Outi Priha

Microbial community structure and characteristics of the organic matter in soils under Pinus sylvestris, Picea abies and Betula pendula at two forest sites

Abstract Microbial biomass C (Cmic), C mineralization rate, phospholipid fatty acid (PLFA) profiles and community level physiological profiles (CLPPs) using Biolog were determined from the humus and mineral soil layers in adjacent stands of Scots pine (Pinus sylvestris L.), Norway spruce [Picea abies (L.) Karst.] and silver birch (Betula pendula Roth) at two forest sites of different fertility. In addition, the Fourier-transformed infrared (FTIR) spectra were run on the samples for characterization of the organic matter. Cmic and C mineralization rate tended to be lowest under spruce and highest under birch, at the fertile site in all soil layers and at the less fertile site in the humus layer. There were also differences in microbial community structure in soils under different tree species. In the humus layer the PLFAs separated all tree species and in the mineral soil spruce was distinct from pine and birch. CLPPs did not distinguish microbial communities from the different tree species. The FTIR spectra did not separate the tree species, but clearly separated the two sites.

Nitrogen transformations in soil under Pinus sylvestris, Picea abies and Betula pendula at two forest sites

Abstract Microbial biomass N, net ammonification, net nitrification, denitrification potential, numbers of nitrifiers and the pH-dependency of nitrification were measured from the humus layer and mineral soil layers in adjacent stands of Scots pine ( Pinus sylvestris L.), Norway spruce ( Picea abies (L.) Karst.) and silver birch ( Betula pendula L.). The trees had been established at two forest sites of different fertility approximately 60 years ago. The aim was to see whether the microbial and chemical characteristics of the soils differed under different tree species. The soil pH(H 2 O) varied from 3.8 to 5.0 and was lowest in spruce soil at both sites in all soil layers. Microbial biomass N, ammonification, nitrification and denitrification all differed in soils of pine, spruce and birch. The flush of N from fumigation varied from 36 to 67 μg N cm −3 fresh soil in the humus layer, and from 13 to 50 μg cm −3 in the mineral soil layers. Denitrification potential with added nitrate was 2–29 ng N 2 O-N cm −3 soil h −1 in the humus layer and 0–28 ng in the mineral soil layers. Both tended to be lowest under spruce and highest under birch, at the fertile site in all soil layers and at the less fertile site in the humus layer. In the mineral soil layers of the fertile site and in the humus layer and upper mineral soil layer of the less fertile site the content of mineral N was highest under birch. Different populations of nitrifiers existed in the soils, regarding numbers, activity and pH-dependency. Only the nitrifier community in pine humus layer from the fertile site was adapted to acidic (pH 4.1) conditions. In an aerobic soil suspension the cumulative nitrate production of it was 32 μg cm −3 soil in three weeks, compared to negligible production in the other soils. When the pH of the suspensions was raised to 6.0, all soils from the fertile site produced nitrate, but the production at the less fertile site was still negligible. Higher C-to-N ratios probably explained the low nitrification activity and numbers of nitrifiers at the less fertile site. Thus, there were differences in N transformations under pine, spruce and birch, but the changes depended also on the fertility of the site.

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.

Microbial biomass and activity in soil and litter underPinus sylvestris, Picea abies andBetula pendula at originally similar field afforestation sites

Microbial biomass C and N, and activities related to C and N cycles, were compared in needle and leaf litter, and in the uppermost 10 cm of soil under the litter layer in Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies L.) and silver birch (Betula pendula L.) stands, planted on originally similar field afforestation sites 23–24 years ago. The ground vegetation was differentiated under different tree species, consisting of grasses and herbs under birch and pine, and mosses or no vegetation with a thick layer of needles under spruce. The C:N ratio of the soils was 13–21 and the soil pHCaCl2 3.8–5.2. Both showed little variation under different tree species. Microbial biomass C and N, C mineralization, net ammonification, reduction) did not differ significantly in soil under different tree species either. Birch leaf litter had a higher pHCaCl2 (5.9) than spruce and pine needle litter (pH 5.0 and 4.8, respectively). The C:N ratio of spruce needles was 30, and was considerably higher in pine needles (69) and birch leaves (54). Birch leaves tended to have the highest microbial biomass C and C mineralization. Spruce needles appeared to have the highest microbial biomass N and net formation of mineral N, whereas formation of mineral N in pine needles and birch leaves was negligible. Microbial biomass C and N were of the same order of magnitude in the soil and litter samples but C mineralization was tenfold higher in the litter samples.

