Phosphorus transformations in a toposequence of lithosols and cambisols from semi-arid northeastern brazil

Geoderma, 62 (1994) 345-362

345

Elsevier Science B.V., Amsterdam

Phosphorus transformations in a toposequence of Lithosols and Cambisols from semi-arid northeastern Brazil J.O. Agbenin" and H. Tiessenb "Department of Soil Science, Ahmadu Bello University, Zaria, Nigeria bDepartment of Soil Science, Universityof Saskatchewan, Saskatoon, Sask., Canada (Received January 29, 1993; accepted after revision June 23, 1993)

ABSTRACT Concepts of phosphorus transformations in soils and landscapes have largely been developed in temperate regions on moderate slopes. Little is known about the P dynamics and availability in semiarid tropical soils where rainfall extremes cause limited but periodically intensive leaching and erosion. We therefore studied the different forms of inorganic P (Pi) and organic P (Po) as related to mineralogy and landscape position in semi-arid northeastern Brazil. Samples were collected from a catena of Lithosols at upper, and Cambisols at mid and lower slopes. All soils were derived from syenite which had a high total P content near 5500 mg kg-m. Phosphaterich particles were identified by X-ray microprobe as fluor-apatite. The apatite contents of the A horizons were reduced in the weathering sequence from Lithosols to the older Cambisols, while the total P contents diminished to between 1200 and 2300 mg kg-~. The lower-slope soils had also lost two thirds of the Ca, Mg and Fe contents from sand and silt fractions. Chemical fractionation of P showed the lowest Ca-P contents in the C horizons of the Cambisols, indicating a maximum transformation of primary Ca-P in these lowermost horizons. Deposition of partly weathered materials from upper slopes explained some of the differences in the Ca-P contents between R and C horizons and the overlying solum at the mid and lower slope. Similarly, some Lithosols showed an exceptionally high Ca-P contents suggesting that these soils have been replenished with unweathered material from rock outcrops above. Microprobe analysis revealed P-rich silt particles containingFe, AI and Ca, which may be explained by the impregnation of primary Ca-P with Fe-oxyhydroxides. This absorption of Ire by mineral particles would represent a short-cut in the transformations of primary P to secondary, resistant P forms. The observation of Fe-oxyhydroxide coatings in feldspars from saprolite indicates that such transformations did occur and may have general importance where primary and highly weathered minerals are intimately mixed through erosion, limited leaching and high weathering intensities typical of semiarid tropical environments. Only 5% of the total P was found in organic forms, mostly in stable forms of low availabilitywith little contribution to P fertility.

INTRODUCTION

Phosphorus undergoes significant changes in form and lability during pedogenesis (Smeck, 1973; Walker and Syers, 1976). During early stages of soil 0016-7061/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved. SSDI 0 0 1 6 - 7 0 6 1 ( 9 3 ) E 0 0 8 2 - 7

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J.O. AGBEN1N AND H. TIESSEN

development, the weathering of apatites supplies inorganic P (Pi) to the soil solution. The released P may be taken up by plants and microbes, leached, or may form secondary P minerals by interacting with mineral surfaces and free cations such as Ca, Mg, Al or Fe depending on the pH of the soils. As basic cations are leached during pedogenesis, pH decreases and there is the relative accumulation of free Fe and AI oxides, which can combine with P to form AlP and Fe-P forms of low solubility. Smeck ( 1985 ) summarised the relationships between Pi fractions, weathering intensity and soil taxonomic groups. Weakly to moderately developed soils have high primary Ca-P contents, while secondary labile Pi and non-labile Pi dominate the Alfisols, Spodosols and Ultisols. Total P (PT) decreases from Entisols to Oxisols due to the effects of weathering and leaching. The decreases in PT during soil development indicate that P is mobile in soil profiles and landscapes. Although phosphate has a low solubility and strong affinity for soil mineral components, P readily moves within soil horizons (Tiessen et al., 1990), and significant amounts can be leached from the solum (Frossard et al., 1989) even in semi-arid climates. Differences in weathering found between soil classes are also reflected within soil profiles: primary Ca-P contents increase with depth, while the surface horizons have a higher secondary Pi contents, reflecting increased weathering at the soil surface (Walker and Syers, 1976). During the course of soil development organic P (Po) accumulates, and may range from 20 to 80% Of PT in A horizons. Often, the Po accumulation near the soil surface is accompanied by a net increase in PT as a result of the biological cycling of P through plant litter to the soil surface (Walbridge et al., 1991 ). At later stages of soil development, when Pi solubility is low, P availability may be largely controlled by Po mineralisation (Tiessen et al., 1984), and Po is considered important for P availability in many weathered soils. In addition to in situ transformations, P can also move across landscapes. Using a mass balance approach on a Prairie toposequence, Smeck and Runge ( 1971 ) showed that P lost from the upper slope profiles was gained by those in the lower slope. Such P redistribution in a landscape has been attributed to surface and subsurface flows (Ryden et al., 1973). Given the low solubility and rapid sorption of phosphate, long-distance subsurface flow of P is likely to occur largely as Po (Ryden et al., 1973 ). Surface runoff and erosion are likely to be more important for the redistribution of Pi in a landscape (Ward et al., 1990), particularly since erosion selectively removes silt and clay-size particles which contain much of the soils' P (Day et al., 1987 ). The study of P transformations in soils has been facilitated by sequential extractions which separate Pi and Po into major chemical groupings, and into pools of different bio-availability. Both the procedures described by Chang and Jackson (1957) and Hedley et al. (1982) chemically separate acid-extractable Ca- versus hydroxide-extractable Fe- and Al-associated Pi (Tiessen,

