NITROGEN-FIXATION CAPACITY IN THE COMPONENT SPECIES OF CONTOUR HEDGEROWS - HOW IMPORTANT

The choice of an appropriate hedgerow species is one of the most criti cal decisions in exploiting the value of a contour hedgerow system. Th e implications of hedgerow species with nitrogen (N)-fixation capacity on hedgerow-crop competition and crop productivity have been widely d ebated. We examined the agronomic significance of N-fixation by compar ing the performance of species representing three classes of hedgerow vegetation: A nitrogen-fixing tree legume Gliricidia sepium), a non-ni trogen fixing tree (Senna spectabilis syn. Cassia spectabilis), and a forage grass (Pennisetum purpureum). The 4-year study investigated the hedgerow biomass and nutrient yields, and their relative effects on t he performance of two annual crops commonly grown in alley farming sys tems, with emphasis on hedgerow-crop interference. The work was done o n an Ultic Haplorthox (pH 4.8, organic C 1.9%, total N 0.18%). Senna p roduced 46% more pruning biomass on an annual basis than did Gliricidi a; N supplied to the alley crops was similar to Gliricidia in the firs t year of observation, but 20-30% higher in the succeeding years. Upla nd rice and maize grain yields and total dry matter were unaffected by tree species, but the nitrogen-fixing tree exerted less competitive e ffects on the annual crops growing in adjacent rows. Grass hedgerows r educed maize yields 86% by the second year, indicating an unsustainabl e drawdown of nutrients and water. We conclude that hedgerow systems c omposed of a nitrogen-fixing tree did not exert significant advantages compared to a non-fixing tree species, and that factors other than N- fixation were more important determinants for the choice of hedgerow s pecies.