Autotrophic Respiration Is More Sensitive to Nitrogen and Phosphorus Supply Than Heterotrophic Respiration in Semiarid Grassland

Quantification of autotrophic (Ra) and heterotrophic (Rh) components of soil respiration (Rs) could greatly improve our understanding of terrestrial carbon cycling. Here, we investigated the effect of nitrogen (N) and phosphorus (P) supply on soil respiration rates and its components, and seasonal cumulative soil CO₂ efflux in a semiarid grassland. A two-factor experiment of N (0, 50 and 100 kg N ha⁻¹ yr⁻¹) and P (0, 18 and 36 kg P ha⁻¹ yr⁻¹) supply was conducted on the Loess Plateau in 2018–2021. Our results indicated that the promoting effect of combination of N and P supply on soil respiration and its components was greater than that of N/P supply alone. N and/or P supply increased the cumulative soil CO₂ efflux in both non-growing and growing seasons, and the non-growing season CO₂ efflux accounted for ∼25% of the growing season CO₂ efflux. The increase of cumulative CO₂ efflux from Ra in response to N and/or P supply was significantly greater than that of Rh in 2018 and 2019, while it shifted to the opposite, that is, a greater increase in Rh than in Ra in 2020 and 2021. Our results suggest that 100 kg N ha⁻¹ yr⁻¹ supply alone is an optimum fertilization scheme to trade-off grassland productivity and soil CO₂ emissions. Our results highlighted that the promoting effects of N and P fertilization on different soil CO₂ efflux components could be reversed in a relatively short period (4 years), and this should be considered when nutrient addition is adapted to restore degraded grasslands.