PHOSPHORUS ALONG A SEMI-ARIDTROPICAL CATENA

347

1991 ). The Ca-bound P fraction is minor in weathered soils of the humid tropics (Vieira, 1988) but its proportion may be greater on some parent materials (Lekwa and Whiteside, 1986) and in the semi-arid tropics (Goedert, 1983). Bio-availability is only approximated by "extractability", and the sequential extraction of Hedley et al. (1982) (Fig. 1 ) identifies resin and bicarbonate Pi as the most labile pools. The stability of the Po fractions (Fig. 1 ) is less clearly defined (Tiessen, 1991 ). Fractionation of 168 benchmark soils covering eight taxonomic orders, and path analysis of the relationships between labile and stable P forms indicated that Pi fractions were largely responsible for buffering labile P pools in weakly weathered soils, while in Ultisols, 80% of the variance in labile P was explained by Po (Tiessen et al., 1984). This confirmed the importance of Po mineralisation as a determinant for P availability in weathered soils (Adepetu and Corey, 1976). The utility of the Hedley fractionation for studying long-term transformations in tropical soils has been limited by the large proportions of Pi and Po that remain unextracted and are lumped in the residual fraction (Tiessen, oil

l

Residue

0.5 g Shake for 16 h in 30 mL deionized water with anion exchange resin membrane in chloride form. Centrifuge at 10 000 rpm determine Resin-Pi Shake for 16 h in 30 mL 0.5 M Na-bicarbonate. Centrifuge at 10 000 rpm y

Residue

determine Bicarb-Pi and Po

Shake for 16 h in 0.1 M NaOH. Centrifuge at 10 000 rpm determine NaOH-Pi and Po

Residue

Shake for 16 h in 1.0 M HC1. Centrifuge at 10 000 rpm

v

Residue

determine HC1-Pi

Heat in 10 mL concentrated HCI in waterbath for 10 min. at 82 deg.C, vortex, allow to stand for lh at room temp. Centrifuge at 10 000 rpm determine conc. HC1-Pi and Po

Residue

Digest in 5 mL sulfuric acid and hydrogen peroxide determine Residual P

Fig. 1. The sequential extraction o f phosphorus ( m o d i f i e d from Hedley et al. 1982).

348

J.O. AGBENIN AND H. TIESSEN

1991 ). Condron et al. (1990) extracted nearly all Po and Pi from tropical soils using hot concentrated HC1 (Anderson, 1960), and this extraction may provide a method to further characterise the residual P. The moderately to highly undulating landscapes of semi-arid northeastern Brazil, characterised by soils of varying degrees of weathering at different landscape positions, afford an opportunity to integrate the concepts of P transformations in hill slope soils. This also provides a test of these concepts for a semi-arid tropical region with a typical combination of low total rainfall and high rainfall intensities which can cause substantial erosion and periodic leaching. The objectives of this study were therefore to determine the size of labile and non-labile inorganic and organic P fractions on a catenary sequence of Lithosols to Cambisols from northeastern Brazil in relation to chemical composition and other soil properties. MATERIALS AND METHODS

Site selection and soil sampling The study site was at the Agricultural Research Institute (IPA) station at Serra Talhada, 450 m above sea level, between 39 ° and 38°W, and 7 ° and 8 °S in Pernambuco State of northeastern Brazil. The mean annual rainfall is 750 mm, and is concentrated between February and April with high rainfall intensities. August through October is the driest period of the year, normally with negligible precipitation. The potential evapotranspiration is calculated to be about 1400 ram, giving a substantial water deficit over the year. The actual evapotranspiration is about 600 mm, and therefore the average leaching potential is approximately 150 mm (750 minus 600) annually. The mean annual temperature is 25 °C with less than 5 °C range between the coolest and warmest months. The vegetation is a xerophilous caatinga consisting mainly of the Leguminoseae, Euphorbiaceae, Bromeliaceae and Cacti. On higher slopes, the caatinga forms a closed forest canopy that gradually thins into open wooded savanna on the lower slopes, due possibly to past slashing and burning activities. The soil parent material consists of peralkaline rocks which intruded into the Precambrian granite and granitic gneisses during the Brasiliano age (Ferreira, 1986). Three landscape positions (upper, middle and lower) which represented changes in geomorphology, topography and soil characteristics were selected for soil sampling. The upper slope of the toposequence is steep (>20%) with frequent rock outcrops and shallow (<50 cm) Lithosols (Entisols). The A horizons are mixed with and underlain by syenitic saprolite. Mid and lower slopes are weakly undulating with gradients of less than 5%, with deeper Cambisols (Inceptisols) showing distinct horizonation. There was little evidence of recent erosion, but deep soil pockets between rocks in-

PHOSPHORUSALONGA SEMI-ARIDTROPICALCATENA

349

dicated that soil creep, i.e. the slow down-slope movement of superficial soil or rock debris (Graham et al., 1990), may have influenced soil profiles. At each landscape position, three to four profiles were sampled by genetic horizons. The soils were air-dried, passed through a 2 mm sieve for physical analyses, and subsamples were ground to 0.5 mm for chemical analyses.

