Effect of topography on selected soil pr

0038-0717/92 55.00 + 0.00
Copyright 0 1992 Pcrgamon Press plc zyxwvut

Soil Bid. Biochem. Vol. 24, No. 2, pp. 145-150,1992
Printed in Great Britain. All rights nwwd

EFFECT OF TOPOGRAPHY ON SELECTED SOIL
PROPERTIES AND NITROGEN MINERALIZATION IN A
DRY TROPICAL FOREST
A. S. RAGHU~ANSHI

Department of Botany, Banaras Hindu University, Varanasi 221 005, India zyxwvutsrqponmlkjihgfedc
(Accepted 25 August 1991)

Snmmary-Soil nutrients and nitrogen mineralization rates were measured along three topographic
sequences in a seasonally dry tropical forest. Soil organic C, total N and P decreased downslope.
Accumulation of soil organic C and N was related to the P content which, in turn, was controlled by the
proportion of fine particles in the soil. Net N mineralization and nitritication rates ranged from 0 to 33 pg
g-l month-’ and 0 to 19pg g-’ month-‘, respectively,within the annual cycle and among the topographic
positions. Annual mineralization increased from 125pg g-l dry soil at the hillbase to 203 pg g-l dry soil
at the hilltop. These differences in mineralization rates were obviously related to the distribution of

substrate.

INTRODUCTION

The composition and productivity of an ecosystem
is markedly affected by the physical and chemical
properties of its soil. Soil properties vary across the
landscape, due to the modifying effects of topography
on soil-forming and geomorphic processes (Coleman
et nl., 1983). Relationships between topography and
soil properties have been investigated by Milne
(1935), Aandahl (1948), Ruhe and Walker (1968),
Schimel er al. (1985) and Aguilar and Heil (1988).
Topography can affect the organic matter and nutrient storage in soil by influencing microclimate,
runoff, evaporation and transpiration. These factors,
in turn, affect N-mineralization processes. Release of
nutrients by biological mineralization is crucial for
maintaining the cycling of essential nutrients immobilized in dead plant material and is essential for
continued productivity
of terrestrial ecosystems.

Increased understanding of factors influencing mineralization process may lead to management practices
which enhance microbial nutrient mineralization
without addition of fertilizers (Elliott and Coleman,
1988).
I have investigated the variations in selected soil
properties and N-mineralization rates as a function of
topography in an Indian dry tropical forest.
MATERIALS AND METHODS
Study sites

The study area is located on the Vindhyan Plateau
(225-525 m altitude) in the Marihan range of East
Mirzapur Forest Division of Uttar Pradesh, India,
between 24” 55’ and 25” 1O’N lat and 82” 32’ and
82” 45’E long. The area experiences a seasonally dry
tropical climate dominated by a typical monsoonal

character. The year is divisible into a cold winter
(November-February),
a hot summer (April-June)

and a warm rainy season (July-September). October
and March constitute transitional months between
rainy and winter, and between winter and summer
seasons, respectively. Warm temperatures (2436°C)
and high relative humidity (70-95%) prevail during
the rainy season. During the winter season, temperatures fall between 10 and 25°C and January is the
coldest month. Summer is dry and hot with temperature ranging between 30 and 45°C during the day.
The annual rainfall averages 820 mm of which 86%
occurs during the rainy season. In the annual cycle,
there is an extended dry period of about 9 months.
The Vindhyan Plateau is largely an erosional
surface. Red coloured and fine textured sandstone
(Dhandraul orthoquartzite) is the most important
rock (K. K. Tandon, unpublished M.Sc. thesis,
Banaras Hindu University, 1973). The soils are ultisols, having a hyperthermic temperature regime typical Plinthustults with Ustorthents, leached, shallow,
low in nutrients and organic matter, and having
moderate water holding capacity.
Three forest sites were selected along a topographic
sequence, with about 30” slope, the first site located
at the hillbase, the second at the midslope, and the

third at the hilltop position. Large boulders and
pebbles are found in the hilltop and midslope positions. 28% of the hilltop and 30% of the midslope
is covered with rock outcrops and large boulders. The
soil depth at these two sites is variable and averages
15 cm. The hillbase site has depositional soil with no
rock outcrops.
The natural vegetation of the area is a dry mixed
deciduous forest. At the hillbase site the depleted
natural forest cover has been enriched (by the State
Forest Department) by plantations of Acacia cute&
145

