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Published in Soil Sci. Soc. Am. J. 68:1635-1644 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

Inorganic and Organic Phosphorus Fertilizer Effects on the Phosphorus Fractionation in Wetland Rice Soils

M. A. Salequea, U. A. Nahera, A. Islama, A. B. M. B. U. Pathana, A. T. M. S. Hossaina and C. A. Meisnerb,*

a Soil Science Division, Bangladesh Rice Research Institute, Gazipur 1701
b Natural Resource Group, CIMMYT, Bangladesh

* Corresponding author (c.meisner{at}cgiar.org)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Long-term effects of rice (Oryza sativa L.) cultivation with varying nutrient management on soil P fraction are important to understand from soil nutritional and environmental point of view. Soil P fractionation gives an idea about the soil P supplying capacity to plants. The present experiment was conducted to evaluate the effect of different nutrient management in wetland rice on the changes of soil P fraction at different depths. Soil samples from five depths (0–5, 5–10, 10–15, 15–30, and 30–50 cm) were collected from a long-term experimental field classified as a Chhiata clay loam, hyperthermic Vertic Endoaquept. The field received six treatments for 10 yr: absolute control with no fertilizer applied (T1), one-third of recommended fertilizer doses (T2), two-thirds of recommended fertilizer doses (T3), full doses of recommended fertilizers (T4), T2 + 5 Mg cow dung (CD) and 2.5 Mg ash ha–1 (T5), and T3 + 5 Mg CD and 2.5 Mg ash ha–1 (T6). The apparent balance of P compared with the initial P status after 10 yr varied from –115 kg ha–1 under T1 to 348 kg ha–1 under T6. The P fractionation study was conducted over the treatments and soil depth. Treatment and depth had no significant effect on solution P. Larger concentrations of NaHCO3 soluble P, NaOH extracted inorganic P (Pi), and acid P were observed under treatments with organic fertilizers (T5 and T6) than with other treatments at 0- to 5-, 5- to 10-, and 10- to 15-cm depths. The concentrations of NaHCO3–P, NaOH-Pi and acid P fractions were lowest under T1 and T2 treatments. At 15 to 30 cm or lower soil depths, none of the P fractions were affected by treatments. The change in NaOH organic P (Po) and residual P (extracted with HNO3 + HClO4) with soil depth was not significant, and the differences in these P fractions under the tested P treatments were not large. The depletion of NaHCO3–P and NaOH-Pi at the 0- to 15-cm depth under control and T2 suggests that the rice plant depends upon these fractions of P. The P depletion profile in wetland rice appears to be confined within the first 15-cm depth. The mean P uptake by rice showed a polynomial relationship with NaHCO3–P and NaOH-Pi (average of 0–15 cm) and it was linearly correlated with acid P (0–15 cm).

Abbreviations: CD, cow dung • HYV, high yielding varieties • Pi, inorganic phosphorus • Po, organic phosphorus • T1, absolute control with no fertilizer applied • T2, one-third of recommended fertilizer doses • T3, two-thirds of recommended fertilizer doses • T4, full doses of recommended fertilizers • T5, T2 + 5 Mg cow dung and 2.5 Mg ash ha–1 • T6, T3 + 5 Mg CD and 2.5 Mg ash ha–1


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PHOSPHORUS DEFICIENCY PROBLEMS are frequently reported in well-weathered Oxisols and Ultisols because of strong acidic reactions and abundance of Al3+ and Fe3+. Relatively young soils, like Inceptisols, have been found to contain a greater portion of their total P in the available form than do more mature soils. Rice soils, which remain anaerobic for most of the rice-growing season, have a greater P availability than an aerobic soil. Nevertheless, inappropriate P fertilizer management coupled with increasing cropping intensity with modern high yielding varieties (HYV) causes P depletion in soils and thus, P deficiency occurs in many alluvial Bangladesh soils (Ali et al., 1997), despite the notion of increased P availability after wetting of a dry soil. Soil available P content decreased over the 8 yr to as low as 3 mg P kg–1 soil due to inappropriate P management in a rice–rice cropping pattern and the application of less P than the crop removal (Saleque et al., 1998). Such a low level of soil P causes acute P deficiencies and a yield reduction in lowland rice by 50% or more (Saleque et al., 1998). Despite these yield reductions, a response of rice to P fertilizer is usually not obtained in most soils of Bangladesh, because they are not deficient in P.

