Abstract: This study investigated the distribution of soil carbon in two slope aspects of a hilly landscape in Southeastern Nigeria. A transect was used to align soil profile pits dug on three physiographic positions of crest, midslope and footslope on both slope aspects. Soil and core samples were collected from pedogenic horizons based on soil profile differentiation. Standard analytical and statistical tools were used to analyze soil samples and data, respectively. Results showed that less values of soil carbon were recorded in soils of the windward side despite its forested nature. Organic carbon (OC) decreased with depth in both slope aspects while inorganic carbon (IOC) had higher values in deeper layers. Bulk density decreased as organic carbon increased in soils of the study site. Soils on the windward landscape had higher erodibility values based on calculated Dispersion Ratio (DR) values. Better relationship existed between soil carbon and DR on leeward aspect of the hilly landscape when compared with values of the same parameters in soils of the windward side of the study area.
INTRODUCTION
Topographic changes influence pedogenesis and soil properties of any given location. Birkeland (1999) reported marked variabilities in soil properties due to slope aspect. Topography affects absorbance of solar energy such that south-facing slopes in the northern hemisphere are more perpendicular to the sunrise and are warmer hence soils on such slopes have lower organic matter content (Esu, 2005). This is possibly due to higher temperatures which increase rate of decomposition and consequent loss of soil carbon (Eshett et al., 1989).
Although greatest influence of slope aspect is found between latitudes 40-60°N (Hunkler and Schaetzi, 1997), steep slopes encourage erosion of surface horizons and soil carbon contained therein irrespective of latitudinal position. Bajracharya et al. (1998) noted that past erosion affects the distribution of organic matter in the soil profile while Pennock et al. (1994) had earlier reported loss of soil organic carbon in shoulder complexes and gain of soil and soil organic carbon on footslope complexes after a prolonged cultivation. In addition to this, decline in OC following cultivation and the detrimental effects of decreased of OC have been well documented (Follett, 2001; Mann et al., 2002; Wilhelm et al., 2004; Hooker et al., 2005).
Soil carbon reflects the plant source of carbon (Landl et al., 2004). Among soil properties, total organic carbon is a sensitive indicator suggesting that within a narrow range of soil, it may serve as a suitable indicator of soil quality (Murage et al., 2000). Soil carbon expressed as soil organic matter is indeed considered as one of the most important estimators of soil quality (Gregorich et al., 1994). If soil quality can be inferred from the quality of soil carbon (Seybold et al., 2004), then knowledge of about the quantitative soil carbon circulation is needed to understand soil development and ecosystem function (Warembourg and Kummerow, 1991). This need becomes more imperative in a densely populated and fragile ecosystem as demographic pressure may promote exploration of marginal lands.
Igwe (2003) reported heightened soil degradation by soil water erosion in some parts of the study area which Mbagwu and Obi (2003) attributed to reduction in soil organic matter and consequent deterioration of soil quality in terms of physical, chemical and biological properties. The situation is worsened in a hilly topography moreso where soils of the region have high erodibility potentials (Onweremadu, 2006). Given that there is shortened fallow due to population increase in the study area (Onweremadu, 1994), farmers report to cultivating fragile lands including steep landscapes. The hilly study area supplies urban population of Okigwe as well as nomadic settlements of the suburbs with farm products. As a consequence of the above farmers tend to extend farming to steep slopes without corresponding organic matter inputs resulting to soil carbon insufficiency and reduced yield. The major objectives of this study were to estimate the distribution of soil carbon in the study site and to compare the soil carbon distribution in the windward and leeward aspects of a hilly landscape at Okigwe, southeastern Nigeria.
MATERIALS AND METHODS
Study Area
The study was conducted before the on-set of the rainy season in 2006 on
a hilly uncultivated landscape at Okigwe, southeastern Nigeria. It is located
on latitude 5° 481 4611. 97oN, longitude
7° 351 5411. 810E and with an altitude of 300 m (Handheld
Global Positioning System Receiver readings). The study area has a rainforest
agroecology characterized with 2000-2250 mm mean annual rainfall and 80-82°F
mean temperature (Federal Department of Agricultural Land Resources, 1985).
