N-2 fixation in cowpea plants grown in farmers' fields in the Upper West Region of Ghana, measured using N-15 natural abundance

Few studies have assessed the levels of symbiotic N nutrition in legumes grown by farmers in Africa. In this study, the shoots of cowpea plants were sampled from 63 farms in 12 villages within 5 districts of the Upper West Region of Ghana, and assessed for growth and symbiotic N nutrition. The data revealed considerable differences in cowpea plant density per m(2), plant growth, N-15 natural abundance (delta N-15), %Ndfa, and N-fixed among different farms under one village, and between villages under the same district, and between districts in the Upper West Region. In farms where there were fewer cowpea plants per m(2), plant growth was better and dry matter yield per plant significantly greater, leading to strong variations in delta N-15 values. Except for four farms at Bamahu which had cowpea shoot Ndfa values of 12.1%, 30.0%, 36.5% and 46.6%, one farm at Babile with Ndfa value of 58.1%, and three farms at Silbelle with Ndfa values of 56.8%, 57.9% and 68.7%, the remaining 55 out of the 63 farms studied showed high shoot Ndfa values, ranging from 70.6% to 99.7%, which clearly indicates that cowpea cultivated by farmers in the Upper West Region of Ghana meet a large proportion of their N requirements from symbiotic fixation. At the district level, isotopic analysis showed that, on average, the N-15 natural abundance values (parts per thousand) of cowpea shoots were -0.496 +/- 0.04 for Jirapa, -0.083 +/- 0.06 for Nadowli, 0.368 +/- 0.08 for Lawra, 1.333 +/- 0.29 for Wa and 0.365 +/- 0.09 for Sissala district. Estimates of the legume's N derived from fixation were 66.3% for Wa district, 89.9% for Nadowli, 79.4% for Lawra, 78.9% for Sissala and 80.9% for Jirapa district. The amount of N-fixed ranged from 402.3 mg.plant(-1) for Nadowli, 176.5 mg.plant(-1) for Wa, 235.4 mg.plant(-1) for Sissala, 179.0 mg.plant(-1) for Lawra to 249.2 mg.plant(-1) for the Jirapa district. Expressed on per-hectare basis using cowpea density per m(2), the total amount of N-fixed was around 16.6 kg ha(-1) in the Nadowli district, 19.1 kg ha(-1) in Wa, 23.0 kg ha(-1) in Sissala, 21.1 kg ha(-1) in Lawra and 17.6 kg ha(-1) in the Jirapa district. Averaged across all 5 districts, N-fixed by cowpea was about 19.5 kg ha(-1) in the Upper West Region of Ghana. These data suggest that, increasing N-2 fixation in farmers' fields in Ghana would require optimization of cowpea plant density rather than biological manipulation of the symbiotic process (as %Ndfa values were generally very high).