environmental influences on soil chemistry in central semiarid tanzania

DIVISION S-5-SOIL GENESIS, MORPHOLOGY,& CLASSIFICATION

Environmental Influences on Soil Chemistry in Central Semiarid TanzaniaJulia Alien Jones*

ABSTRACTThis study assessed the effectiveness of landscape features for

predicting the variation in a set of chemical properties from 81 soilprofiles sampled in a 236-km2 site in central Tanzania. Soil sampleswere treated as points and coded by horizon and by groups, includinglandform, parent material, vegetation type, vegetation percent cover,and presence or absence of hypogeal termites. The variation cap-tured by these groups in eight soil chemical properties (organic C,total N, acid-extractable P, exchangeable cations, and extractableAl) was assessed using analysis of variance, Duncan's multiplerange test, and discriminant analysis. Soil development fits the ca-tena model along hillslope transects, but lateral subsurface transportof dissolved Fe and cations has produced plinthite and highly base-saturated horizons just above the lithic or paralithic contact down-slope of transitions between ferruginous sediments and granitic sed-iments. Nevertheless, biotic factors (vegetation type and density oftermites) captured more variation than other groups at the regionallevel. Vegetation species composition and the density and type oftermites in central Tanzania reflect soil depth, texture, clay miner-alogy, and drainage characteristics influenced by landform and par-ent material. Root symbioses of native miombo and Acacia spp.woodlands may also modify local soil chemistry via selective uptakeof C arid exchangeable cations. Termites contribute to the depletionof organic C, total N, and associated nutrients, throughout the soilsthey occupy in this region.

SOIL SURVEYS provide basic information on soil fer-tility for agricultural planning at the regional level

in the semiarid tropics. Although a wide range of land-scape features could be used to design soil map unitsfor sampling of representative peddns, the compara-tive effectiveness with which different landscape fea-tures capture variation in soil chemistry has not beentested at the regional scale in the dry tropics. In Tan-zania, Milne's catena concept (Milne, 1935), originallyintended as a description of soil sequences on a hill-slope, has been extrapolated as the basis for map unitson a regional or national scale (Hathout, 1972; Scott,1972). Other soil maps of Tanzania (Baker, 1970)have been based on the land systems concept, treatingsoils and slopes as integral units (Moss, 1968). How-ever, sources of landscape information such as geo-logic maps or remotely sensed imagery are availableas a basis for soil map unit generalization in centralTanzania. These provide information on vegetationtype and percent cover, and even on the distributionand density of termite mounds. On the basis of fieldand laboratory observations from a detailed soil sur-Dept. of Geography, Univ. of California, Santa Barbara, CA 93106.Contribution from the Dep. of Geography and the EnvironmentalStudies Program, Univ. of California at Santa Barbara/Received 5Feb. 1988. 'Corresponding author.

Published in Soil Sci. Soc. Am. J. 53:1748-1758 (1989).

vey of a 236 km2 portion of central Tanzania (Jones,1989, unpublished data), it was hypothesized that veg-etation type and termite-mound occurrence incorpo-rate the most information about pedogenesis in cen-tral Tanzania, and would therefore be most closelyrelated to soil variability. This hypothesis was testedby comparing the variation in soil chemical data fromthe soil survey, captured by a series of groups basedon hillslope position, landform, parent material, veg-etation, and termite activity.

MATERIALS AND METHODSStudy Site

The study site has a topography, geologic history, climate,vegetation, and biota similar to much of East and CentralAfrica. The landscape consists of steep mountains rising outof a gently sloping, soil-mantled pediment (Fig. 1). Complexpedogenetic factors have influenced soils in the 236 km2

study site, which is on the "immense pedimented land-scape" of the central plateau of Tanzania (King, 1962). Theplateau has been subject to repeated cycles of erosion sinceplanation in the early Tertiary period, and the study sitenow lies at the extreme southeast corner of the dischargelessEast African Rift basin (King, 1962). The underlying rockconsists of the Dodoman system: a belt of schist, ferruginousquartzites, amphibolites, and hornblende gneisses much in-truded by granite and pegmatite, and dated at older than 2.5billion yr (Cahen and Snelling, 1984). A wide variety of par-ent materials (Fig. 2) (Government of Tanzania, 1963, 1964)have been warped and tilted to form an irregular basin some20 km southeast of the site (Wade and Gates, 1938). Local-ized faulting follows the preexisting northeast trend of thecountry rock (King, 1962). Although underlain by Precam-brian rocks, the soils of the study site Have formed on veryyoung sediments (Wade and Gates, 1938), which are beingcontinually reworked by faulting and uplift associated withthe East African Rift.

This complex landscape history is compounded by evi-dence of change since 12 000 yr BP toward a warmer, drierclimate (Livingstone and Van der Hammen, 1978). Presentclimate is characterized by mean annual precipitation ofabout 550 mm and mean monthly temperatures rangingfrom 20 to 25 °C. The soil moisture regime is ustic and thesoil temperature regime is isohyperthermic (Van Wambeke,1982).

The natural vegetation of the study site includes three dry-deciduous formations: miombo woodland, dominated byBrachystegia spiciformis (Benth.) and Julbernardia globi-flora (Benth.) Troupin; Commiphora woodland dominatedby Commiphora spp., and Acacia wooded bushland domi-nated by Acacia spp., especially Acacia tortilis (Forssk.)Hayne subsp. spirocarpa (Hochst. ex A. Rich.) Brenan andCombretum apiculatum Sond. (Jones, 1989, unpublisheddata). Miombo woodland extends from Kenya to Zambia,and wooded bushland dominated by Acacia and Combre-tum spp. ranges from northern Ethiopia to Zimbabwe(White, 1983).

