Nutrient and organic-matter accumulation in Acacia senegal fallows over 18 years

Abstract Amounts of biomass, soil organic matter, soil and tissue N, P and K were assessed using regression techniques in widely spaced Acacia senegal plantations, aged between 3 and 18 years in northern Senegal, in order to provide quantitative information on the rate of site nutrient enrichment in tree fallows. Tree biomass increased linearly with time from Age 3 to Age 18 and was linearly related to stem cross-sectional area at 30 cm height. Between ages 3 and 18 years, above- and below-ground biomass accumulation averaged ca. 1770 kg ha −1  year −1 for trees evenly spaced at 6 m. By Age 18, the average tree had accumulated ca. 945, 38 and 420 g of N, P and K, respectively. As expected, concentrations of N, P, and K were greater in leaves and fruits than in woody tissues, and nutrient concentrations in wood decreased as tissue diameter increased. Phosphorus concentrations in wood decreased as tissues aged and there was evidence to suggest P retranslocation from woody tissues as trees grew older. In general, P concentrations in roots were similar to those found in above-ground woody tissues, but N and K concentrations in woody roots were, respectively, greater and smaller than concentrations found in above-ground woody tissues of similar diameter. Soil organic matter, N, P and K concentrations were always greatest in surface horizons close to stems. However, unlike the case for N and K, there was little evidence of increasing P in surface soil as plantations aged. Correlations of amounts of N, P and K with amounts of soil organic matter were only significant in surface horizons and there was no evidence of soil amelioration other than near the soil surface. Increases of N and K in surface soil could be predicted by multiple regression equations based on tree age and inter tree spacing. N and K increased in surface soil by ca. 24 and 4 kg ha −1  year −1 , respectively, in plantations at 6-m spacing and soil organic matter increased by ca. 0.05% and 0.035% year −1 under tree canopies and in open ground, respectively. Felling the trees and harvesting all wood >2 cm diameter at Age 15 would remove ca. 60, 2.7 and 35 kg ha −1 of N, P and K, respectively, from a site with trees at 6-m inter-tree spacing. Harvesting a single years' production of leaves and fruits at Age 15 would remove ca. 75%, 80% and 40% of those N, P and K amounts. Regular large-scale fodder harvesting appears to pose a greater threat to site nutrient budgets in low input agroforestry systems than harvesting of wood at the end of tree rotations. However, the threat to nutrient budgets was less serious for N than it was for K and P because, in contrast to K and P at Age 15, there was more N in surface soil than in tree tissues.