Relationships between on-farm tree stocks and soil organic carbon along an altitudinal gradient, Mount Kilimanjaro, Tanzania

Understanding above-ground tree biomass carbon (AGC) and relationships to soil organic carbon (SOC) stocks across a landscape provide opportunities for better management of the carbon pools. This study determined relationships between on-farm AGC and SOC stocks along an altitudinal gradient on the slopes of Mount Kilimanjaro. Fifty plots (100 × 100 m) were established, whereby all trees ≥5 cm dbh, were recorded. Soil samples from top (0–20 cm) and subsoils (21–50 cm) were collected at the centre of the plots using four subplots. Tree inventory and soil analyses were performed and statistical tests were conducted to understand relationships between AGC and SOC stocks. Results indicated that stem density increased with altitude, however the upland and the midland did not differ significantly while the lowland differs with both the midland and the upland. A similar pattern was observed for basal area and above-ground tree biomass (AGB), with no significant difference between the midland and upland whereas the lowland differed significantly from both the upland and the midland. SOC stocks varied significantly, being the largest in the upland, amounting to almost twice the size recorded in the midland or the lowland. SOC stocks indicated poor correlation (Pearson’s: r = 0.327, df = 47, p = 0.023) and poor interaction (Wald = 0.0008, df = 1, p = 0.977) with AGC. This study concludes that the relationship between AGC and SOC stocks was masked by other factors including soil types, precipitation and land management. The protocol used to test the relationships might also have contributed further to current observation. Overall, the lowland area, having low AGC and SOC stocks, requires management interventions aimed at increasing SOC stocks.