DECOMPOSITION OF COWPEA AND MILLET AMENDMENTS TO A SANDY ALFISOL IN NIGER

Current inputs of organic materials to cropped lands on sandy Alfisols and Entisols in Sahelian West Africa are insufficient to arrest soil organic matter (SOM) decline. Crop residues and green manures require proper management in order to maximize their contribution to nutrient supply and SOM maintenance. The objectives of this study were to quantify the rates of C and N mineralization from cowpea (Vigna unguiculata (L.) Waip.) green manure, cowpea residue, and millet (Pennisetum glaucum (L.) R.Br.) residue under field conditions in Niger and to determine the effect of these organic amendments on pearl miller yield. Miller was grown (1) as sole crop, (2) as intercrop with cowpea, (3) as intercrop with cowpea that was incorporated as green manure during the second half of the growing season, (4) with incorporated cowpea residue (2000 kg ha(-1)), (5) with millet residue mulch (3000 kg ha(-1)), and (6) with N fertilizer. Carbon loss as CO2 from soil with and without organic amendment was measured three times per week during the growing season. Nitrogen fertilizer increased miller yield only in a year with a favorable rainfall distribution. Cowpea grown in intercrop with miller during the first part of the sowing season and subsequently incorporated as green manure between miller rows increased miller grain yield in a year with sufficient early rainfall, which could be attributed to the rapid rate of decomposition and nutrient release during the first 3 weeks after incorporation. In a year with limited early rainfall, however, densely planted green manure cowpeas competed for water and nutrients with the growing miller crop. Incorporated cowpea residue and miller residue mulch increased miller yield. Surface applied miller residue had high rates of decomposition only during the first 3 days after a rainfall event, with 34% of the miller residue C lost as CO2 in one rainy season. Recovery of undecomposed millet residue at the end of the rainy season was related to presence or absence of termites, but not to seasonal C loss. Miller residue mulch increased soil organic C content of this sandy Alfisol in Niger. Cowpea and millet residues had a greater effect on SOM and miller yield than cowpea green manure due to their greater rate of application and slower rate of decomposition.