Ulrich’s soil acidification hypothesis on forest decline

In 1979, Bernhard Ulrich was the first researcher to discover the connection between air pollution and the forest decline or dieback (Waldsterben) in Germany. He hypothesized that acid rain results in soil acidification which in turn causes the forest dieback phenomenon. According to his soil acidification hypothesis, as soil becomes more acidic there is a release of aluminum that damages the roots of the trees. This leads to the following effects: reduction in uptake and transport of some cations, reduction in root respiration, damage to fine feeder roots and root morphology, and reduction in elasticity of the cell walls. The discovery was first published by Ulrich and co-workers in "Deposition von Luftverunreinigungen und ihre Auswirkungen in Waldökosystemen im Solling." Schriften Forstl. Fak. Univ. Goettigen 58, Sauerländer Verlag, Frankfurt a.M., 291pp. 

Photo source: www.museumplatkow.de

In 1986, Ulrich put forward his 8 theses on soil acidification which appeared in the Journal of Plant Nutrition and Soil Science 149: 702-717 (1986) as follows:

1. Rocks contain only bases and no acid precursors. Therefore, with the exception of sulfide containing rocks, soils cannot acidify as a result of atmospheric rock weathering.

2. A consumption of protons in rocks and soils results in a decrease of their acid neutralizing capacity and can result in the buildup of a base neutralizing capacity.

3. Weak acids (carbonic acid) lead in geological times to the depletion of bases without a larger accumulation of labile cation acids. Strong acids (HNO3, organic acids, H2SO4) can lead within a few decades to soil acidification.

4. The acid input caused by the natural emission of SO2 and NOx can be buffered by silicate weathering even in soils low in silicates.

5. The cause of soil impoverishment and soil acidification is a decoupling of the ion cycle in the ecosystem.

6. Acid deposition in forest ecosystems which persists over decades leads to acidification.

7. Formation and deposition of strong acids with conservative anions (SO4, NO3) shifts soil chemistry into the Al or Al/Fe buffer range up to a great soil depth.

8. In the long run, soil acidification by acid deposition results in the retraction of the root system of acid tolerant tree species from the mineral soil, and in water acidification.

Bernhard Ulrich was professor of forest soil science and forest nutrition at the University of Goettingen, Germany, from 1965 until his retirement in 1991. He obtained his PhD in agricultural science from the University of Hohenheim, Stuttgart, in 1953 based on a dissertation on the rapid determination of soil cation sorption capacity. He was widely recognized as the leading expert of soil acidification and forest ecosystem research.