Rice removes about 2 to 3 kg P for 1 Mg of grain produced (Timsina and Connor, 2001; Saleque et al., 2001). Although the rice requirement for P is much less than that for N, the continuous removal of P exploits the soil P reserve if the soil is not replenished through fertilizer or manure application. Chemical P fertilizer is a costly agricultural input for rice farmers of the developing world, and sometimes the material is not available in the local village market. Cattle manure may be considered as an alternative to chemical P fertilizer. Many studies have shown that cattle manure can be a potential source of P (Agabenin and Goladi, 1998; Reddy et al., 2000). Eghball and Power (1999) reported that manure application of 92 Mg ha–1 in 4 yr increased soil available P at the 0- to 15-cm depth from 49 to 116 mg kg–1. The greater accumulation of P due to manure application may increase the potential of P loss through run-off water (McDowell and Sharpley, 2001), but mixing the manure with soil may potentially decrease the problem of P loss (Kleinman et al., 2003; Sharpley, 2003).

Biosolid applications to soil not only increase available P, but also decrease P binding affinity by soils and increase P desorption capacity (Sui and Thompson, 2000). Ekholm et al. (1999) have shown that the application of large amounts of organic matter increased soil P movement, because introduction of organic matter at the surface promotes the build up of soil macropores. These macropores are the critical element for the movement of organic P (Akhtar et al., 2003). However, in perpetual saturated conditions, like wetland rice, the growth and movement of worms and micropore production would be less, so the organic P movement down to the soil profiles would be also less.

Crop plants can only take up available P, but other fractions of P, such as NaOH-extracted inorganic and acid-extracted P are depleted due to crop growth (Saleque and Kirk, 1995). Sui and Thompson (1999) found that the application of biosolids increased the soil concentrations of water soluble, acid, and alkali soluble inorganic P. The latter two fractions of P, though not readily available to plants, are biologically dynamic (Schmidt et al., 1996). Thus, these latter fractions may replenish the available P when depleted.

Lowland rice under saturated conditions increased solubilization of acid and alkali soluble Pi fractions through root-induced pH changes, and increased their absorption (Saleque and Kirk, 1995). However, all portions of solubulized P were not absorbed by rice. Thus, some of the root-induced solubilized P may move away from the root through diffusion (Kirk and Saleque, 1995). Under a controlled experiment, the P depletion profile for lowland rice was found to be confined to within several millimeters of roots. Rice was found to rely on P within a resin and the Pi fraction, which can be extracted by 0.1 M NaOH (Saleque and Kirk, 1995). There are only a few studies that have shown the actual P depletion profile in rice soils under field conditions.

Growing rice on native P fertility may deplete P from different soil pools while the application of P fertilizer in large excess of crop needs may contribute to build-up of the different P fractions (Reddy et al., 2000). Fractionation of P in soils in a long-term experimental field with various P sources and rates might provide answers about the depletion or accumulation of P in different soil layers. The objective of this investigation was to understand the long-term effects of different levels of nutrient application (N, P, K, and S) from chemical fertilizer alone or in combination with organic materials on the different soil P fractions and the extent of P depletion/accumulation within a soil profile.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil and Location
A long-term (10-yr) field experiment consisting of a two-rice cropping systems per year, was conducted at the experimental farm of the Bangladesh Rice Research Institute, Gazipur, Bangladesh, located at 23°59'N latitude, 90°24'E longitude. The site is about 30 m above the mean sea level and has a subtropical climate, which is strongly influenced by a southwestern monsoon. The average annual rainfall is 2000 mm with more than 80% of it occurring from mid-June to the end of September. Mean temperature is lowest (15°C) in January and highest (30°C) in May.

The soil of the experimental field is Chhiata clay loam, a member of fine, hyperthermic Vertic Endoaquepts. Initial properties of the surface soil (0- to 15-cm depth) were as follows: clay loam texture, pH 6.40, organic C content 11.3 g kg–1, total N 0.80 g kg–1, available P 9.0 mg kg–1 (Olsen P-recommended method for P extraction in Bangladesh), exchangeable K 0.18 cmol kg–1 soil, and available S 14 mg kg–1.

