Nitrogen deposition magnifies the positive response of plant community production to precipitation: Ammonium to nitrate ratio matters.

Abstract The impacts of atmospheric nitrogen (N) deposition amount on plant communities have been extensively explored. However, the responses of plant communities to the ratio of reduced (NH4+–N) and oxidized (NO3−–N) forms remain unclear in natural ecosystems. A field N enrichment experiment using different NH4+–N/NO3−–N ratios was conducted in a natural semi-arid grassland in northern China from 2014 to 2019. Nitrogen addition tended to reduce plant species richness and significantly enhanced plant community aboveground net primary productivity (ANPP). Neither plant species richness nor plant ANPP at species and community levels was significantly affected by NH4+–N/NO3−–N ratios. At the plant functional group level, ANPP of grasses was not significantly affected by the NH4+–N/NO3−–N ratios examined, whereas ANPP of forbs was significantly increased at 1:1 NH4+–N/NO3−–N. Regardless of N supplied using the different ratios of NH4+–N/NO3−–N examined, plant community ANPP was positively associated with growing season precipitation. Unexpectedly, 1:1 NH4+–N/NO3−–N (NH4NO3) significantly improved the positive response of plant community ANPP to precipitation (it had the biggest slope value). Our results suggest that precipitation was the main determinant of the influence of NH4+–N/NO3−–N ratios on plant community ANPP. Therefore, the results of our study showed that without referring to NH4+–N/NO3−–N ratios and precipitation, models using NH4NO3 enrichment may overestimate the positive effect of atmospheric N deposition on ecosystem ANPP in semi-arid ecozones.