Using a five-year factorial experiment (control, N, P, and NP additions) in a grassland, we show that NP co-addition synergistically promoted the community productivity, carbon uptake, water use, and light use, but not the N use and P use. Moreover, the net interactive effect (NIE) of N and P on productivity was closely related to their NIEs on carbon uptake, water use, and light use, but not to those on N use and P use.