Urochloa grasses swap nitrogen source when grown in association with legumes in tropical pastures

The degradation of tropical pastures sown with introduced grasses (e.g., Urochloa spp.) has
dramatic environmental and economic consequences in Latin America. Nitrogen (N) limitation to
plant growth contributes to pasture degradation. The introduction of legumes in association with
grasses has been proposed as a strategy to improve N supply via symbiotic N2 fixation, but the fixed
N input and N benefits for associated grasses have hardly been determined in farmers’ pastures.
We have carried out on-farm research in ten paired plots of grass-alone (GA) vs. grass-legume (GL)
pastures. Measurements included soil properties, pasture productivity, and sources of plant N uptake
using 15N isotope natural abundance methods. The integration of legumes increased pasture biomass
production by about 74%, while N uptake was improved by two-fold. The legumes derived about
80% of their N via symbiotic N2 fixation. The isotopic signature of N of grasses in GA vs. GL pastures
suggested that sources of grass N are affected by sward composition. Low values of δ
15N found in
some grasses in GA pastures indicate that they depend, to some extent, on N from non-symbiotic
N2 fixation, while δ
15N signatures of grasses in GL pastures pointed to N transfer to grass from the
associated legume. The role of different soil–plant processes such as biological nitrification inhibition
(BNI), non-symbiotic N2 fixation by GA pastures and legume–N transfer to grasses in GL pastures
need to be further studied to provide a more comprehensive understanding of N sources supporting
the growth of grasses in tropical pastures.