Soil analyses Particle size analysis was performed by the pipette method after pro-treatment with 35% H 2 0 2 and 1.0M HC1 to remove organic matter and carbonates. Sand and silt fractions were separated by sedimentation and centrifugation after ultrasonic dispersion at 300 W for 8 min in deionised water. The soil pH was determined in 1 : 1 soil: 0.01M CaCI2 suspension. The amount of organic C was determined by dry combustion (Tiessen etal., 1981 ). The effective cation exchange capacity was determined by the summation of basic and acidic exchangeable cations. The cations were displaced with 1.0M NH4C1. The Ca, Mg, A1, Fe and Mn in the extracts were determined by atomic absorption and Na and K by flame emission. Oxalate Fe and A1 (Feo and Alo) were extracted according to the method of McKeague and Day (1966). Dithionite-citrate Fe and A1 (Fed and Ah) were extracted by the method of Mehra and Jackson as described by McKeague et al. ( 1971 ). Selected properties of the main horizons of the soils are given in Table 1. Total elemental analyses of particle size fractions were determined on samples from representative (median) profiles by hydrofluoric acid digestion, and A1, Ca, Fe, K, Mg, Na, Si and Ti in the digests were determined in a directly coupled plasma spectrospan. TABLE1 Selected properties averaged across all horizons from the profiles at the different slope positions Soil properties

Upper slope (n=6)

Mid slope (n= 14)

Lower slope (n= 13)

Sand (g 100g - I ) Silt (g 100g -~ ) Clay (g 100g - l ) pH /do (gkg -1) Aid (skg -1) Feo (g k8 -~ )

72.3_+3.4 14.7-+2.3 13.0_+2.8 5.7_+0.3 1.7_+0.1 3.0_+0.6 5.1 _+1.4 32.0_+4.5 5.5-+2.5 5.5_+3.3

72.1 _+2.3 13.6-+ 1.0 14.3_+ 1.6 6.4_+0.2 1.2_+0.3 2.8_+0.3 3.1 +0.6 32.3_+3.3 4.5_+ 1.1 3.9_+2.9

74.7-+3.7 14.2-+2.6 11.1 _+1.5 5.8_+0.4 0.9_+0.2 2.4-+0.3 2.2 +0.3 26.7-+ 1.8 4.0_+0.8 2.9_+2.1

Fee (skg -I) ECEC (cmolc kg -~) Ors. C (gkg - t )

350

J.O. AGBENINAND H. TIESSEN

Fractionation of phosphorus Total phosphorus (PT) was determined by H2SO4-H202 digestion as described by Hedley et al. ( 1982 ) and also by sodium carbonate fusion (Syers et al., 1968), followed by colodmetry (Murphy and Riley, 1962). Total P recovered by fusion was consistently 95-100% of that determined on the acidperoxide digest. Total Po of the soils was determined by the method described by Bowman ( 1989 ) for weakly weathered soils, and found suitable for weathered tropical soils by Condron et al. (1990). The sequential extraction ofHedley et al. ( 1982 ) was modified to fractionate P (Fig. 1 ). In the modified sequence, the dilute acid extraction was followed by an additional hot (80°C) concentrated acid ( 11.3M HC1) extraction to remove the more chemically resistant Pi and Po fractions. This was necessary to further characterise the residual P which constitutes a substantial part of PT in tropical soils (Tiessen, 1991 ). The resulting sequence of NaOH and concentrated acid extractions is similar to the total Po extraction method of Mehta and Anderson as modified by Anderson (1960), and removes practically all Po and some chemically stable Pi (Condron et al., 1990). The residue from this treatment was digested with concentrated HESO4-H202to recover the unextractable, residual P fraction (Fig. 1 ). The sum of P fractions determined by the modified Hedley fractionation was compared with PT determined by NaECO3 fusion (Syers et al., 1968). A regression of the results obtained by the two methods showed a slope of 0.994 at an R2=0.996 ***, indicating that the soil PT was fully recovered by the fractionation. Phosphorus fractionation was performed on all horizons of all l0 profiles sampled, upon which all statistical analyses were based. Detailed fractions were tabulated for 4 representative profiles only, in order not to obscure relationships between horizons and profiles by reporting averages for slope positions. RESULTS AND DISCUSSION

Element concentrations in sand and silt The relative abundance of the elements in the sand and silt fractions followed the order Si > AI > K > Fe > Na > Ca > Mg (Table 2 ). Total K levels were high at all sites (Table 2), consistent with the abundance of K-feldspars in the sand and silt fractions of the soils (Agbenin, 1992). The uniform K distribution in sand and silt fractions along the slope suggests the common origin and uniformity of the parent and colluvial materials along the slope. This uniformity is further supported by the similarity of textures (Table 1 ), and by the similar Ti concentrations (Hutton, 1977) with soil depth at all sites which averaged 2.56+_0.26 nag Ti g-~ soil and 8.9+_0.4 mg Ti g-~ clay over

A A

A BA B C

5

Ap BA B1 B2 C

Lower slope

13

Mid slope

1A 1B

Upper slope

0- 15 15- 40 40- 85 85-152 152-200

0- 8 8 - 30 30- 60 60- 65

0-20 0-30

Soil Horizon Depth (cm)

