Analysing resilience in dryland agro‐ecosystems: a case study of the Makanya catchment in Tanzania over the past 50 years

Dryland agro‐ecosystems in sub‐Saharan Africa provide the resource base for some of the fasted growing populations today. However, rainfall variability and poor soils make these systems inherently vulnerable, and land degradation reduces their capacity to cope with disturbances. In this paper we propose a theoretical framework for interpreting dynamics and resilience in such systems, where two aggregate variables, the agricultural soil water index and the ecosystem insurance capacity, are particularly important. We apply the framework to the case of the Makanya catchment in Tanzania and conclude that the studied area has moved towards an increasingly degraded state, where ecosystem services other than food have been lost, over the past 50 years. Three main drivers behind this are identified; a) institutional changes affecting strategies for natural resource management, b) increased dry‐spell frequency, and c) high population growth. We suggest that the reason for the dramatic effects is that these changes occurred simultaneously, reducing the adaptive capacity of the local population. However, several trends in the area today indicate that there is a window of opportunity for positive change. Copyright © 2007 John Wiley & Sons, Ltd.     

Soil organic matter decline and compositional change associated with cereal cropping in southern Tanzania

The spatial analogue method and ¹³C analytical techniques were used to reveal medium‐ to long‐term changes in soil organic matter (SOM) in farmers' fields under maize in southern Tanzania. Aerial photography and detailed farmer interviews were used to relate land‐use history to declines in SOM concentration and changes in composition. The research attempted to measure the rate of SOM decline and the extent to which farmers' residue management practice was allowing cereal residues to contribute to SOM. The combination of research methods employed in this study proved to be highly complementary. Results indicate that native SOM decreased by on average 50 per cent; after 25 years of cultivation. Under current residue management with cereal residues mostly grazed and burnt there is only a relatively modest contribution from cereal residues to SOM. When cereal residues are retained in the field it is likely they will contribute significantly to SOM but they are much less likely to build SOM in the medium to long term. The paper concludes that in many situations it is probably best for farmers to allow the majority of the residues to be eaten by cattle in these systems rather than attempt to build SOM or risk nitrogen immobilization in cropped fields. The greater importance of inputs of high‐quality (e.g. legume) residues for nutrient supply in the short term is highlighted, in contrast to inputs of poor‐quality (e.g. cereal) residues in an attempt to build SOM in the longer term. Copyright © 2001 John Wiley & Sons, Ltd.