Veikko Kitunen

Structure of the Microbial Communities in Coniferous Forest Soils in Relation to Site Fertility and Stand Development Stage.

A bstractThe structure, biomass, and activity of the microbial community in the humus layer of boreal coniferous forest stands of different fertility were studied. The Scots pine dominated CT (Calluna vulgaris type) represented the lowest fertility, while VT (Vaccinium vitis-idaéa type), MT (Vaccinium myrtillus type), and OMT (Oxalis acetocella–Vaccinium myrtillus type) following this order, were more fertile types. The microbial community was studied more closely by sampling a succession gradient (from a treeless area to a 180-years-old Norway spruce stand) at the MT type site. The phospholipid fatty acid (PLFA) analysis revealed a gradual shift in the structure of the microbial community along the fertility gradient even though the total microbial biomass and respiration rate remained unchanged. The relative abundance of fungi decreased and that of bacteria increased with increasing fertility. The structure of the bacterial community also changed along the fertility gradient. Irrespective of a decrease in fungal biomass and change in bacterial community structure after clear-cutting, the PLFA analysis did not show strong differences in the microbial communities in the stands of different age growing on the MT type site. The spatial variation in the structure of the microbial community was studied at a MT type site. Semivariograms indicated that the bacterial biomass, the ratio between the fungal and bacterial biomasses, and the relative amount of PLFA 16:1ω5 were spatially autocorrelated within distances around 3 to 4 m. The total microbial and fungal biomasses were autocorrelated only up to 1 m. The spatial distribution of the humus microbial community was correlated mainly with the location of the trees, and consequently, with the forest floor vegetation.

Soil microbial activities and characteristics of dissolved organic C and N in relation to tree species

Abstract The aim of this study was to compare soil microbial biomass and activities with the characteristics of water-extractable organic matter in different types of forest stands. The study was carried out in adjacent 70-year-old stands dominated by silver birch (Betula pendula Roth.), Norway spruce (Picea abies (L.) Karst.) or Scots pine (Pinus sylvestris L.) growing in originally similar soil on Hylocomium–Myrtillus site type in northern Finland. The birch stand was a pure single-species stand, but the coniferous stands were mixed with birch. Soil samples were taken from the humus layer. Water extracts were analyzed for the concentrations of dissolved organic C and N (DOC and DON) and characterized according to chemical composition using a resin fractionation technique and molecular size distribution. C and N in the soil microbial biomass and rates of C mineralization, net N mineralization and net nitrification were measured. Soil pH varied from 4.0 to 4.3 and was highest in the birch stand. The C-to-N ratio of the soil organic matter was higher in the pine stand than in the other stands. Per unit organic matter, the amounts of C and N in the soil microbial biomass were highest in the birch stand. In laboratory incubation experiments, both the rate of C mineralization and that of net N mineralization were much higher in soils from the birch and spruce stands than in soil from the pine stand. This was also the order for concentrations of both DOC and DON. The DOC-to-DON ratio was similar in the birch and spruce soils but higher in the pine soil. The distributions of both different chemical and molecular size fractions were relatively similar in all soils, except for differences in some fractions of DON. The hydrophobic acid fraction was the major carrier of both DOC and DON, accounting for about 60% of DOC and 40–55% of DON. The DOC-to-DON ratio was especially low in the hydrophilic neutral fraction and in the smallest molecular size fraction (

Microbial biomass C and N, and respiratory activity in soil of repeatedly limed and N- and P-fertilized Norway spruce stands

Abstract Our aim was to assess long-term effects of repeated liming and N and P additions on soil microbial biomass and activity. The experimental sites were four, 40–60 yr old, Norway spruce (Picea abies L.) stands growing on mineral soil sites. During the 30 yr study period, the cumulative amount of finely-ground limestone totalled 6000 kg ha−1, and fertilizer N and P totalled 530–950 and 70–115 kg ha−1, respectively. The main effects of the additions, expressed on an organic matter basis, were as follows: liming increased, and N addition decreased, both the fumigation-extraction (FE) and substrate-induced-respiration (SIR) derived microbial biomass C, microbial biomass N and its proportion of total soil N, and microbial respiration rate. Nitrogen addition increased slightly the soil ergosterol content, indicating an increase in fungal biomass or a change in fungal population structure. There were no significant differences in microbial C:N ratio and respiration: biomass ratio between the treatments. No clear effects of P addition were observed.