Orographic rainfall characterizes the landscape with the windward portion of
the terrain receiving higher amount and intensity of rainfall than the leeward
side of the study area. The windward side faces the south while the leeward
part has a northern orientation. Windward area was forested while grasses with
patches of shrubs dominated the leeward side of the study area. Soils are derived
from Falsebedded Sandstones (Ajalli Formation) (Orajaka, 1975). Hillside farming,
hunting, stone mining, quarrying and excavation activities constitute the major
socio-economic activities of the study area.
Soil Sampling
A transect was used to locate and align soil profile pits in both windward
and leeward slope portions of the landscape at an equidistance of 20 m. The
vegetation along the transect line was cut to enhance contact with the soils.
Three soil profile pits were dug at three identified physiographic positions,
namely crest, midslope and footslope on both the windward and leeward sides
of the hill. Soil samples were collected from the profile pits based on the
degree of horizon differentiation. Core samples were collected from each pedogenic
horizon of the soil profile pits in both core and soil sampling and three samples
were collected per horizon. Soil samples were air-dried at room temperature,
crushed and sieved using 2 mm sieve in preparation for laboratory studies.
Analytical Methods
Particle size distribution dispersed in both water and sodium hexametaphosphate
(calgon) was determined by hydrometer method according to the procedure of Gee
and Or (2002). Bulk density was obtained using the procedure of Grossman and
Reinsch (2002). Soil organic carbon was measured by combustion at 840°C
while total soil carbon was estimated at 1140°C using a Leco CR-12 C analyzer
(Lec Corp, St. Joseph, MI) (Wang and Anderson, 1998). Inorganic soil carbon
was calculated as a difference between total soil carbon and soil organic carbon.
Total soil nitrogen was measured using microkjeldahl method (Bremner and Mulvaney,
1982). Soil pH was estimated electrometrically on a 2:1 soil/water solution
(Hendershot et al., 1993).
Dispersion Ratio (DR) was calculated and used for estimating erodibility of soil the study site. It was computed as a ratio of silt-clay dispersed in water to silt-clay dispersed in sodium hexametaphosphate multiplied by 100 (Middleton, 1930). Although DR was computed for all horizons only two topmost horizons were used in the determination of erodibility since soil erosion is a surficial prienomenon.
RESULTS AND DISCUSSION
Physical Properties
Results of soil physical properties on the windward and leeward slope aspects
of the study area are shown in Table 1 and 2,
respectively. Soils of the area were texturally dominated by sand-sized particles
which is attributable to the parent material from which soils were formed. Although
sand-sized particles were of higher values on the leeward side of the landscape,
sandiness decreased downslope whereas the reverse was the case for clay-sized
fractions. Silt-sized particles followed the same trend as observed in clay
especially in the windward side of the landscape. Silt and clay are lighter
than sand leading to easy transportability from the crest towards the footslope.
These results are similar to the findings of Obi and Asiegbu (1980) that highest
values of sand were obtained at the crest when compared with middle slope and
footslope physiographies. Higher bulk density values were recorded on the windward
side of the hilly landscape. Rainfall amount and intensity were higher on the
windward slope side and this tends to impact on the soil despite the thicker
vegetation.
Carbon Distribution and Selected Chemical Properties
Table 3 and 4 show the distribution
of carbon forms and selected chemical properties in the windward and leeward
slope aspects, respectively. Total carbon decreases with depth in all the profile
pits irrespective of landscape position. Organic Carbon (OC) content increased
with depth. Greater values of all forms of carbon were recorded on the leeward
landscape soils. Mean values of total carbon as recorded on the leeward soils
were 7.5 g kg-1 (crest), 10.3 g kg-1 (midslope) and 10.4
g kg-1 (footslope) when compared with 4.6, 4.8 and 7.7 g kg-1
total carbon contents of crest, midslope and foot slope soils of the windward
portion of the landscape.
