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Keywords:

  • 15N tracer;
  • alpine Kobresia meadow;
  • ammonium;
  • inorganic N;
  • nitrate;
  • plant–microbe competition;
  • plant–soil (below-ground) interactions;
  • rhizosphere interactions;
  • spatio-temporal pattern

Summary

1. Plant–microbe competition for available nitrogen (N) has been suggested to be an important mechanism controlling N limitation of plants in a variety of ecosystems. However, spatio-temporal patterns of competition between plants and microbes for soil N remain unclear.

2. Short-term 15N tracer experiments were conducted during a growing season (July, August and September) in an alpine meadow on the Tibetan Plateau to unravel spatio-temporal patterns of plant–microbe competition for NH4+ and NO3.

3. Alpine plants were poorer competitors than soil microorganisms for inorganic N in July compared with August and September. Occupation of soil volume by roots and root density (high in August and September) played a greater role in plant–microbe competition than air temperature or precipitation (high in July).

4. In topsoils (0–5 cm, highest root density), alpine plants effectively competed with soil microorganisms for N and showed a preference for 15NO3, while soil microorganisms that preferentially took up 15NH4+ out-competed plants below 5 cm soil depth (lower root density). Competition between plants and soil microorganisms for inorganic N strongly depended on root density (< 0.0001, R2 = 0.93, exponential decay model).

5.Synthesis. Plant–microbe competition for inorganic N showed a clear spatio-temporal pattern in alpine meadows depending on (i) root density and therefore soil depth, (ii) inorganic N form, and (iii) different periods during the growing season. These findings have important implications for our understanding of above-ground–below-ground interactions and plant–microbial competition for available N.