Higher response of terrestrial plant growth to ammonium than nitrate addition

Abstract. Terrestrial plant growth and ecosystem productivity are strongly limited by availability of nitrogen (N). Atmospheric deposition of wet N as nitrate and ammonium has been rapidly increased since the industrial revolution, associated with a high spatial variation of changes in the ammonium- to nitrate-N ratio (i.e., NH₄⁺-N / NO₃⁻-N). However, whether and how terrestrial plants respond differently to NH₄⁺-N and NO₃⁻-N addition have never been quantitatively synthesized. Here, we first did a literature survey and analysis on the model projections of future changes in NH₄⁺-N / NO₃⁻-N in atmospheric N deposition. Most models predicted an increase in the global average of NH₄⁺-N / NO₃⁻-N ratio, but decreasing trends in western Europe and eastern China. Then, a meta-analysis was applied to compare the different growth responses of 402 plant species to NH₄⁺-N and NO₃⁻-N addition from 217 N fertilization studies. In general, a greater response of plant growth to NH₄⁺-N (+6.3 % g⁻¹ N) than NO₃⁻-N (+1.0 % g⁻¹ N) addition was detected across all species. The larger sensitivity of plant growth to NH₄⁺- than NO₃⁻-N was found in all plant functional types except for grasses. In addition, the NO₃⁻-N addition promoted terrestrial plants to allocate more biomass to above-ground, whereas NH₄⁺-N addition significantly enhanced below- but not above-ground growth. These results imply that the global accelerating N deposition could stimulate plant growth more in regions with increasing (e.g., North America) than decreasing (e.g., eastern China) NH₄⁺-N / NO₃⁻-N ratio. The findings suggest future assessments and predictions on the vegetation response to atmospheric N enrichment could benefit from a better understanding of plant strategies for acquiring different forms of N.