Effects of UV-B on litter quality and decomposition

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Summary

The objective of this project was to test the following hypotheses on silver birch, white birch, grey alder and bilberry:

  1. The effects of UV radiation on phenolics in live plant tissues are carried through to the fresh litter, resulting in significant changes in the phenolic chemistry of litter.
  2. In turn, UV-induced changes in the phenolic chemistry of litter (a) inhibit microbial and faunal activity during decomposition, (b) have direct effects on N loss from litter via increases in tanning action, and (c) alter the balance of soluble phenolics in solutions leaching from litters early in decomposition, resulting in changes in the flora and fauna of soil underlying decomposing litter.
  3. These effects mediated by (a) soil fauna, (b) bacteria, (c) saprophytic and mycorrhizal fungi, and/or their interactions affect the rate of litter decomposition and nutrient release.
  4. The effects on soil biota and chemistry, in turn affect the growth of plants.

Stratospheric ozone depletion is causing an increase in UV-B at ground level, especially at high latitudes. Recovery of ozone levels is expected to occur not before the second half of the current century. More importantly, UV irradiance varies with the seasons of the year, latitude and altitude, and with the position in a canopy and the bearing towards which the foliage is exposed. UV affects plant chemistry, especially concentrations and relative composition of phenolics. Plants also respond to ‘normal’ levels of UV-B when compared to reduced UV-B, in addition to responding to increased UV-B. Changes in phenolic plant metabolites can affect interactions with herbivores and pathogens. Similar effects can happen in the decomposition of dead plant materials, the litter. However, in different experiments with UV-B, effects in different directions have been observed. Decomposer microbes (fungi and bacteria) have been shown to be affected, but effects on soil fauna have been rarely studied. Soil fauna, which includes both species that consume litter and species that consume microbes, has been shown to affect litter decomposition, soil microbes, and chemistry and in turn plant growth. The litter can also affect soil biota and chemistry at a distance, because substances including phenolics are leached and transported by water.

We have used filters to exclude UV-B and/or UV-A from branches of white birch and grey alder. We have studied litter decomposition in microcosms in controlled environment conditions. Litter produced under different UV-B and UV-A levels was compared. Extensive chemical characterization of the litter before decomposition was done, including flavonoids, phenolic acids, tannins, lignin, cellulose, hemicellulose, starch and sugars. Concentrations of nitrogen were also measured.  Indirect effects of litter on soil fauna were assessed. More general studies on other soil biota are planned for the future.

Boreal forests, and the organic matter in boreal forest soils, are a major pool of organic carbon globally, and it is important to get information on the effects of normal UV levels on soil organic matter. On the long term the amount of organic matter in the soil is not only affected by the size of litter inputs of organic matter into the soil, but more directly by the balance between the inputs and decomposition.

Funding and status. Eexperiments related to soil fauna were done in 2004 and 2005. Some funding was granted by the Nessling Foundation for writing up, as part of Titta Kotilainen’s PhD thesis. Currently concluded and no longer funded.  This project was the precursor of the later project about action spectra.

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