| 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. 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.
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