Finnish Forest Research Institute  Metla

Press release 28.07.2006

Wood ash application does not pollute berries or mushrooms

Metla conducted a study to determine the short- and long-term effects of wood ash application by measuring heavy metal concentrations from berries and mushrooms growing in upland forests and on drained peatlands. A follow-up study showed that heavy metal concentrations in berries and mushrooms growing on test plots treated with ash fertilization remained mostly at the same level or was even reduced. Fertilization of forests with wood ash was shown to be safer than expected for forest berries and mushrooms.

Wood ash is generally known to increase tree growth in peatland forests. The effect of wood ash as an additional nutrient is also known to last for several decades. Intensified recovery of residue wood in loggings reduces the soil’s nutrient composition, and hence it has been suggested that recovery of ash back to the natural cycle would be ecologically justified. However, in addition to plant nutrients, wood ash also contains heavy metals. In particular, the heavy metal cadmium is considered harmful. It has been suggested that cadmium might migrate into berries and mushrooms growing in forests and thus cause a threat to human health. Very little research data relating to the adverse effects of wood ash is available. Since 1997 Metla has studied the possible environmental effects of ash, such as concentrations of chemical elements and heavy metals in berries and mushrooms. The Universities of Oulu, Kuopio, and Helsinki as well as Metsäteho Ltd have also participated in these studies.

During 1946-1998 samples of forest berries, mushrooms and soils were collected from test plots fertilized with wood ash. When the samples were analyzed in the laboratory, the results showed that ash fertilization had caused a major increase in chemical element concentrations in surface soil: for example, compared to the reference plots, the amount of calcium had increased by many tens of times and cadmium levels were almost ten times greater. At 14-20 years after the application, the cadmium concentration in cloud berries (Rubus chamaemorus) in all studied cases was either at the same level or significantly lower than in samples collected from reference test plots. Heavy metal concentrations in lingonberries (Vaccinium vitis-idaea) were reduced, when four years had passed from the wood ash application. Heavy metal concentrations in mushrooms (for aluminium, arsenic, nickel and titanium) were elevated in certain cases, after one year from the application. However, the calcium concentration in Paxillus involutus and northern milk-cap (Lactarius trivialis) was reduced and the effect remained in the northern milk-cap even 50 years from the application.

Migration of heavy metals into plants is reduced when their solubility is reduced. Increased soil pH prevents metals from dissolving. Wood ash increases the pH value of soil, reducing acidity. This liming effect of wood ash may explain why heavy metal concentrations remained either the same or were reduced, even if the amounts of metals in the surface layer were significantly increased. The temporary increase of heavy metals observed in mushrooms after one year from wood ash application was probably due to the fact that the mushrooms came into direct contact with ash as they grew through the wood ash layer. The samples thus contained traces of ash.

In spite of the changes observed in the study, application of wood ash as a forest fertilizer on drained peatland soils or upland forest soils does not accumulate metals in edible berries and mushrooms. The heavy metal concentrations measured from the samples did not differ from the values presented in previous studies on the same species of forest berries and mushrooms. However, picking berries and mushrooms from areas where wood ash has been applied should be avoided during the summer following the fertilization, especially if fly ash has been used instead of granular ash.

The study is part of the master project “Environmental effects of wood ash application and heavy metals contained in it”.

Publication: Moilanen, M., Fritze, H., Nieminen, M., Piirainen, S., Issakainen, J. & Piispanen, J. 2006. Does wood ash application increase heavy metal accumulations in forest berries and mushrooms? Forest Ecology and Management 226: 153-160.

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