| Table 1: | Mean values of some physical properties of soils measured on the windward side (n = 57) |
| BD = Bulk Density, c = The letter ‘c’ attached as subscript shows that values were calgon dispersed | |
| Table 2: | Some physical properties of soils measured on the leeward side (n = 54) |
| Table 3: | Some chemical properties of soils measured on the windward side (n = 57) |
| OC = Organic Carbon, IOC = Inorganic Carbon, Total C = Total carbon, Total N = Total nitrogen, C/N = Carbon-nitrogen ratio | |
Higher values of carbon on the leeward landscape soils is attributable to
lesser rainfall and consequently lower runoff water hence minimal erosion and
inter-pedon translocation of soil carbon. This promotes soil formation as can
be seen in the thickness of A horizons of leeward pedons (crest:0-22 cm and
midslope: 0-25 cm) when compared with windward pedons (crest: 0-11 cm and 0-13
cm).
| Table 4: | Some chemical properties of soils measured on the leeward side (n = 54) |
This is in consonance with the assertion of Schimel et al. (1991) that climate, land use and landscape position are important variables in predicting rates of ecosystem processes.
Organic and total carbon contents increased downslope in both the windward and leeward slope aspects. Earlier, Pierson and Mulla (1990) found that soils on footslope and toeslope topographic positions had a higher organic carbon content than those on summit positions. Similar results were also observed for prairie soils of Canada (Gregorich and Anderson, 1985).
Carbon-nitrogen ratios were higher on soils lying on the windward side of the hilly landscape. The carbon-nitrogen ratio of West African forest topsoils usually stabilizes itself at about 10 to 12:1 but much lower in deeper horizons (Ahn, 1979). The results tend to suggest that soils on the windward portion of the study area are prone to greater immobilization by soil organisms.
Soils of the windward slope aspect had lower soil pH values (strongly acidic) which could be due to interactive activities of pedogensis and climate on soils. Soils of the windward side received more rainfall which increased leaching and runoff of nitrates, suggesting that available soil nitrogen will be utilized by soil microbes to build up their tissues thereby inducing N-deficiency. In addition to this, soils of the windward side contain organic acids due to plant litter and this supports leaching through solubilization of substance and add soil acidity.
Land Degradation Potential
Values on vulnerability of soils to erosive forces of runoff are shown in
Table 5. Soils on the windward side of the hilly terrain were
more vulnerable to soil erosion by the agency of water. This trend holds despite
the forested nature of the ecosystem. However, climate and land use history
tend to account for this variability. Because of the forest vegetation on the
windward side, local farmers believe that it can sustain their crops more than
the grassy part of the hilly ecosystem. Consequently, the windward side is commonly
devegetated for farming activities and allowed to regenerate its fertility naturally
after harvest.
| Table 5: | Vulnerability of soils to erosion (n = 6) |
| DR = Dispersion Ratio | |
| Table 6: | Relationship soils carbon and DR (p = 0.05) |
Generally, the subsoils had higher DR values, suggesting that their exposure may promote rapid removal of soils by runoff water. Soils having DR values grater that 15% are highly vulnerable to soil erosion (Middleton, 1930). Lower values of DR in soils of the leeward side of the study area could be due to higher soil carbon content coupled with less rainfall amount and intensity. Again, results of correlation studies (Table 6) indicate better relationship between soil carbon and DR while R2-values suggest greater prediction certainty in soils of the leeward-oriented landscape.
CONCLUSIONS
It was concluded based on the findings of the study that soil carbon and its various forms vary in very short distances in a hilly ecosystem. As total soil carbon and soil organic carbon decrease with depth vertically, soil inorganic carbon tend to have appreciable values in subsurface horizons. Results also indicated high susceptibility to soil degradation of soils of both the windward and leeward slope aspects given the values of DR. With the available soil data, soils of the leeward side had better correlation and can be predicted with more certainly than soils of the windward slope aspect.
ACKNOWLEDGMENT
The author is highly thankful to Dr. Ezekiel Izuogu for financial assistance in the study.