5YR4/6 2.5YR4/6 2.5YR4/8 2.5YR4/6 5YR 5/6

5YR3/4 2.5YR3/6 2.5YR4/6 2.5YR4/6

5YR4/6 5YR 4/4

Colour Na20

62 62 63 63 60

63 63 63 64

63 60

16 15 17 17 17

16 17 16 16

16 14

2.8 2.6 2.9 3.0 3.5

2.8 2.7 2.3 2.3

3.2 4.5

0.2 0.2 0.2 0.2 0.2

0.6 0.5 0.5 0.4

0.9 2.8

0.30 0.23 0.32 0.30 0.13

0.70 0.55 0.62 0.72

1.23 0.25

13 13 12 12 13

13 14 13 12

14 11

0.72 0.61 0.63 0.71 0.57

0.76 0.72 0.74 0.77

1.2 1.2

60 60 59 61 59

61 62 61 61

61 62

16 17 16 17 16

15 16 15 15

14 12

6.2 6.5 6.2 5.4 12.7

6.9 8.0 6.4 6.2

8.8 10.7

A1203 Fe203

SiOz

CaO MgO K20

SiO2

A1203 Fe203

Silt fraction ( g 100 g- 1)

Sand fraction ( g 100 g- ~)

Elemental composition of sand and silt fractions at different landscape positions

TABLE 2

0.5 0.3 0.3 0.3 0.7

0.8 0.9 0.8 0.7

1.5 2.8

0.53 0.33 0.23 0.43 0.28

0.72 1.05 0.35 0.86

1.8 2.1

10.5 11.0 10.7 10.9 10.2

8.6 8.5 9.9 10.6

8.8 6.0

CaO MgO K20

0.98 1.32 0.57 0.61 0.42

0.81 0.84 0.62 0.80

1.21 0.89

Na20

L~

rn

2 >

z ~3 >

352

J.O. AGBENIN AND H. TIESSEN

Fig. 2. Photomicrographof a thin section through saprolite obtained from the bottom of lower slope profile 5. Note the dark Fe oxides in all fractures. The entire width of the plate corresponds to 0.6 mm. all profiles and horizons without discernible trends with depth or slope position. Properties of sand and silt fractions can be used to follow in situ mineral weathering since they are relatively immobile in soil profiles and their rates of weathering decrease with increasing particle size. There was a depletion of Ca in the sand, and of Ca and Mg in the silt at the mid and lower slope soils in comparison to the upper slope soils (Table 2 ), indicating increased weathering and leaching at the mid and lower slopes even in the C horizons. Total Fe was also more depleted in sand and silt fractions of the mid and lower slope soils than the upper slope soils, consistent with the significant down slope decrease in Feo and Fe~ (Table 1 ). In contrast, the Fe contents of the silt fraction in the C horizon in profile 5 at the lower slope were considerably higher, although the horizon colour was yellower (Table 2). This suggested an accumulation of Fe leached from upper horizons. Light microscopy of a thin section of a saprolite fragment from profile 5 showed feldspars that were heavily coated and impregnated with Fe oxides (Fig. 2), which were likely leached from the solum.

Total P and inorganic P fractions The total P contents in all soils were high (Table 3 ) compared to the mean PT contents reported for temperate or tropical soils (Larsen, 1967 ). The par-

Hor.

5

Ap BA BI B2 C

Lowerslope

13

A BA B C R

A

IB

Midslope

A R

1A

Upperslope

Profile No.

66 29 22 18 24

69 73 60 53 57

66

61 89

40 31 32 33 32

46 48 46 44 12

52

53 28

191 220 209 193 170

302 328 358 352 28

270

376 115

260 190 140 93 67

850 970 670 540 5340

4880

1560 5120

HCI

760 750 790 780 2000

760 810 900 1100 200

720

800 150

C.HC1

53 39 46 24 29

96 90 104 125 17

22

51 13

Resid. Pi

1400 1300 1200 1100 2300

2100 2300 2100 2200 5600

6000

2900 5500

~Pi

6.3 6.1 4.9 1.7 1.9

2.3 19 0.1 7.3 0.0

13

0.3 0.0

51 43 9 2 5

53 56 18 15 0

48

232 0

OH

HCO 3

OH

Resin

HCO 3

Po fractions

Pi fractions

Typical examples of the distribution of P T and P fractions in soil profiles (values in mg kg- ~)

