Changes in soil organic carbon and soil nutrient levels of a natural semi-arid savanna brought under cultivation

Abstract

The study was conducted between January 1997 and February 2000 in a semiarid area located at the University of Nairobi's Institute of Dryland Research, Development and Utilisation, located at Kibwezi. The purpose was to determine the changes that occur in soil carbon and nutrients (total soil nitrogen and available phosphorus) when a natural ecosystem is cleared to make way for cultivation of under local agronomic practices. The experimental design used was a 5x 3 Latin square design witha slight modification in which the treatment plots were spaced over the entire fenced off area to take care of the heterogeneity of soil texture and nutrients common to semi arid and arid regions. Plots measuring 20 x 20 m were marked in an enclosed area 2 hectares in size. Fifteen of plots were randomly assigned to five treatments in 3 blocks of replicates. The treatments were natural ecosystem, maize sown with goat manure, maize intercropped with pigeon peas and maize sown alone. At the beginning of each of the six growing seasons, soil samples were collected using a soil augur at Q - 20, 20 -40 and 40- 60 cm depths. The levels of total soil organic carbon, total soil nitrogen and available phosphorus were then determined. Soil moisture content was determined at this point of the season and every fortnight thereafter. Net nitrogen mineralisation was determined in.seasons 2, 3 and 6 using the aerobic laboratory incubation from soil samples collected-;,over the growing season. At the end of each growing season, dry maize yield (kg ha-I