To measure nutrient uptake in trees to my knowledge there is no direct method. FIrst of all you must know that you should differentiate total uptake (growth of new wood, bark and leaves and foliar leaching or exudation) and net uptake (immobilisation of nutrients in the ligneous parts of the tree: wood and bark in stem and branches). Because one cannot measure (yet) foliar exchanges (leaching, exudation and absorption), direct measurements total nutrient uptake are yet not possible. However, it can be estimated. Most nutrient uptake studies focus on the net uptake. This is how you measure net uptake:

Currently the most widely used method is to establish allometric equations which relate tree diameter or circumference to the total amount of a given nutrient in a tree. You can then apply these equations to forest inventories carried out each year. By doing so you are measuring the change in the pool of nutrients immobilized in the trees over the years. Net uptake is simply the difference (the increment) between two years.

Establishing the allometric equations requires a lot of time and sampling: you need to fell a certain number of trees trying to respect the species and tree-size distribution in your stand. Your felled trees need to be thoroughly measured: diameters at different heights, all the different tree compartments weighed (fresh biomass), samples from each compartment must be taken for two reasons: 1) oven-dried to measure fresh biomass moisture and 2) for mineral analysis (to determine the concentrations of the nutrients which interest you).

There are many existing protocols in the literature for this.

I hope this answer was useful!

Good luck

If it is not in a lab Situation it is difficult but maybe with some isotope ratio/radioactivity measurements. You just have to have a controlled and defined media/Substrate/soil the tree is growing form

Just for argument sake...! All the stated methods above by Du Enzai are of course valid!

However, if you use a budget method, the nutrient uptake which is the end-member of the budget equation will suffer from uncertainties on all the flux measurements. For example, how can one measure N mineralization, N fixation or N leaching in the field? Furthermore, in the budget equation there is also "change in soil N pools". Again, this is very difficult to measure because changes are often smaller than spatial variability. Otherwise, you need to work with a heavy assumption: N pools in the soil are constant over the time period considered.

The allometric equation method is good for trees but it's a destructive method and if you don't have that many trees in your study plot, this becomes problematic. Also, the allometric equations are only valid for a short period of time before and after their establishment: trees change shape with age, stand density, disturbances (thinnings, storms...)

The isotope approach is difficult in the field because the soil won't be homogeneously labelled with 15N or other isotopes. You would need to monitor the isotope ratios at different depths and over time. You would still have difficulties in estimating total N uptake because there will be multiple solutions to your isotope mixing model (uptake from different depths). This is where Norbert Bittner is right: you need to be in controlled conditions to be effective. But you can't deny that in the field with all the uncertainties, isotopes will clear the picture for you!

So I would have to say that in my opinion the best method is the allometric equations when studying uptake in trees.

Cheers

One direct method is to sample the transpiration stream. It is possible to extract xylem sap from excised twigs using vacuum. The extract can then be analysed for nutrients in the usual way. I have used this method with tropical trees. This provides a direct measure of uptake, which by the way is highly dynamic during 24-hour cycles as well of course seasonally.

There is now quite a large body of literature out there, for both tropical and (mostly) temperate trees. I would you suggest that you do a literature search using key words such as 'xylem sap nutrients'.

The other (indirect) approach would be to analyse foliage. This is of course a well-established approach that can provide an insight into the nutrient status of trees.

Good luck! Martin

@ Gregory Do you think its that "arbritary/hard to do" with stable isotopes? With good controls e.g. soil microorganism istope uptake (which would be any way nice to monitor the flow from soil morganisms to plants :) i think its possible..

Currently i am working on this issue. there are some methods regarding this measurement. For more details please see my colleagues publications. I hope these can help you.

Gessler A, Schneider S, Von Sengbusch D, Weber P, Hanemann U, Huber C, Rothe A, Kreutzer K, Rennenberg H (1998) Field and laboratory experiments on net uptake of nitrate and ammonium by the roots of spruce (Picea abies) and beech (Fagus sylvatica) trees. New Phytol 138:275–285.

Gessler A, Kreuzwieser J, Dopatka T, Rennenberg H (2002) Diurnal courses of ammonium net uptake by the roots of adult beech (Fagus sylvatica) and spruce (Picea abies) trees. Plant Soil 240:23–32.

Gessler A, Jung K, Gasche R, Papen H, Heidenfelder A, Borner E, Metzler B, Augustin S, Hildebrand E, Rennenberg H (2005) Climate and forest management influence nitrogen balance of European beech forests: microbial N transformations and inorganic N net uptake capacity of mycorrhizal roots. Environ J For Res 124:95–111.