Sustainable intensification of tropical pastures: Optimize nitrogen supply through integration of legumes and grasses with biological nitrification inhibition?

In the northwestern Amazon region livestock farming typically involves extensive grass-only pastures that do not receive nitrogen (N) fertiliser. Over time, nutrient depletion leads to degradation in vast areas of pasture, with severe economic and ecological consequences. Our project investigates the impact of integrating legumes (e.g., Arachis pintoi) and grasses with biological nitrification inhibition (BNI) capacity on the N cycle in the soil–plant system. The study was conducted on seven farms in the Caquetá department of Colombia, in pasture plots that had been under the present pasture type for 5 to 30 years. We quantified plant biomass yields, N uptake and biological N₂ fixation. In topsoil (0-0.1 m) the ammonium and nitrate content was measured and the gross N fluxes quantified using the ¹⁵N pool dilution method. The results revealed that the presence of legumes and the species of grass significantly affected the N cycle in the soil-plant system. Higher forage yield and higher mineral N in soils were observed in grass-legume than grass-alone pastures. This was likely due to the N₂ fixation capacity of the legumes, which derived more than 70% of their N from atmosphere. The yield benefit in grass-legume pastures was more pronounced when the legumes were combined with the high BNI capacity grass (Urochloa humidicola). Ammonium was the dominant soil mineral N form in all pasture types, and gross and net nitrification tended to be lower in soils from pastures with high BNI capacity grass (P ≤ 0.10). Reduced nitrate production indicates a lower risk of nitrate leaching and N₂O emissions. Data on the impact of pasture type on total soil organic carbon and N contents are under evaluation. To our knowledge, this is the first study to examine the role of high-BNI grasses under low-input farming conditions, combined with legumes to mitigate N deficiency in pastures. Our findings illustrate a pathway towards sustainable intensification through biological interventions, with the potential to reduce soil degradation and harmful N losses across large areas of tropical pastureland.