1748

JONES: ENVIRONMENTAL INFLUENCES ON SOIL CHEMISTRY 1749

Fig. 1. Study site location, topography, and location of pedons. Dotted lines indicate elevation in m, solid lines indicate boundaries of soilunits defined by landform, parent material, vegetation, and presence or absence of termites. A = Lithic Ustropepts on summits, backslopes,and toeslopes of granite or gneiss mountains with miombo woodland and no termites. B = Lithic Ustropepts on summits, backslopes, andtoeslopes of amphibole schist or ferruginous quartzite schist mountains with Commiphora woodland and no termites. C = Oxic Ustropeptsand Ustoxic Dystropepts on red sediments on the upper pediment, with Acacia wooded bushland and cultivated fields, no Macrotermitinae,but other termites. D = Ustic Dystropepts, Ustoxic Dystropepts, Typic Ustipsamments, Ultic Paleustalfs, Plinthustalfs, and Typic Ha-plustalfs on reworked granite sediments on the upper and lower pediment, with Acacia wooded bushland and cultivated fields, and Ma-crotermitinae and other termites. E = Fluventic Ustropepts and Fluventic Dystropepts on granitic sediments in narrow floodplains andchannels of nonperennial streams, with riparian woodland and cultivated fields, and no termites. Unit E occupies only the upper segmentsof drainages shown in Fig. 2. Pedons marked with a "p" have one or more layers in which plinthite occupies >50% of the matrix.

1750 SOIL SCI. SOC. AM. J., VOL. 53, NOVEMBER-DECEMBER 1989

Reworked Granitic Sediments

Red Sediments

Amphibole Schist

Ferruginous Ouartzite, SchistMicaceous Quartzite, SchistBiotitlc Quartzo-Feldspathic Gneiss, w/ Amphibolite

Synorogenic Granite

Basic, Ultrabasic Intrusive (w/ Soapstone & Talc Schist)

AmphiboliteQuartz Tourmaline-Serlcite Schist

Inclined Foliation

Vertical FoliationObserved Faults

_ Inferred Faults

— — Concealed FaultsSeasonal Streams

Fig. 2. Study site geology. Source: Government of Tanzania, 1963, 1964.

Much of the site is occupied by Macrotermitinae or othertermites, with up to 200 large termite mounds km-2 (Jones,1989). Soils at the site are Lithic Ustropepts, Lithic Dystrop-epts, Oxic Ustropepts, Ustoxic Dystropepts, Ustic Dystrop-epts, Typic Ustipsamments, Ultic Paleustalfs, Plinthustalfs,Fluventic Ustropepts, Ruventic Dystropepts, and Typic Ha-plustalfs (Jones, 1989, unpublished data).Experimental Design

Nine groups of landscape and soil features observablefrom maps, aerial photographs, and satellite imagery wereconstructed and compared to determine which group cap-tured the most variation in soil chemical properties. Eightof the groups were based on landform (LF), hillslope posi-tion (HP), parent material (PM), vegetation type (VT), veg-

etation cover (VC), termite occurrence and density (T), land-form and parent material (LPM), and termite occurrenceand vegetation type (TVT). A ninth group, based on soilparticle size and mineralogy differentiae (PSM), capturedmost of the variation among the 11 family-level taxonomicclasses in the data set (Jones, 1989, unpublished data). Fig-ure 3 specifies the hypothesized causal links between thesegroups and soil chemistry. Because of pedogenic processes,all of these groups are interdependent, rather than indepen-dent. This means that causal relationships implied by thestatistical comparisons among groups must be substantiatedby examination of all interrelated factors shown in Fig. 3.

Each group is composed of classes. The three LF classesare: mountains, pediment, and channels. The seven HPclasses are: summit, shoulder, backslope, toeslope (on the


soil depth

soil chemistryFig. 3. Schematic diagram of the relationships tested between groups

and soil chemistry, and the interrelation among groups.

mountains) and upper pediment, lower pediment, and nar-row floodplains and channels (on the warped and tilted pe-diment), following Ruhe (1975) and Gerrard (1981). Thepediment is an extremely long, flat planar surface, lackingsummits or backslopes, so the HP and LF groups are some-what redundant. The six PM classes are: synorogenic gran-ite, amphibole schist, ferruginous quartzite schist and mi-caceous quartzite, gneiss, red sediments, and reworkedgranitic sediments, following terminology in Government ofTanzania (1963, 1964). The five VT classes are: miombowoodland, Commiphora woodland, Acacia wooded bush-land, riparian woodland, and cultivated fields, determinedfrom field sampling (Goldschmidt and Jones, 1989). Thefour VC classes are: 0 to 10, 11 to 40, 41 to 80, and 81 to100%, photointerpreted from 1978 aerial photographs andverified during field sampling (Goldschmidt and Jones,

