Laura Paavolainen

Inhibition of nitrification in forest soil by monoterpenes

Nitrate production was detected in untreated soil of a Norway spruce (Picea abies L.) stand only after clear-cutting the stand. The aim of this study was to determine whether allelochemical inhibition of nitrification by monoterpenes played any role in inhibiting nitrification in the stand. Therefore, soils from a clear-cut plot and from a forest plot were studied. In the field, monoterpenes (mostly α- and β-pinenes), measured by soil microair diffusive samplers, were intensively produced in the forest plot, but not in the clear-cut plot. In the laboratory, soil samples taken from the forest plot produced only small amounts of monoterpenes, indicating that monoterpenes were mainly produced by the roots and not to great extent by the soil microbial population. The effect of a mixture of monoterpenes (seven major monoterpenes detected in the field) on net nitrification, net N mineralization and denitrification activities of soil from the clear cut plot, and on carbon mineralization of soils from both the forest and clear-cut plots, was studied in the laboratory. In both aerobic incubation experiments and in soil suspensions with excess NH4-N, nitrification was inhibited by exposure to the vapours of monoterpenes at similar concentrations at which they had been detected in forest plot. This indicates direct inhibition of nitrification by monoterpenes. Exposure to monoterpenes did not affect denitrification. However, it increased respiration activity of both soils. This could also indicate indirect inhibition of nitrification by monoterpenes, due to immobilization of mineral N. Thus it seems that monoterpenes could play a role in inhibiting nitrification in the forest soil.

Nitrogen and carbon transformations before and after clear-cutting in repeatedlyN-fertilized and limed forest soil

Abstract Nitrogen and carbon transformations were monitored in a Norway spruce ( Picea abies L.) stand in the summer before clear-cutting, and for the following three summers. During 30 y before the clear-cutting the stand had been repeatedly limed (total 6 t limestone ha −1 ), fertilized with N (total about 900 kg N ha −1 ), and both treatments were combined. Aerobic incubation experiments in the laboratory showed that, before clear-cutting, nitrification took place only in the soil that had been both limed and N-fertilized. Clear-cutting increased soil pH and net formation of mineral N, and initiated nitrification in all soils. These effects were observed throughout the study period. The only exception was the soil that had been both limed and N-fertilized, where the effect of clear-cutting on these N transformations was negligible or even suppressive. Generally, the greatest response in N transformations to clear-cutting was observed in the control soil. There was a small increase in microbial biomass C and N, and C mineralization in the first summer after clear-cutting. Net formation of mineral N correlated positively with pH at a lower pH range (pH 3.9–4.9) and negatively at a higher pH range (pH 4.9–6.9). C mineralization correlated positively with microbial biomass C, but there was no linear relationship between net formation of mineral N and microbial biomass N. C mineralization and net N mineralization were not correlated.

Rapid PCR-based method for the direct analysis of fungal communities in complex environmental samples

Abstract Fungal community analysis using 18S rDNA primer pairs and denaturing gradient gel electrophoresis of PCR products (Vainio, E.J., Hantula, J., 2000. Mycological Research 104, 927–936) was applied to field studies of the forest ecosystem. We report a DNA extraction method producing high quality DNA allowing successful PCR amplification from problematic samples without use of nested polymerase chain reaction (PCR) procedures. The analysis was found to be applicable for samples from environments of varying fungal diversities and high organic matter content: wood samples from fallen branches of trees, laboratory mini-ecosystems and forest humus samples. When the method was tested using replicate forest soil samples, it was shown to be highly reproducible.

C and N transformations in forest soil after mounding for regeneration.

Abstract Carbon and nitrogen transformations were monitored in mounded and unmounded forest soil for 3 years after mounding. The mounding was done 1 year after the clear-cutting of a Norway spruce stand where a forest fertilization experiment had been carried out for 30 years with the following treatments: control, N fertilization, liming and combined N fertilization and liming. In the mounding plots, soil was sampled from the buried double humus layer of the mounds and from the undisturbed humus layer on the corresponding plots without soil preparation. Mounding decreased carbon mineralization in the humus layer, but had little effect on microbial biomass C and N, and net formation of mineral N and nitrification. Microbial biomass and activities were always higher in the humus layer than in the mineral soil layers. This was observed in both the mounded and unmounded plots. Net formation of mineral N and nitrification were, however, intensive also in the mineral soil layers of the unmounded, previously limed plots. Higher N concentrations, especially NO 3 -N and total N, in soil solutions were found in the mounded plots compared to the unmounded plots. The discrepancy between net formation of mineral N, nitrification and soil solution results can be explained to a great extent by the doubled amount of organic matter in the mounds. Mineral nitrogen concentrations of soil solution decreased during the 2 years after mounding, obviously due to the development of ground vegetation. The planting of birch seedlings increased microbial biomass C and carbon mineralization in the mineral soil of the mounds.

Decomposition of Norway spruce and Scots pine needles : Effects of liming

The effect of liming on the decomposition of Norway spruce needle litter was studied in 40–60-year-old Norway spruce stands. Finely-ground limestone had been spread about 30 years ago at a dose of 2 t ha−1 and reliming was carried out about 20 yr later at a dose of 4 t ha−1. Needle litter was collected from both control and limed plots, and it was placed in litter bags in the middle of the humus layer of the plot from which they originated, and similarly to the other plot in May. Litter bags were sampled after 4, 12 and 16 months. The site of origin of the needle litter, whether from control plot or from limed plot, affected mainly the early stages of decomposition. Initially the effect of liming was seen as decreased concentration of water soluble material and then, during decomposition, as decreased mass loss and decreased degradation of lignin, and increased C/N ratio. The incubation site, whether the control or the limed plot, did not affect decomposition significantly.Decomposition of Scots pine needles in a young Scots pine plantation was also studied. The treatments were: 2 t ha−1 of finely-ground limestone and 2.5 t ha−1 of bark ash spread 8 months before this study. The treatments did not affect decomposition much, but some stimulation of the treatments on decomposition was observed. Compared to spruce needles, the C/N ratio of pine seedles was lower, they contained less lignin and more water soluble material, and decomposed faster in the first summer.