146

A. S. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH
RAGH U BAN SH I

and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
Emblica oficinalis which comprise the top storey.
NH,-N were determined at time zero and after 30

days of field exposure. The increase in the concenThe understorey (second stratum) comprises small
trees of Ougeiniu oojeinensis and shrubs Ziziphus
tration of NH,-N plus NO,-N during the course of
fieId exposure is defined as net N-diction
glaberrima, Ziziphus oe~pl~ and Carissa opaca. The
midslope site is dominated by A. catechu and
(Pastor et al., 1984). The increase in NO,-N during
field exposure is referred to as nitrification.
Lannea coromandelica with an understorey
dominated by the shrubs Ny ctanthes arbor- tristis,
All results are expressed on an oven dry soil
(lOS”C, 24 h) basis. The statistical analyses were
Holarrhena antidy senterica and Z. glaberrima. The
vegetation on the hilltop is dominated by Boswelliu done using a SPSSlpC statistical package for
(SPSS/PC, 1986).
serrata and Acacia catechu. The understorey
is ~cr~ompute~
predo~nantly
N. arbor- trist~ and 2. g~~erri~.
RESULTS

Soil sampling and analyses

All of the soils studied were of sandy loam texFive samples were collected randomly from each
ture and total ties (silt + clay) ranged from 31
site from the upper IOcm soil layer at monthly
to 44% (Table 1). The highest percentage of silt f
intervals for two annual cycles (May 1987-May
clay occurred in hiiltop and the lowest in hillbase
1989). Samplings during the periods when soils were
soils. Silt + clay content was significantly different
excessively wet were avoided. Large pieces of plant
among slope positions (P < 0.05). Fine-textured soils
material were removed and the field-moist soil was
such as clay, clay loams and silt loams generally
sieved through a 2 mm mesh screen. Field-moist soil
have Iower bulk densities than sandy soils (Brady,
samples were divided into two parts. One part was
1984). Soil bulk density was negatively correlated
used for the analysis of gravimetric moisture, and

with the proportion of fine particles (silt +clay)
inorganic nutrients. The second part was used for
(r = -0.98, P c 0.03). There was a negative correassessing N-mineralization
rate. Once during the
lation between bulk density and total organic C
study period air-dried soil samples were analysed for
(r = -0.94,
P < 0.05), bulk density and total N
texture, organic C, total N and P.
(r = -0.95, P < O.OS), and bulk density and total P
Particle size distribution (texture) was analysed
(r = -0.98, P < 0.03). Bulk density influences soil
using sieves of different mesh sizes (Indian Standards,
nut~ents since it is inversely related to soil organic
1965) and the pipette method (Piper, 1944). Bulk
matter (Davidson et al., 1967).
density was determined by measuring the weight of
Soil organic C ranged from 0.47 to 2.00%, and
dry soil of a unit volume to a 10 cm depth. Soil pH
the values were significantly different (P < 0.05)

was determined by using a glass electrode (1: 2,
among slope positions. The most soil organic C was
soil: water ratio). Water holding capacity (WHC) was
measured in the hilltop soil, followed by the r&slope
determined using perforated circular brass boxes zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ
soil. Per cent soil organic C values are lower than the
(Piper, 1944). Total C and N were determined by
range reported in other studies in the dry tropics. The
Perkin-Elmer CHN Autoanalyser and organic C was
organic C in tropical ultisol soils of Puerto Rico and
calculated by subtracting inorganic C content deterBrazil ranged from 1.3 to 4.9% in 0-10cm soil
mined following Jackson (1958), from the total C.
horizon (Sombroek, 1967; FAO-UNESCO,
1971).
Total P was measured by an ammonium molybRapid
organic
matter
decomposition
at
the

present
date-stannous
chloride method after HCQ digestion (Jackson, 1958). NO,-N was measured using
Table 1. Phy~~~~~
characteristics of forest soils investigated.
the phenol disulphonic acid method, with CaSO,
DataanXlMoflSE
as the extractant (Jackson, 1958). NH.,-N was
Hillbase
Middope
Hilltop
extracted by 2 M KC1 and analysed by the phenate
Bulk density
I .68 f 0.002
I .48 f 0.003
1.51 f 0.02
method (APHA, 1984). Plant available phosphate
(kxcm-2)
P (NaHCO,-Pi) was determined by the ammonium
Soil texture

