Litter bags with natural mixed litter were incubated until °60—70% mass loss in two oak—hornbeam and two pine—beech forest stands in southern Poland. At the same stands the input of chemical elements with throughfall was followed. Decomposition constants k for the oak—hornbeam litters were —0.57 and —0.55, and for the pine—beech litters —0.30 and —0.27. Chemical elements (except for Cu and Mn) revealed similar relative mobility in the four litters. On average the elements could be ordered by decreasing mobility as follows: K > Mg > Ca > S > Cu > Na > Mn = N > Cd > Pb = Zn > Fe. Instead of the two presupposed factors controlling litter decomposition, biological and chemical, three factors were specified: (1) biological, dominating the decay of organic matter and the dynamics of N, Ca, Mg, Mn, and S; (2) physical, dominated by leaching and atmospheric deposition, and controlling the dynamics of organic matter, K, Na, Pb, Cd, and Zn; and (3) chemical, determining the dynamics of Fe, Zn, Pb, and Cd through the fixation of metal ions to humic substances. Potassium was the only element that decreased in concentration in all litters, while the concentrations of N, Na, Fe, Zn, Pb, and Cd increased in all litters. S. Ca, Mg, and Mn concentrations revealed different patterns in different litters, presumably due to the differences in initial concentrations and soil acidity. No clear trend was found for Cu. In all litter types, Fe, Zn, Pb, and Cd significantly increased in absolute amounts at the end of litter—bag incubation. In all four stands the input with throughfall was high enough to explain the increases in amount of elements, with the exception of Fe in the oak—hornbeam litters.
Reaction of soil bacteria to drought and rewetting stress may depend on soil chemical properties. The objectives of this study were to test the reaction of different bacterial phyla to drought and rewetting stress and to assess the influence of different soil chemical properties on the reaction of soil bacteria to this kind of stress. The soil samples were taken at ten forest sites and measured for pH and the contents of organic C (Corg) and total N (Nt), Zn, Cu, and Pb. The samples were kept without water addition at 20 – 30 °C for 8 weeks and subsequently rewetted to achieve moisture equal to 50 – 60 % of their maximum water-holding capacity. Prior to the drought period and 24 h after the rewetting, the structure of soil bacterial communities was determined using pyrosequencing of 16S rRNA genes. The drought and rewetting stress altered bacterial community structure. Gram-positive bacterial phyla, Actinobacteria and Firmicutes, increased in relative proportion after the stress, whereas the Gram-negative bacteria in most cases decreased. The largest decrease in relative abundance was for Gammaproteobacteria and Bacteroidetes. For several phyla the reaction to drought and rewetting stress depended on the chemical properties of soils. Soil pH was the most important soil property influencing the reaction of a number of soil bacterial groups (including all classes of Proteobacteria, Bacteroidetes, Acidobacteria, and others) to drought and rewetting stress. For several bacterial phyla the reaction to the stress depended also on the contents of Nt and Corg in soil. The effect of heavy metal pollution was also noticeable, although weaker compared to other chemical soil properties. We conclude that soil chemical properties should be considered when assessing the effect of stressing factors on soil bacterial communities.Electronic supplementary materialThe online version of this article (doi:10.1007/s13213-014-1002-0) contains supplementary material, which is available to authorized users.
Despite the global importance of forests, it is virtually unknown how their soil microbial communities adapt at the phylogenetic and functional level to long-term metal pollution. Studying 12 sites located along two distinct gradients of metal pollution in Southern Poland revealed that functional potential and diversity (assessed using GeoChip 4.2) were highly similar across the gradients despite drastically diverging metal contamination levels. Metal pollution level did, however, significantly impact bacterial community structure (as shown by MiSeq Illumina sequencing of 16S rRNA genes), but not bacterial taxon richness and community composition. Metal pollution caused changes in the relative abundance of specific bacterial taxa, including Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, Firmicutes, Planctomycetes and Proteobacteria. Also, a group of metal-resistance genes showed significant correlations with metal concentrations in soil. Our study showed that microbial communities are resilient to metal pollution; despite differences in community structure, no clear impact of metal pollution levels on overall functional diversity was observed. While screens of phylogenetic marker genes, such as 16S rRNA genes, provide only limited insight into resilience mechanisms, analysis of specific functional genes, e.g. involved in metal resistance, appears to be a more promising strategy.
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