Nitrogen Cycling and N₂O Emissions in Brachiaria-Based Grass-Alone Pastures and Silvopastoral Systems in a Grazing Trial in Colombia

Silvopastoral systems provide a number of environmental and productive benefits compared to grass-alone pastures in terms of i) increased forage biomass offer and quality, ii) nutrient cycling, iii) biodiversity, iv) cash flow, among other ecosystem services. With the aim of evaluating the nitrogen (N) cycling differences in grass-alone pastures and silvopastoral systems we established a field trial in the CIAT campus in Palmira, Colombia consisting of two grassonly, and two silvopastoral treatments: i) Brachiaria hybrid cv. Cayman, ii) Brachiaria hybrid cv. Cayman + Leucaena diversifolia, iii) B. brizantha cv. Toledo, and iv) B. brizantha cv. Toledo + L. diversifolia. Over a six months period we evaluated every 50-55 days the aboveground biomass production after standardization of the Brachiaria grass, the L. diversifolia foliage, forbs, and plant litter. The dry matter production per treatment for each botanical fraction, and nutrition quality parameters of the grasses and the legume were determined. The isotopic ¹³C and ¹⁵N natural abundance of all samples were measured to estimate N fixation by the legume and transfer to the associated grass. The soil-borne nitrous oxide (N₂O) emissions and inorganic N transformations in the soil were monitored over a period of 50 days after simulation of urine deposition during the rainy season. Soil chemical and physical characterization were performed at the beginning and at the end of the experiment. Our hypothesis is that plant biomass and N concentration in forage is higher in the silvopastoral systems compared to the Brachiaria alone pastures via N fixation by the legumes, and that N₂O emissions intensity is lower in the silvopastoral treatments. Currently diverse technologies have been proposed pointing for sustainable livestock production, however, silvopastoral systems stand out among them for the potential of dual benefit increasing livestock productivity while providing ecosystem services, including reduction of the carbon balance of the system.