Janna Pietikäinen

Clear-cutting and prescribed burning in coniferous forest : comparison of effects on soil fungal and total microbial biomass, respiration activity and nitrification

Abstract The effects of clear-cutting (CC) and clear-cutting followed by prescribed burning (CC-B) on humus chemical and microbiological variables and quality were compared in a Norway spruce dominated stand in North-Eastern Finland. The pattern of chemical changes in humus was similar after both treatments but CC-B caused greater changes than CC. Treatments raised the pH, cation exchange capacity and base saturation compared to an untreated standing forest control (Ctr). Total microbial carbon (C mic ) measured by substrate-induced respiration (SIR) and fumigation-extraction (FE) methods decreased following treatments. CC caused a 21% reduction of C mic compared to Ctr (10,890 μg g −1 dry wt), as measured by SIR, and a 27% reduction compared to Ctr (7281 μg g −1 dry wt) as measured by FE. CC-B resulted in 53 and 67% lower C mic than Ctr as measured by SIR and FE, respectively. Reasons for this decline in C mic are proposed. Fungal biomass determined as humus ergosterol concentration fell even more steeply than total C mic . Humus quality was analysed by near infrared reflectance spectroscopy (NIR) which revealed differences in humus structure between treatments. The NIR data could be interpreted to explain 75–82% of the variation in C mic -FE, C mic -SIR and ergosterol concentration. CC and CC-B lowered soil basal respiration, but not proportionally with the reduction in C mic since the specific respiration rate (CO 2 -C evolved per unit C mic ) was clearly higher with CC-B than CC or Ctr. CC and CC-B both resulted in a higher concentration of NH 4 + but only the humus from CC-B showed nitrification during a 6 week laboratory incubation.

Does short-term heating of forest humus change its properties as a substrate for microbes?

Abstract Prescribed burning is known to reduce the size of the microbial biomass in soil, which is not explained by preceding clear-cutting or the effects of ash deposition. Instead, burning induces an instant heat shock in the soil, which may either directly kill soil microbes or indirectly alter the soil organic matter. We heated dry forest humus at temperatures from 45 to 230°C, inoculated them to ensure equal opportunities for microbial proliferation and incubated the heated humus samples at 14°C. After 1, 2, 4 and 6 months we studied the microbial community structure of the samples by determining the phospholipid fatty acid pattern (PLFA), microbial substrate utilization pattern using Biolog Ecoplates and total microbial biomass ( C mic ) by substrate-induced respiration (SIR). The chemical structure of humus was scanned by Fourier-transform infrared (FTIR) and 13 C NMR spectroscopy. Heating at 230°C caused changes in the chemical structure of the humus as indicated by FTIR spectroscopy, increased the pH of the humus by 1.1 units, reduced C mic by 70% compared with the control and caused changes in substrate utilization patterns and proportions of PLFAs. More interestingly, the heat treatments from 45 to 160°C, which did not increase humus pH, resulted in differences in both microbial community structure and substrate utilization patterns. The severely heated samples (120–160°C) were relatively richer in 16:1 ω 7 t , cy19:0 and 18:1 ω 7, while the mildly heated samples (45–100°C) showed higher proportions of 16:1 ω 5, 16:1 ω 9, 10me16:0 and a15:0. The t / c ratio calculated from trans and cis configurations of 16:1 ω 7 increased from 1 to 6 months in the severely heated humus, possibly indicating nutrient deprivation. The control showed a decreasing t / c ratio and a stable amount of C mic indicating sufficient amount of decomposable organic matter. After incubation for 1 month, similar amounts of C mic had reestablished in 160°C-treated and control samples. However, the C mic in 160°C-treated samples decreased over 5 months. This might have been caused by a heat-induced flush of easily decomposable carbon, which was later exhausted. We conclude that changes in chemical properties of humus during dry heating at 230°C were capable of causing changes in microbial community structure of the humus.

