Spatio‐temporal variations determine plant–microbe competition for inorganic nitrogen in an alpine meadow

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 15 N 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 NH 4 + and NO 3 − . 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 15 NO 3 − , while soil microorganisms that preferentially took up 15 NH 4 + 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 ( P < 0.0001, R 2 = 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.