1. Phosphorus: High demand – because … Supply relatively low … Available forms derived 1° from apatite Ca 5 (PO 4 ) 3 (F,Cl,OH) = Calcium (Fluoro,Chloro,Hydroxyl) Phosphate Availability pH dependent – highest at pH range of 6-7 (per text) See also Brady&Weil: Fig 14.16)

2. Phosphorus: Limits primary (plant) production – where less limiting? … why there? … High P content geology Arid systems What kind of ecosystems expect P limitation? … “Old”, wet systems – tropics Also some aquatic systems

3. weathered mineral P (total potentially available P) organic P soluble P occluded P Time (”geologic” scale) Soil P [concentration] (after Walker and Syers 1976) Development of P limitation P: Changes in soil P availability with time: …

4. Plant Adaptations to low nutrient supply A. Biomass allocation Greater allocation to roots over shoots … Roots of alpine plants - exposed by erosion

5. B. Root proliferation High number of fine roots in fertile soil patches – Especially for less mobile resources zone of high soil resource availability } “Proteoid” or Cluster Roots  P capture Rootlets produce organic acids, chelating agents Proteaceae, …

6. C. Root uptake kinetics – Biochemical capacity to take up nutrients Greater production of nutrient carriers on surface of roots (cost?) Benefit varies depending on the mobility of the nutrient …  best for highly mobile elements (e.g. NO 3 – ) more readily diffuse in soil water  On other hand, increased root allocation [last slide] better for less mobile (e.g. P) D. Low growth rates, small size Reduce nutrient demand by growing at a slow rate Potential tradeoffs?… lower competitive ability

7. Nutrient Limitation How determine what’s limiting? Liebig’s Law of the Minimum + N & P control +P +N Nutrient addition experiments Plants require same elements in roughly same proportions – Some variation by plant type & environment Niwot Ridge LTER – alpine tundra

8. Example of results: What is your interpretation of the results?? Physiological compensation Differing species response Competitive exclusion