Thursday, 10 November 2005 - 8:15 AM
306-2

Nutrient distribution in relation to landscape position and tree abundance in a small agricultural watershed in Southern Malawi.

Rebbie Harawa, Cornell University, Cornell University, 205 Cypress Court Apt 6, Ithaca, NY 14850

Abstract

In Malawi, marginal lands are increasingly under food production due to human population increase. In the southern part, the forested watersheds have been converted to agrarian watersheds where sediment and nutrient losses are accelerated. Understanding the distribution of soil nutrients is a critical component for implementing soil management practices in these watersheds. A study was conducted to determine the effect of landscape position on soil nutrient distribution and to examine the relationship between trees and nutrient accumulation in a watershed. Soil chemical properties were measured on a 20m grid in a 15 ha agricultural watershed with two distinct landscape positions: upper slope and dambo (lower slope). A total of 235 soil samples were taken from the topsoil (0-20 cm) and each sampling point was geo-referenced using a Global Positioning System (GPS). Total N, organic C, available Ca, Mg, and pH were higher in the dambo than the upper slope. Available P was low in the dambo and highest concentrations were observed around homesteads in the upper slope. Trees significantly influenced the distribution of soil properties in the watershed. Medium to high concentrations of organic C, total N, available K, and soil pH were observed under high tree counts. In contrast, high concentrations of available Ca, Mg and P corresponded with low tree counts. The results show that distribution of organic C, total N, available Ca and Mg, and soil pH were related to landscape position. The distribution of available P was rather related to anthropogenic activities. Trees improved soil properties with the exception of available Ca, Mg and P. The increase in total N in relation to high tree counts is very important in maintaining the productivity of the upper slope soils since N is the nutrient limiting crop productivity the most in agricultural watersheds.


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