Different impacts of external ammonium and nitrate addition on plant growth in terrestrial ecosystems: A meta-analysis

Terrestrial plant growth is strongly limited by the availability of nitrogen (N). Atmospheric deposition of N has been rapidly increasing since the industrial revolution, associated with fast compositional shifts between ammonium- (NH₄ ⁺) and nitrate-N (NO₃ ⁻) globally. However, whether and how such composition changes of deposition will affect the response of terrestrial plant growth to N deposition remains unclear. To fill the gaps, this study quantified the different responses of terrestrial plants to external NH₄ ⁺-N and NO₃ ⁻-N additions. A meta-analysis was applied to compare the growth responses of 367 plant species to different forms of N addition from 210 N-fertilization experiments. In general, a greater response of plant growth to NH₄ ⁺- N (+6.3% per g N) than NO₃ ⁻-N (+1.0% per g N) addition was detected across all species. The larger response of plant growth to NH₄ ⁺-N than NO₃ ⁻-N addition was found in trees and forbs but not in shrubs and grasses. The NH₄ ⁺-N and NO₃ ⁻-N additions had contrasting effects on biomass allocation. For example, the NO₃ ⁻-N addition increased biomass allocation to above-ground tissues, whereas the NH₄ ⁺-N addition enhanced below- but not above-ground growth. These results generally reveal a higher response of plant growth to NH₄ ⁺- N than NO₃ ⁻-N addition in terrestrial ecosystems. The findings suggest that future predictions on the vegetation response to atmospheric N enrichment could benefit from a better understanding of plant strategies for acquiring different forms of N.