Trade-offs and synergies between seed yield, forage yield, and N-related disservices for a semi-arid perennial grassland under different nitrogen fertilization strategies

A ¹⁵N labeling and lysimeter experiment was conducted with mesocosms of a semi-arid Leymus chinensis grassland. The aim of the study was to evaluate the effects of N fertilization timing (fertilization in fall or spring) and rate (0, 56, and 112 kg-N ha⁻¹ year⁻¹) on ecosystem services (seed yield and forage yield), ecosystem disservices (N leaching surveyed during 1 year and emissions of NH₃ and N₂O integrated over 76 days after fertilization), and recovery of added fertilizer N in plants and soil. Seed and forage yields increased more under fall than spring N fertilization. Further, N fertilization was linked to higher soil NH₃ and N₂O emissions, particularly under high N rate for both NH₃ (2.0 and 1.6 kg-N ha⁻¹ under fall and spring N fertilization, respectively) and N₂O (0.24 and 0.21 kg-N ha⁻¹, respectively). N leaching was never observed. A significant N fertilization timing × rate interaction effect was observed on plant recovery efficiency of added fertilizer N (Plant-_NRE). Plant-_NRE was higher for high than moderate N rate, with + 13.2% (from 22.8 to 36%) and + 16.4% (from 28.2 to 44.7%) for fall and spring fertilization, respectively. Fertilizer N recovered in soil was highest for moderate N rate in fall (68% of total N fertilizer) and lowest for high N rate in spring (46%). Our results show synergies among the ecosystem services (seed and forage yields) and among the disservices (NH₃ and N₂O emissions), and trade-offs between the services and disservices, some of these synergies and trade-offs being modulated by N fertilization timing and rate. Our study is the first one analyzing the possibly interactive effects of the N fertilization timing and rate on this range of ecosystem services and disservices in semi-arid perennial grasslands, which can be useful for N risk: benefit analysis when evaluating N fertilization strategies.