Anders Wirén

Effects of light and nutrient stress on leaf phenolic chemistry in salix dasyclados and susceptibility to Galerucella lineola (Coleoptera)

The amounts of phenolic compounds present in Salix dasyclados Wimm. leaves, and the susceptibility of the tissue to the leaf beetle Galerucella lineola F., were determined in laboratory grown plants stressed with respect to light and nutrient availability. Plants of clonal origin were grown in units with circulating nutrient solutions under (1) low light with free access to nutrients (low carbon supply), (2) high light with free access to nutrients (optimal conditions), and (3) high light with suboptimal nutrient supply (low nutrient supply). Concentrations of phenolic compounds in plants with low carbon supply were only about one-third compared with the other two treatments. The relative availability of carbohydrates for construction of carbon-based defensive compounds, such as phenolics, may explain the different susceptibility of willow leaves when grown under the specified environments. Beetles consumed about five times as much leaf tissue on plants low in carbon and rich in nitrogen as in the other two treatments.

Effects of acidification and liming on carbon and nitrogen mineralization and soil organisms in mor humus

The aim was to determine if changes in C and N mineralization after acidification and liming could be explained by changes in the soil organism biomass. Intact soil cores from F/H layers in a Norway spruce (C:N=31) and a Scots pine (C:N=44) stand in central Sweden were treated in the laboratory for 55 days with deionized water (control), weak H2SO4 (successively applied as 72 mm of acid rain of pH 3.1), strong H2SO4 (applied as a single high dose of pH 1), and lime CaCO3. Strong acidification reduced C mineralization and increased net N mineralization in both soils. Weak acidification resulted in similar but less pronounced effects. Liming initially stimulated C mineralization rate, but the rates declined, indicating that an easily available C source was successively used up by the microorganisms. Liming also increased net N mineralization in the C:N=31 humus, but not significantly in the C:N--44 humus. Strong acidification generally affected the amounts of FDA-active fungal hyphae, nematodes and enchytraeids more than the other treatments did. The increases in net N mineralization after acidification and liming could only partly be explained by the decreases in biomass N in soil organisms. Mineralization of biomass N from killed soil organisms could at the most explain up to about 30% of the increase in net N mineralization after strong acidification. Most of the effects on N mineralization seemed to depend on the fact that acidification reduced and liming increased the availability of C and N to the microorganisms. Furthermore, acidification seemed to reduce the incorporation of N from dead organisms into the soil organic matter and, thereby, make the N compounds more readily available to microbial decomposition and mineralization.

Effect of nitrogen and carbon supply on the development of soil organism populations and pine seedlings - microcosm experiment

Series of pots containing Scots pine seedlings and a humus-sand mixture were watered twice weekly for 398 d with different nutrient solutions (control treatment with complete plant nutrient solution less nitrogen; glucose addition; nitrogen addition; glucose and nitrogen addition). Analyses were made of organic and nitrogen contents in the substrate, plant weights and nitrogen contents, nitrogen fixation and respiration rates, abundance, biomass and in some cases species composition of different soil organism populations. The growth rate of pine seedlings was lowest in series supplied with glucose, which is most easily explained by a deficiency of nitrogen due to microbial immobilization. The fungi and yeast were stimulated by addition of an easily available carbon source whereas the bacteria needed both nitrogen and carbon to maintain high biomass. A positive correlation between fungal feeding soil organisms and amount of fungal mycelium was found while a more complex situation prevailed with regard to bacterial numbers and bacterial feeding nematodes. The systems are evaluated in relation to the different treatments and compared with the field situation. (Less)

Pools and fluxes of carbon and nitrogen in 40-year-old forest liming experiments in Southern Sweden

Soil samples were collected from litter, humus and mineral soil layers to a depth of 50 cm in 37–42 year-old limed and unlimed plots in one beech and three spruce stands in S Sweden for determination of carbon (C) and nitrogen (N) pools, C and N mineralization rates and nitrification rates. The samples were sifted while still fresh and incubated at a constant temperature (15°C) and soil moisture (50 % WHC) for 110–180 days with periodic subsamplings. The C and N pools in the uppermost soil layers were significantly lower in plots limed with 9–10 t CaCO3 ha−1 than in unlimed plots, whereas the pools in the deeper mineral soil did not differ markedly between the treatments. In the whole soil profile, the C and N pools had, on average, decreased by 16% (P<0.05) and 11% (P>0.05), respectively, after 40 yrs. The smaller reduction in N pools resulted in significantly lower C:N ratios and increased N immobilization in the limed spruce plots but not in the limed beech plot. C and net N mineralization rates were increased in some of the limed plots and decreased in others. This indicates that liming can still have a stimulatory effect after 40 yrs in some soils. The nitrification potential was increased in the limed plots. Liming did not increase tree growth in the stands investigated. We conclude that liming with high doses of CaCO3 is likely to reduce pools of soil C and possibly even soil N in relation to unlimed areas in spruce and beech forests in S Sweden. If trees in limed stands do not respond with better growth, the treatment will thus result in a net ecosystem loss of C and N in relation to unlimed areas. It was not possible to conclude whether the effects of low doses of lime would be similar to those of high doses.

SOIL ORGANISMS AND LITTER DECOMPOSITION IN A SCOTS PINE FOREST - EFFECTS OF EXPERIMENTAL ACIDIFICATION

The effects of increasing acidification in different ecosystems is today a topic under debate. Several investigations are being carried out to evaluate different questions within this relatively new problem area. In forest ecosystems the research efforts are focused mainly upon effects on vegetation and soil chemistry. Other areas, e.g. the soil biota, have been given little attention, perhaps owing to the fact that soil biological processes are relatively poorly understood but also because the study of soil organisms is more laborious than is the study of vegetation or soil chemistry.