Responses of decomposer community to root-isolation and addition of slash

Abstract We studied the causal relationships between forest harvesting and the soil decomposer community focusing on suppression of energy inputs from trees to the soil through root–mycorrhizal network and increased energy input to the soil in the form of slash left on site. We hypothesised that both of these factors would affect the decomposer community, since the soil food web has been regarded as a system in which the amount of resources controls the numbers of consumers. To study the importance of these factors without changes in microclimate, like in sunshine and shade, taking place in clear-felled areas, the experiment was performed in a mature spruce ( Picea abies (L.) Karst.) forest. The experiment with factorial design included two factors, isolation of the tree roots in the study plots from the surrounding soil and addition of slash. The isolation increased soil NH 4 content and promoted growth of herbs. It had no effect on the total microbial or bacterial biomass, but the amount of soil fungi decreased by 40% and community structure of bacteria changed in the isolated plots. At the level of microbivores, fungivorous microarthropods (collembolans and oribatid mites) decreased in their numbers while bacterivorous nematodes slightly increased. In addition, the isolation increased the numbers of enchytraeids with a 2 year delay. No effects on decomposers by slash addition could be detected. The responses of decomposers to the isolation were similar to those observed earlier in clear-felled areas. It was concluded that, in short-term, elimination of root–mycorrhizal connections is one of most important factors affecting the soil decomposer community after harvesting of northern coniferous forest. The effect of slash is, to a large extent, due to its effects on micro-climate and soil moisture.

Reaction of forest soil microflora to environmental stress along a moderate pollution gradient next to an oil refinery

Twenty-five study sites were established along a 57-km-long transect in order to estimate the impact of an oil refinery, mainly emitting sulphur dioxide (24000 t yr−1), on forest soil (F/H-horizon) chemistry and microbiology. The study demonstrated the existence of a pollution gradient which was best represented by the logarithm of the concentration of vanadium in the analyzed F/H soil layer. Of the soil microbial characteristics measured, including length of fungal hyphae, soil respiration, microbial biomass C and N, and percentage mass loss of Scots pine (Pinus sylvestris) needle litter, only fungal hyphal length was suppressed by the pollution load. No reduction in basic cations (Ca, Mg, K, and Na) in the F/H-horizon, or enrichment of soluble aluminum in the F/H-horizon of the Scots pine forest could be detected to result from the deposition.

Leaf and root litter of a legume tree as nitrogen sources for cacaos with different root colonisation by arbuscular mycorrhizae

Traditionally cacao (Theobroma cacao L.) is cultivated under legume shade trees, which produce N-rich litter that improves soil organic matter content, microbial activity, and recycles N to the crop. Arbuscular mycorrhiza forming fungi (AMF) are known to play an important role in plant nutrient uptake, yet their role in plant N uptake from organic residues in tropical agroforestry systems is not clear. We studied root and leaf litter of the legume shade tree Inga edulis Mart. as a source of N for cacao and the importance of AMF colonisation in the uptake of litter N under controlled conditions. Leaf and root litter of I. edulis enriched with 15N was added to cacao pots filled with field soil. Half of the cacao saplings were AMF-inoculated and the soil of non-inoculated saplings was treated with fungicide to suppress AMF. During the 10-week experiment, young cacao leaves were sampled for 15N analyses and at the end of the experiment whole plants were harvested. Microbial populations in the soil were determined using phospholipid fatty acid (PLFA) analysis, and AMF structures in the roots were quantified. Fungicide treatment decreased AMF structures in roots and increased bacterial populations, but did not affect the decomposition rate of either litter type. Inoculated and non-inoculated cacao saplings used 2.6 and 2.1%, respectively, of N added to the pots in leaf litter and 12.1 and 7.1% of N available in root litter indicating that root litter of I. edulis may be a more efficient N source than leaf litter for cacao. Although the fungicide treatment did not completely suppress AMF in non-inoculated pots, it created sufficient contrast in root AMF colonisation for concluding that AMF significantly enhanced cacao N use from both litter types. The role of root litter of shade trees as a N source in agroforestry should not be neglected.

Some observations on the copper tolerance of bacterial communities determined by the (3H)-thymidine incorporation method in heavy metal polluted humus

Abstract Changes in pH after filtration of bacterial suspensions are important when applying the radioactive thymidine incorporation method to heavy-metal polluted soils with low microbial activity. In the original method (Baath 1992; Soil Biology and Biochemistry 24, 1157–1165) the blended and centrifuged suspension was filtered through glass wool to remove humus particles from the suspension. When we filtered the bacterial suspension through glass wool the pH increased by 2 units and the thymidine incorporation rate decreased. This made the community copper tolerance measurement ambiguous. When using soil samples with very low activity, we recommend the use of acid-washed glass wool or polyester net filtration which eliminates changes in pH.