BEATE FORMOWITZ1, ANDREAS BUERKERT1, RAINER GEORG JOERGENSEN2
1University of Kassel, Organic Crop Production and Agroecosystems Research in the Tropics and Subtropics, Germany
2University of Kassel, Department of Soil Biology and Plant Nutrition, Germany
To investigate whether root residues of legumes and their specific decomposition are responsible for the yield increases on the subsequent cereal in a rotation, 2 g kg-1 soil of dry roots of cowpea (Vigna unguiculata Walp.; CP), groundnut (Arachis hypogaea L.; G), pearl millet (Pennisetum glaucum L.; PM), maize (Zea mays L.; M) and sorghum (Sorghum bicolor Moench; SO) were applied to pre-incubated monoculture soils from Fada (Burkina Faso, F) and Koukombo (Togo, K). Other treatments comprised mineral phosphorus (P) application in an amount equal to the applied P through legume root residues and a control (Con). At 5 sampling times over a period of 189 days (0, 7, 21, 63 and 189 days after incubation) microbial biomass C (Cmic) and N (Nmic) were determined and the soil respiration was measured initially every two days and later weekly. During the first 3 weeks Cmic concentrations were almost 41% higher in FG and 38% higher in FCP compared to FCon. In the Koukombo soils 43% higher Cmic concentrations were measured in KPM than in KP. Significantly higher Nmic concentrations were determined in FG (11.6 11#11g g-1) and FSO (10.5 11#11g g-1) compared to FCon (6.2 11#11g g-1). Nmic concentrations measured in KPM (7.0 11#11g g-1) were significantly higher compared to KP (4.7 11#11g g-1) and KCon (3.7 11#11g g-1) and soil respiration was up to 11-fold higher in FG than in FCon, whereas it was up to 16 fold higher in KP compared to KCon. Even if no legume-specific effects on microorganisms were observed in Koukombo soils, the system"=specific effects in the Fada soils indicate that influences of legume root residues and their decomposition on the microbial population might contribute to the yield"=enhancing effect of legume rotations. A subsequent experiment with analyses of adenylates, microbial P, ergosterol, pH, and POM finalized by September 2006 will allow more insights into soil biological and biochemical factors.
Keywords: Legume-rotation, microbial biomass , root residues, sahel