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.

Nitrification, denitrification and microbial biomass N in soil from two N-fertilized and limed Norway spruce forests

Abstract Laboratory measurements of ammonification, nitrification, denitrification and microbial biomass N (N mic ) were made on samples of forest soil humus layer from two Norway spruce ( Picea abies L.) experimental sites which had been repeatedly fertilized over the past 30 years with liming (Ca, total 6000 kg ha −1 ) or with N (N, total approx. 900 kg ha −1 ) or both together (CaN). N plots showed the greatest NH 4 + N production. Net formation of mineral N (accumulation of NH 4 + N + NO 2 − N + NO 3 − N), was greatest in N and CaN plots. Nitrification and denitrification were shown in both CaN plots and in one Ca plot. Overall, N fertilization alone decreased the immobilization of N in microbes and increased the net N mineralization, whereas combined liming and N fertilization stimulated both nitrification and denitrification, possibly leading to loss of N.

Fumigation-extraction and substrate-induced respiration derived microbial biomass C, and respiration rate in limed soil of Scots pine sapling stands

The effect of liming on microbial biomass C and respiration activity was studied in four liming experiments on young pine plantations. One of the experimental sites had been limed and planted 12 years before, two 5 years before, and one a year before soil sampling. The youngest experimental site was also treated with ash fertilizer. Liming raised the pHKCl of the humus layer by 1.5 units or less. Microbial biomass was measured using the fumigation-extraction and substrate-induced respiration methods. Liming did not significantly affect microbial biomass C, except in the experiment which had been limed 11 years ago, where there was a slight biomass increase. Basal respiration, which was measured by the evolution of CO2, increased in the limed soils, except for the youngest experiment, where there was no effect. Ash fertilization raised the soil pHKCl by about 0.5 unit, but did not influence microbial biomass C or basal respiration. Fumigation-extraction and substrate-induced respiration derived microbial biomass C values were correlated positively with each other (r=0.65), but substrate-induced respiration gave approximately 1.3 times higher results. In addition, the effect of storing the soil samples at +6 and -18°C was evaluated. The effects were variable but, generally, the substrate-induced respiration derived microbial biomass C decreased, and the fumigation-extraction derived microbial biomass C and basal respiration decreased or were not affected by storage.

Mycorrhizas and C and N transformations in the rhizospheres of Pinus sylvestris, Picea abies and Betula pendula seedlings

Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies L.) and silver birch (Betula pendula L.) seedlings were grown in a greenhouse for four months in three different soils. The soils were from a field afforestation site on former agricultural land: soil from a pine site, soil from a spruce site and soil from a birch site. Pots without seedlings were included. The aim was to discover, independent of the effects of the different quality of aboveground litter and microclimate under the tree species, whether the roots change the microbial activities and chemical characteristics of the soil, whether the changes are dependent on the tree species, and whether the changes vary in different soils.Pine, spruce and birch had, on average, five, one and six meters of roots, respectively. Birch had by far the highest number of short root tips, on average 11 450 per seedling, compared to 1900 and 450 in pine and spruce seedlings, respectively. The majority of the short roots of pine and spruce were brown sheathed mycorrhizas, and those of birch were mycorrhizas in an early stage of development.The seedlings caused no major changes in either the soil pH or the concentrations of nutrients in the soils, but did affect the microbial characteristics of the soils. The effect of the tree species did not differ in different soils. Microbial biomass C and N, C mineralization rate and the concentration of ergosterol were all higher under birch and pine than under spruce and in plantless soils. Nitrate concentrations were lowest under pine and birch, but rates of net N mineralization, nitrification and denitrification did not differ under different seedlings.The stimulative effect of pine and especially birch on soil microbes was possibly due to them having more roots and releasing more root exudates to soil. There were, however, indications that not only the length/mass of roots determined the changes in microbial activities, but also differences in root activities per unit of root or in the quality of root exudates.

Role of Tree Species in Determining Soil Fertility

Although different tree species tend to establish themselves in different types of soil, trees also modify the soil in which they grow. Trees affect the soil through the microclimate that develops under the tree cover, through their above- and below-ground litter, and through the activity of their roots. These mechanisms modify the physical, chemical and biological properties of the soil.