4.1 ANPP
As we expected, the response of ANPP to both N and P inputs varied between years, possibly because of differences in precipitation. The amount of precipitation is the main limiting factor constraining plant productivity in arid grassland (Bai et al., 2004; Irisarri et al., 2016; A. K. Knapp & Smith, 2001; Ma et al., 2020). Plant communities are sensitive to fluctuation in precipitation and N availability in dryland ecosystems (Ma et al., 2020). Furthermore, increased soil moisture can diminish the negative impacts of N input on plant communities (Ma et al., 2020). In accordance with the co-limitation theory (Harpole et al., 2011), N and P inputs can synergistically increase ANPP only when water is not limited, as was the case in the present study in 2019, which was a normal year in terms of precipitation. Consistent with our results, nutrient addition only enhanced ANPP when there was sufficient water in the desert steppe (Ma et al., 2020; Qiao et al., 2019).