Nitrification and denitrification in forest soil subjected to sprinkling infiltration.

Abstract In Ahvenisto esker, southern Finland, artificial recharging of groundwater has been done by sprinkling infiltration, i.e. by sprinkling lake water directly onto forest soil. Due to infiltration, the pH of the humus layer rose from about 5 to 6.5, nitrification was initiated and the fluxes of N 2 O and leaching of nitrate from the soil increased. Our aim was to study nitrogen transformations in different soil layers and to determine the response of nitrification to pH. Nitrification in ammonium-enriched soil suspensions was pH-dependant in a gradient from 4.7 to 6.7. In the soils subjected to infiltration, the production of (NO 2 +NO 3 )-N was inhibited by decreasing the pH to 5.3 or lower. Low pH also led to decreased numbers of nitrifiers. In the soils not subjected to infiltration (control soils), (NO 2 +NO 3 )-N production initiated at pH 6.7 and the numbers of nitrifiers increased. In incubation experiments, with no added ammonium, the adjustment of pH to 6.7 also initiated nitrification in the control soils. Thus, increase in soil pH was the main reason for initiation of nitrification at this site. During infiltration, N 2 O was produced mainly by denitrification and approximately 75% of the denitrification products was N 2 . In the samples from the humus layer, the concentrations of (NO 2 +NO 3 )-N, the net production of mineral N and net nitrification were in general less, whereas denitrification enzyme activity and denitrification potential were higher than in the samples from the mineral soil layer. The mineral soil may therefore contribute substantially to the leaching of nitrate.

Pyrenopeziza betulicola and an anamorphic fungus occurring in leaf spots of birch.

We isolated ascospores and conidia of fungi associated with leaf spots common in birch leaves, and prepared mycelial cultures from them. The taxonomic positions of the fungi were determined by analysing morphological characteristics with photomicroscopy. The relationship between the teleomorph and anamorph was tested using random amplified microsatellite fingerprints, in which the variation was not explained by the origin of isolates (e.g. ascospores or conidia). This suggested that the ascospores and conidia would have been produced by the same biological species. In pathogenicity tests birch leaves inoculated with mycelia derived from ascospores developed spots showing that the fungus could be the cause of the leaf spots. The anamorph was isolated from the developing spots. Based on these results and on review of the literature we concluded that the causative agent of the leaf spot disease of birch in Finland is Pyrenopeziza betulicola, the anamorph of which belongs to Cylindrosporium.

The Effect of Lake-Water Infiltration on the Acidity and Base Cation Status of Forest Soil

In this article, the effects of the artificialrecharging of groundwater by infiltrating surface water throughforest soil, i.e. sprinkling infiltration, on the acidity andbase cation status of the soil are described. The study wascarried out in the Ahvenisto esker area, Hämeenlinna,southern Finland, during 1996–1998. The sample plots werelocated in a 110- to 160-yr-old Scots pine (Pinussylvestris L.) stand. The site was classified as the Oxalis-Maianthemum type. The soil consisted of a mixture oftill and glaciofluvial sediments. The pH of the organic layerincreased from 4.7 to >6.5 soon after the start of irrigationon the infiltration plot. The pH of the 0–10 cm mineral soillayer also increased from 4.9 to 6.4 as a result ofinfiltration. Sprinkling infiltration increased theexchangeable Ca and Mg concentrations in the organic anduppermost mineral soil layers. The output of Ca and Mg inpercolation water from the 0–100 cm thick layer was lower thanthe input to the soil surface via irrigation in 1996. Theretention of Ca and Mg on cation exchange sites took placewithin a relatively short period of time, since retention wasobserved only in 1996 but no longer in 1997 or 1998 indicatingsaturation of the cation exchange sites by base cations. Lakewater infiltration leads to the neutralisation of forest soilacidity, and increases the capacity of the soil to withstandacidic inputs by increasing the concentrations of exchangeablebase cations on cation exchange sites in the soil.

Changes in nitrogen transformations in forest soil as a result of sprinkling infiltration

Sprinkling infiltration through forest soil is a relatively new technique for generating artificial groundwater reserves to supplement urban water supplies in Finland. The aim of this study was to determine the effects of infiltration and surface runoff, resulting from sprinkling infiltration, on the acidity status and nitrogen transformations in forest soil and in percolation water. The amount of infiltrated water applied annually during the study period was more than 2000 times greater than natural annual recharge in the area. The study area is located on an esker in southern Finland (Ahvenisto, Hameenlinna). The tree cover on the relatively fertile site consisted of Scots pine and Norway spruce. The results obtained during the first two years of the project indicated that the pH(H2O) of the organic layer of the soil had increased strongly due to sprinkling infiltration. The original pH of the organic layer was 4.7–5.1, and infiltration increased the pH to 6.5. The pH of percolation water collected below the organic layer increased strongly as a result of infiltration (>6.7 during infiltration). The pH of the infiltration water is about 7.0, which is higher than that of precipitation in the area (4.5). Sprinkling infiltration initiated net nitrification due to the elevated pH and increased ammonium availability. Infiltration increased emissions of N2O from the soil. During breaks in the infiltration treatment the leaching of NO3 from the topsoil was considerable compared to that on the control plots.