Distribution of Phosphorus Fractions down the Soil Depth in the Savanna Zone of Nigeria

The distribution of phosphorus (P) fractions down the soil depth varies depending on the soil type as being affected by the surrounding soil management practices. A 10 km by 10 km site was chosen in the savanna zone of Nigeria for the study. Modal soil profile pits were dug to identify the different soil types while soil sampling was done at 0-20 cm, 20-40 cm, 40-80 cm and 80-120 cm depth. Resin P was the largest extractable P fraction while residual P was the least in the soil series characterized, down the soil depth. Fractionation results showed that there was variation down the soil depth. The conc. HCl-Pi was not observed at the 0-20 cm depth, NaHCO3-Pi and Po was small while NaOH-Pi and Po was observed at this depth. Initial decrease at the 20-40 cm depth was observed for most of the P fractions while more of the P fractions were observed at the 40-80 cm depth. The values observed at the 80-120 cm depth was however small compared to other soil depth. Resin P was the sink of P and would serve as a reserve of P in the soil series. However, organic and inorganic P extractable with NaOH i.e. NaOH Pi and Po which were high at the 0-20 cm depth would be mainly responsible for the availability of P in the soil solution for plant uptake.


Introduction
Phosphorus (P) is the one major element in soil which must be supplied almost entirely by the parent material of the unfertilized soils because of low atmospheric returns.Also, P is a main limiting nutritional factor in high weathered soils, like most tropical areas and is characterized by a low content of available P with high P fixation.The usefulness of phosphorus fractions as an indicator of soil fertility and the degree of pedogenesis have long been established (Ridley, 1984;Loganathan & Sutton, 1987).Transformation and losses of P has been found to be influenced by the nature of the P in the parent material (Raji, 2006) and the relative abundance of the various fractions phosphorus fractions reflects the degree of soil weathering and development.Soil P is generally categorized into three types-solution P, labile P and non-labile P. the labile P fractions are more than the soluble Ps and they are not strongly adsorbed in the soil and may enter the soluble phase relatively quickly.Stable or the non-labile P fractions on the other hand are unavailable to plants and constitutes the greatest fraction of the total soil P. With time, due to weathering among several factors like land use/cropping systems and the past fertilizer use, a small amount of the non-labile P reacts chemically to become labile P and soluble P. In a study on soil profiles in selected soils of the derived savanna and northern guinea savanna zones of Nigeria, total P was found to vary among the soil profiles and decrease as the soil depth increases while resin P was significantly increased by organic matter (Nwoke et al., 2004).The increase or decrease in the earlier stated forms of P fractions could be attributed to various factors such as fertilizer application, land use, organic matter build-up, tillage practices among various factors (Bunermann et al., 2006).An example was found in the research of Scherer and Sharma (2002) where they discovered that residual P for instance decreased or increased due to application of mineral and organic fertilizers after 38 years of cultivation.Beck and Sanchez (1994) reported that NaOH-Pi was the dominant fraction related to availability of P to plants in a continuously cultivated and fertilized cropping system for over 18 years in an Ultisol.Also, increasing application or deposition of organic residues allowed P to be retained in more labile P fractions for a longer period (Uygur & Karabatak, 2009).Land use and soil management practices have been found to influence soil P through processes such as oxidation, mineralization

Fractionation of P
Soil to extractant used was what Tiessen and Moir (1993) recommended i.e. 1:60.Two resin strips were added to 10 ml of distilled water and shaken for 16 hrs.After retrieving the strips and washing adhering soil with distilled water into the centrifuge tube, P was extracted from the resin with 10 ml 0.5 M HCl for 16 hrs.The soil suspension was centrifuged at 2,000 rpm for 10min and the supernatant was discarded.Consecutively, extractions were made with 10 ml of 0.5 M NaHCO 3 (adjusted to pH8.5), 0.1 M NaOH, and 1 M HCl, each separately shaken for 16 hrs and centrifuged at 2,000 rpm.The supernatant was carefully decanted and stored in the refrigerator for analysis.Two milliliters of concentrated HCl was added to the soil and the tubes were placed in a water bath at 80 °C for 10 min.After removal, the suspension was allowed to cool down for 1 h, while shaking the tubes every 15 min by hand.After adding 1 ml of concentrated HCl to the same tubes, they were centrifuged at 2,000 rpm for 10 min and the supernatant was poured into a 10-ml volumetric flask.This was repeated with 2 ml of distilled water.The volumetric flask was then made up to volume with distilled water.
After adding 3 ml of concentrated HNO 3 and 1 ml of concentrated HClO 4 the soil was transferred to a crucible and placed in a sand bath at 200 °C for 16 hrs.After the soil cooled down, 2 ml of 5 M HNO 3 was added to the crucible, filtered into a 10-ml volumetric flask and made up to volume with HNO 3 .
Inorganic P in the resin-, dilute-acid-, acid-, and residual-P extracts was measured directly using the molybdate ascorbic acid method (Murphy & Riley, 1962) on a spectrophotometer at 712 nm.For the alkaline extracts, 1.2 and 0.3 ml of 0.9 M H 2 SO 4 was added to 2 ml of bicarbonate and hydroxide extract, respectively.After centrifugation at 4,000 rpm for 20 min and cooling in a refrigerator for 30 min, inorganic P was measured as described above.This procedure separates any organic precipitates from the solution (Tiessen & Moir, 1993).Organic P in the bicarbonate and hydroxide extracts was calculated as the difference between total and inorganic P, since organic P cannot be determined directly.Total P was analyzed as inorganic P after digestion with ammonium persulfate and sulfuric acid, followed by autoclavation (Tiessen & Moir 1993).An aliquot of 1 ml bicarbonate or hydroxide extract was digested with 0.1 g ammonium persulfate and 2 ml 0.9 M H 2 SO 4 in an autoclave for 2 hrs.Inorganic P was determined as described above for the resin and acid extracts.