1989). The three T classes are: without termites, withoutMacrotermitinae (i.e., fungus-growing, mound-building ter-mites of Macrotermes and Odontotermes spp.) but withother termites (i.e., Hodotermes, Amitermes, and Microcer-otermes spp.), and with Macrotermitinae and other termites.Areas with termite mounds were photointerpreted followingHoward (1959), and termite species collected during fieldsampling were identified by J.P.E.C. Darlington. The sixLPM classes are: granite and gneiss mountains, amphiboleschist and ferruginous quartzite schist mountains, red sed-iments on the upper pediment (>1090 m), reworked gran-itic sediments on the upper pediment, reworked granitic sed-iments on the lower pediment (<1090 m), and graniticsediments in floodplains and stream channels. The sevenTVT classes are: miombo woodland with no termites, Com-miphora woodland with no termites, riparian woodlandwith no termites, Acacia wooded bushland with no termites,Acacia wooded bushland with no Macrotermitinae but withother termites, miombo woodland with Macrotermitinaeand other termites, and Acacia wooded bushland with Ma-crotermitinae and other termites. The spatial distribution ofthe factors used to define the nine groups is shown in Fig.1 and 2, along with the pedon locations. To provide a basisfor comparing these groups with taxonomic classes, fivePSM classes were designated based on family differentiae ofthe soils: loamy-skeletal, mixed; coarse-loamy, mixed; fine-loamy, mixed; mixed (Psamments); and fine, kaolinitic(Jones, 1989, unpublished data).

Statistical MethodsData on eight soil chemical properties from 81 pedons

(211 samples) representing 11 taxonomic family-levelclasses (Jones, 1989, unpublished data) were subjected toanalysis of variance (ANOVA), Duncan's multiple range test(DMR), and discriminant analysis. The properties were or-ganic C, total N, acid-extractable P, exchangeable cations,and extractable Al (Jones, 1989, unpublished data). The AN-OVA was supplemented by DMR, which is a significancetest for differences between each pair of class means (Alderand Roessler, 1977; Snedecor and Cochrane, 1967). How-ever, DMR is only applicable to a single soil property at onetime, whereas soil chemical properties, particularly organicC, total N, acid-extractable P, and exchangeable cations inthis study are strongly correlated with each other. Therefore,discriminant analysis was used to assess the variation cap-

Table 1. Means of eight chemical properties in A, B, and C horizons of soil classes based on termites and vegetation type. These data wereused in the analyses presented in Table 2 to 7. Classes are explained in the text.

Soil chemical propertyClass Horizon Ca Mg Na Al

No termites,miombo woodland

No termites,Commiphora woodland

No termites,riparianwoodland

No termites,Acaciawooded bushland

No Macrotermitinaebut other termites,Acacia wooded bushland

Macrotermitinaeand other termites,Acacia wooded bushland

Macrotermitinaeand other termites,miombo woodland

AACA

ACABCABCABCABCABC

31050525322

191316393026632

21.1-t7.9—6.83.01.42.31.30.74.81.51.13.31.61.32.30.91.2

.,,-1*2.2—1.6-

0.80.30.20.30.20.30.90.40.40.60.40.30.40.20.2

- mg kg-'

16.5_5.0_

24.614.78.66.5

15.93.92.81.00.75.11.32.61.30.21.0

8.3_

11.4—6.03.62.27.6

14.94.63.42.74.42.82.43.51.11.53.1

2.4—2.9—1.31.20.81.01.00.61.52.02.01.41.52.00.40.60.9

cmolc kg~'

0.5_0.3—

0.70.30.30.10.10.10.60.20.30.40.30.50.20.10.4

0.1—0.1—0.10.10.13.79.7

11.10.10.10.30.20.41.00.10.20.3

0.01—

0.01—

0.010.020.010.010.020.030.500.880.850.621.350.970.541.240.18

t Data not used.


tured by a set of landscape features using all eight chemicalproperties at once. Discriminant analysis is frequently usedfor vegetation classification (Barbour et al., 1987), and hasrecently been applied to soil classification (Edmonds andLentner, 1987).

The following brief description of discriminant analysis(or discriminant ordination) is modified from Pielou (1984)and Becker and Chambers (1984). Data for a discriminantordination is ordered in a matrix X with s + k ~ I rowsand n columns, where 5 = number of observed variables(eight soil properties in this analysis); k = number of classes(ranging from three to seven); and n = number of obser-vations (soil samples). The entries in the first k - 1 rows areinteger (dummy) variables indicating an observation's mem-bership in a class. The matrix X is post-multiplied by itstranspose to produce the sum of squares and cross-products(SSCP) matrix S. The matrix S is then subdivided into foursubmatrices: S,, (the k — 1 by k - 1 matrix of the SSCPof dummy variables [classes]); S22 (the sby s matrix of SSCPof observed variables [soil properties]); S12 (the k — I by smatrix of sums of cross-products of dummy variables[classes] and observed variables [soil properties]); and S2i(S',2). The inverses of Su and S22 (87! and S22) are used tocalculate the matrix product D

the eigenvalues X corresponding to the rows of W(fc_t)equivalent to

are

P\c

D = 822From an eigenanalysis of D, the eigenvectors correspondingto the eigenvalues X become the k — 1 by s matrix W^_1}.This matrix is pre-multiplied by the matrix X/s) (the originalobservations [soil properties] stripped of the k — 1 dummyvariables) to determine the matrix Y

The first two columns of Y are the first two discriminantvariables, each of which is a linear combination of the dataX grouped by k. The discriminant ordination routine usedhere is more general than described above, in the sense that

1 - PYCwhere pYC is the correlation between the discriminant vari-able (column of Y) and the variable C that describes thecorresponding contrast between the classes k (Becker andChambers, 1984). The values of pYC, C, and Y can be inter-preted to indicate how well the discriminant variable dis-tinguishes among the classes k, which of the classes k aremost strongly contrasting, and which of the eight soil chem-ical properties are responsible for the contrast.

