Precipitation differentially regulates above- and belowground productivity in response to nitrogen enrichment in an alpine meadow

The fertilization effect of nitrogen (N) deposition on global productivity may be limited by water availability due to the coupling effect of N and water on ecosystem carbon cycling. However, it remains unclear whether this limitation is synchronized between above- and belowground productivity. Based on a long-term field manipulation experiment in an alpine meadow of the Qinghai-Tibetan Plateau, we found that precipitation played differential roles in regulating above- and belowground productivity responses to N addition. In experimental years with higher precipitation, N addition increased belowground productivity more. The main underlying mechanism was that higher precipitation mitigated excessive accumulation of soil inorganic N under N addition and shifted background plant community composition towards more N-favored species, which increased plant biomass allocation belowground and consequently stimulated the return of belowground productivity from N addition. In contrast, while N addition increased aboveground productivity, its effect didn't change with precipitation. This was mainly due to the trade-off between positive and negative effects of precipitation on the N response of aboveground productivity. Specifically, the precipitation-induced changes in soil inorganic N accumulation and background plant species composition reduced the return of aboveground productivity from N addition, while precipitation also offset this effect by mitigating the N-induced plant species loss, resulting in an overall insignificant effect of precipitation on the N response of aboveground productivity. Our findings imply that the trends of belowground productivity under N deposition can't be inferred solely from the easily measured aboveground parts, especially in the context of climate change. The work underscores an urgent need for more belowground measurements to improve the assessment of terrestrial carbon sinks.