The results of a long-term study were published in the journal Land, by a team of researchers from the Tartu-based Estonian University of Life Sciences (Eesti Maaülikool), the University of Tartu, and the Tallinn Botanical Gardens (Tallinna Botaanikaaed), as reported by associate professor Jürgen Aosaar of the University of Life Sciences.

According to the study's lead author, forestry researcher Katri Ots, also from the University of Life Sciences: "Vegetation establishment in abandoned peatlands reduces fire risk, and the formation of vascular and moss layers improves the microclimate of peat fields, creating conditions for new plant species to emerge."

The research team involved in the study said it was most surprised by the positive impact of the wood and oil shale ash mixture.

Since oil shale ash contains significantly fewer nutrients compared to wood ash, researchers expected the mixed ash to be less effective than the nutrient-rich wood ash.

However, in reality, things turned out to be the opposite.

"The nutrient ratio in the mixed ash is likely ideal for plant growth and promotes the development of active microbiology in the peat," Ots said.

On ash-fertilized plots, tree growth rates and biomass far exceeded those of unfertilized control plots.

The study provided practical insights by addressing two issues simultaneously: Estonia's 10,000 hectares of abandoned, vegetation-sparse peatlands—only up to 20 percent naturally re-vegetated and serving as major carbon sources and fire risks—and the annual production of 35,000 tonnes of unused, nutrient-rich wood ash from boiler houses.

The Puhatu abandoned peatland area, in Ida-Viru County, was used for peat extraction from 1963 to 1996, and had seen limited vegetation growth up to 2011, with its water regime still unrestored, when ash fertilization began.

Plant cover developed sluggishly in areas treated with pure oil shale ash, where ongoing peat decomposition and wind erosion exposed tree roots, unlike treatments with wood or mixed ash, which earlier studies by Katri Ots showed significantly benefit birch, spruce, and pine growth in abandoned peatlands.

Forestry researcher Katri Ots and her team found that mixing nutrient-rich ash into peat significantly enhanced vegetation diversity and biodiversity in abandoned peatlands, transforming barren areas into well-vegetated landscapes over time, unlike untreated regions where no vegetation developed.

Over a decade ago, experimental plots were established in the Puhatu abandoned peatland in Ida-Viru County to study the effects of wood and oil shale ash on plant species richness and biomass, revealing that herbaceous vegetation began emerging just months after wood ash was mixed into the peat in 2011, while untreated areas remained barren.

Nine years after ash treatment, researchers identified 23 vascular plant species, including the protected common spotted orchid (Dactylorhiza fuchsii), and various moss species such as Marsh helleborine (Epipactis palustris). Wetland-specific plants like round-leaved sundew (Drosera rotundifolia), hemp-agrimony (Eupatorium cannabinum), and bog haircap moss (Polytrichum strictum) also emerged.

These have been able to overcome the harsh conditions of abandoned peatlands, where growth is hindered by nutrient deficiencies, extreme temperatures, and moisture variability.

The highest average species richness of vascular plants, about four species per area, was observed in plots treated with 15 tonnes of wood ash per hectare, while areas treated with half as much oil shale ash supported roughly half as many species, highlighting the nutrient-rich ashes' role in improving peat fertility and plant nitrogen absorption.

The study, published in the journal Land, was supported by various Estonian and EU institutions, including the Environmental Investment Center Foundation (Keskkonnainvesteeringute Keskus), the Estonian Research Council (Eesti Teadusagentuur), state energy firm Eesti Energia, state forestry commission the RMK, and the EU Horizon program (the ECOLOOP project).

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