Statistical Analysis
Data collected were subjected to analysis of variance (ANOVA) using GenStat Discovery Edition 4, 10.3DE statistical software, and where the F-value was found to be significant, the means were separated at P ≤ 0.05 level of significance using Fisher least significant differences (Genstat, 2011).

Dilute Acid Pi
At the 0-20 cm depth, the largest accumulation of Ca-P fraction was observed in Katcha series.At the 20-40 cm depth, increases in this P fraction was observed in Okpanam and Afia Mpko series while at the 40-80 cm depth, accumulation of Ca-P was observed in Okpanam and Afia Mpko series and decreased at the 80-120 cm depth (Figure 2).The recalcitrant P fraction was initially low at the 0-20 cm depth while there was no significant increase in the P fraction at the 20-40 cm depth except in Katcha series which had the largest accumulation of the recalcitrant P fraction.At the 40-80 cm depth, decreases in the P fraction were observed except in Afia Mpko series with a slight increase in the P fraction.However, at the 80-120 cm depth, the recalcitrant P fraction accumulated in Okpanam series but decreased in the other two soil series (Figure 3).Variability in the P fractions was observed for the inorganic Fe and Al-P fraction extractable with NaOH i.e. the moderately available P fraction across the soil depth.An initial increase was observed in Katcha and Okpanam series at the 0-20 cm depth.However, accumulation of the P fraction was observed at the 20-40 cm depth in Katcha series while at the 40-80 cm depth, the P fraction tend to accumulate in Afia Mpko seires and decreased in the three soil series characterized at the 80-120 cm depth (Figure 4).Accumulation of the organic P fraction extractable with NaOH i.e. the moderately available p fraction was observed in Okpanam and Afia Mpko series at the 0-20 cm depth and decreased at the 20-40 cm depth.Increase in the P fraction was later observed at the 40-80 cm depth and then decreased at the 80-120 cm depth except in Katcha series which had an accumulation of the organic P fraction (Figure 5).The readily available P fraction extractable with bicarbonate was initially low at the 0-20 cm depth except in Katcha series.At the 20-40 cm depth, decrease in the P fraction was observed except in Okpanam series with a slight increase in the P fraction.Accumulation of the inorganic P fraction was observed at the 40-80 cm depth in Afia Mpko and Katcha series while the P fraction was observed to later decrease at the 80-120 cm depth but with accumulation of the readily available P fraction in Okpanam series (Figure 6).The moderately available P fraction i.e.NaHCO 3 -Po was also low initially at the 0-20 cm depth as observed in the inorganic P fraction extractable with bicarbonate except in Afia Mpko series which had higher values of the moderately available P fraction than the readily available P fraction at the 0-20 cm depth.The moderately available P fraction continued to increase in Afia Mpko series at the 20-40 cm depth while further increases in the P fraction were observed at the 40-80 cm depth in Okpanam and Katcha series.Decrease in the P fraction was observed at the 80-120 cm depth, except in Afia Mpko series (Figure 7).Irrespective of the soil series characterized in this agro-ecological zone, residual P was the smallest of the P fraction determined.At the 0-20 cm depth, accumulation of the P fraction was observed in Okpanam series while at 20-40 cm depth, residual P accumulation was observed in Afia Mpko and Katcha series.Reduction in the P fraction was later observed down the soil depth to the 80-120 cm depth (Figure 8).The readily available P fraction i.e. resin P was the largest P fraction observed and this was evident down the soil depth.However, at the 0-20 cm depth, the P fraction was low when compared to the values obtained for the subsurface soil depths.Increase in the resin P fraction was observed at the 20-40 cm depth, with a little decrease at the 40-80 cm depth and with the largest increase in the P fraction at the 80-120 cm depth, which tend to act as the sink of resin P fraction in this agro-ecological zone (Figure 9).

