Abstract
AbstractAsh from combustion of biofuels, for example wood chips, is often deposited as waste, but due to its high content of essential plant nutrients and alkalinity, it has been proposed to recycle ash as a fertilizer and liming agent in biofuel production forest. However, current legislation sets rather strict limitations for wood‐ash application in biofuel production systems. The soil microfood web, that is microorganisms and their microfaunal grazers, protozoa and nematodes, is pivotal for essential ecosystem processes such as decomposition and plant nutrient release. Therefore, a thorough assessment of the impacts on microfood web structure and functioning must precede actions towards raising the currently allowed application rates. In a Danish Norway spruce plantation, we evaluate the impact of wood ash applied at dosages from 0 to the extreme case of 90 t ash ha−1 on the microfood web, the bacterial community structure, soil content of inorganic nitrogen, organic matter, dissolved organic carbon and nitrogen. Using structural equation modelling (SEM), we disentangled the direct effect of the disturbance imposed by ash per se, the associated pH increase and changes in prey abundance on individual organism groups in the microfood web. The SEM showed that the pH rise was the main driver of increasing abundances of culturable heterotrophic bacteria with increasing ash doses, and via trophical transfer, this also manifested as higher abundances of bacterial grazers. Fungal‐feeding nematodes were unaffected by ash, whereas carnivorous/omnivorous nematodes decreased due to the direct effect of ash. Increasing ash doses enhanced the difference between bacterial communities of control plots and ash‐amended plots. The ash‐induced stimulation of culturable heterotrophic bacteria and bacterial grazers increased inorganic nitrogen availability at ash doses of 9 t ha−1 and above. Hence, raised limits for ash application may potentially benefit tree growth via enhanced N mineralization activity of the soil food web.
Highlights
Wood ash – waste product or a valuable fertilizer?Renewable energy sources such as wood chips play an increasing role in the global energy production
Our aim is to assess the effects of ash application across a wide range of doses on the bacterial community structure and the microfood web that is based on microbial production in a spruce plantation, which represents a system where enhanced application of wood ash as fertilizer is most relevant
At 90 t ash haÀ1, the variation within both labile organic pools increased even further, but there was an overall decline in dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) compared to the maximum values found at lower ash doses (Fig. 2a,b)
Summary
Wood ash – waste product or a valuable fertilizer?Renewable energy sources such as wood chips play an increasing role in the global energy production. Wood ash – waste product or a valuable fertilizer? Renewable energy sources such as wood chips play an increasing role in the global energy production. The European Council has set a mandatory target of 20% renewable energy of the total energy consumption by 2020 (COM, 2006). Long-term sustainable use of wood biomass for energy production calls for. The major components of wood ash are calcium, potassium, magnesium and phosphorus, while it contains virtually no nitrogen (N) (Demeyer et al, 2001). The efficiency of ash as a fertilizer depends on the existing N in the system and/or the rate of N deposition. In spite of the negligible N content in wood ash, the increased pH can indirectly increase microbial activity and N mineralization and availability
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