Analyses were carried out using the S statistical package(Becker and Chambers, 1984) and UNIX software (Ker-nighan and Pike, 1984) on an IBM VAX 750. Data from alleight soil properties were log-normally distributed, and weretransformed to fit a normal distribution following Alien(1985). Statistical analyses were performed on subsets ofsamples by horizon, with n = 108 for surface horizon, n =51 for B horizon, and n = 52 for C horizon subsets.

RESULTSComparison of Groupings

Means of the soil chemical data set and numbers ofsamples from A, B, and C horizons are grouped ac-cording to TVT in Table 1. In ANOVA analyses, A-horizon chemical properties were found to differ sig-nificantly (P < 0.01) for all nine groups (Table 2).Only two groups (PSM and TVT) capture significantvariation in all eight chemical properties of A hori-zons. The variation in chemical properties of B andC horizons is less well explained in a statistical senseby any of the groups. Two groups (HP and T) capturesome variation in acid-extractable P in B and C ho-

Table 2. Significance levels (excedence probabilities) of analysis of variance results from nine groups used to capture variation in eight soilchemical properties. Groups are explained in the text.

Soil chemical propertyGroup Horizon N Ca Mg Na AlHillslope position

Landform

Parent material

Vegetation type

Vegetation cover

Termites

Particle size,mineralogy

Landform, parentmaterial

Termites, vegetationtype

ABCABCABCABCABCABC

ABC

ABC

ABC

<0.00001—_

<0.00001—_

<0.00001——

<0.00001_—

<0.00001—_

<0.00001—_

<0.00001——

<0.00001__

<0.00001—-

<0.00001—_

<0.00001——

<0.00001——

<0.00001——

<0.00001——

<0.00001——

<0.00001——

<0.00001——

<0.00001—-

<0.00001—

0.00005<0.00001

—_———

0.0001—_

0.0001—_

<0.000010.0002

_

<0.00001——

<0.00001——

<0.00001—-

<0.00001—_

<0.00001—_

<0.00001——

<0.00001—_

<0.00001—_

<0.00001——

<0.00001——

<0.00001——

<0.00001—-

-t__——————-—————

0.00006——

<0.00001<0.00001

0.0001

0.00009——

0.00002—-

— . __ —_ __ _— —_ _— —— —— —- -— —_ _— —— —_ _— —— —_ _

<0.00001 <0.000010.0001

— —

— —— —_ _

0.00002 <0.00001<0.00001

0.00007

<0.00001—_

<0.00001——

0.00003——

<0.00001——

<0.00001——

<0.00001——

<0.00001——

<0.00001——

<0.00001—-

t Indicates excedence probability is greater than 0.0002, which is .05/216 ANOVAs.


rizons. The PSM group captures variation inexchangeable Na and Mg in B and C horizons. TheTVT group also captures some variation in exchange-able Na in B and C horizons, and produces the greatestnumber of significant results. The PM group gives thepoorest results (Table 2).

The results from DMR support these findings. TheT group captures no variations in A-horizon soil prop-erties significant at P < 0.01, although organic C issignificantly lower in soils with Macrotermitinae andother termites than in soils without termites at P <0.05. The DMR results using the VC group indicatethat soils under dense vegetation have significantlygreater organic C (P < 0.01), total N, and exchange-able Ca (P < 0.05) than some sparser categories, butthese results are weak and inconsistent, suggesting thatother factors cause these patterns. The VT group isslightly more effective than the VC group in capturingvariation in the A horizon. Soils under Acacia woodedbushland have significantly less organic C than undermiombo woodland (P < 0.01), less total N thanmiombo or Commiphora woodland, less acid-extract-able P than riparian woodland, and less exchangeableCa than Commiphora woodland (P < 0.05, data notshown). The PM group captures more variations inA-horizon properties, particularly extractable Al (P <0.05) and in organic C, total N, exchangeable Ca andMg (P < 0.01) (Table 3). The A horizons of soils ongranite have significantly greater organic-C contentsthan those on gneiss, red sediments, or reworked gran-itic sediments, while A horizons of soils on granite oramphibole schist have significantly greater total-Ncontents than soils on reworked granitic sediments.Results for ferruginous quartzite schist and gneisshave little meaning, since there was only one obser-vation in each of these classes. Grouping soils by HPexplains variance in more A-horizon chemical prop-

erties than grouping by PM (Table 4). The A horizonsof soils on summit, shoulder, and backslope hillslopepositions have significantly higher organic-C and to-tal-N contents than soils on toeslopes or pediments (P< 0.01). Acid-extractable P contents in A horizons ofsoils on summits, shoulders, backslopes, and channelswere significantly greater than on toeslopes, and thosein channels were significantly greater than on summitsor on the pediment. Results for summits have littlemeaning as there were only two observations in thisclass. When soils are grouped by both LF and PM,the classes capture significant variation in the sameproperties as captured by HP. As might be expected,the PSM group captures considerable variation in Ahorizons, including differences at P < 0.01 in all prop-erties except acid-extractable P, which is significant atP < 0.05 (Table 5). The TVT group is the most ef-fective discriminator, capturing significant variationin all eight soil properties in A horizons at P < 0.01(Table 6).