The long-term experiment (10 yr) was conducted with the following six treatments: absolute control, (no chemical fertilizer or manure) (T1), one-third of recommended fertilizer doses, (40-8-12-5 kg ha–1 N, P, K and S in dry season rice and 30-6-9-4 kg ha–1 N, P, K, and S in wet season rice, respectively) (T2), two-thirds of recommended fertilizer doses, (80-16-24-10 kg ha–1 N, P, K and S in dry season rice and 60-12-18-8 kg ha–1 N, P, K, and S in wet season rice, respectively) (T3), full doses of recommended fertilizers (120-24-36-15 kg ha–1 N, P, K and S in dry season rice and 90-18-27-12 kg ha–1 N, P, K, and S in wet season rice, respectively) (T4), T2 + 5 Mg CD and 2.5 Mg ash ha–1 (T5), and T3 + 5 Mg CD and 2.5 Mg ash ha–1 (T6) for 10 yr. The N, P, K and S content in the CD was 12.9, 05.8, 17.6 and 2.6 g kg–1, respectively. The ash contained 0.20 g kg–1 N, 1.0 g kg–1 P, and 10.0 g kg–1 K. The treatment details of the long-term experiment are presented in Table 1. The total P inputs in treatments T1 through T6 were 0, 14, 28, 42, 45.5, and 59.5 kg ha–1 yr–1. At maturity the crop was harvested manually at 15 cm above the ground level, however, 16 hills from each plot were harvested at the ground level for straw yield data. After harvest, the crop residue was incorporated into the soil by spading. Two rice crops were grown annually. Dry season rice (variety BR3) was transplanted in first week of January and was harvested in May; wet season rice (variety BR11) was transplanted in the first week of August and harvested in third week of November. The experiment was conducted as a randomized complete block design with four replications. Rice yields and nutrient removal by rice in this long-term experiment have been reported by Saleque et al., (2004). The long-term rice cropping with various nutrient management practices created apparent soil P balance varied from –115 kg ha–1 to 348 kg ha–1 (Table 2).


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  		Table 1. Amount of N, P, K, and S added through fertilizer, cow dung, and ash each year.{dagger}

 

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  		Table 2. Apparent balance of P in rice–rice cropping system with different nutrient management practices over the period of 1990–1999.

 
The soils for the study were collected from five depths: 0 to 5, 5 to 10, 10 to 15, 15 to 30, and 30 to 50 cm from each 5 by 4 m plot. The soil samples were air-dried, crushed, and passed through 2-mm sieve and stored in polyethylene bags at room temperature, before P fractionation.

Sequential Phosphorus Fractionation
Fraction of inorganic and organic P was performed on each soil by a modified P fractionation scheme of Sui and Thompson (1999). The following soil P fractions were measured in sequence:

  1. Solution P, by shaking 1 g soil in 30 mL of 0.05 M CaCl2 for 16 h, centrifuging, filtering, and measuring P in the filtrate.
  2. NaHCO3–P, by shaking the residue from (1) in 30 mL of 0.5 M NaHCO3 for 16 h, centrifuging, filtering, and measuring P in the filtrate.
  3. NaOH-Pi–P, by shaking the residue from (2) in 30 mL of 0.1 M NaOH, centrifuging, filtering, and measuring P in the filtrate after acidifying 5 mL (with concentrated HCl) and centrifuging.
  4. NaOH-Po–P, by digesting 5 mL of the filtrate from (2) in 6 mL of concentrated H2SO4 for 1 h, cooling, adding 5 mL of H2O2, and reheating until the residue became white, determining P in the digest, and subtracting the NaOH-Pi–P from it (Hedley et al., 1982).
  5. Acid P, by shaking the residue from (3) in 30 mL of 1:1 mixture of 1 M HCl/1 M H2SO4, centrifuging, filtering, and measuring P in the filtrate.
  6. Residual P, by refluxing the soil residue from (5) in 6 mL of a 5:2 mixture of concentrated HNO3 and HClO4, and determining P from the digest (Hedley et al., 1982).

All P was determined colorimetrically (Murphy and Riley, 1962) after neutralization when necessary with dilute HCl and NaOH and the neutral pH indicated by the light yellow color of the solution in the presence of P-nitrophenol indicator. Absorbance for P was determined at a wavelength of 712 nm by spectrophotometer (Hitachi, Model U–1100, No. 118–0102, Hitachi Ltd, Tokyo, Japan).

