Louise Hindborg Mortensen:
Implications of wood ash application on soil decomposer food webs

Date: 25-04-2019    Supervisor: Søren Christensen, Mette Vestergård

In recent years there is an increased demand for biofuel to replace fossil fuel. To meet this demand whole-tree biomass is harvested from biofuel plantations, subsequently intensifying removal of nutrients from the ecosystem. The wood ash produced by biofuel combustion contains all nutrients necessary for plant growth, except for nitrogen which evaporates. Returning ash to the plantations can thus amend soil nutrient status and facilitate sustainable forest management. However, ash is highly alkaline and contains heavy metals which can affect the forest ecosystem. Center for Bioenergy Recycling (ASHBACK) studies the consequences of applying wood ash to an ecosystem. This thesis will evaluate the effects of wood ash application on a broad array of soil organisms in a coniferous forest plantation. Our aim is to contribute with general conclusions on the effects of wood ash on the decomposer soil food web and the potential propagating effects to the ecosystem, with the perspective of the possibilities for wood ash recycling in a Danish context.

In 2014 we established two experimental sites in the same coniferous plantation: 1) a realistic experiment which used ash dosages of 0, 3, 4.5 or 6 t ash ha-1 and; 2) an extreme experiment which used ash dosages of 0, 3, 9, 15, 30 or 90 t ash ha-1. The sites were sampled for soil organisms 2 months, 1 year and 2 years after ash application. Abundance of bacteria, protozoa, nematodes, mites, collembola, enchytraeids and earthworms, as well as fungal biomass was used in structural equation modelling to detangle the direct and indirect effects of ash application on organisms in the decomposer food web and on nitrogen (N) availability. Stable isotope values (δ15N) in organisms were used to determine relative change of recalcitrant organic matter decomposition. Cadmium concentration in organisms was used to give an indication of cadmium accumulation risk. To extend the results further than to the conditions of our field site, we also included a literature review which was used to synthesise the main parameters affecting the risk of cadmium accumulation after ash application.

The short term effect of ash was a stimulation of the basal decomposer food web which added to an increased availability of ammonium, subsequently facilitating the limiting nutrient in ash. During the study period, the ash effect on the basal food web subsided, but was still significant. Most other studied organisms remained unaffected. The available pool of nitrogen was markedly increased with extreme ash dosages, but at realistic ash dosages the ammonium pool decreased with ash application. Organism δ15N signatures in organisms increased with realistic ash dosages. This suggests that ash stimulates the incorporation of N from recalcitrant OM in organisms, which can unlock an otherwise inaccessible pool of nitrogen for the system. The observed decrease in ammonium pool is likely a combination of the relative lessened stimulation of mineralization and a high nitrogen demand with increasing availability of other nutrients facilitated by ash. After two years, the earthworm abundance increased with ash application. Compared to the other organisms, earthworms also contained the highest concentration of cadmium, which is consistent with our findings in the literature review; earthworms are generally cadmium accumulators. As cadmium is added with the ash, this could cause concern for risk of cadmium accumulating in the food web. However, only 3 out of 11 organisms tested showed an increase in cadmium concentration with ash application. Furthermore, at our experimental site, ash with low cadmium content was applied to a soil system with very low pH, high organic matter content and low earthworm abundance, which minimizes the risk of cadmium accumulation and biomagnification.

In conclusion, the soil system experienced some turbulence with ash application, but the effect on organisms was limited to the basal food web and it subdued with time. Ash stimulated the decomposer food web by increasing inorganic nitrogen, the missing element for ash to work as a fertilizer. If ash acts as an efficient fertilizer in biofuel plantations, recycling ash back to the system would not only be a sustainable way of disposing of combustion “waste”, but also facilitate long term sustainable replacement of tree biomass. As continuous stress to the soil organisms can disrupt their role in vital ecosystem processes, e.g. mineralization, we recommend minimizing the number of disturbances in the system, i.e. applying higher ash dosages (<9 t ash ha-1) fewer times.