Long-Term Human and Biophysical Dynamics of Soil Degradation in the Kenyan Highlands.

Abstract

Agroecosystems are among the most tightly coupled of human and natural systems, as farmers make conscious decisions regarding land use and improvement, cropping systems, livestock management and labor allocation. These decisions can profoundly impact the natural resource base, which can then lead to changes in farmers' behaviors. The focus of this study is to understand the long term human and biophysical dynamics of soil degradation. We are especially interested in the role that soil degradation plays in creating poverty traps and in interventions that will strongly impact the dynamics of these systems. We have developed an integrated economic and biophysical systems dynamic model to understand and predict the long term behavior of farms in the Kenyan highlands. Additionally, we have established a chronosequence in western Kenya of farms converted from primary forest to agriculture 100, 70, 50, 30, 15, 5, and < 3 years ago. This chronosequence includes three blocks that contain all time conversions, with 3 farms per conversion. Soil chemistry and soil organic matter fractions have been measured from fields that have never received fertilizer additions. An extensive set of fertility experiments to examine the response of maize to amendment with nitrogen and phosphorus fertilizers, manure and green manure have been established on these soils. Socioeconomic data for these farms has been collected. The chronosequence data is being used to both parameterize and evaluate the model. Preliminary findings indicate that both soil organic matter and maize yields decline after conversion from primary forest, but not at the same rates. As the soil degrades and maize yields decrease, farms become more diversified by shifting some land into perennials. This change in land use is associated with a stable, though decreased, soil fertility level. The relationship of these changes in cropping systems and soil fertility to off farm activities and income will be discussed, as well as the implications of these dynamics for preventing soil degradation and restoring fertility.