All measurements of P were done in triplicate, and the data were analyzed by ANOVA using the statistical software IRRISTAT 3.0 (Windows version; Bartolome et al., 1998).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Solution Soil Phosphorus
The mean concentration of solution P at the 0- to 5-cm depth varied from 0.19 to 0.25 mg kg–1 and at the 5- to 10-cm depth it varied from 0.13 to 0.20 mg kg–1 soil (Fig. 1) . The range of solution P pool at the 10- to 15-cm depth was 0.15 to 0.22 mg kg–1, and a similar narrow range of solution P was also found in the 15- to 30- and the 30- to 50-cm soil layers. The variation in solution P due to previous P treatment and between the depths within a treatment was not significant (Table 3). However, the application of CD and ash tended to increase solution P at the 0- to 5- and 5- to 10-cm depths, though not significant. Griffin et al. (2003) reported from an incubation study that fertilizer P increased the CaCl2–P fraction more than manure P. However, there was no removal of solution P by plants in their experiments. The insignificant variation in solution P in soil is probably attributed to plant P uptake of this readily available P source during the growing season. The solution P pool is the immediate source of P for plant uptake, and this P pool was probably depleted due to crop removal in all the treatments.



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  		Fig. 1. Effect of P inputs on the solution P fraction after 10 yr of rice–rice cropping. Initial = Soil collected after 10 yr from fallow portion of the experimental field; T1 = no chemical fertilizer or cow dung (CD) (control); T2 = 40-8-12-5 kg ha–1 N, P, K, and S in dry season rice and 30-6-9-4 kg ha–1 N, P, K, and S in wet season rice; T3 = 80-16-24-10 kg ha–1 N, P, K, and S in dry season rice and 60-12-18-8 kg ha–1 N, P, K, and S in wet season rice; T4 = 120-24-36-15 kg ha–1 N, P, K, and S in dry season rice and 90-18-27-12 kg ha–1 N, P, K, and S in wet season rice; T5 = T2 + 5 Mg CD and 2.5 Mg ash ha–1; T6 = T3 + 5 Mg CD and 2.5 Mg ash ha–1.

 

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  		Table 3. Summary of analysis of variance results for different P fractions after 10 yr of rice cultivation with different fertilizer management practices.

 
NaHCO3–P
The long-term differential P fertilizer management yielded considerable changes in NaHCO3–P in soil (Fig. 2) . The concentration of NaHCO3–P varied significantly due to treatment, with depth and the interaction effect of treatment x depth (Table 3). Figure 2 shows that at the 0- to 5-cm depth, the NaHCO3–P under the control plots (T1) was only 5.1 mg kg–1, which was slightly greater with T2 (9.3 mg kg–1). The concentration of NaHCO3–P in T3 and T4 plots at the 0- to 5-cm soil depth was about 16 and 18 mg kg–1, respectively. The NaHCO3–P under T5 and T6 was more than 40 mg kg–1 at the 0- to 5-cm soil depth. The large accumulation NaHCO3–P in T5 and T6 was certainly due to CD application. The distribution of NaHCO3–P under different treatments at the 5- to 10-cm soil depth was similar to that at the 0- to 5-cm depth. At the 10- to 15-cm depth, NaHCO3–P under treatments T1 through T3 was similar, which was slightly lower than that of initial soil. There was slight build up of NaHCO3–P at the 10- to 15-cm depth with T4, T5, and T6. The change in NaHCO3–P at soil depths below 15 cm was not significant (Fig. 2). Zhang and Mackenzie (1997) also reported an increase in NaHCO3–P in temperate soil with the application of P fertilizer. Long-term application of liquid pig manure increased the NaHCO3–P fraction in maize soil (Hountin et al., 2000). Our results suggest that the NaHCO3–P can be increased by the combination of fertilizer and organic manure application. The increase in NaHCO3–P due to the application of manure was several folds greater than the increase that occurred due to fertilizer, which further suggest that manure may fully substitute for P fertilizer.



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  		Fig. 2. Effect of P inputs on the NaHCO3–P fraction after 10 yr of rice-rice cropping (bar represent LSD value at 5% level). Initial = Soil collected after 10 yr from fallow portion of the experimental field; T1 = no chemical fertilizer or cow dung (CD) (control); T2 = 40-8-12-5 kg ha–1 N, P, K, and S in dry season rice and 30-6-9-4 kg ha–1 N, P, K, and S in wet season rice; T3 = 80-16-24-10 kg ha–1 N, P, K, and S in dry season rice and 60-12-18-8 kg ha–1 N, P, K, and S in wet season rice; T4 = 120-24-36-15 kg ha–1 N, P, K, and S in dry season rice and 90-18-27-12 kg ha–1 N, P, K, and S in wet season rice; T5 = T2 + 5 Mg CD and 2.5 Mg ash ha–1; T6 = T3 + 5 Mg CD and 2.5 Mg ash ha–1.

