Assessment of carbon stock and selected soil fertility indicators in a bush encroached savanna at Erichsfelde farm, Namibia
Bush encroachment is a form of land degradation common predominantly in semi-arid areas of the world. The resulting habitat degradation and loss of resource productivity make bush encroachment a serious environmental and economic problem in Namibia. Despite these negative effects, bush encroachment offers potential woody carbon stock storage, which can render Namibia a net carbon sink. This study was, therefore, aimed at quantifying potential woody carbon stock storage from different pools and assessing selected soil fertility indicators in low, medium and high bush-encroached sites at Erichsfelde farm in Otjozondjupa region. This was a quantitative study employing a stratified random sampling with systematic sampling along transects. For vegetation surveys in each site, five 20m x 10m plots were set up along two line transects, while established allometric equations were applied to related measured vegetation variables to estimate carbon stock. Additionally, five 1m x 1m soil pits were nested within the vegetation plots and soil samples collected at different soil depths for analysis. Across the three sites, the Kruskal-Wallis test showed no significant difference in the total (P=0.294>0.05) and above-ground (P=0.718>0.05) carbon stock. However, the below-ground carbon stock was significantly higher in the medium encroached site (P<0.001). This can be attributed to the fact that savanna vegetation accumulates most of its biomass underground as a protection against fire and herbivory. Additionally, the dense shrubs in the high encroached site do not invest in the below-ground biomass and carbon stock because of low herbivory as livestock will find it difficult to roam in there as oppose to the medium encroached site that feature higher below-ground carbon storage to protect the vegetation from herbivory and fire. The fact that themedium encroached site sequestered more below-ground carbon than the high encroached sites, suggest that bush encroachment does not positively contribute to carbon sequestration. Soil analyses showed no significant differences in the SOM, EC, pH, total N, Mn and ions (Cl-, Ca2+, Mg2+, and Na+) across the three sites. Contrariwise, the high bush encroached site was found to have greater amounts of soil P and K+ (Kruskal-Wallis P=0.009<0.05 & 0.049<0.05) and lower amounts of K, Ca, Mg, Cu and SAR. The difference in these major nutrients content indicated a soil nutrient deficiency due to the fact that savanna bushes have less biomass that returns low nutrients to the soil. It can be concluded that despite the known ecological importance of invader bushes, they do not sequester significant amounts of carbon nor do they positively contribute to soil nutrients. Thus, both farmers and decision makers need to put in place interventions to control invader bushes.
Carbon stock, Soil fertility, Bush encroachment
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