Discriminant variables produced using the TVTgroup account for the contrast among its classes betterthan discriminant variables produced using othergroups account for the contrast among their classes.The higher the value of /oYc m Table 7, the better thediscriminant variable from that group accounts for thecontrast in soil properties between its classes (Beckerand Chambers, 1984). Discriminant variables pro-duced using the TVT group reflect strong contrasts inA horizons between soils without termites undermiombo woodland or Acacia wooded bushland vs.soils with Macrotermitinae under Acacia woodedbushland, especially with respect to organic C, totalN, and exchangeable Na, Mg, and K (data not shown).Organic C content is significantly higher in A horizonsof soils without termites under miombo woodlandthan in those with Macrotermitinae or other termites

Table 3. Significance of the difference of means of eight chemical properties in A horizons captured by classes based on parent material.Means with the same letter are not significantly different from others in that column at P < 0.01. Classes are explained in the text.

Soil chemical propertyClass

GraniteAmphibole schistFerruginous quartzite schistGneissRed sedimentsReworked granitic sediments

n

29511

1656

C

6 R&

19.9a19.5abll.Oabc3.5cdS.Obcd3.6d

N

2. lab2.5al.Sab0.5c0.9bc0.6c

Table 4. Significance of the difference of means of eight chemicalMeans with the same letter are not significantly different

P

mg kg'1

16.48.96.51.63.16.4

properties infrom others in that

Ca

7.6ab15.2a9.0ab1.7c3.7bc3.2bc

A horizonscolumn at P

Mg

2.2ab4.5a2.3ab0.6b1.6ab1.2ab

K

kg-0.50.50.70.50.40.7

Na

0.0.0.0.0.0.3

Al

0.010.010.010.010.550.51

captured by classes based on hillslope position.< 0.01. Classes are explained in the text.


SummitShoulderBackslopeToeslopeUpper pedimentLower pedimentFloodplain, channel

n

214185

48165

C

& K&28.0a22.1a17.9a4.lb3.8b2.8b6.8b

N

2.6a2.4a2.0aO.Sb0.7b0.6b0.8b

P

mg kg-1

7.5b26.6abt8.2ab1.2c4.2bc4.5bc

24.6a

Ca

8.4ab10.8aS.Oab1.8b3.0ab3.3ab6.0ab

Mg

3.3a3.1a2.1abO.Sb1.4bI.Ob1.3ab

Kkg-0.80.60.40.30.50.30.7

Na

0.10.10.10.10.20.80.1

Al

0.010.010.010.050.60.60.01

f Although this mean appears higher, it is not significantly different from those with letters a and b in that column, because log-transformed data were used inthe analysis.


Table 5. Significance of the difference of means of eight chemical properties in A horizons captured by classes based on particle size andmineralogy. Means with the same letter are not significantly different from others in that column at P < 0.01. Classes are explained in thetext.

Soil chemical propertyClass Ca Mg Na Al

Loamy-skeletal, mixedFine, kaoliniticCoarse-loamy, mixedFine-loamy, mixedMixed (Psamment)

332333127

6 K

20.6a4.5b4.0b3.1b1.9b

E-i

2.2a0.8b0.6b0.6b0.5b

~ mgkg-'16.12.86.4

10.21.6

9.1a3.1a3.2a4.9a2.3b

2.6a1.4a1.2a1.8a0.7b

0.5a0.6a0.4a0.4aO.lb

O.lbO.lbO.lbl.laO.lb

O.Olb0.7abt0.3ab0.6abf0.6a

t These means are not significantly different from others in this column, although they appear equal or higher, because log-transformed values were used in theanalysis.

Table 6. Significance of the difference of means of eight chemical properties in A horizons captured by classes based on termites and vegetationtype. Means with the same letter are not significantly different from others in that column at P < 0.01. Classes are explained in the text.


No termites, miombo woodlandNo termites, Commiphora woodlandNo termites, riparian woodlandNo termites, Acacia wooded bushlandNo Macrotermitinae but other

termites, Acacia wooded bushlandMacrotermitinae and other termites,

Acacia wooded bushlandMacrotermitinae and other termites,

miombo woodland

n

31553

19

39

6

C

—————— g kg"1

21. la7.9ab6.8bc2.3cd

4.8bc

3.3bcd

2.3d

N

2.2a1.6aO.Sbc0.3c

0.9ab

0.6bc

0.4bc

P

mg kg'1

16.5abS.Ob

24.6a6.5ab

2.8b

5. lab

1.3b

Ca

8.3ab11. 4a6.0ab7.6ab

3.4bc

2.8bc

l.lc

Mg

2.4a2.9a1.3al.Oa

l.Sa

1.4a

0.4b

K

0.6a0.3ab0.5aO.lc

0.7a

0.4a

0.2bc

Na

O.lbO.lbO.lb3.7a

O.lb

0.2b

O.lb

Al

O.OlbO.OlbO.OlbO.Olb

0.5a

0.5a

0.6a

under miombo woodland or Acacia wooded bushland(Table 6). The A horizons of soils without termitesunder Acacia wooded bushland also have significantlylower organic-C contents than other classes withouttermites, but there were only three observations forthis class (Table 6). The A horizons of soils withouttermites under riparian woodland and soils withoutMacrotermitinae under Acacia wooded bushland alsohad significantly higher organic-C contents than soilswith Macrotermitinae under miombo woodland. To-tal-N content is significantly higher in A horizons ofsoils without termites under miombo or Commiphorawoodland than in those with Macrotermitinae undermiombo woodland or Acacia wooded bushland (Table6). These differences in surface-horizon chemistrymay reflect the removal of litter by termites and theconsequent lack of incorporation of organic matterinto soil surface horizons.

