P status in soil and leachate
In the control, relative P components were Fe-Pi > Fe-Po > Al-Pi, Al-Po > soluble P in the three layers of soils, and Fe-Pi accounted for at least 50% of easily-available P and moderately-available P components (soluble P + Al-Pi + Al-Po + Fe-Pi + Fe-Po; Table S3, Fig. 2). With both warming treatments, the soluble P concentrations and microbial biomass P were not changed (Table S3). However, the +2.1°C treatment significantly decreased moderately-available organic P concentrations in 0–10 cm soil depth by 15.5±7.3% (P < 0.05) relative to the control (28.4±1.5 mg kg-1; Fig. 2a). The Fe-Pi concentrations were significantly decreased by +2.1°C treatment in 0–10 cm soil depth by 6.8±2.7% relative to the control (34.0±0.8 mg kg-1), while Al-Pi concentrations were not changed by warming (Fig. 2b, Table S3). There was no significant response of moderately-available organic P and Fe-Pi to warming in the deeper soils (10–20 cm and 20–40 cm). Soil potential acid phosphatase activity in all three depths was significantly increased in the +2.1°C treatment (P < 0.05, Fig. 3a).
Averaged monthly total leachate P in the control was 0.02 kg hm-2 month-1 during 2013–2019 (Fig. 4). Warming had no significant effect on total leachate P throughout the experiment. However, relative to the control, leachate P of surface runoff was decreased by 55.0±37.4% in the +2.1°C treatment, while leachate P in underground water was increased by +21.4±9.2% and 36.8±11.2% in the +1.0°C and +2.1°C treatments, respectively. There was no significant interaction between warming treatments and times.