Wood-ash fertilization and fire treatments in a Scots pine forest stand: Effects on the organic layer, microbial biomass, and microbial activity

We studied the reactions of humus layer (F/H) microbial respiratory activity, microbial biomass C, and the fungal biomass, measured as the soil ergosterol content, to the application of three levels of wood ash (1000, 2500, and 5000 kg ha-1) and to fire treatment in a Scots pine (Pinus sylvestris L.) stand. Physicochemical measurements (pH, organic matter content, extractable and total C content, NH4+and total N content, cation-exchange capacity, base saturation) showed similarity between the fire-treated plots and those treated with the lowest dose of wood ash (1000 kg ha-1). The ash application did not change the level of microbial biomass C or fungal ergosterol when compared to the control, being around 7500 and 350 μg g-1 organic matter for the biomass C and ergosterol, respectively. The fire treatment lowered the values of both biomass measurements to about half that of the control values. The fire treatment caused a sevenfold fall in the respiration rate of fieldmoist soil to 1.8 μl h-1 g-1 organic matter compared to the values of the control or ash treatments. However, in the same soils adjusted to a water-holding capacity of 60%, the differences between the fire treatment and the control were diminished, and the ash-fertilized plots were characterized by a higher respiration rate compared to the control plots. The glucose-induced respiration reacted in the same way as the water-adjusted soil respiration. The metabolic quotient, qCO2, gradually increased from the control level with increasing applications of ash, reaching a maximum in the fire treatment. Nitrification was not observed in the treatment plots.

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.

PHENOLIC COMPOUNDS AND TERPENES IN SOIL ORGANIC HORIZON LAYERS UNDER SILVER BIRCH, NORWAY SPRUCE AND SCOTS PINE

The aim of this study was to monitor the concentration of some plant secondary metabolites, such as low- and high-molecular-weight phenolics, condensed tannins (proanthocyanidins), and sesqui-, di- and triterpenes, in litter (L), fermentation (F) and humified (H) layers of the soil organic horizon in stands dominated by silver birch (Betula pendula Roth.), Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.), and from samples taken from birch leaves and spruce and pine needles. Concentrations of low- and high-molecular-weight phenolics and terpenes from the four most dominant species of ground vegetation taken from the stands were also determined. In general, the L layer showed higher concentrations of both phenolic compounds and terpenes than the F and H layers did. Concentrations of terpenes decreased relatively more with soil depth than did concentrations of total phenolics (=low + high) or condensed tannins. Of the total phenolics, the proportion of low-molecular-weight phenolics increased from the L to the H layer with all tree species. Concentrations of all terpenes were highest under pine and lowest under birch. Concentrations of the studied secondary metabolites in the ground vegetation species were similar under different tree species. Blueberry (Vaccinium myrtillus L.) and lingonberry (Vaccinium vitis-idaea L.) contained considerably higher concentrations of total phenolics than did feather moss (Pleurozium schreberi (Brid.) Mitt.) and wavy hair-grass (Deschampsia flexuosa (L.) Trin.). Concentration of total phenolics in soil correlated positively with soil respiration and microbial biomass C, and terpenes showed positive correlation with soil C-to-N ratio.

Temperature sensitivity of soil carbon fractions in boreal forest soil.

Feedback to climate warming from the carbon balance of terrestrial ecosystems depends critically on the temperature sensitivity of soil organic carbon (SOC) decomposition. Still, the temperature sensitivity is not known for the majority of the SOC, which is tens or hundreds of years old. This old fraction is paradoxically concluded to be more, less, or equally sensitive compared to the younger fraction. Here, we present results that explain these inconsistencies. We show that the temperature sensitivity of decomposition increases remarkably from the youngest annually cycling fraction (Q10 < 2) to a decadally cycling one (Q10 = 4.2-6.9) but decreases again to a centennially cycling fraction (Q10 = 2.4-2.8) in boreal forest soil. Compared to the method used for current global estimates (temperature sensitivity of all SOC equal to that of the total heterotrophic soil respiration), the soils studied will lose 30-45% more carbon in response to climate warming during the next few decades, if there is no change in carbon input. Carbon input, derivative of plant productivity, would have to increase by 100-120%, as compared to the earlier estimated 70-80%, in order to compensate for the accelerated decomposition.