TABLE 3

41 31 36 32 21

171 12 36 8 0

49

86 0

C.HCI

98 80 49 35 28

226 86 55 31 0

110

318 0

YPo

1460 1330 1280 1170 2340

54 28 34 36 33

36 55 63 49 0

190

6120

2340 2400 2200 2210 5650

26 0

C:Po ratio

3090 5510

PT

tJ~

0

>

354

J.O. AGBENINAND H. TIESSEN

ent saprolite (R horizons) of the lithosols contained 5500 nag P kg -l, while the P contents of the A horizons ranged between 3000 and 6000 mg kg- ~. The total P contents of these upper-slope soils were highly variable, which reflects the complexity of the landscape with its rock outcrops, deep soil pockets, and highly variable leaching and weathering losses. At the mid slope, PT concentrations in the solum varied between 2200 and 2400 mg kg- 1, and at the lower slope between 1200 and 2300 mg kg- ~. An analysis of variance of P contents of similar horizons along the slope was used to assess the role of topography in the redistribution of the main forms of P in the landscape (Table 4). The PT and the sum of Pi fractions were significantly higher in the A horizons of the upper slope soils than in the mid and lower slope soils. Similarly, PT and the sum of inorganic P fractions were significantly higher in the B horizons of the mid slope soils than the lower slope, but the differences in the amounts of P in the C horizons between the mid and lower slope soils were not significant. The decreasing amounts of Pi from the upper to the lower slope indicate that the total P losses from the sola of mid and lower slope positions were greater than the gains by coUuviation from upslope. Absolute gains by the lower slope, as described by Smeck ( 1985 ), were not observed. Three factors may explain the decreasing amounts of P downslope: ( 1 ) the mid and lower slopes are formed from pre-weathered colluvial deposits with a lower P content from the upper slope; the P weathered out would have remained at the TABLE 4

Analysis of variance for the effect of landscape position on the distribution of P in main soil horizons Slope position

YPi mg kg- mb

~Po mg kg- ~

PTa mg kg-

3875 + 1567 1817 + 258 1221 + 135 1874

178 + 103 148 + 81 104+ 10 145

4059 + 1493 1946 + 296 1325 +- 127 1793

1834+_ 313 1164 + 91 296

81 _+21 80 +-22 28

1913 +_310 1243 _+95 294

1683 + 286 1368 + 629 738

52 + 20 56 _+29 38

1723 _+280 1423 + 609 716

A horizon

Upper slope Mid slope Lower slope LSD ( P = 0 . 0 5 ) B horizon c

Mid slope Lower slope LSD ( P = 0 . 0 5 ) C horizon c

Mid slope Lower slope LSD ( P = 0 . 0 5 )

aSeparately determined by H2SO4-H202 digestion. bWeighted mean (mass of P divided by the mass of soils in the horizon ). CB horizon includes BA subhorizon, and C horizon includes BC subhorizon.

PHOSPHORUS ALONG A SEMI-ARID TROPICAL CATENA

355

upper slope, and might account for the slightly higher P contents of the remaining topsoils than the parent rock (Table 3, profile 1B); (2) higher weathering and leaching intensity at the mid and lower slope positions have reduced the P contents in situ, but no accumulation zone has been observed within the depth sampled; (3) the upper slope soils are continuously rejuvenated by the addition of P-rich colluvial material and slope wash from the steep rocky slopes above. The elemental composition of the sand and silt fractions provided evidence for leaching losses of Ca, Mg and Fe in the mid and lower slope soils in comparison to the upper slope (Table 2). These are the main elements to which P is bound (Ryden et al., 1973 ), and their losses are likely to be accompanied by P losses. The net losses of several elements including P from the sola of the lower slopes indicate that processes of leaching and transformations must have occurred below the soil profile. This is confirmed by the weathering and Fe deposition seen in the saprolite sampled below the lower slope profiles (Fig. 2 ). Such deep weathering has been described for soils of the humid part of northeastern Brazil (Tricart, 1972 ). Calcium phosphate levels extractable with dilute HC1 were higher in these soils than in most tropical soils (Lekwa and Whiteside, 1986; Vieira, 1988 ) (Tables 3 and 5 ), but values were consistent with the results from other semiarid soils in Pernambuco (Naercio, 1981 ). The relatively high Ca-P levels are due to the semi-arid climate and potential rejuvenation from unstable slopes, which result in much less weathered soils than the "typical" tropical Ferralsols (Oxisols and Ultisols). Calcium phosphate in the A horizons decreased from the upper to the lower slope, showing the expected reduction of Ca-P contents from the younger Lithosols to the older Cambisols. This resulted in a greater proportion of non-labile Pi in the mid and lower slope A horizons (Table 5), although no net increases in these fractions were observed (Table 3 ). Within profiles, the distribution of P fractions did not conform to the expected trends discussed in the Introduction (Smeck, 1973, 1985 ). To facilitate the discussion, P fractions were grouped into Po, primary Ca-Pi, labile Pi and non-labile Pi, and the relative percentages of the conceptual P pools of the A, B and C horizons of the soils were computed (Table 5 ). The A and C horizons usually are respectively the most and least weathered of a soil (Jackson and Sherman, 1953 ). Therefore, primary Ca-P contents are expected to decrease from the C to the A horizon. In contrast, the C horizons of the mid and lower slopes showed a marked decline in the quantity and proportion of Ca-P (Tables 3 and 5 ) relative not only to the upper slope profiles but also to the overlying A horizons, with B horizons showing intermediate values. Sudden changes in Ca-P with depth may be due to lithological discontinuity (Meixner and Singer, 1985 ), but in the present study, the gradual changes, and textural, chemical and mineralogical properties (Agbenin, 1992) point to different causes.

356

J.O. AGBENINAND H. TIESSEN

TABLE 5 Conceptual P pools in soil horizons at different landscape positions. Values (% of Pr) are means for each landscape position Landscape position

Labile Pi (Resin + H C O 3 + O H - P i )

Ca-Pi (HC1)

Non-labile Pi (conc. H C l + Resid.)

