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).