 
Regression analysis of yearly mean P uptake by rice with the soil NaHCO3–P (average over 0–15 cm) showed polynomial relationship (Fig. 3) . The mean P uptake increased with the increase in NaHCO3–P up to 20 mg P kg–1, but at 35 mg kg–1 P, there was not further increase in P uptake. Huguenin-Elie et al. (2003) asserted that only 10% of the P taken up by rice crop was drawn from readily available pools; the bulk was drawn from sparingly soluble pools as result of solubulization induced by roots.



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  		Fig. 3. Relationship between different fractions of soil P (mean over 0- to 15-cm depth) and mean annual P uptake by rice (mean of 10-yr P uptake) (* = significant at 5% level, ** = significant at 1% level, ns = not significant).

 
The NaHCO3–P pool is readily available to plants (Enwezor, 1977; Bowman and Cole, 1978). The lower value of NaHCO3–P at the 0- to 10-cm than at the 10- to 15-cm in T1 indicates that this P pool was depleted due to crop uptake. The NaHCO3–P depletion up to 15 cm deep was also apparent in case of T2. The depletion of the NaHCO3–P pool in the 0- to 15-cm depth accounted for 6% to the total P uptake by rice in T1 while the depletion of this P pool in T2 accounted for 3% of total P removal by crop.

There was an accumulation of NaHCO3–P when larger doses of P fertilizer (28 or 42 kg P ha–1 annually in T3 and T4) were applied. The application of CD and ash contributed to the large amount of NaHCO3–P build up from the 0- to 15-cm depth. About 1% of the applied P in T3 was accumulated as NaHCO3–P at the 0- to 15-cm soil depth while it was 3% in the case of T4. The build-up of NaHCO3–P in T5 and T6 was 7 and 6% of the total P inputs through organic and inorganic sources. About 12% of the P applied through CD and ash accumulated as NaHCO3–P in the 0- to 15-cm soil depth.

The results further indicate that P additions did not influence the concentration of NaHCO3–P below 15 cm. Aulakh and Pasricha (1991) demonstrated the accumulation of NaHCO3–P at a soil depth of 15 to 30 cm, but their experimental soil was coarser in texture.

NaOH-Pi
The control plots had only 48 mg kg–1 NaOH-Pi at 0 to 5 cm, which increased to 63 mg kg–1 at 5 to 10 cm and 76 mg kg–1 at 10 to 15 cm deep (Fig. 4) . At the 0- to 5-cm depth, the NaOH-Pi in the T2 plots was greater than that of control, but at 5 to 10 cm or deeper soil layers the difference between T1 and T2 was not significant. The application of greater annual P rates (28 kg ha–1) in T3 significantly increased NaOH-Pi at 0- to 5-, 5- to 10-, and 10- to 15-cm soil depths to 120, 97, and 100 mg kg–1, respectively. Increasing fertilizer P rates to 42 kg ha–1 yr–1 (T4) increased NaOH-Pi at the 0- to 5- and 5- to 10-cm depths more than obtained with T3. The concentrations of NaOH-Pi in T4 at 0- to 5-, 5- to 10-, and 10- to 15-cm depths were 146, 153, and 107 mg kg–1, respectively. The application of fertilizer seemed to increase NaOH-Pi concentration down to 15 cm. The build up of NaOH-Pi was largely due to CD and ash application. A similar increase in NaOH-Pi in soils due to organic matter addition was reported by Sharpley et al. (1984) and Iyamuremye et al. (1996). Schmidt et al. (1996) reported that fertilizer P applied in excess of plant uptake accumulated in the NaOH-Pi pool.



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  		Fig. 4. Effect of P inputs on the NaOH-Pi fraction after 10 yr of rice-rice cropping (bar represent LSD value at 5% level). Initial = Soil collected after 10 yr from fallow portion of the experimental field, T1 = no chemical fertilizer or cow dung (CD) (control); T2 = 40-8-12-5 kg ha–1 N, P, K, and S in dry season rice and 30-6-9-4 kg ha–1 N, P, K, and S in wet season rice; T3 = 80-16-24-10 kg ha–1 N, P, K, and S in dry season rice and 60-12-18-8 kg ha–1 N, P, K, and S in wet season rice; T4 = 120-24-36-15 kg ha–1 N, P, K, and S in dry season rice and 90-18-27-12 kg ha–1 N, P, K, and S in wet season rice; T5 = T2 + 5 Mg CD and 2.5 Mg ash ha–1; T6 = T3 + 5 Mg CD and 2.5 Mg ash ha–1.

