Can nitrate-based fertilization be recommended for the cultivation of ammonium-preferring species in a salty ecosystem? The case for Spartina alterniflora

Nitrogen (N) is an indispensable mineral nutrient for plant growth and development. Proper nitrogen (N) management, in terms of quality and availability, can increase plant productivity and reduce soil N losses. This study investigated the impact of N availability (form and dose) on the growth, water relations, and nitrogen status and assimilation of smooth cordgrass (Spartina alterniflora) in the presence of 200 mM NaCl. At the optimal N concentration, ammonium (NH₄⁺) application improved biomass production and total leaf area more than nitrate (NO₃^–) application—the leaves-free NH₄⁺ contents remained very low and did not reach the plant toxicity threshold, while those of NO₃⁻ were low, regardless of N form. Compared to NO₃⁻, the use of NH₄⁺ as the sole N source increased water economy. However, at low N concentration, S. alterniflora was sensitive to NH₄⁺ deficiency. Media containing 0.25 or 0.5 mM N as NH₄⁺ inhibited plant growth, causing general chlorosis and cell death, particularly on the lower leaves. In contrast, the response of S. alterniflora to NO₃⁻ dose did not significantly differ, indicating their preference for NH₄⁺. The lower glutamine synthetase activity in NH₄⁺-fed plants also suggests NH₄⁺ tolerance. Our findings question the use of NO₃⁻-based fertilizers for species that prefer NH₄⁺ as an N source in salt-affected soil.