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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) 15N‐labelled tracer (15N‐NO3, 15N‐NH4, 2‐13C‐15N‐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 13C and 15N was assessed in the two species receiving glycine.The ratio 13C:15N 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 15N was significantly affected by species, month, soil depth and N form. The two species absorbed most 15N from the 0–5 cm soil layer, and the absorption rate in July was higher than that in June. The absorption of 15N‐NO3 and 15N‐NH4 was significantly higher than that of 2‐13C‐15N‐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. [ABSTRACT FROM AUTHOR]