Analysis of the Spatio - Temporal Dynamics of Soil Carbon and Nitrogen in a Tropical Bush - Fallow Chronosequence

The quantity of carbon (and nitrogen) that can be sequestered in soils is of particular interest to environmental scientists, ecologists, economists, and politicians amidst the changing climate, mainly driven by high levels of atmospheric carbondioxide. It seems logical, therefore, that bush fallows, which accounts for some of the largest land use in tropical environments, can maintain a reasonable store of carbon for long periods, thus the need to study and elucidate the soil nutrient status and interactions in such agro - ecosystems. Soil samples were collected in 2010 (Y1) and 2011 (Y2) from six different parts of the country separated by distance of at least 60 km. Each site is characterized by a different combination of geographic features and cultural setting of the local people. A total of 540 soil samples was taken from bush fallows one to 10 years old (denoted as 1Y, 2Y, 3Y, 4Y, 5Y, 6Y, 7Y, 8Y, 9Y and 10Y, respectively), at depths of 0cm, 2cm, 10cm and 20cm. The Elementor Vario EL and TruSpec CN analyzers were used to analyze the carbon and nitrogen content of the soils, in Y1 and Y2 sampling respectively, in accordance with published procedures. The relationship between C and N was consistent at all sampling depths, with very strong correlation (r2 values greater than0.85) in all cases (Figure 2). The highest mean percentages at the 2cm depth in the Y1 samples was obtained from 5Y fallows for both C (5.35?3.15) and N (0.36?0.18), whist 9Y fallows accounted for the highest mean %C (5.48?0.94) and %N (0.44?0.02) in the Y2 samples. The largest, but negative change in %C occurred in fallow plots that were 5 - 6 years (mean =1.905?3.489), but reasonable increases also occurred on the 2 - 3 years (mean = 1.707?2.142) and 9 - 10 years (mean = 1.337?3.183) fallows, respectively (Figure 4). Mean %N change was highest for 2 - 3 years (0.140 ? 0.082) and 9 - 10 years (0.134 ? 0.197) fallows. The results from a one - way ANOVA (Table 3) indicate a strong significant difference in overall mean %C between the study sites in the Y1 samples (F = 42.11; p less than0.001) and a reasonable significant difference in the Y2 samples (F = 3.09; p less than 0.05). In the case of %N, one - way ANOVA show very strong significant difference between sites in both Y1 samples (F = 11.6; p less than 0.001) and Y2 samples (F = 10.18; p less than 0.001), respectively. The general deduction from the nature of the graphs in Figure 3, especially at the 2 cm depth, is that C increased with 5 to 6 years of fallowing and declined by 7 to 8 years of fallowing and increased again towards 10 years of fallowing. Further research should throw light on the effect of location characteristics and management regimes on C and N dynamics in fallow soils.