Popular Science: How does vegetation influence water chemistry?
With the increasing industrialization since the second half of the 20th century, (not only) forest soils have been strongly influenced by acidification and eutrophication. This was initially mainly due to the increased concentration of sulphates, later also by nitrogen compounds. Forest soils have their buffering capacity, sulfate and nitrate anions are neutralized by cations in soils (especially Ca2+, Mg2+, Na+, K+), but the cation exchange complex is depleted when the buffer capacity is exceeded. In addition to the lack of nutrients for vegetation after the loss of base cations, there is also a decrease in pH, which in turn leads to the release of dissolved Al3+, which is toxic to plant roots. With the decreases of acid imputs in the 1990s, the ecosystems of lakes and rivers have gradually started to recover from acidification; for forest ecosystems, this is taking a longer time than was expected, mainly due to soil degradation.
The type of forest stand influences the intensity of the acidification process and nitrogen saturation. A coniferous forest has, due to its higher leaf area index and due to its evergreen character, a greater ability to scavenge atmospheric compounds. At the same time, it is characterized by a higher interception (precipitation that does not reach the soil), so throughfall is less diluted and has a greater effect on changes in the soil chemistry. There are, of course, more influencing factors on soil chemistry such as the distribution of the root system in the soil profile, the quality/degradability of organic matter, the amount of topsoil and others.
The researchers studied changes in the cycle of elements within the ecosystem of two different forest stands, the main types of forest ecosystem in Central Europe, the Norway spruce (Picea abies) and the beech (Fagus sylvatica) between 2005 and 2017 with monthly sampling. Both sites were in the Ore Mountains at an altitude of about 800 m near the German border and close to the village of Načetín. During the monitored period, bulk precipitation, throughfall and soil water from the topsoil and at depths of 30, 60 and 90 cm in the mineral soil were collected on a monthly basis.
Between 2005 and 2017 there was a decrease of rainfall acidity detected, the most significant decrease was of the throughfall under the beech forest. The pH increase was driven by the decrease of the concentration of acidifying anions, in particular SO42- and the less significant decrease of NO3-. There was also a decrease in the dissolved organic carbon (DOC) concentration in the open area, there was no change in throughfall precipitation. Under the spruce forest, however, the concentration of both DOC and base cations (Ca2+, Mg2+, Na+, K+) was higher than under the beech forest. In soil water there was a decrease in the concentration of sulphates, which corresponds to a change in precipitation chemistry, with higher concentrations always registered in the spruce forest. Due to the lower pH and higher concentration of sulphates, higher concentrations of aluminum were also present in spruces. There was no significant decrease in nitrates, at the same time higher concentrations were measured in beech forests.
The results show that the type of forest stand has a fundamental influence on the state and changes of the throughfall and soil water chemistry. Although there has been a decrease of SO42- emissions and pH registered in the past, unfavorable conditions in the rooting zone of spruce stands are still observed, especially in the upper mineral soil characterized by reduced levels of base cations, important nutrients for vegetation, and increased concentrations of aluminum, which is toxic for the root system.
Růžek, M., Myška, O., Kučera, J., Oulehle, F. (2019): Input-Output Budgets of Nutrients in Adjacent Norway Spruce and European Beech Monocultures Recovering from Acidification. Forests, 10, 68; doi:10.3390/f10010068.