Soil property changes over a 120-yr chronosequence from forest to agriculture in western Kenya

摘要

Much of the native forest in the highlands of western Kenya has been converted to agricultural land in order to feed the growing population, and more land is being cleared. In tropical Africa, this land use change results in progressive soil degradation, as the period of cultivation increases. Both rates and variation in infiltration, soil carbon concentration and other soil parameters are influenced by management within agricultural systems, but they have rarely been well documented in East Africa. We constructed a chronosequence for an area of western Kenya, using two native forest sites and six fields that had been converted to agriculture for up to 119 yr. We assessed changes in infiltrability (the steady-state infiltration rate), bulk density, proportion of macro- and microaggregates in soil, soil C and N concentrations, as well as the isotopic signature of soil C (δ13C), along the 119-yr chronosequence of conversion from natural forest to agriculture. Infiltration, soil C and N decreased within 40 yr after conversion, while bulk density increased. Median infiltration rates fell to about 15% of the initial values in the forest, and C and N concentrations dropped to around 60%, whilst the bulk density increased by 50%. Despite high spatial variability, these parameters have correlated well with time since conversion and with each other.