N and P addition increase soil respiration but decrease contribution of heterotrophic respiration in semiarid grassland

Fertilization is an effective measure to decrease soil erosion by promoting vegetation restoration, while it affects soil respiration (Rs) greatly through influencing soil properties and plant root growth. A factorial combination of N addition (0, 50 and 100 kg N ha-1 yr-1 defined as N0, N50, and N100) and P addition (0, 40 and 80 kg P2O5 ha-1 yr-1 defined as P0, P40, and P80) experiment was conducted in a farming-withdrawn grassland to investigate the Rs and its components (heterotrophic respiration, Rh; autotrophic respiration, Ra) in responses to different nitrogen (N) and phosphorus (P) addition levels from May 2018 to October 2019 on the semiarid Loess Plateau. Results showed that Rs, Rh and Ra presented single-peak curves both in diurnal and seasonal variations across all N and P addition treatments. The highest Rs, Rh and Ra values during the day occurred at 12:00–14:00 and the highest values during the year appeared in July, which were consistent with soil temperature changes. Rs, Rh and Ra increased significantly under N and/or P addition from May to September. Compared with no fertilization, Rh and Ra significantly increased by 4.2–5.1% and 43.9–46.4%, and by 13.2–15.8% and 23.0–34.2% under N or P addition alone, respectively. N and P combined addition increased Rh and Ra by 25.4–30.7% and 62.0–111.4% than N or P addition alone. Rs and Ra reached the highest values under N100 combined with P80 addition, but Rh reached the highest values under N50 combined with P80 addition treatment. Annual cumulative CO2 emission increased by 24.4%−26.5% under N addition alone, 17.0–27.4% under P addition alone and 42.2–68.7% under N and P combined addition. Compared with no fertilization, root biomass and soil microbial biomass carbon significantly increased by 15.8–122% and 5.2–92.5% after N and P additions, respectively. However, the positive effect of N and P addition on Rh was weaker than that of Ra, which decreased the contribution of Rh to Rs. Our findings point to the need of rational N and P combination to tradeoff biomass production improvement and soil carbon emission when applying fertilization to promote grassland restoration in semiarid Loess Plateau.