In B horizons, the first discriminant variable fromthe TVT group expresses a contrast between soils withtermites under Acacia wooded bushland vs. soils with-out termites under Acacia wooded bushland. The lat-ter have greater exchangeable Na, reflecting seasonallywaterlogged conditions that prevent the establishmentof termite colonies (Table 1). The second discriminantvariable in B horizons expresses a contrast betweensoils with termites under Acacia wooded bushland vs.soils with no termites under riparian woodland; thelatter have greater organic C and exchangeable Ca (Ta-ble 1). In C horizons, the first discriminant variableexpresses a contrast between soils without termitesunder riparian and miombo woodland vs. soils with-out termites under Acacia wooded bushland with re-spect to exchangeable Na. The second discriminant

variable expresses a strong contrast in C horizons be-tween soils with termites under Acacia wooded bush-land vs. soils without termites under riparian wood-land; the latter have greater acid-extractable P and lessexchangeable Ca, Mg, Na, and Al (Table 1). Differ-ences in subsurface soil chemistry among classes de-fined by the TVT group reflect seasonally high watertables that appear to prevent the establishment of ter-mite colonies. These soils have greater accumulationof organic C, total N, and related cations than otherdeep well-drained soils, which are colonized by ter-mites. However, a few samples from pedons underAcacia wooded bushland in internally drained depres-sions that also lack termite colonies have highexchangeable Na contents and other cations that re-flect seasonal waterlogging, but do not have high or-ganic-C or total-N contents.

DISCUSSIONAnalyses of soils grouped by termites and vegeta-

tion type (TVT) indicate that, in general, surface ho-rizons of soils without termites have greater organicC and total N and often higher exchangeable cationsthan soils with termites, while subsurface horizons ofsoils without termites either have greater organic Cand total N, or greater exchangeable Na and othercations, than soils with termites. Two possible expla-nations for this outcome are (i) that termites and veg-etation modify soil chemistry, or (ii) that the occur-rence of termites and vegetation reflects soil chemicalvariations caused by other factors.

The TVT group captures soil chemical variationscaused by other factors, because certain vegetationtypes and termites occur only on certain parent ma-


Table 7. Correlation (values of prc) between the discriminant vari-able Y and the variable C that describes the corresponding con-trast between the classes for the first and second discriminantvariables obtained using nine different groups of data on eight soilchemical properties. Groups are described in the text.

HorizonA B

Group dlf d2}: dl d2 dl d2Hillslope position 0.88 0.44 0.67 0.49 0.75 0.51Landform 0.87 0.42 0.69 - 0.77Parent material 0.82 0.46 0.66 0.39 0.68 0.47Vegetation type 0.80 0.62 0.65 0.51 0.64 0.53Vegetation cover 0.79 0.55 0.77 0.58 0.73 0.57Termites 0.82 0.34 0.67 0.51 0.67 0.47Particle size, mineralogy 0.87 0.73 0.79 0.69 0.73 0.56Landform, parent

material 0.88 0.48 0.71 0.57 0.75 0.64Termites, vegetation type 0.89 0.83 0.87 0.59 0.77 0.72t First discriminant variable.$ Second discriminant variable.

terials, which in turn tend to occupy certain landformsand certain hillslope positions (Fig. 1, 2, and 3). Forexample, miombo and Commiphora woodland occuralmost exclusively on mountains formed of granite,gneiss, amphibole schist, or ferruginous quartziteschist, which occupy summit, shoulder, backslope, ortoeslope positions in the study site. Acacia woodedbushland is restricted to the pediments, which aremantled with red or granitic sediments. Riparianwoodland is restricted to stream channels that arefilled with granitic sediments (Fig. 1 and 2).

Field observations indicate that the distribution ofvegetation types reflects land-use patterns, and topo-graphic position and parent material effects on soildepth, drainage, and texture, as suggested in Fig. 3.Miombo and Commiphora woodland covered largeareas in 1956 aerial photographs, but by 1985 it hadbeen mostly cleared from the pediment by local farm-ers. In its place, there apparently has been an expan-sion of Acacia wooded bushland, particularly Acaciatortilis subsp. spirocarpa, perhaps due to greater fireresistance and dispersal by large ungulates (Lampreyet al., 1974).

The distribution and density of termites reflectslandform characteristics (Fig. 3). Macrotermitinae re-quire deep soils and prefer well-drained locations (Leeand Wood, 1971), in order to construct mound en-vironments that maintain high humidity and constanttemperature (Singh and Singh, 1981). Thus, no Ma-crotermitinae occur in the shallow soils of the moun-tains (Lithic subgroups), or in the seasonally floodedstream channels. Mound-building termites favor soilswith a sandy loam, sandy clay loam, or sandy claytexture, because they cement individual sand grainstogether with clay and saliva to construct theirmounds (Lee and Wood, 1971). Thus, Macrotermi-tinae also do not occur on the deep, fine-textured (clayloam, silt loam, or clay) soils on red sediments in thesite.

However, termites and vegetation also modify soilchemical characteristics (Fig. 3). In deep, well-drained,medium-textured soils where Macrotermitinae occur,large mounds are very common and termite densitymay reach 400 individuals m~2 (Jones, 1989). The

lower organic-matter content observed in surface ho-rizons of soils with termites could be explained by thehypothesis that litterfall throughout these areas is ex-haustively partitioned among neighboring termite col-onies, where it is thoroughly decomposed (Jones,1989). Many studies have reported significantly higherconcentrations of N, P, and exchangeable bases in ter-mite mounds compared with surrounding soils in EastAfrica (Arshad, 1982; Wielemaker, 1984; Hesse,1955). One source of these elements is plant litter for-aged from the soil surface (Arshad, 1982; Schaefer andWhitford, 1981). Also, termites may bring soil parti-cles to the surface from a subsurface horizon rich inexchangeable bases (Lal, 1987; Watson, 1976). Ter-mite mounds and soils beneath them have pellety,masepic and lattisepic fabric and papules of formerferriargillans (Mermut et al., 1984). A dense termitepopulation, continually reworking soils surroundingmounds, could partly explain the absence of clay skinsin argillic horizons, and the absence of structure indeep, well-drained pedons (Jones, 1989, unpublisheddata). Removal of litter from large areas by termitesalso could account for the significantly lower organicC, total N, acid-extractable P, and exchangeable cat-ions observed in soils with termites than in soils with-out termites (Table 6), or in soils on reworked sedi-ments on pediments (Table 2 and 3).