31 f 1.1
37 f 1.4
44* 1.7
(% silt + clay)
molybdate+stannous chloride method by using alka% Rock area
0
30
28
line sodium bicarbonate (NaHCO,, pH 8.5) as the
Water holding
capacity (%)
extractant (Jackson, 1958).
38 f0.3
44* 1.7
46*1.5
Nitrogen mineralization

N-mineralization
was measured using the buried
bag technique of Eno (1960). A portion of the fresh,

field-moist soil sample (about 150 g) was incubated
in the soil at 1Ocm depth using a large sealed
polyethylene bag. Coarse roots and large fragments
of organic debris were removed in order to avoid any
marked immobilization during incubation (Ross et
al., 1985; Schimel and Parton, 1986). NO,-N and

PH
Organic C
(%)
T;z k-1)
0Gg-?

OLBK’)
(kg ha-‘)
Inorganic N

6.3 f 0.06

6.2 f 0.04

6.2 f 0.09

0.47 * 0.07
7896

0.98 f 0.09
10,192

2.00 f 0.09
19.720

MO* 10
840

9OOf 10
936

1600*23
1578

210f0
353
4.53 f 0.40

25Ofl
260
5.24 f 0.33

310*3
306
4.7s * 0.35

1.77 f 0.25

2.04 f 0.23

2.17 f 0.25

N-mineralization

in a dry tropical forest

147

N MINERALIZATION

(Ng g-’ month-‘)

5
0
-5

Fig. 1. Rainfall, soil moisture and air temperature for the
study area. (m) rainfall; (a) air temperature; (0) soil
moisture.

JJASONDJFMA
MONTHS

Fig. 3. Variations in nitrifiation rates (bars 1 SE). Legends
same as Fig. 2.

quantities of total P, mostly below 0.02% (Nye and
sites is a possible reason for these low organic matter
Bertheux, 1957; Westin and de Brito, 1969).
contents.
In the soils studied N-mineralization and nitrificaThe N content of the soil was low and it ranged
tion rate, across sampling intervals and sites, ranged
from 0.05 to 0.16%. Accumulation of soil N closely
from 0 to 33 and 0 to 19 pg g-i month-‘, respectively
follows that of soil organic matter because, on
(Figs 2 and 3). The seasonal pattern of N-mineralizaverage, about 99% of the N in terrestrial ecosysation and nitrification were similar at all the sites, the
tems is organically bound (Rosswall, 1976). The
present study also showed a strong positive correvalues being highest during the rainy and lowest
lation between total N content and organic C during the summer season. Differences in N-mineralization and nitrification due to season and sites were
content of soils (r = 0.99, P < 0.001).
significant (P < 0.01). zyxwvutsrqponmlkjihgfedcbaZYX
The soils exhibited a marked P-poverty with total
P concentration varying within a relatively narrow
range of 0.021-0.031%. Values for P were significantly different (P < 0.05)
among the three sites. The
DISCUSSION
highest values were recorded for the hilltop samples
and lowest for the hillbase samples. Several studies on
The content of organic matter in soil changes
tropical ultisols and alfisols report generally low zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCB
systematically down the hillslopes or “toposequences”, and lower slope positions and depressions
are typically reported to have higher amounts of
NITRIFICATION
(,ug g-’ month-’ )
organic matter and N and P than slopes or ridgetops
20
(Schimel et al., 1985; Aguilar and Heil, 1988). Since
topography directs the course of runoff water, it often
determines the distribution of erosion products; sol15 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
uble salts, mineral colloids and organic matter. In
effect, the local climate, primarily the effective precipitation, varies with slope. Runoff causes rapid
ia
erosion of the surface soil, which generally contains
the highest percentage of N. On the other hand,
accumulation of runoff products from surrounding
E
areas in depressions increases the effective precipitation and the storage of N in the soil (Black, 1968).
(
In contrast to the above generalized trend, in the
present study organic C and total N and P decreased
downslope in the forest sites. Vegetation of the sites
-! jand microrelief affected the soil C, N and P contents
J JASONDJFMAM
through the deposition and accumulation of organic
MONTHS
matter. Though midslope forest sites received 14%
Fig. 2. Variations in N-mineralization rate (bars 1 SE). (0)
hillbase forest; (0) midslope forest; (A) hilltop forest.
more litterfall than hilltop site, litter movement from

