions in both fractionation schemes was observed (Table 5); this indicated that the increase in organic P was a result of the P enrichment in the soil surface. Nonsignificant differences between total P in the soil samples as determined by microwave digestion and the combination of P fractions according to both fractionation schemes were observed (Table 2), which was consistent with previous findings by Saavedra and Delgado (2005b). When only the NT treatment was considered, however, the amounts determined by digestion were significantly greater (P < 0.01) than those determined as the combination of P fractions in both fractionation schemes. This difference could probably be ascribed to a higher organic P content related to crop residues under NT, which can only be mineralized using more effective digestion techniques.

Chemical fractionation usually does not discriminate between organic forms differing in their ease of hydrolysis. Sodium hydroxide in mR must extract essentially organic P related to humic and fulvic acids (Schlichting et al., 2002) with hot NaOH extraction being more efficient (Table 3). Citrate is effective at extracting most phytates from soil and usually extracts greater amounts of enzyme-labile organic P than do basic extractants (Hayes et al., 2000). The combination of organic P extracted by the four reagents containing citrate accounted for most extracted organic P in mR, particularly with the CT and MT treatments (Table 3). The efficiency of citrate–ascorbate and citrate–bicarbonate–dithionite was perhaps related to the dissolution of oxides capable of acting as adsorbent surfaces. The dominance of citrate-extractable organic P can be ascribed to the fact that phytates are the dominant form of organic P in soil (Condron et al., 1985). Although phytates can be considered a fairly stable form in soils, they may account for a substantial portion of organic P in eroded sediments, which can be mineralized in anoxic environments and contribute to P release from sediments (Suzumura and Kamatani, 1993; Golterman et al., 1998). In the mG scheme, NaEDTA proved the most efficient step extracting organic P, the extracted amount being significantly greater in NT (Table 4). This extractant has proved effective in extracting organic matter and organic P from soil (Bowman and Moir, 1993).

Phosphorus Release Potential
The amounts of P released by the paper strip impregnated with Fe oxide (FeO strip, Table 2) and those released by the resins at different times (Table 6, Fig. 1 ) were significantly greater (double) in NT than in CT and MT, thus indicating that the P release potential in the soil surface was significantly increased under NT. The amounts extracted by the FeO strip were similar to those extracted by the resins at 1 d (Table 6). The proportion of inorganic P in soil released by the resins was not significantly different at 1 d; however, at 90 d, it was significantly higher in NT than in CT and MT. This could also be related to the increase in the P fractions including the more labile inorganic P forms. No significant differences in WP between treatments were observed (Table 2); this indicated that the P concentration in solution at a 1:10 soil/solution ratio was buffered, yielding similar values. This is in agreement with previous findings by Delgado et al. (2002), who observed nonsignificant increases in WP in calcareous soils, where the organic amendment promoted a greater proportion of applied P being recovered by NaOH and citrate–bicarbonate.


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  		Table 6. Kinetic desorption paramenters (resin extraction) as affected by soil tillage.{dagger}

 

Figure 1
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  		Fig. 1. Phosphorus desorbed by resins as a function of time for the three tillage treatments: conventional tillage (CT), minimum tillage (MT), and no-till (NT). Error bars indicate standard deviation.

 
A logarithmic equation provided an accurate fit of the kinetic data (R2 > 0.98, P < 0.001, in all cases; Fig. 1). The parameters in the equation (Table 6) were significantly higher in NT than in CT and MT; the increased parameter B is consistent with an increased effect of time on P release (viz. with a slower kinetics) under NT.

The best estimate of P released by resins in the long term (90 d) was provided by the combination of inorganic P fractions according to mR, including the more labile P forms (NaOH + citrate–bicarbonate, R2 = 0.98, P < 0.001, n = 12). On average, the amount of P released by the resins at 90 d accounted for 77% of the combination of these two fractions. This percentage was similar to that reported by Delgado and Torrent (2000) as the proportion of the inorganic P released by NaOH and citrate–bicarbonate that could be potentially released in the long term to a dilute electrolyte with a near-zero P concentration in similar soils from southern Spain. Therefore, most P included in the combination of these two fractions in mR, which was significantly increased in the soil surface under NT relative to CT and MT (Table 3), can be potentially released to water surrounding soil particles through desorption or dissolution processes. Moreover, P occluded in poorly crystalline Fe oxides, which also increased under NT, can be potentially released through reduction of Fe oxides under anoxic conditions in the bottom of water reservoirs (Saavedra and Delgado, 2005a). These conditions can also promote the hydrolysis of organic P forms (phytates), which were found in significantly increased amounts in the surface of the NT plots. Thus, the inorganic P potentially releasable to water increased from 244 (CT and MT) to 418 mg kg–1 of surface soil under NT, and that of potentially hydrolyzable organic P from 180 (CT and MT) to 238 mg kg–1 in NT if one assumes most citrate-extractable organic P to be mineralized under anoxic conditions. Thus, although NT can help to reduce soil losses, one should bear in mind that the amount of potentially releasable P per unit mass of soil loss was roughly twice as much under this tillage system.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The total P content and most of the P fractions studied in the surface soil were found to be higher with NT than with MT or CT. Also, the ratio of available P and P fractions including labile forms to total inorganic P in soil was increased as a result not only of surface enrichment with P, but also changes in processes affecting P dynamics in soil. The P release potential of surface soil estimated by using P sinks (resins and Fe-oxide-impregnated paper) increased significantly under NT relative to MT and CT, which was consistent with the increase in the fractions including the more labile P forms (NaOH and citrate–bicarbonate in the mR scheme, and CaEDTA in the mG scheme). Although NT can help to reduce soil losses, the increased P release potential revealed that the dissolved P concentration in runoff and the amount of potentially releasable P per unit mass of surface soil lost through erosive processes can be greater with NT than with MT or CT.


   ACKNOWLEDGMENTS
 
This work was funded by Spain's R & D Program (Plan Nacional I+D), Project AGF99–0574–CO1, and, supplementarily, by AGL2002–04134–CO2–01. The Institute of Agriculture and Fisheries Research of Andalusia (IFAPA) provided working facilities and access to the experimental site.


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Abbreviations: CT, conventional tillage; EDTA, ethylenediaminetetraacetic acid; MT, minimum tillage; NT, no-till; WP, water-extractable phosphorus.

Received for publication April 18, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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