Quantifying Inhibitory Impacts of Soil Nitrogen on Biological Nitrogen Fixation Potential in Guar.

Guar (Cyamopsis tetragonoloba L. Taub.) is a leguminous crop cultivated in semi-arid regions around the world with high demand for its products, especially guar gum. As a legume, guar roots can form a symbiotic relationship with Rhizobia bacteria in the soil to form nodules in which the bacteria fix atmospheric nitrogen (N) that is assimilated by the plant. Elevated levels of soil N, especially NO₃, are known to inhibit nodulation in many legumes, but the nature of this relationship is completely unknown in guar. To ensure effective N management and optimization of ecosystem services in guar production, there is a need to understand the relationships among soil N and root nodulation and plant productivity, which is the objective of this research. A greenhouse study was conducted with four levels of added N (0, 10, 30, 90 mg N kg^-1 soil) and three soil types (loamy fine sand, loam, clay loam). The results showed that soil type, added N, and their interaction affected total plant N assimilation and plant productivity. Added N had an inverse relationship with nodule weight per plant, while having a relatively modest positive impact on plant productivity. From 0 to 90 mg N kg^-1 soil, nodule weight per plant decreased by 80 to 96%, while plant biomass increased by just 21 to 60% across soils. Guar producers typically do not fertilizer their crop, but this research shows that maximizing the N fixation benefit of guar will require avoiding planting at sites with high soil N levels, which should be confirmed by preplant soil tests.