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Carla Cruz Paredes:
Effects of wood ash on soil fungi

Date: 14-03-2017    Supervisor: Rasmus Kjøller

Reutilizing biomass ash on soil has been proposed to counteract soil acidity and to save fertilizer inputs by recycling valuable nutrients contained in biomass ash such as calcium (Ca), magnesium (Mg) and phosphorus (P). However, the heavy metal content of biomass ashes, such as cadmium (Cd), copper (Cu) and nickel (Ni), is a major environmental concern. This work is part of the project ASHBACK (www.ashback.dk) which addresses the potentials and possible problems in re-distributing wood ash to the forest. The aim of this thesis was to determine the effects of biomass ash application in agricultural and forest soils focusing on soil microbial communities’ composition and function, particularly mycorrhizal fungi.

Two study sites were used for this study, one in an agricultural field where different biomass ashes were evaluated as replacements for P fertilizers in barley, and a second one in a Norway spruce forest where different amounts of wood ash were spread on the soil to study the effects on ectomycorrhizal (ECM) fungi, bioaccumulation of metals in sporocarps, and microbial communities. Laboratory microcosm experiments were run in parallel to the field studies, to compare the effects of wood ash with factorial additions of lime and Cd to disentangle the pH and Cd effects of wood ash amendments using community trait distributions.

Barley yield, P content, and Cd content were not affected by biomass ashes. Some arbuscular mycorrhizal (AM) fungal species were reduced when biomass ashes were used. However, the AM colonization was as abundant and active as in the untreated plots. In the Norway spruce experiment, ECM mycelial production and nitrogen (N) retention capacity remained unaffected. Moreover, ECM fungal richness, diversity and community composition did not change with wood ash additions. Sporocarps collected from wood ash treated plots did not bioaccumulate more heavy metals than control plots. When analysing the soil microbial community structure, changes only occurred with extremely high wood ash additions (30 and 90 t ha-1). With these extreme treatments bacterial growth increased while fungal growth decreased. Finally, the bacterial community trait distributions revealed that the microbial responses to wood ash were induced by soil pH changes rather than the increase in Cd.

In conclusion, biomass ashes can replace P fertilizers in agriculture without presenting any harmful effects on barley or associated AM fungi. Furthermore, wood ash can also be re-distributed to the forest without representing any risks for ECM fungal communities, N leaching, Cd bioaccumulation in sporocarps or the general activity of the soil microbial community.