Soil nutrient impoverishment, particularly very lowlevels of total N and acid-extractable P in deep pe-diment soils (Table 4) may contribute to the domi-nance of Acacia wooded bushland. Acacia spp. haveroot symbioses, including associations with N2-fixingbacteria and endomycorrhizae, which may allow themto outcompete other woody plants in N- and P-poorsoils (Hogberg, 1986). Deep soils with termites thatsupport relict patches of miombo woodland have evenlower organic C, and exchangeable Ca and Mg thansoils with termites under other vegetation types (Table6). The dominant tree species in miombo woodlandare ectomycorrhizal (Hogberg and Nylund, 1981;Alexander and Hogberg, 1986). Dighton and Mason(1985) suggest that mycorrhizal hyphae are able to en-hance decomposition and directly recycle nutrientsfrom litter. This mechanism could account for the dif-ferences in organic C and exchangeable cations be-tween soils with termites under miombo vs. other veg-etation types. However, there are no consistentdifferences attributable to vegetation type among soilswithout termites (Table 6).

Catenas and Contrasting Parent MaterialsThe influence of termites and vegetation on pedo-

genesis in this landscape reflects underlying topo-graphic and parent-material factors, as indicated bythe trends in soil chemistry captured by the HP, LF,and LPM groups. Soils show features of Milne's soilcatena (Milne, 1935, 1936, 1947) along a hillslopetransect (Fig. 4) of a similar slope length to those ex-amined in most studies of toposequences in East andCentral Africa (Morison et al., 1948; Berry and Rux-ton, 1959; Ruxton, 1958; Stocking, 1979; Watson,1964, 1965).

The six pedons shown in Fig. 4 are located along a


n P• ND C

PEDON PEDON49 50

PEDON 51 PEDON 52 PEDON 53 PEDON 54

PROFILES BY HORIZON

Fig. 4. Selected chemical properties of six pedons along a 1-kmhillslope transect on granite and granite sediments, (a) Soil organicC (g kg-'), total N (g kg-'), and acid-extractable P (mg kg-'), (b)Exchangeable cations and extractable Al (cmol,. kg'1).

1-km hillslope transect on granite or granitic sedi-ments (Fig. 1 and 2), and occupy the backslope andtoeslope of a hill (Pedon 49 and 50), the pediment(Pedon 51 and 52), the floodplain (Pedon 53), and asediment-filled stream channel (Pedon 54). The soilson the backslope and toeslope positions have the leastwell-developed profiles. Pedon 49 on the backslope isa Lithic Ustropept with near-neutral pH, weak tomoderate granular structure in A and AC horizons,and relatively high organic C, total N, acid-extractableP, and exchangeable cations. Pedon 50 is a Lithic Dys-tropept located at the toeslope, where there is an ab-rupt change from a steep to a gentler slope. It has weakto massive structure in A and AC horizons and lowerC, N, P and cations than Pedon 49; this portion of thehillslope showed signs of surface erosion associatedwith the slope break. Pedon 51, a Ustoxic Dystropeptfurther downslope on the pediment, contains themost-acid cambic horizon and the lowest exchangea-ble cations of this sequence. Pedon 52, a Ustic Dys-tropept located 200 m further downslope, shows evi-dence of sediment accumulation from upslopeerosion, with higher C, N, and P in the upper threehorizons. It has a less-acid cambic horizon than Pedon51. Pedon 53, a Fluventic Ustropept on the floodplain,and Pedon 54, a Fluventic Dystropept in the season-ally flooded channel, are composed of layered sedi-ments with the characteristic irregular decrease in C,N, and P and cations, and near-neutral pH (Fig. 4).Soil differences along this transect are attributable todrainage conditions, differential transport of eroded

PEDON 6 PEDON 45 PEDON 44. PEDON 43 PEDON 42 PEDON 7

PROFILES BY HORIZON

Fig. 5. Selected chemical properties of six pedons along a 4-kmtransect across contrasting parent materials:.red sediments andgranitic sediments, (a) Soil organic C (g kg-'), total N (g kg'1),and acid-extractable P (mg kg'1), (b) Exchangeable cations andextractable Al (cmoic kg'1)-

material, and leaching, translocation, and redeposi-tion of mobile chemical constituents cited by Milne(Gerrard, 1981).

