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

Amounts of biomass, soil organic matter, soil and tissue N, P and K were as sessed using regression techniques in widely spaced Acacia senegal plantati ons, aged between 3 and 18 years in northern Senegal, in order to provide q uantitative information on the rate of site nutrient enrichment in tree fal lows. Tree biomass increased linearly with time from Age 3 to Age 18 and was line arly related to stem cross-sectional area at 30 cm height. Between ages 3 a nd 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 fruit s than in woody tissues, and nutrient concentrations in wood decreased as t issue diameter increased. Phosphorus concentrations in wood decreased as ti ssues aged and there was evidence to suggest P retranslocation from woody t issues as trees grew older. In general, P concentrations in roots were simi lar to those found in above-ground woody tissues, but N and K concentration s in woody roots were, respectively, greater and smaller than concentration s found in above-ground woody tissues of similar diameter. Soil organic matter, N, P and K concentrations were always greatest in surf ace horizons close to stems. However, unlike the case for N and K, there wa s little evidence of increasing P in surface soil as plantations aged. Corr elations 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 amel ioration 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 k g ha(-1) year(-1) respectively, in plantations at 6-m spacing and soil orga nic matter increased by ca. 0.05% and 0.035% year(-1) under tree canopies a nd in open ground, respectively. Felling the trees and harvesting all wood >2 cm diameter at Age 15 would re move 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 t o 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. (C) 1999 Elsevier Science B.V. All rights reserved.