Ash 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 À1 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 ashinduced stimulation of culturable heterotrophic bacteria and bacterial grazers increased inorganic nitrogen availability at ash doses of 9 t ha À1 and above. Hence, raised limits for ash application may potentially benefit tree growth via enhanced N mineralization activity of the soil food web.
Soil pH is probably the most important variable explaining bacterial richness and community composition locally as well as globally. In contrast, pH effects on fungi appear to be less pronounced, but also less studied. Here we analyze the community responses of bacteria and fungi in parallel over a local extreme pH gradient ranging from 4 to 8. We established the pH gradient by applying strongly alkaline wood ash in dosages of 0, 3, 9, 15, 30, and 90 t ha–1 to replicated plots in a Picea abies plantation and assessed bacterial and fungal community composition using high throughput amplicon sequencing 1 year after ash application. At the same time, the experiment investigated if returning wood ash to plantation forests pose any immediate threats for the microbial communities. Among the measured environmental parameters, pH was by far the major driver of the microbial communities, however, bacterial and fungal communities responded differently to the pH increment. Whereas both bacterial and fungal communities showed directional changes correlated with the wood ash-induced increase in pH, the bacterial community displayed large changes at wood ash dosages of 9 and 15 t ha–1 while only higher dosages (>30 t ha–1) significantly changed the fungal community. The results confirm that fungi are less sensitive to pH changes than bacteria but also that fertilizing plantation forests with wood ash, viewed through the lens of microbial community changes, is a safe management at standard dosages (typically 3 t ha–1).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.