~Po fractions

13+5 24±3 22 _+1

60±18 24+15 17 _+3

23±9 45+10 54 ± 4

4±4 7+1 8+ 2

22+2 20+1

17+ 12 10+3

57+ 11 65+4

4+ 1 6+2

21 + 3 13+6

8 + 11 6+ 1

67 ± 11 78+ 11

3± 1 3+5

A Horizon

Upper slope Mid slope Lower slope B Horizon

Midslope Lower slope C Horizon

Mid slope Lower slope

A possible cause for the high Ca-P contents of the lower-slope top soils is the past deposition of partly weathered or unweathered materials derived from the same parent material from higher elevations. If the time since deposition was insufficient for significant weathering of Ca-P to occur, the contents would still be high (Meixner and Singer, 1985 ). The large differences in Ca-P contents between the C horizon and the underlying saprolite of profile 13 (Table 3 ), support this theory if most of that profile was formed from colluvial material. By a similar process, profile 1B, which shows an exceptionally high CaP content even for the upper slope, may have received Ca-P rich unweathered material from steep slopes and rock outcrops above, or from overlying materials that have now eroded away. An additional explanation for the low Ca-P contents in the subsoil is given by the increase in hot concentrated HC1 extractable Pi in the C horizons of profiles 13 and 5. The concentrated HCI-Pi fraction is part of the relatively stable secondary Pi (Tiessen, 1991 ) which may represent an end-product of pedogenic P transformations (Tiessen and Stewart, 1984). Accordingly, the concentrated HC1-Pi concentrations were low at the upper slope, and the relative contents increased down-slope, until it constituted the dominant Pi fraction in the more weathered mid and lower slope soils. Contrary to expectations from normal weathering concepts the concentrated HC1-Pi contents increased with depth, reaching between 58 and 85% of PT in the C horizons of the mid and lower slope profiles (Table 3 ). This suggests that a transformation of dilute acid-extractable primary Ca-P minerals to more resistant Pi fractions, extractable only in hot concentrated HC1, has occurred in lower horizons which have not been exposed to intensive weathering. The uniform

PHOSPHORUS ALONG A SEMI-ARID TROPICAL CATENA

3 57

sandy texture of all soils with depth ( < 4% variation in sand content, Table 1 ) and the moisture surplus during the rainy seasons (several rains > 50 mm expected during each season) make it unlikely that B and C horizons are exposed to greater weathering than the top soil. It is therefore probable that the normal process of dissolution of Ca-P and formation of secondary and resistant Pi forms has not occurred to a greater extent in the subsoil, and that the formation of hot concentrated HC1 extractable Pi in the subsoil at the expense of the dilute HC1 extractable Ca-P fraction (Table 3, profiles 5 and 13 ) represents a short-cut in the transformations of primary to secondary, resistant P forms. The Fe required for this process is released in the early stages of weathering of the amphibole-rich parent material (M.C.D. Santos, pers. commun., 1992). The impregnation and coating of feldspars with Fe-oxyhydroxides observed in the saprolite from below profile 5 (Fig. 2), suggests that a similar process might have occurred with apatite particles, changing their composition and extractability. The potential role of free Fe (and A1) in the formation of concentrated HC1-Pi is supported by a highly significant correlation of concentrated HC1-Pi with Fed (rs=0.59**) and Ald (rs= 0.60"*), and further corroborated by Fe leaching and accumulation in the C horizon in profile 5 (Table 2). Scanning electron microscopy and X-ray microprobe analysis showed pristine, crystalline, silt-sized fluor-apatite particles obtained from the upper slope Lithosols (Fig. 3A). Phosphate-rich silt particles from the C horizon of lower slopes were rounded, irregularly weathered (Fig. 3B) and contained significant amounts of Fe, A1 and Ca. In the absence of Fe nodules in these profiles, which would sorb and accumulate P, it is probable that these particles were formed by in situ modification of primary phosphates. The concentrated HC1-Pi fraction may be considered inactive because of its low solubility, and therefore it does not readily contribute to P availability. Despite the high contents in stable Pi, labile Pi (resin and HCOa-Pi) pools were also large, accounting for between 2 and 5% of PT, and decreasing only slightly with depth towards the saprolite (Table 3 ). The OH-Pi contents were fairy uniformly distributed in the profiles, but decreased from the upper to the lower slope positions. The proportion of OH-Pi ranged from 5 to 20% of soil PT. In temperate soils (Tiessen et al., 1984), only resin- and HCO3-Pi are taken to be labile Pi while OH-Pi is more resistant to transformation. In the tropical soils studied here, HCO3-Pi and OH-Pi together explained 51% of the variance in resin Pi in a stepwise regression, indicating that both HCO3and OH-Pi participate in the maintenance of the plant available Pi pool. A summation of resin-, HCO3- and OH-Pi as the "labile Pi pool" may therefore be appropriate, while only the concentrated HC1-Pi and the residual P were designated as non-labile.

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Fig. 3. Backscattered electron image of phosphate rich particles from (A) the A horizon of a Lithosol and (B) the C horizon ofa Cambisol. White P-rich particles are marked with a P. The entire width of the plate corresponds to 310/~m.

