Carbon and nutrient budgets of the Chagga home garden system in the Kilimanjaro highlands, Tanzania

Sustainable land management of smallholder farms is crucial for ensuring food security in sub-Saharan Africa. However, little is known about the nutrient dynamics of smallholder farming systems at the farm level based on primary data. In this study, carbon (C) and nutrient budgets of the home garden system in the Kilimanjaro highlands, where Andosols predominate, were quantified at the farmer's field. We evaluated (1) the soil C and nutrient flow in the main three land-use blocks (banana (Musa spp.) garden, maize (Zea mays L.) field and grassland) in one representative home garden and (2) the internal flow between farmland and livestock and the external nutrient flow across the inside and outside of the six home gardens. Intensive applications of livestock dung to the banana trees resulted in a positive C budget (7.2 Mg C ha(-1) year(-1)) in the banana garden. Nitrogen loss through the harvesting of feed and crops was almost balanced with the livestock dung application, while nitrogen loss through leaching only accounted for 3% of that applied. Banana productivity has been maintained despite a negative potassium budget (-241 kg K ha(-1) year(-1)), probably owing to the replenished exchangeable potassium from Andosols. In the maize field, the C budget was negative (-1.7 Mg C ha(-1) year(-1)) owing to high organic matter decomposition. Carbon and nutrient budgets in the grassland were all negative. Our results revealed that the village average livestock density (4.4 TLU ha(-1): TLU means tropical livestock unit) was sufficient for P, Ca and Mg balance in the home garden, whereas it was not sufficient for N and K balance. Increasing livestock density improved the nutrient balance of the system. However, it is noteworthy that 33%-47% of the feed supplied as C and nutrients was collected from outside the home gardens, suggesting a high reliance on external inflow to fulfil feed demands. In conclusion, intensive livestock dung application to banana cultivation was fundamental for maintaining agricultural productivity to replenish the nutrients lost from the system. At the same time, this system was sustained not only by C and nutrient cycling within the system, but also by transporting resources from the external environment into the system.

Automated summary

Sustainable land management of smallholder farms is essential for food security in sub-Saharan Africa. This study focused on quantifying the carbon and nutrient budgets of a home garden system in the Kilimanjaro highlands, revealing the importance of intensive livestock dung application for maintaining agricultural productivity and the reliance on external inflow for meeting feed demands. Increasing livestock density improved nutrient balance, but nitrogen and potassium balance were still insufficient. The results highlight the need for sustainable and efficient nutrient management in smallholder farming systems for long-term food security.