Recently, scientists have concentrated their efforts on the study of ecological stoichiometry, particularly on the most dynamic elements in soil. Most researchers (including Mulvaney et al, 2007; Christoph et al, 2013) have considered carbon and nitrogen sequestration, and phosphorous distribution (Osodeke et al, 2006) in soils around the world that dominate soil ecology.
In our study of ecological stoichiometry, carbon, nitrogen, sulphur and phosphorous are very important because during the decay and synthesis of organic matter in the soil, transformation and mineralisation of these elements that chelate as compounds are usually involved. Because of our advancing land-use it is not clear, for example, how carbon sequestration may change in response to the changing land use. Sterner and Elser (2002) stated that ecological stoichiometry implied that plant communities with low biomass carbon to nutrient ratio have fast turnover rates, high nutrient cycling and low carbon sequestration, while those with high biomass carbon to nutrient ratios have slow turnover rates, slow soil nutrient cycling and high carbon sequestration in organic soils.
The transformation of landscapes from non-urban to urban land use has the potential to greatly modify soil carbon (C) pools and fluxes (Pouyat et al, 2002). They stated that for urban ecosystems very little data exists to assess whether urbanisation leads to an increase or decrease in soil C pools. Urban areas play a significant role in the global carbon cycle as source of carbon emission due to the effects of urban sprawl against other land use types. It is expected that by 2030, an additional 1.2 million square kilometres of land will be converted to urban land use, which is expected to result in a loss of carbon storage in natural vegetation of about 138 PgC (Seto et al, 2012). During the decomposition of organic matter, mostly made up of C, nitrogen is usually mineralised. Because of the inter-link in biochemical reactions between C, nitrogen and phosphorous, the study is thus necessary for a proper understanding of ecological stoichiometry, especially carbon, in our ecosystem. One such stoichiometry that signifies the breakdown of carbon in the soil is given thus; CO (NH2)2 + 2H2O ® (NH4)2CO3. In this research, carbon was given an important priority and was converted to a proper SI unit for a good understanding of the concentrations sequestered. Leaf litter from trees and shrubs are mainly the organic matter components of natural soils. Organic matter is usually decomposed by soil inhabiting organisms and the nutrients and energy they contain are released for utilisation by the organisms themselves or the vegetation. Soils in urban and peri-urban areas are interrupted from the cycles by various factors; leaf litter is often swept up as trash, or very little litter falls on urban soils because of the low amount of biomass produced by the plants. Assessment of sequestered carbon and other ecological stoichiometry along urban and rural gradients are important to be able to give an account of changes in carbon pool with consequent land use change since carbon has been recorded as the most important element to sequester if we can check the global climate change.
The objective of the study was therefore to use and detect the soil carbon stock as a key element of the ecology in defining soils that were highly sequestered with carbon and other ecological stoichiometry along urban-rural gradients.
Description of the study site
The study was conducted in three urban-rural gradients, namely urban, suburban and rural soils. Soils were delineated in this order based on ongoing activities and evidence of changing land uses. Soils in the urban areas had some parts with gullies as a limitation, the suburb soils were mainly used for arable crop production while that of the rural had thick vegetation that were made up of shrubs and trees. The whole study areas were located in Imo State, a Southeastern part of Nigeria. Imo state is located approximately between longitudes 6° 50’E and 7° 25’E and latitudes 4° 45’N and 7° 15’N. The state lies within a tropical climate characterised by rainy season (February/March – November) and dry season (November – February/March). Annual rainfall in the state ranges from 3,000 mm along Atlantic coast to 2,000 mm in the hinterland. Average annual temperature of the state ranges from 25 to 27° C.
Table 1 shows the result of the descriptive statistics of the soil properties of the studied soils. The normality of distribution of the measures of central tendency of the soil properties was mostly symmetric in the urban soils with the exception of clay, K, exchangeable acidity and moisture content. In the suburban soils, the distribution of soil carbon stock, bulk density, hydraulic conductivity and phosphorous were as well asymmetric. It was obvious that the changing land use gradient from urban to a suburb affected the carbon stock of the latter soils (mean = 6.59 Mg C ha-1) because of the trending presence of organic matter as we travel in space off from the urban areas. According to Craul (1985) soils in urban and peri-urban areas are interrupted from the cycles by various factors; leaf litter is often swept up as trash, or very little litter falls on urban soils because of the low amount of biomass produced by the plants. This was not the case for the suburb soils. The trending presence of organic matter that affected the carbon stock of the suburb soils as well affected the phosphorous contents (mean = 23.39 g kg-1) which could be because of the availability of phosphorous from the organic phosphates. In the rural soils, most of the soil properties were neither skewed nor kurtosis with the exception of phosphorous, Ca, Mg, K, Na and bulk density. The distribution of the carbon stock of the rural soils were not skewed but kurtosis, which could be adduced to availability of more carbon pool (mean = 8.63 Mg C ha-1) in the whole area as a result of the thick vegetation made up of trees and shrubs. The soil properties varied in the areas. Moisture content, hydraulic conductivity and K varied highly (CV ≥ 45% ≤ 150%) in most of the soils, base saturation, Mg, Ca, organic carbon, nitrogen, clay and silt varied moderately (CV ≥ 17% ≤ 39%) while the bulk density, ECEC, exchangeable acidity, P, pH and sand had a low variability (CV ≥ 3% ≤ 13%). Soil carbon stock varied moderately in the suburb and rural soils. The carbon stock (SCS) in the rural soil was highest (mean SCS = 1.65 Mg C ha-1) followed by suburb (mean SCS = 6.59 Mg C ha-1) and then urban soils (mean SCS = 8.63 Mg C ha-1) in that order.
“ecological stoichiometry has had a diverse study; its importance is in the mitigation of the global climate change and for exactitude in soil management”
The graphs adjacent plot soil carbon stocks against depths in the urban-rural land use gradients. It could be seen that at the depth of 20 cm, soil carbon stock in the rural soils (8.5 Mg C ha-1) were highest when compared among the other gradients. When monitored at the depth of 40 cm, the carbon stock in all the areas neared equals (8.0 Mg C ha-1 – 8.2 Mg C ha-1). This could be because the land use in the urban and suburban areas had not so deteriorated the carbon pool in these areas. The soil carbon stock showed a great difference having at least 4 Mg C ha-1 greater pool in the rural soils than the other two areas.
Most of the soil properties of the three land use gradients had a symmetric distribution. The changing land use of the suburb soils might have resulted in the asymmetric distribution of the soil carbon stock, phosphorous and bulk density in the areas. The rural soils had the greatest carbon pool, which affected the availability of phosphorous in a direct proportion. Ecological stoichiometry has had a diverse study; its importance is in the mitigation of the global climate change and for exactitude in soil management.