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Organic P The sum of the three Po fractions from the modified Hedley fractionation were highly correlated (R2=0.92 ***) with total Po determined by the method of Bowman (1989). A tendency for high recoveries by summation at low total Po and low recoveries at high total Po as determined by Bowmans method reduced the slope of the regression to 0.64. Walker and Syers (1976) showed that the build up of organic matter and Po in soils is influenced by the amount of available Pi. We observed a weak correlation between PT and organic carbon (rs = 0.49"), and amounts of Po were low, accounting for less than 5% of PT (Table 5 ), despite the high levels of labile Pi. The low amounts of organic matter and Po are likely to be due to moisture deficits which limit the productivity and organic matter accumulation in this semi-arid ecosystem. The uniformity in the PT distribution with depth in these profiles indicated that the biocycling of P was negligible in comparison to the total soil P stores. The HCO3-Po fraction accounted for only a small fraction of the total Po, and Po was predominantly present in resistant fractions (0.1M NaOH-Po and concentrated HC1-Po), i.e. in humified organo-mineral complexes. This stabilization might explain the persistence of Po in materials with C: Po ratios of less than 30, despite suggestions that C:Po ratios less than 200 are an indication of potential Po mineralisability (Lekwa and Whiteside, 1986). In contrast to other, highly weathered tropical soils, P availability at this site is largely controlled by Pi transformations. CONCLUSIONS

The results presented here indicated that the phosphorus distribution and transformation pathways within the profiles and along the slope of a semiarid tropical landscape are more complex than soil-P-landscape relationships in temperate landscapes (Smeck, 1973). Observed down-slope decreases in P content contrast with down-slope increases of P in temperate landscapes reported by Smeck ( 1973, 1985). This can partly be attributed to a methodological problem, since in these tropical soils, the C horizon may not be the limit of weathering with substantial transformations of the underlying saprolite indicating that "soil" processes progressed well below the recognisable profile. Phosphate losses that occurred from the mid and lower slope soils, could not be accounted for within any of the profiles, but no lower limit for potential leaching and weathering could be defined in the profile and parent material. These semi-arid tropical soils are characterised by substantial weathering, seasonal leaching and potentially important down-slope movements. As a resuit, both highly weathered and unweathered soil components co-exist in any

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one profile which make the interpretation of soil mineralogy and chemistry, and the resulting P transformations difficult.The mixing of old and young materials could also provide for unusual transformations, not envisaged in conventional models of weathering sequences. As an example, there may have bccn a directtransformation of Ca-P minerals to more resistantforms through absorption of Fe-oxyhydroxides, facilitatedby the mixing of Fc-rich materialswith primary Ca-P minerals in the C horizons of these soils. The high proportion of non-labile Pi at the lower slope positions implies, based on the conceptual relationship between P forms, soiltypes and weathering intensity, that these soils arc weathered to a stage comparable to the Alfisols or Ultisols described by Smeck (1985 ). But at the same time, total amounts of non-labile P were no higher at the lower than at thc mid slope positions, labile P was abundant, and even primary minerals co-existed with weathered materials. The example of these P transformations indicated that the complex processes occurring in tropical landscapes with steep slopes where leaching and erosion intensities may bc high arc difficult to describe adequately with concepts of soil development and mineral transformations dcvcloped on temperate soils. ACKNOWLEDGEMENTS

The authors express their gratitude to M.C.D. Santos and M.R. Ribeiro for their guidance in the field.This study was facilitatedby Canadian N S E R C Grant No. O G P 2274 and C I D A project No. 01842 S 23239.

REFERENCES

Adepetu, J.A. and Corey, R.B., 1976. Organic phosphorus as a predictorof plant availablephosphorus in soilsof southern Nigeria. Soil Sci., 122: 159-164. Agbenin, J.O., 1992. Phosphorus fractions,mineralogy and transformation in soilsof two catcnary sequences from northeastBrazil.Ph.D. thesis,Department of SoilScience,Univ. of Saskatchewan, Saskatoon. Anderson, G., 1960. Factors affectingthe estimation of phosphate estersin soil.J. Sci. Food Agric., 9: 497-503. Bowman, R.A., 1989. A sequential extractionprocedure with concentrated sulphuricacid and dilutebase for soilorganic phosphorus. Soil Sci. SOc. Am. J., 53: 362-366. Chang, S.C. and Jackson, M.L., 1957. Fractionationof soilphosphorus. Soil Sci.,59: 39-45. Condron, L.M., Moir, J.O., Tiessen, H. and Stewart,J.W.B., 1990. Criticalevaluation of rncthods for determining organic phosphorus in tropicalsoils.Soil Sci. Soc. Am. J., 54: 12611266. Day, L.D., Collins,M.E. and Washer, N.E., 198i7. Landscape positionand particlesizeeffects on soilphosphorus distribution.Soil Sci. Soc. Am. J., 5 I: 1547-I 553. Ferreira,V.P., 1986. Petrologia e geoquimica de rochas peralinas do cinturo de dobrarncntos Cachocirinha-Saigueiro, Nordeste do Brasil.Dissertationpresented to the Centro dc Tccnologia da Universidade Federal de Pernambuco, 177 pp.