148

A. S. RAGHUBANSHI

slopes left about 5% less forest floor mass on the
correlation in the present study, between the promidslope than on the hilltop site (Singh and Singh,
portion of fine particles (silt + clay) and total P
199 1). Runoff assisted in the accumulation of organic
content (r = 0.99, P < 0.001). Sharpley (1980) and
matter in depressions found abundantly at the hillSchimel et al. (1985) found fine particles to be
top. The hillbase sites had low soil nutrients due to
enriched in P relative to whole soils. Evidence from
15 and 27% lower litter input than the hilltop and
the soil chronosequences suggests that the P content
hillbase sites, respectively. Further the free drainage
of the parent material and the amount of P remaining
and high rates of runoff and erosion do not permit the
in weathered soils may be major factors governing the
accumulation of organic matter on the hillbase site.
accumulation of soil organic C, N and S within
However, soils from the three sites may not be strictly
ecosystems (Syers and Walker, 1969; Walker and
comparable because of the rocky and stony nature of Adams, 1958). Cole and Heil (1981) emphasized the
importance of P in controlling aspects of organic
two of them.
Very few studies are available from tropical
matter production and N cycling. In the soils studied
accumulation of soil organic C and N was dependent
environments on the effect of topography. A study
by Livingston et al. (1988) in Amazonian forests of on the P content of the soil in accordance with the
Brazil shows a trend similar to that found in the
proposals of Syers and Walker (1969). Lajtha and
Schlesinger (1988), on the other hand, found that the
present study. In their study total N varied with
total P content of desert soils was not related to
topographic positions and was greatest on the ridges
and lowest in the valley bottoms (Livingston et zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH
al.,
organic C content and argued that both slope position and vegetation controlled the C content of soils
1988). Yanker et al. (1988) argued that expectation
of higher amounts of organic matter and N and P along the transect they studied.
In the soils studied, peak N-mineralization
and
at lower slope position and in depressions is only
nitrification rates were substantially
lower than
true in the landscape which act as closed catchments
and have less water erosion. The results of this study
the values reported for other tropical ecosystems.
Vitousek and Matson (1988) reported N-mineralizare similar to those reported for sandhill topography
ation rates ranging from 8 to 19, 12 to 16, and 26
(Barnes and Harrison, 1982) and in ecosystems comto 68 pg g-i 10 d-’ for Amazonian forests of Brazil,
posed of stable uplands, eroded slopes, and lowlands.
semi-deciduous forests of Panama, and old growth
In all these ecosystems highest quantities of organic
tropical forests of Costa Rica, respectively. Old field
matter and N and P reside in the upland and lowest
and repeatedly-disturbed experimental plots of Costa
in the slopes because material is removed as quickly
Rica also showed very high mineralization rates,
as it is accumulated.
from 30 to 99pgg-’ month-’ (Robertson, 1984;
The concentrations of C, N and P in the present
Werner, 1984). For Serengeti grasslands of Tanzania,
soils increased from hillbase to hilltop, following the
Ruess and McNaughton (1987) reported N-mineraltrend in silt + clay content. In the forest and savanna
ization rates to vary from 1.04 to 64 pg N g-i 20 d-‘.
soils of Ghana, Asamoa (1980) showed that coarse
For nearby savanna soils, Singh et al. (1991) reported
textured soils were lower in N status than soils with
0.8-15 pg N g-i month-’ nitrification rates. In tropfiner texture. The variations in amounts of P and fine
ical forests of Brazil, Panama and Costa Rica, nitrifiparticles along hillslopes would be expected to influence the in situ turnover and steady-state levels of cation rates are very high which range from 6 to 54 pg
N g-’ 10 d-’ (Vitousek and Matson, 1988). Thus it
C and N (Schimel et al., 1985). In the present
seems that the present dry tropical forest is extremely
study, among the sites, there was a strong positive
nutrient poor because of low nutrient pools and low
correlation between proportion of fine particles and
mineralization rates.
soil organic C content (r = 0.98, P < 0.01). Similar
Nitrification
and N-mineralization
rates were
results were observed in North Dakota rangeland
maximum in the rainy season and minimum in the
toposequences where Aguilar and Heil(1988) showed
that fine textured soils had greater quantities of summer season. In semi-arid ecosystems, nutrient
organic C, N and P. A sandy soil usually carries less dynamics are closely linked to seasonal variation in
temperature and moisture (Burke, 1989). Temperaorganic matter and nutrients than one of a finer
ture is seldom limiting for these biological protexture because of lower moisture content and more
cesses in this region but soil moisture drops substanrapid oxidation occurring in lighter soils. Uehara and
tially after the end of rainy season and during sumGillman (1981) and Sollins et al. (1984) showed that
mer soils become very dry (Fig. 1). Evidently
the widespread amorphous clays of tropical soils
N-mineralization is moisture-limited in these ecosysstrongly bind organic matter and reduce its decompotems and observed seasonality is not or little consition. Van Veen and Paul (1981) also argued that the
trolled by temperature. The flush of N-mineralization
soils with high clay contents are able to protect the
observed at the onset of the rainy season following
organic matter from degradation.
the summer is consistent with several reports of
Phosphorus distribution is controlled by the longincreased N-mineralization of a dry soil after rewetterm movement of fine materials across the landting (Birch, 1958; Marumoto et al., 1977; Singh et al.,
scape, hence P content may correlate with clay
1989).
content (Burke, 1989). This is supported by a positive

