Plant P status
During the period of 2013–2019, foliar N:P at ecosystem level in the control averaged 23.8±0.6. In each treatment, foliar N:P was negatively correlated with foliar P concentration but was uncorrelated with foliar N concentration (Fig. S3). In the +2.1°C treatment, foliar N:P was significantly decreased by 4.7±1.5% without changing foliar P concentrations throughout the experiment (P < 0.05; Fig. 1a, b). The +2.1°C treatment significantly increased P concentrations by 23.4±4.3% in stems, 14.4±5.0% in fine roots and 32.4±6.2% in coarse roots in 2018 (P < 0.05), but it did not affect tissue P concentrations in 2015 (Fig. S4a). The +2.1°C treatment significantly increased fine roots and decreased abundance of arbuscular mycorrhizal fungi in 2015, but it had no effect on these in 2018 (Fig. S4b, c).
Plant P uptake was significantly increased in both 2015 and 2018 under warming (33.8±7.4% in the +1.0°C treatment and 90.6±11.8% in the +2.1°C treatment on average; Fig. 1c). During 2014–2015 and 2017–2019, plant P resorption efficiency was significantly increased by 5.8±2.9% in the +2.1°C treatment relative to the control (56.8±2.3% on average) (Fig. 1d). Plant resorption was negatively correlated to foliar P concentrations, soil NH4Cl-P concentrations, and soil moisture (Fig. S5). Litter C:P was significantly increased by 26.0±9.7% in the +2.1°C treatment during 2017–2019 (P < 0.01; Table S2). Litter P stock and litter phosphatase activity were not changed under warming during 2017–2019.