Low nodulation and nitrogen fixation of mungbean reduce biomass and grain yields

Abstract

Apparent nodulation failures and associated low grain yields have been reported for commercial mungbean (Vigna radiata) crops in southern Queensland and northern New South Wales. We therefore conducted on-farm surveys of 40 commercial mungbean crops in the region in which symbiotic traits, i.e. nodulation and nitrogen fixation, and biomass and grain yield were monitored. Effects of bradyrhizobial inoculation and inoculation methods on mungbean and soybean (Glycine max) symbiosis and yield were determined in experiments at 3 sites in northern New South Wales. Thirty-four of the 35 mungbean crops assessed for nodulation were nodulated. The relationship between soil nitrate to a depth of 90 cm at sowing and mungbean nodulation was not significant. However, at low-to-moderate soil nitrate levels (<100 kg N/ha), the mean nodule score was 1.6, compared with 0.5 at high (>100 kg N/ha) soil nitrate levels. Soil nitrate had a negative effect on the percentage of mungbean nitrogen derived from nitrogen fixation (%Ndfa). Mean %Ndfa values for soil nitrate levels <50, >50–100 and >100 kg N/ha were 35, 22 and 19% respectively. Grain yields of the surveyed mungbean crops varied from 0.3 to 2.1 t/ha, and were correlated with shoot dry matter. Grain yield was not significantly correlated either with sowing soil nitrate, nodule score or %Ndfa. In the inoculation experiments, mungbean did not nodulate as well as soybean, producing about one-third the number of nodules. Both species responded to inoculation with increased nodulation, although data from one of the sites suggested that responses during early growth of mungbean were not maintained during pod-fill. Effects of inoculation on mungbean %Ndfa were marginal. Average increases were 9%, based on natural 15N abundance, and 6%, based on the ureide method. Soybean %Ndfa, on the other hand, responded strongly to inoculation, with increases of 56 (15N) and 77% (ureide). Inoculation increased mungbean crop N by an average of 10% and grain yield by 6%, compared with responses to fertiliser nitrogen of 31% (crop N) and 10% (grain yield). For soybean, inoculation increased crop nitrogen by 43% and grain yield by 7%, similar to responses to fertiliser nitrogen of 45 (crop N) and 5% (grain yield). These results suggest that inoculated mungbean was N-limited and that inoculation of mungbean using current technology may be somewhat ineffectual. We concluded that low nodulation and nitrogen fixation of commercial mungbean most likely results from the suppressive effects of nitrate and/or insufficient numbers of bradyrhizobia in the soil. When low symbiosis and low soil nitrate are combined, N is likely to limit crop growth, and potentially grain yield. Suggested strategies for improving mungbean nodulation and nitrogen fixation in the northern grains belt include selection of more symbiotically competent plant and bradyrhizobial genotypes and more effective utilisation of established soil populations of mungbean bradyrhizobia.

Assessment of Nitrogen Fixation by Mungbean Genotypes in Different Soil Textures Using 15N Natural Abundance Method

“…Increase of %Ndfa values on the loamy sand soil could be due to the relatively lower fertility status compared with the silt loam soil (Giller 2001). Herridge et al (2005) also found that lower availability of soil nitrate resulted in higher %Ndfa values in mungbean. The study conducted by Romanyà and Casals (2019) revealed that chickpea (Cicer arietinum) and bitter vetch (Vicia ervilia) fixed higher amounts of N (17-22 mg kg −1 ) in the low-fertility soils compared with high-fertility conditions.…”

Chasmogamous mutant, a novel character enabling commercial hybrid seed production in mungbean

“…Besides being one of the shortest duration field crops in the world (maturing within 2 months), mungbean can symbiotically fix nitrogen gas from the air and thus makes it a popular component in cropping systems. However, seed yield in the farmers field is still low, varying from 0.3 to 2.1 t/ha (Herridge et al 2005) with the norm of 0.5-0.7 t/ha. Mungbean breeders have used pure line breeding strategies such as pedigree selection, bulk selection, single seed descent and early generation testing with limited success in improving seed yield.…”

Symbiotic performance of grain and wild herbaceous legumes in the Okavango Delta and Tswapong region of Botswana

“…Thus so far, very few studies have measured N fixed under African farm conditions (Adu-Gyamfi et al 2007 ; Adjei-Nsiah et al 2008 ; Naab et al 2009 ; Pule-Meulenberg et al 2010 ). Other studies have been done in Australia (Peoples et al 1995 a; Herridge et al 2005 ) and other parts of the world under very different conditions to those on African farms. Also, to our knowledge, no study has assessed the level of N 2 fixation on herbaceous wild leguminous flora of Botswana as well as their dependency on symbiotic fixation.…”