Where hillslopes cross adjacent contrasting parentmaterials, soil development demonstrates the influ-ence of subsurface flow and lateral transport of ma-terials. The chemical characteristics of six pedonssampled along a gently sloping 2.5-km transect areshown in Fig. 5. Pedon 6, 45, and' 44, upslope, aredeveloped on red sediments; and located within 1000m of each other; Pedon 43 is on a mixture of red andgranitic sediments located 500 m downslope of Pedon44, just below the abrupt transition from red to gran-itic sediments, and Pedon 42 is 1 km further down-slope on granitic sediments (Fig. 1). Pedon 7 was in-cluded to illustrate soil chemistry of a profile ongranitic sediments in a hillslope position similar toPedon 42 and 43, but which has no red sedimentsupslope (Fig. 1). Pedpn 6 is typical of the Ustoxic Dys-tropepts (Jones, 1989, unpublished data), with clay toclay loam texture throughout, a gradual decrease in C,N, and P and an increase in pH and exchangeablecations with depth, and occasional fragments of rottedschist in the BC horizon (Fig. 5). In contrast, Pedon45 and 44 have virtually no exchangeable Ca exceptin A horizons, and are very acid (pH < 4.5) from thetop of the cambic horizon to the lithic contact. Pedon43, developed on a coarser transitional parent mate-rial, has a similarly acid subsurface horizon lackingexchangeable Ca but showing evidence of clay trans-location. Below the Bt horizon, Pedon 43 has severallayers ranging from 90- to 200-cm depth in which plin-


thite occupies >50°/o of the matrix. Below the Bv ho-rizon is a wet layer with pH 7.5, and accumulationsof exchangeable Ca and Na and acid-extractable P.Pedpn 42, developed on granitic sediments, has someCa in the exchange complex throughout the profile,and a marked accumulation of exchangeable Na in theBt, Bv, and BC horizons. Like Pedon 43, this profilehas several layers ranging from 90- to 180-cm depthin which plinthite occupies > 50% of the matrix, un-derlain by a wet layer with higher pH, but it has noP accumulation in the wet layer. Pedon 7 has an acidcambic horizon gradually transitional to a moistweathering zone with pH of 5.5; this profile has someexchangeable Ca in all horizons, but no plinthite andno accumulations of cations along the weatheringfront (Fig. 5).

Plinthite only occurs downslope of red sediments.It is a surprising occurrence in granitic sediments withweakly developed argillic horizons, silt and sand min-eralogy dominated by feldspar and mica, and dithion-ite-citrate-bicarbonate-extractable Fe < 2 g kg-1.There was no evidence of surface transport downslopeacross the abrupt formation contacts between red sed-iments and granitic sediments. This suggests that sub-surface flow has transported dissolved Fe oxides,some acid-extractable P, and exchangeable cationsdownslope from red sediments. Red sediments havetotal P of 200 to 500 mg kg-1, compared to 50 to 200mg kg"1 total P in granitic sediments (Jones, 1989,unpublished data). The occurrence of high acid-ex-tractable P in the BC horizon of Pedon 43, but not inother pedons, suggests that soluble Fe and P may betransported short distances from the red sedimentsand then immobilized in the granitic sediments.

In stable landscapes in the humid tropics and sub-tropics, subsurface drainage and landscape evolutionhave been associated with lateral translocation andformation of Fe minerals and concentration of basesdownslope (Daniels et al., 1975; Moniz et al., 1982;Macedo and Bryant, 1987). Ruxton (1958) suggestedthat subsurface flow is a pedogenic factor south ofKassala in the Sudan. In central Tanzania, multiplelayers of segregated Fe—up to four distinct layers inPedon 42 and 43—suggest fluctuating or seasonallyperched water tables. Subsurface flow may have beenmore marked under a previously wetter climate (Liv-ingstone and Van der Hammen, 1978; Milne, 1947).Water tables also may have been perched during struc-tural warping and tilting of the Central Plateau. Be-cause of anisotropy at the lithic contact (Zaslavskyand Rogowski, 1969), present-day wet-season lateralsubsurface transport of reduced Fe probably occurs indeep soils on the pediment. Such a combination offactors would explain the formation of plinthite ingranitic sediments containing significant amounts ofweatherable minerals but little Fe, as well as accu-mulation downslope of exchangeable bases in plin-thite and just above the lithic contact.

Implications for Soil MappingPoor understanding of the spatial distribution of

soil chemical properties has had some disastrous con-sequences for agricultural projects in central Tanzania

(Acland, 1971). However, the high cost and time re-quired for detailed soil surveys means that regionalsoil chemistry must be generalized from a limitednumber of sampled points. At this site, vegetationtype and the occurrence and density of termites ex-plain, in a statistical sense, more of the variation ineight soil chemical properties than any other group.This is because vegetation and termites both respondto pedogenic factors and also exert their own influenceon soil formation. However, pedogenesis also appearsto be strongly affected by subsurface lateral flow andthe occurrence of contrasting parent materials alonghillslope sequences. These results do not indicate thatvegetation and termites are dominant pedogenetic fac-tors, but they do suggest that the distribution of veg-etation types and termites incorporates the most in-formation about pedogenesis of the groups tested inthis study. If this result is confirmed by other studiesin dry Africa, prediction of soil chemical properties insecond- and third-order surveys may be improved byincorporating information on vegetation and termitedistribution.

ACKNOWLEDGMENTSThis study was partially supported by the Academic Sen-

ate of the Univ. of California, Santa Barbara and conductedwith the cooperation of J.A. Maghembe, the Tanzania Nat.Scientific Research Council, G. Mashurano, H. Macha, andother colleagues in Tanzania. I thank staff of the Royal Bo-tanic Gardens at Kew, J.P. Darlington, S. Jones, A. Gold-schmidt, N. Dudley, and F. Setaro for specimen identifica-tion, field, and laboratory work, and J.G. Cady, R. Southard,and S.W. Buol for thoughtful comments on the manuscript.

environmental influences on soil chemistry in central semiarid tanzania

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