N-mineralization

in a dry tropical forest

Table2. Net N-mineralizationand nitrifioationin the surface10cm
of the forestsoils (valuesare mean of 2 yr)
N-mincrahition
Site
Hillbase
Midslope
Hilltop

bg g-’ yr-‘)
125
164
203

(kg ha-’ yr-‘)
210
170
200

Nitrification
(as % of
N-mineralization)
61
57
52

Relatively little information is available on the
effect of topography on soil N transformations
@chime1 e* cl., 1985). MY study helped to understand
the pattern of N-mineralization along a soil catena.
Within the forest ecosystem nitrification and N-mineralization
eralization

decreased downslope. Annual net N-minwas estimated by summing the monthly

estimates for in situ mineralization (summarized in
Table 2). Annual mineralization
increased from
125 pg N g-l dry soil at the hillbase to 203 pg N g-i
dry soil at the hilltop. When data of N-mineralization
was corrected for the area occupied by the rocks, the
hillbase site had the highest N-mineralization rates.
Thus the soil on the hilltop may be a better growth
medium for plants but there is not much of it on an
area basis, compared to the poorer soil at the hillbase. Studies from temperate toposequences generally show an opposite trend. In a shortgrass steppe
catena studied by Schimel et al. (1985), N availability
increased downslope. On the other hand, for tropical
forests in Brazil, Livingston et al. (1988) reported
results similar to the present

study. They showed that

N-mineralization rates increased from 7.6 pg N gg’
10 d-l at the hill bottom to 19.3 pg N g-i 10 d-l at
the ridge. Tanner (1977) also showed 28-53% lesser
nitrification at slopes than on ridges. Differences are
obviously

related

to the distribution

of substrate

as

total C and N contents of soil decreased downslow.
. ,
as opposed to the trend in the shortgrass steppe.
Acknowledgements-I
thank Professor J. S. Singh for
research guidance and a review of the manuscript. The
research was supported
by the University Grants
Commission, New Delhi.

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