BEATE FORMOWITZ1, RAINER GEORG JOERGENSEN2, ANDREAS BUERKERT1
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
Numerous reports have shown legume-rotation induced increases in cereals on nutrient"=poor West African soils, however, their mechanisms are still debated. In this study differences in P and mineral N concentrations between continuous cereal (CC) and legume rotation (R) soils from the two West African sites Fada (Burkina Faso, F) and Koukombo (Togo, K) were determined and taken as the basis for nutrient application rates for a growth chamber experiment. Sorghum (Sorghum bicolor Moench) was planted on Fada soils and maize (Zea mays L.) on Koukombo soils. Treatments for CC soils consisted of five times the difference of N (N5), five times the difference of P (P5), and five times the difference of both nutrients (N5P5). Treatments for rotation soils consisted of four times the difference of the respective nutrient concentrations (N4, P4 and N4P4). These treatments were compared to the unamended soils (CC and R). Shoot length was measured daily. After harvest shoots and roots were analysed for their nutrient concentrations and total root length and mycorrhizal infection determined.
The combined application of P and N increased plant height significantly (FCC-P5N5 = 65 cm; FR-P4N4 = 59 cm; both FCC-P5 and FR-P4 = 55cm) compared to N application only and to the unamended soil (FCC-N5 = 35 cm; FR-N4 = 40 cm; F-CC = 42 cm; F-R = 41cm). Irrespective of the N and P level applied, for the same nutrient input shoot dry matter was significantly higher on rotation soils than on continuous soils (FR-P4N4 = 7.4 g; FCC-P5N5 = 4.7 g). Sorghum shoots had higher concentrations of P, K and Na for all CC treatments compared to the respective rotation treatments. In contrast, shoot N concentrations were significantly higher for FR-N4 (18.5 mg g-1) and FR (18.8 mg g-1) compared to FCC-N5 (9.6 mg g-1) and FCC (6.7 mg g-1).
Mycorrhizal infection rates were higher on FR soils even if this difference was only significant for FR-N4 (17%) compared to FCC-P5 (7%) and FCC-P5N5 (6%). Nematode counts are conducted and will be presented in the final version of the abstract.
Keywords: Mineral N, mycorrhiza, phosphorus, root length, Sahel