Legume Nitrogen Fixation and Symbioses in Low-Inputs Rainfed Rice Rotations

Harimenja Razafintsalama,Marie Sauvadet,Thierry Becquer,Aude Ripoche,Patrice Autfray,Jean Trap

doi:10.3390/su132212349

Abstract

Cropping systems with legumes play key roles in farming systems in sub-Saharan Africa. However, how commonly legume associations perform in low input-systems is not well-known. Here, we studied four legume species used in three systems in rotation with upland rice, i.e., groundnut monocropping, sorghum–cowpea intercropping, and velvet bean–crotalaria intercropping, in two fertilization managements on the previous rice, i.e., manure alone or complemented with mineral fertilization. Legume suitability was assessed using rhizobial and mycorrhizal colonization rates, plant biomass production, shoot N and P content, and biological N2 fixation based on their δ15N natural abundance. Shoot and root biomasses varied significantly between legume species and were positively correlated with nodule number (r = 0.49 and 0.74, p-value < 0.05 and <0.001, respectively) and the amount of fixed N (r = 0.73 and 0.50, p-value < 0.001 and <0.05, respectively). The proportion of plant N derived from N2 fixation also varied significantly between species, with a higher percentage for velvet bean (66%), compared to the other three species (50 to 60%). Legume roots were weakly colonized by AM fungi, with similar levels between species. Overall, fertilization management did not significantly impact legume biomass, symbioses, or N2 fixation, yet the organo-mineral fertilization significantly increased legume shoot P content. The lack of effect of mineral fertilization on N2 fixation and biomass could be due to other nutrient deficiencies (Ca, Mg, micronutrients), which can hamper symbioses with rhizobia and mycorrhizae.

Highlights

IntroductionThe conventional agriculture model, based on the use of synthetic inputs and nonrenewable natural resources, has made possible an increase in productivity but at the cost of damage to the environment [1]
A field experiment was established in November 2015 and hereby we focus on three two-year rotations including solely legumes or a crop mixture, namely (i) rice after groundnut (RG), (ii) rice after sorghum–cowpea (Vigna unguiculata) intercropping (RSC), and (iii) rice after velvet bean (Mucuna pruriens)–crotalaria (Crotalaria spectabilis) intercropping (RVC) (Figure 1)
The highest values of shoot biomass were found for groundnut and crotalaria, were found for groundnut and crotalaria, while while the lowest ones were observed for velvet bean and cowpea

Summary

Introduction

The conventional agriculture model, based on the use of synthetic inputs and nonrenewable natural resources, has made possible an increase in productivity but at the cost of damage to the environment [1]. Ecological intensification has been proposed as a nature-based alternative that sustains agricultural production while minimizing adverse effects on the environment [2]. Among the key practices to promote ecological intensification, crop diversification has shown positive impacts on both plant production and the environment [2,3,4]. It can contribute significantly to livelihoods, improved health and nutrition, household food security, climate resilience, and ecological sustainability [5]. It can contribute significantly to livelihoods, improved health and nutrition, household food security, climate resilience, and ecological sustainability [5]. 4.0/).

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Conclusion