Nitrogen rhizodeposition from soybean (Glycine max) and its impact on nutrient budgets in two contrasting environments of the Guinean savannah zone of Nigeria

Nitrogen (N) rhizodeposition by grain legumes such as soybean is potentially a large but neglected source of N in cropping systems of Sub-Saharan Africa. Field studies were conducted to measure soybean N rhizodeposition in two environments of the Guinean savannah of Nigeria using N-15 leaf labelling techniques. The first site was located in Ibadan in the humid derived savannah. The second site was in Zaria in the drier Northern Guinean savannah. Soybean N rhizodeposition in the top 0.30 m of soil varied from 7.5 kg ha(-1) on a diseased crop in Ibadan to 33 kg ha(-1) in Zaria. More than two-thirds of soybean belowground N was contained in the rhizodeposits at crop physiological maturity, while the rest was found in the recoverable roots. Belowground plant-derived N was found to constitute 16-23% of the total soybean N. Taking rhizodeposited pools into account led to N budgets close to zero when all residues were removed. If residues were left in the field or recycled as manure after being fed to steers, soybean cultivation led to positive N budgets of up to +95 kg N ha(-1). The role and potential of grain legumes as N purveyors have been underestimated in the past by neglecting the N contained in their rhizodeposits.