 
The concentration of NaOH-Pi at 15 to 30 and 30 to 50 cm was low in the control and tended to increase with P application. At the 15- to 30-cm layer, the NaOH-Pi concentration ranged between 69 mg kg –1 in the control and 95 mg kg–1 in T5, but there were no differences in NaOH-Pi concentration for T2 to T6.

The NaOH-Pi is chemisorbed P (Williams et al., 1980) and largely reacts with Al and Fe (Iyamuremye et al., 1996). Enwezor (1977) reported that this fraction of P is less related to plant uptake than NaHCO3–P but in our studies, the depletion of NaOH-Pi from the surface soil compared with the subsurface soil of the control plots suggested that this fraction of P may become mobilized when NaHCO3–P was depleted from the soil. Our previous studies showed the NaOH-Pi contributed about 10% of the total P depletion by rice plants (Saleque and Kirk, 1995). Sharpley (1996) observed a depletion of NaOH-Pi after repeated extraction of P by a Fe-oxide strip, which supports the thesis that depletion of labile P pools by plant root uptake may mobilize the relatively nonlabile NaOH-Pi. Beck and Sanchez (1994) reported that NaOH-Pi was the dominant fraction related to availability of P to plants in an 18-yr continuously cultivated and fertilized cropping system on an Ultisol.

NaOH-Po Fraction
The NaOH-Po pool or organic P fraction, constituted the largest fraction of P in the studied soils, but the difference in this P fraction among the treatments and soil depths was not significant (Table 3). The mean concentration of NaOH-Po ranged from 2107 to 2255 mg kg–1 over the depths and treatments. Despite the increased concentration of the NaOH-Pi pool due to CD addition, there was not a significant increase in NaOH-Po pool probably because of slow incorporation of P into organic macromolecules or the microbial biomass under flooded conditions. Oberson et al. (1993) found no differences in organic P fractions under biological farming systems imposed for 13 yr compared with conventional systems. Our previous greenhouse experiment showed that rice did not absorb P from the NaOH-Po pool in some soils of the Philippines (Saleque and Kirk, 1995). Paniagua et al. (1995) investigated the distribution of P pools as affected by the addition of organic amendments for 10 yr in a maize and bean rotation on a typical volcanic soil. They found no differences in the size of the organic P pools as a result of the addition of organic amendments. Sui et al. (1999) observed a slight increase in NaOH-Po due to biosolid application, but the increase was not significant. NaOH-Po contains P associated with humic compounds and P sorbed to Fe and Al oxides (Cross and Schlesinger, 1995). However, Sharpley (1985) reported that mineralization of organic P during the growing season, which contributed similar amounts of P (20–74 kg P ha–1) as added in fertilizer (13–100 kg P ha–1), was not inhibited by fertilizer P addition. On the other hand, McGill and Cole (1981) reported that the mineralization of organic forms of P occurred only when the supply of inorganic P is limited. In the flooded rice culture of this study, the anaerobic soil conditions were more likely to limit microbial activity and release of P from CD and the organic pool, than was the supply of Pi.

Acid Phosphorus
The application of inorganic and organic P increased acid-P pools significantly from 0–15 cm (Table 3 and Fig. 5) . At the 0- to 5-cm depth, acid-P concentration in the control plots was 49 mg kg–1, which increased to 73 mg kg–1 with T2. With an increase in P application rates, the acid-P concentration increased progressively, though not always significantly. The application of CD and ash resulted in a greater accumulation of acid P. The increase in acid P with T5 was 58 mg kg–1 greater than T2 and the increase in T6 was 28 mg kg–1 greater than T3. However, difference in acid P between T4 and T5 was not significant at 5% level (Table 3).