Nitrogen transformations in limed and nitrogen fertilized soil in Norway spruce stands

Nitrogen transformations in the soil, and the resulting changes in carbon and nitrogen compounds in soil percolate water, were studied in two stands of Norway spruce (Picea abies L.). Over the last 30 years the stands were repeatedly limed (total 6000 kg ha−1), fertilized with nitrogen (total about 900 kg ha−1), or both treatments together. Both aerobic incubations of soil samples in the laboratory, and intact soil core incubations in the field showed that in control plots ammonification widely predominated over nitrification. In both experiments nitrogen addition increased the formation of mineral-N. In one experiment separate lime and nitrogen treatments increased nitrification, in the other, only lime and nitrogen addition together had this effect. In one experiment immobilization of nitrogen to soil microbial biomass was lower in soil only treated with nitrogen. Soil percolate water was collected by means of lysimeters placed under the humus layer and 10 cm below in the mineral soil. Total N, NH4-N and NO3-N were measured, and dissolved organic nitrogen was fractioned according to molecular weight. NO3-N concentrations in percolate water, collected under the humus layer, were higher in plots treated with N-fertilizer, especially when lime was also added. The treatments had no effect on the N concentrations in mineral soil. A considerable proportion of nitrogen was leached in organic form.

Nitrogen transformations in boreal forest soils—does composition of plant secondary compounds give any explanations?

Two major groups of plant secondary compounds, phenolic compounds and terpenes, may according to current evidence mediate changes in soil C and N cycling, but their exact role and importance in boreal forest soils are largely unknown. In this review we discuss the occurrence of these compounds in forest plants and soils, the great challenges faced when their concentrations are measured, their possible effects in regulating soil C and N transformations and finally, we attempt to evaluate their role in connection with certain forest management practices. In laboratory experiments, volatile monoterpenes, in the concentrations found in the coniferous soil atmosphere, have been shown to inhibit net nitrogen mineralization and nitrification; they probably provide a C source to part of the soil microbial population but are toxic to another part. However, there is a large gap in our knowledge of the effects of higher terpenes on soil processes. According to results from laboratory experiments, an important group of phenolic compounds, condensed tannins, may also affect microbial processes related to soil C and N cycling; one mechanism is binding of proteins and certain other organic N-containing compounds. Field studies revealed interesting correlations between the occurrence of terpenes or phenolic compounds and C or net N mineralization in forest soils; in some cases these correlations point in the same direction as do the results from laboratory experiments, but not always. Different forest management practices may result in changes in both the quantity and quality of terpenes and phenolic compounds entering the soil. Possible effects of tree species composition, clear-cutting and removal of logging residue for bioenergy on plant secondary compound composition in soil are discussed in relation to changes observed in soil N transformations.

Dissolved soil organic nitrogen and carbon in a Norway spruce stand and an adjacent clear-cut

Abstract The aims of this study were to characterize dissolved soil organic N (DON) and C (DOC) in a coniferous stand and an adjacent clear-cut, and to evaluate the importance of DON in N leaching. The study was carried out in a Norway spruce stand and a clear-cutting treatment in the same forest stand. Concentrations of DON in soil solution were monitored for 5 years after clear-cutting with gravity lysimeters. In the Norway spruce stand DON comprised 62–83% of the total N in soil solution over the 5-year period. The concentrations of DON in the clear-cut were higher than in the forest stand, but the proportion of total N was lower. To characterize dissolved organic matter, soil samples were aerobically incubated for 6 weeks in the laboratory, and the quantity, molecular size distribution and chemical nature of both DON and DOC were determined from water extracts made before and after the incubation. In the soil samples from the Norway spruce stand, C-rich compounds with a high C/N ratio and large molecular size were formed. In contrast, after the incubation the major carriers of DON in soil samples from the clear-cut were N-rich organic compounds with a low C/N ratio and a small molecular size. The distribution of different chemical fractions of DOC in soil did not differ much whether recovered from the Norway spruce stand or the clear-cut. It was (from highest to lowest concentration): hydrophobic acids>hydrophilic acids>phenols>hydrophilic neutrals. A major part of DON was also carried by these fractions. During incubation the concentration of N-containing hydrophilic acids increased, especially in the soil from the clearcut. In soil samples from the Norway spruce stand, the rate of net N mineralization was low and no NO3 was formed, whilst the rate of net N mineralization was high and net nitrification was intensive in soil from the clear-cut.