Discussion
Variability in the distribution of all the P fractions determined occurred down the soil depth.Soils under different land use and management practices have been found to vary in pool sizes and in the distribution of P within soil profile (Emiru & Gebrekudan, 2013).Resin P was the largest extractable P fraction in all the soil series characterized while residual P was the least in abundance.The dilute acid Pi i.e.Ca-P tend to accumulate at the 0-20 cm depth specifically in Katacha series while this P fraction increased down to the 40-80 cm depth in Okpanam and Afia Mpko series.The concentrated HCl-Pi fraction, which represents the stable P fraction was low at the 0-20 cm depth and increased down to the 20-40 cm depth in Katcha series while a decrease was observed down to the 80-120 cm depth except in Okpanam series, where conc.HCl-Pi accumulation was observed.The moderately available inorganic P fraction i.e.NaOH-Pi varied down the soil depth.The NaOH-Pi fraction was initially high at the 0-20 cm depth, especially in Katcha and Okpanam series and continued to increase down to the 20-40 cm depth in Katcha series and to the 40-80 cm depth in Afia Mkpo series.A decrease in the NaOH-Pi fraction was later observed at the 80-120 cm depth.The organic P fraction extractable with NaOH was initially observed at the 0-20 cm depth and later decreased down to the 20-40 cm depth.Intermittent increase and decrease was observed down to the 80-120 cm depth.The moderately available organic P fraction may be in abundance at the sub-soil horizons because portions of the P fraction are supplied through weathering of the bedrock (Nobuo et al., 2010).The readily available inorganic and organic P fraction i.e.NaHCO 3 -Pi and Po was initially low at the 0-20 cm depth.At 20-40 cm depth, decrease in the inorganic P fraction was observed but with increase in the organic P fraction.Increases in NaHCO 3 -Pi and Po was later observed at the 40-80 cm depth and decreased down to the 80-120 cm depth.In most tropical soils, with the derived savanna zone inclusive, P deficiency has necessitated large application of fertilizer P and recently manure to overcome P fixation.Past fertilizer application has been observed to be the main factor for the increases observed for the moderately and readily available P fraction.Reddy et al. (1999) has observed that application of fertilizer (fertilizer P and manure) significantly increased NaHCO 3 -Pi, Po as well as the NaOH-Pi and Po fractions.The residual P fraction was low irrespective of the soil series.However, accumulation of the P fraction was observed at the 0-20 cm depth and down to the 20-40 cm depth, varying among the soil series.The resin P fraction was the largest P fraction and this tends to be the sink of P in this agro-ecological zone.The resin P fraction was initially low at the 0-20 cm depth but increased down the soil depth.The large amount of resin P observed would probably be due to past application of inorganic and organic fertilizers in the different soil types.Although, residual P was the least fraction observed, the sink of P has been found to be either more in the labile P fraction (Iyamuremye et al., 1996;Liliefein et al., 1999;Oberson et al., 1999) or in the stable fractions as acid extractable or even residual P (Linquist et al., 1997;Daroub et al., 2000).

Conclusion
Appropriate management of soil phosphorus fertility relies on sound knowledge about the phosphorus reserve and its bioavailabilty.Phosphorus availability to plants depends on both the inorganic and organic pools in the soil.However, in this study, it was discovered that the pattern of P distribution down the soil depth varied among the soil types in the agro-ecological zone.Resin P was the largest extractable P and therefore serves as the sink of P while residual P was the smallest P fraction.Variability was observed down the soil depth in all the soil series characterized.However, NaOH-Pi and Po was in abundance at the 0-20 cm depth while other P fractions were observed down the soil depth especially at the 40-80 cm depth.

Figure 2 .
Figure 2. Distribution of dilute acid P down the soil depth across the soil series

Figure 3 .
Figure 3. Distribution of conc.HCl Pi down the soil depth across the soil series

Figure 4 .
Figure 4. Distribution of NaOH-Pi down the soil depth across the soil series

Figure 5 .
Figure 5. Distribution of NaOH-Po down the soil depth across the soil series

Figure 6 .
Figure 6.Distribution of NaHCO 3 -Pi down the soil depth across the soil series

Figure 7 .
Figure 7. Distribution of NaHCO 3 -Po down the soil depth across the soil series

Figure 8 .
Figure 8. Distribution of residual P down the soil depth across the soil series

Figure 9 .
Figure 9. Distribution of resin P down the soil depth across the soil series

Table 1 .
Identified soil series in the agro-ecological zone and their FAO classifications