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  		Fig. 5. Effect of P inputs on the acid-P fraction after 10 yr of rice-rice cropping (bars represent LSD value at 5% level). Initial = Soil collected after 10 yr from fallow portion of the experimental field, T1 = no chemical fertilizer or cow dung (CD) (control); T2 = 40-8-12-5 kg ha–1 N, P, K, and S in dry season rice and 30-6-9-4 kg ha–1 N, P, K, and S in wet season rice; T3 = 80-16-24-10 kg ha–1 N, P, K, and S in dry season rice and 60-12-18-8 kg ha–1 N, P, K, and S in wet season rice; T4 = 120-24-36-15 kg ha–1 N, P, K, and S in dry season rice and 90-18-27-12 kg ha–1 N, P, K, and S in wet season rice; T5 = T2 + 5 Mg CD and 2.5 Mg ash ha–1; T6 = T3 + 5 Mg CD and 2.5 Mg ash ha–1.

 
In the 5- to 10-cm soil layer, the concentration of acid P in the control was 70 mg kg–1, and the application of P fertilizer increased acid-P concentration. The accumulation of acid P due to organic material application was not significant as is shown by the comparison of the acid P concentrations between T4 versus T5. At the 10- to 15-cm depth, the concentration of acid P was 85 mg kg–1 in the control, and the application of P increased acid-P concentration progressively. However, the magnitude of increase was smaller than that obtained at 0 to 5 cm. The treatment effect on acid-P concentration at the 15- to 30- and 30- to 50-cm depths was not significant (Table 3).

The acid-P profile of different treatments was not similar. In T1 and T2, the acid P increased with soil depths up to 30 cm, while in T3 through T6, this P pool decreased with soil depth. In T1, the acid P at the 0- to 5-cm depth was 49 mg kg–1, which increased progressively with the depth of soil to the 15- to 30-cm depth, where it was 92 mg kg–1. In T2, there were 73 mg kg–1 acid P at the 0- to 5-cm depth, which increased to 95 mg kg–1 at the 15- to 30-cm depth. In T3 the 116 mg kg–1 of acid P at the 0- to 5-cm depth decreased to 100 mg kg–1 at 15 to 30 cm. Similarly, in T4, the acid P at the 0- to 5-cm depth was 123 mg kg–1, which decreased to 91 mg kg–1 at 15- to 30-cm depth. In T5 and T6, the acid-P concentration at the 0- to 5-cm depth was 131 and 144 mg kg–1, respectively, which decreased to 100 and 99 mg kg–1, at the 15- to 30-cm depth. The decrease in acid-P fraction with soil depth indicated that this fraction of P did not move deeper in the soil profile. This lack of acid P movement through the soil profile is good, especially considering the P accumulation associated with manure application, and the environmental implications. Acid P constitutes stable Ca-bound P (Williams et al., 1980); so it would unlikely contribute to the eutrophication of water sources. However, it would be mobilized to labile fraction when the later is depleted. Our previous studies showed that the acid P contributed about 25% of the total P uptake by rice under greenhouse conditions (Saleque and Kirk, 1995).

Residual Phosphorus
Residual P at the 0- to 5- and 5- to 10-cm depths was least in the control and tended to increase with increasing P rates. However, the variation was not statistically significant. The residual P pool is likely in the humus fraction (Stewart et al., 1980) and insoluble inorganic forms. Unlike solution P and NaOH-Pi forms of P, the downward movement of the residual P fraction was much slower.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The results of the present investigation show that the wetland rice crop obtains most of its P from labile P fractions, NaOH-Pi, and acid P pools. These three P pools independently showed good relationship with the mean P uptake. Depletion of these P pools may lead to mineralization and transformation of P from stable P pools of either NaOH-Po and residual P. However, in the 10-yr experimental period, these two P pools did not decrease significantly, even in control plots. In these moderately P deficient soils, the amount of P recovered by rice was about 35 to 42% of the applied P fertilizer, and most of the remaining P accumulated in labile, NaOH-Pi and acid-P pools. The application of CD and ash (5 and 2.5 Mg ha–1) significantly increased soil labile, NaOH-Pi and acid-P pools. Since the quantity of P applied through ash was less than a tenth of the P added through CD, the build up of the P pools was mainly attributed to the CD. The build-up of soil P pools was about 300 kg ha–1 in 10 yr due to the application of 5 Mg CD ha–1 and 2.5 Mg ash ha–1 of which 90% would have come from CD. Five megagrams of CD ha–1 were applied to supplement N fertilizer to provide a total N rate similar to T4. However, much less P would be required to maintain soil P as well as satisfy crop P demand. Further research is needed to find out the quantity of CD required to fulfill rice P demand and maintain P levels in tropical soils.

Received for publication August 11, 2003.


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





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