Attributes affecting residual benefits of N-2-fixing mungbean and groundnut cultivars

Legumes grown for grain may or may not contribute net N benefits to soil and succeeding crops. An experiment was conducted to assess N-2 fixation attributes of six mungbean cultivars and two groundnut cultivars (Tainan 9 and Non-nod), which determine their residual benefit to the subsequent maize. Nodule number and dry weight of mungbeans peaked early (at 45 days) and declined thereafter strongly. In groundnut nodulation peaked later and declined only by 50% towards the final harvest. The N-2-fixing groundnut produced higher total dry matter yield than mungbeans; however, mungbeans produced higher seed yields. Dry matter harvest index and nitrogen harvest index (NHI) were higher in mungbeans (average 0.44 and 0.69) than groundnut (0.23 and 0.47, respectively, in Tainan 9). The percentage of nitrogen derived from air (%Ndfa, N-15 isotope dilution) ranged from 54% to 62% in mungbeans, similar to that of groundnut (64%). However, Tainan 9 fixed more N-2 (82 kg N ha(-1)) than mungbeans (35-50 kg N ha(-1)) and resulted in a positive soil net N balance (+22 kg N ha(-1)) while negative values were found for Non-nod groundnut and mungbeans (-3 to -12 kg N ha(-1)). Maize grown after groundnut Tainan 9 had the highest total dry weight and total N uptake. This was equivalent to maize grown in fallow plots, which received 60-90 kg N ha(-1), while the respective benefits after mungbeans were 30-60 kg N ha(-1). Maize yield was directly related to the amount of residue N returned. Thus, the combination of high N yield, residue quality, %Ndfa and low NHI proved most beneficial to soil fertility and the succeeding crop.