You could install tensiometers?

Dear Mr. Plauborg:

I have measurements of soil suction and water content in different depths (10, 30 and 50 cm). So you recommend me to look at the seasonal behavior of soil suction by soil depth. That will probably work for grassland vegetation but not for crops. There the maximum root length will go definitely deeper than 50 cm.

If I understand your question you are looking for greneral guides on rooting depth.

In general terms -- here is a reference from crop etc in US http://www.soilandhealth.org/01aglibrary/010139fieldcroproots/010139toc.html

In some models the default root distribution is assumed to be 40-30-20-10 depending on depth assumed.

No answer, but very interesting question! I could only suggest to implement another experiment in situ and to dig some pits in order to estimate roots distribution (by the way, that's how we work...)

I guess that you mean effective depth of root activity (rooting depth is poorly defined; it sounds self-explaining but if you think about it for a while you will find out that the maximum depth usually is not what you are interested in and also inactive roots are usually not in the center of interest).

Stable isotopes are always a choice but completely non-destructive measurement would not work. You could either label the water in different depths (seasonal variation in rain isotopic composition is often sufficiently differentiated) and then you analyze stem water to find out where water uptake happens. Or, you could label the CO2 of the canopy atmosphere and measure the soil CO2 at different depths to find out where roots respire.

Dear Jannis, I suppose that you intend to assess the "effective rooting depth" which means the water store which is accessible by roots including the capilalry rise. There exists a very old but effective method presented by Renger in 1976: If you measure a continuous depth profile of water content (TDR-, or FDR- techique) you can evaluate these depth profiles when you determine at the end of the campaign in the depth levels where you measured the water contend time series pF curves in order to determine field capacity (FC) (this last step of couse is destructive). Then you can observe if the water content along the depth profile will be lower than FC which indicates a signal of plant water uptake. The downward reach of plant water uptake will be the point where the actual water content meets FC. This method is referred to in short words in the issue of "Bodenkundliche Kartieranleitung" (/BGR, 1982).

Thank you all for the interesting answers.

The project is a long term experiment (10-15 years), so any destructive methods are not useable.

The idea to dig some pits under same crop conditions near the station is quite reasonable, but in my case not usable, because we are working with a space for time approach. Means, we have different soil types from Germany under same climate conditions in one place, so root depth beside the experiment field are not representative for all lysimeters soil types.

My last contribution may be to ask the real importance of the precision of this roots distribution that mostly depends on the question you want to answer... For example if you're only interested in water fluxes, data from the litteratture might be enough precise?

I agree with the comment of Finn Plauborg. Following your answer to him I suggest first to analyze data, then draw conclusions. If 50 cm is not enough install tensiometers or other probes at deeper depths. Then you have also to consider the effect of soil moisture regime: if crops are well watered generally they do not develop roots too deep (they do not need to look for water)

Thank you Mr Sere and Mr Borin.

In my opinion is the root distribution or seasonal rooting depth of plants an important variable to model adequate the water fluxes. The rooting depth and root distribution gives important information for modeling root water uptake in soils and helps to find a more realistic soil water and nutrition balance on the plot scale. As Mr Borin remarked, roots development depends on the soil moisture regime, so more and deeper measurements of soil suction devices would definitely help me to get more information about root depth for crops in soil lysimeters. But unfortunately I only have information about soil suction and water content until 50 cm.

An interesting relatively new method for the spatial variability of root water uptake and so on also for rooting depth is the use of ERT (electrical resistivity tomography), does anybody have any experience with that kind of technique or others?

The question is a little vague. You must define the variety of the plant you have in mind. There are plants for which the roots grow faster than the stem and also some for which roots has a bushy nature that is it spreads laterally rather than in depth. So first let us consider any given type of plant and if a large number is planted at a given site ie soil preparation (this is important factor), initially roots grow faster and as the plant matures, the growth of the root is reduced so there is no linear relationship except the thumb rule that depth of the root and its lateral spread are generally larger than the weather force on the plant that is why it is held in place