Differential uptake of nitrogen forms by two herbs in the Gurbantunggut desert, Central Asia

Understanding how plants adjust their requirements for different N forms can help elucidate plant coexistence strategies in N-limited desert ecosystems. To understand the mechanisms involved, we investigated whether two desert herbs can directly absorb dissolved organic nitrogen (N) and tested whether the patterns changed over different growth stages. Two dominant herbaceous species, Astragalus arpilobus and Arnebia decumbens, from the southern edge of the Gurbantunggut desert, China, were selected. Short-term (24 h) ¹⁵ N-labelled tracer (¹⁵ N-NO₃ , ¹⁵ N-NH₄ , 2-¹³ C-¹⁵ N-Glycine) treatments were conducted at two soil depths (0-5 cm and 5-15 cm) in the season of rapid growth (June) and in the peak biomass season (July). Enrichment in ¹³ C and ¹⁵ N was assessed in the two species receiving glycine. The ratio ¹³ C:¹⁵ N was 0.21-1.39 at the 24-h harvest, suggesting that approximately 10.5-69.5% of glycine had been absorbed. The amount of absorbed ¹⁵ N was significantly affected by species, month, soil depth and N form. The two species absorbed most ¹⁵ N from the 0-5 cm soil layer, and the absorption rate in July was higher than that in June. The absorption of ¹⁵ N-NO₃ and ¹⁵ N-NH₄ was significantly higher than that of 2-¹³ C-¹⁵ N-Glycine. The results indicate that these herbs could use amino acids in the N-deficient desert ecosystem. The two co-existing species used different forms of inorganic N for their requirements and maintained a specific preference throughout various growth stages.