Quantifying tradeoffs in nodulation and plant productivity with nitrogen in guar

Abstract

Guar (Cyamopsis tetragonoloba L. Taub.) is a leguminous crop with high industrial demand for its products, especially guar gum, and is cultivated in semi-arid regions of the world. Guar roots form a symbiotic relationship with Rhizobia bacteria in the soil, creating nodules in which the bacteria fix atmospheric N that is assimilated by the plant. Elevated levels of soil N, especially NO3, are known to inhibit nodulation in legumes, but the nature of this relationship has not been studied in guar, which is needed to enable effective N management and optimization of ecosystem services in guar production. To better understand the relationships among soil N, root nodulation, and plant productivity, a fully replicated and repeated greenhouse study was conducted with four levels of N (0, 10, 30, 90 mg N kg−1 soil) in three texturally contrasting soils (loamy sand, loam, clay loam) for 50 days. There was a clear distinction in biomass-nodulation tradeoffs between the soils in which nodules were produced (loamy sand, loam) and virtually no nodules were formed in the clay loam. In the loam, relative to no added N, nodule weight declined by averages of −15%, −35%, and −82% with 10, 30, and 90 mg N kg−1, respectively, while biomass productivity was optimized at 30 mg N kg−1. In the generally unfertile loamy sand soil, adding any N drastically reduced nodule weight (−44%, −67%, and −91% on average with 10, 30, and 90 mg N kg−1, respectively), with relatively minor improvements in biomass productivity. This research confirms that guar producers should avoid planting at sites with high indigenous levels of soil N. When N fertilizer is applied to guar, the rate should be based on soil test results, with consideration of producers’ goals for crop productivity and/or N fixation, as plant productivity-nodulation tradeoffs are best optimized with low to moderate levels of N.