Abstract
Lack of organic input is a major factor contributing to the deterioration o f the sandy Alfisols and Entisols in the Sahelian zone of West Africa. Cowpea [Vigna unguiculata (L.) Walp.] has great potential as green manure due to its rapid N accumulation and efficient N 2 fixation. Our objectives were to quantify N mineralization of 5- and 7-week old isotopically labelled cowpea parts, to quantify C and N mineralization of cowpea parts in moist and in frequently airdried and rewetted soil, and to describe decomposition of cowpea green manure and millet [Pennisetum glaucum (L.) R.Br.] residue under field conditions in Niger. Millet was grown with green manure cowpeas incorporated during the second half of the growing season, incorporated cowpea residue, millet residue mulch, N fertilizer, and without amendment on a Psammentic Paleustalf soil in Niger. The amount of N mineralized from 7-week old cowpeas was more than double (235%) that from 5-week old cowpeas due to greater N accumulation (213% ) and a more rapid rate of N mineralization (27% vs. 24%) of the more mature cowpeas. The percentage of N mineralized was linearly related to the tissue N concentration and C/N ratio of the plant parts. Repeated drying and wetting did not reduce C mineralization of cowpea at 68 days, which averaged 62% of initial plant C. Nitrogen mineralization, however, was 46% of initial plant N in moist soil and 29% in dried and rewetted soil. Carbon and N mineralization were linearly related after an initial phase of rapid C loss. Cowpea green manure grown with millet for approximately 6 weeks and subsequently incorporated, increased millet grain yield in a year with sufficient early rainfall due to rapid decomposition during the first 3 weeks after incorporation. In a year with limited rainfall, however, green manure cowpeas competed for water and nutrients with the growing millet crop. Cowpea and millet residue increased millet yield. Millet residue mulch had high rates of decomposition only during the first 3 days after a rainfall event, and 34% of millet residue C was lost as CO>2 in one rainy season. Millet residue mulch can increase the soil organic C content of the sandy Alfisols in Niger.
Franzluebbers, Kathrin (1994). Carbon and nitrogen mineralization from cowpea green manure and millet residue in Niger and under controlled conditions. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1554398.