Soil nitrogen mineralization under a Eucalyptus plantation and a natural Acacia forest in Senegal

Abstract

In-vitro and in-situ N mineralization were studied in a natural Acacia seyal stand and in a Eucalyptus camaldulensis plantation in Senegal. Mineralizable N, measured by 20 days in-vitro incubations, averaged 40–50 ppm in Acacia soil and 11–14 ppm in Eucalyptus soil, and reached 3.5 and 2.3%, respectively, of total N. The coarse light fractions (>0.2 mm) of Eucalyptus soil organic matter did not produce any mineral N; about 80% of the mineral N was supplied by the organo-mineral fraction, as against 30–50% in Acacia soil. In-situ mineralization was related to precipitation, and ranged from 18 to 40 ppm over 4 weeks during the rainy season in the Acacia stand where 7–10% of total N was mineralized each year. Under Eucalyptus, N mineralization reached only 10 ppm over 3 weeks in the beginning of the rainy season and then decreased sharply. It was assumed that this decrease was related to a depressive effect of herbaceous root growth, the possible processes of which are discussed.