Alpine landscape variation in foliar nitrogen and phosphorus concentrations and the relation to soil nitrogen and phosphorus availability

We tested the hypothesis that foliar nitrogen and phosphorus concentrations are correlated with estimates of soil nutrient supply, a common assumption in studies of plant nutrient relations. This hypothesis was tested in an alpine ecosystem characterized by a wide range of soil nutrient availabilities using 3 herbaceous plants with widespread distributions. Rates of soil N and P supply were estimated using ion exchange resin bags deployed during the first half of the growing season, when the majority of plant nutrient uptake occurs. Measurements were made at 3 to 5 landscape positions (vegetation types) at 3 sites: a valley bottom that was glaciated until 12,000 yr ago and 2 ridgetop sites, I with deposits of Tertiary age and I that was not glaciated during the Pleistocene. Foliar N and P concentrations generally were not correlated with rates of soil N and P supply. We present several hypotheses to explain the lack of a correlation between soil N and P supply and foliar N and P concentrations, most notably the probable buffering between soil nutrient supply and foliar nutrient concentrations by belowground nutrient storage in plants and the use of organic N by plants. Foliar N:P ratios reflected the specific nutrient limitation of production for I of the 3 study species. Rates of soil N supply were associated with landscape position, indicating that microclimatic and/or plant species effects were the most important controls over spatial variation in N supply. Rates of P supply differed significantly among valley and ridge locations, but not with landscape position. Soil age and eolian deposition of dust probably differ between these collection sites and may explain the differences in soil P supply. These results suggest that caution should be used in estimating soil fertility and specific nutrient limitations of growth based on foliar nutrient concentrations in herbaceous communities.