Form‐Dependent Roles of Nitrogen in Root Growth and Metabolic Adaptation of Spartina alterniflora to Increasing Water Scarcity

The responses of Spartina alterniflora Loisel. roots to the interactive effects of drought and nitrogen (N) form, and the underlying mechanisms involved, remain poorly understood. We conducted a greenhouse experiment to evaluate the effects of N form (NH₄ ⁺, NO₃ ⁻, and NO₃ ⁻/NH₄ ⁺) and increasing water deficit on root performance, including growth, metabolite profiles, antioxidant activity, and N metabolism. Under well-watered conditions, NH₄ ⁺-fed plants exhibited the greatest root growth, nearly double that of NO₃ ⁻-fed plants. However, this growth advantage was lost under mild (50% field capacity, FC) and severe (25% FC) drought stress. In contrast, drought stress enhanced root growth in NO₃ ⁻-fed plants relative to well-watered conditions. Under well-watered conditions, NH₄ ⁺ nutrition increased the activities of superoxide dismutase, glutathione reductase, and ascorbate peroxidase compared to NO₃ ⁻ nutrition. Although drought stress further stimulated antioxidant enzyme activities in the roots of NH₄ ⁺-fed plants, this response did not mitigate drought-induced growth reductions. Antioxidant enzyme activities in the NO₃ ⁻- and NO₃ ⁻/NH₄ ⁺-fed plants were largely unaffected by drought, except for guaiacol peroxidase. Regardless of N form, glutamine synthetase activity increased under mild drought stress but declined under severe stress. Drought stress also enhanced glutamate dehydrogenase activity across all N treatments, particularly in NH₄ ⁺-fed plants, and was accompanied by increased total amino acid concentrations, especially proline. Despite these metabolic adjustments, drought stress reduced the overall performance of NH₄ ⁺-fed plants. These findings provide insights into N form-dependent drought responses and may help guide fertilizer management strategies to improve S. alterniflora productivity under water-limited conditions.