Nitrogen fertilization allows grazing intensification without degrading soil physical quality

Abstract

Increasing pasture biomass production through nitrogen fertilization enables greater stocking rate and grazing intensification in pastoral livestock production systems. However, grazing intensification can compromise the soil physical quality if stocking rates exceed the soil bearing capacity to support treading. The objective was to quantify the impact of long-term (12 years) intensified grazing of Panicum maximum Jacq. cv. IPR-86 Milênio on the physical quality of a sandy soil (Luvisol Ferric soil) under subtropical climate. The 12-year field experiment was arranged in a completely randomized split-plot design with four replicates. The approach for moving the animals in and out of the paddocks varied across harvests. In the first five harvests, the period of use and rest was fixed with each paddock being occupied for 5 days followed by 35 days of rest (i.e., 40-day grazing cycle). In the subsequent harvests, the timing for moving animals in and out of each paddock was based on pasture height, with entry set at 0.90 m and exit at 0.40 m. Grazing intensification levels (IL) consisted of different animal stocking rates, grazing cycles, and forage biomass, which varied depending on the nitrogen application doses: 0 (IL-0), 150 (IL-150), 300 (IL-300), and 450 (IL-450) kg of N ha⁻¹ year⁻¹. Undisturbed soil samples were taken at four depths (0–0.10, 0.10–0.20, 0.20–0.30, and 0.30–0.40 m) in two sampling positions (under plants vs. between plants) and used to measure several soil physical properties (e.g., bulk density, macroporosity, mesoporosity, microporosity, water and air storage capacity, available water, pore size distribution, and water retention curve). The impact of grazing intensification on soil physical quality was minor, occurring only between plants and at the 0–0.10 m depth, where an increase in soil bulk density and a decrease in microporosity was observed. Grazing intensification also increased the water retention capacity between plants. Regardless of the sampling position and the grazing IL, water and air storage capacity was not detrimental to plant development. Overall, the increased animal stocking rate resulting from the greater pasture biomass production due to improved nitrogen fertilization does not degrade soil physical quality.