PHOSPHORUS ALONG A SEMI-ARID TROPICAL CATENA

361

Frossard, E., Stewart, J.W.B. and St. Arnaud, R.J., 1989. Distribution and mobility of phosphorus in grassland and forest soils of Saskatchewan. Can. J. Soil Sci., 69:401-416. Goedert, W.J., 1983. Avaliaqao de efeito de fosfatos naturals em solos de Cerrado. Pesq. Agropec. Bras., 18: 499-506. Graham, R.C., Daniels, R.B. and Buol, S.W., 1990. Soil geomorphic relations in the Blue Ridge Front. 1. Regolith and slope processes. Soil Sci. Soc. Am. J., 54:1362-1367. Hedley, M.J., Stewart, J.W.B. and Chauhan, B.S., 1982. Changes in inorganic and organic soil phosphorus fractions induced by cultivation and by laboratory incubations. Soil Sci. Soc. Am. J., 46: 970-976. Hutton, J.T., 1977. Titanium and zirconium minerals. In: J.B. Dixon and S.B. Weed (Editors), Minerals in Soil Environments. SSSA Publ., Madison, WI, pp. 673-688. Jackson, M.L and Sherman, G.D., 1953. Chemical weathering of minerals in soils. Adv. Agron., 5: 219-318. Larsen, S., 1967. Soil phosphorus. Adv. Agron., 19:151-206. Lekwa, G. and Whiteside, E.P., 1986. Coastal Plain soils of southeastern Nigeria. I I. Forms of extractable Fe, A1 and phosphorus. Soil Sci. Soc. Am. J., 50: 160-166. McKeague, J.A. and Day, J.H., 1966. Dithionite and extractable iron and aluminum as aids in differentiating various classes of soils. Can. J. Soil Sci., 46: 13-32. McKeague, J.A., Brydon, J.E. and Miles, N.M., 1971. Differentiation of forms of extractable iron and aluminum in soils. Soil Sci. Soc. Am. Proc., 35: 33-38. Meixner, R.E. and Singer, i . J . , 1985. Phosphate fractions from a chronosequence of alluvial soils, San Joaquin Valley, California. Soil Sci., 139: 37-46. Murphy, J. and Riley, J.P., 1962. A modified single solution for the determination of phosphorus in natural waters. Anal. Chim. Acta, 27: 31-36. Naercio, J.L., 1981. Movimentacao e formas de fosforo em solos de Tejucupapo-Pernambuco. M.Sc. thesis, UFRPE, Recife. Ryden, J.C., Syers, J.K. and Harris, R.F., 1973. Phosphorus in run-off and streams. Adv. Agron., 25: 1-45. Smeck, N.E., 1973. Phosphorus: an indicator of pedogenetic weathering process. Soil Sci., 115: 199-206. Smeck, N.E., 1985. Phosphorus dynamics in soils and landscapes. Geoderma, 36: 185-189. Smeck, N.E. and Runge, E.C.A., 1971. Phosphorus availability and redistribution in relation to soil profile development in an Illinois landscape segment. Soil Sci. Soc. Am. Proc., 35: 952959. Syers, J.K, Williams, J.D.H. and Walker, T.W., 1968. The determination of total phosphorus in soils and parent material. New Zealand J. Agric. Res., 11: 757-762. Tiessen, H., 1991. Characterization of soil phosphorus and its availability in different ecosystems. Trends in Soil Science Council for Scientific Research Integration (Trivandrum, India): 1: 83-99. Tiessen, H. and Stewart, J.W.B., 1984. The biogeochemistry of soil phosphorus. In: D.E. Caldwell, J.A. Brierley and C.A. Brierley (Editors), Planetary Ecology: Selected Papers from the 6th Int. Symp. on Environmental Biogeochemistry, Van Nostrand Reinhold, New York, Ch. 39, pp. 463-472. Tiessen, H., Stewart, J.W.B. and Cole, C.V., 1984. Pathways of phosphorus transformation in soils of differing pedogenesis. Soil Sci. Soc. Am. J., 48: 853-858. Tiessen, H., Bettany, J.R. and Stewart, J.W.B., 1981. An improved method for the determination of carbon in soils and soil extracts. Comm. Soil Sci. Plant Anal., 12:211-218. Tiessen, H., Frossard, E., Mermut, A.R. and Nyamekye, A.L., 1990. Phosphorus sorption, and properties of ferruginous nodules from semiarid soils from Ghana and Brazil. Geoderma, 48: 373-390. Tricart, J., 1972. The landforms of the humid tropics, forests and savannas. Longmans, London.

362

J.O. AGBENIN AND H, TIESSEN

Vieira,L.S., 1988. Forms of phosphorus in Amazonian soils.Bol. Fac. Cicnc. Agrarias do Para, BrazilNo. 17. Walbridge, M.R., Richardson, C.J. and Swank, W.T., 199 I. Verticaldistributionof biological and geochemical phosphorus subcyclcs in two southern Appalchian forest soils.Biogcochemistry, 13: 61-85. Walker, W. and Sycrs, J.K., 1976. The fate of phosphorus during pedogcncsis. (}codcrma, 15: 1-19. Ward, J.C.,O'Connor, K.F. and (}an Wei-Bin, 1990. Phosphorus lossesthrough transfer,runoff,and soilerosion.In: Phosphorus Rcquiremcnts for SustainableAgriculturein Asia and Oceania. Int.Rice Res. Inst.,Los Bafios,Philippines,pp. 167-182. Williams, J.D.H., Mayer, T. and Nriagu, J.O., 1980. Extractabilityof phosphorus from phosphatc minerals c o m m o n in soilsand sediments. SoilSci.Soc. Am. J.,44: 462-465.