Soil microbial dynamics and the condition of Norway spruce on the Bothnian land-uplift coast (dissertation - väitöskirja).

Merilä, Päivi
Metsäntutkimuslaitoksen tiedonantoja 877. 55 p.+ 4 appendices.
[ISBN 951-40-1863-X]

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Asiasanat:

Alnus incana, Betula sp., defoliation, discoloration, forest condition, forest soil, land uplift, needle analysis, nitrification, nitrogen mineralisation, phospholipid fatty acids, Picea abies, primary succession, soil fertility, soil respiration s

Tiivistelmä:

The poor condition of Norway spruce (Picea abies (L.) Karst.) forests growing in the coastal area of the Gulf of Bothnia, western Finland, has been a cause of concern for several decades. In this study, the crown condition of spruce in the coastal area was compared with that of spruce growing in other parts of southern Finland. The variability in the crown condition of coastal spruce was evaluated in relation to foliar chemistry, soil type, and the mineral nutrient and moisture status of the organic layer in 30 forest sites. Relationships between the chemical, physical and microbial properties of the organic layer were also studied in a survey covering the same 30 coastal sites and 12 sites in the coastal region of Västerbotten, Sweden. None of the studied stands were growing on acid sulphate soils, which is a type of soil that occurs sporadically in the coastal region of the Gulf of Bothnia. The spruce stands older than 60 years were more defoliated in coastal Ostrobothnia than in other parts of southern Finland. Defoliation and discoloration increased with increasing stand age. Old spruce stands that were strongly defoliated and discoloured also had low needle nitrogen and copper concentrations and the highest boron concentrations. Total nitrogen and extractable sulphur concentrations in the organic layer decreased with increasing stand age, and degree of defoliation and discoloration. The most common soil types in the stands on the Ostrobothnian coast were carbic podzols and dystric gleysols, which develop in sporadically waterlogged soil conditions. Crown condition was found to be the poorest in old stands growing on these soil types. The carbic podzols and dystric gleysols also differed from the ferric podzols as regards certain microbial activities and the physico-chemical properties of the organic layer. The organic layer of the carbic podzols had lower basal respiration (BASAL) and substrate-induced respiration (SIR), and the gleysols had lower SIR than the ferric podzols. The results support the assumption that, especially on carbic podzols and dystric gleysols, poor nutrient status, acidity and a lack of oxygen due to sporadic periods of excess moisture in the organic layer, result in low microbial activity, impaired water and nutrient uptake and, consequently, poor condition of the spruce trees. In this study, attention was also focused on successional changes in a forest ecosystem along a primary successional transect, located in the archipelago of Raippaluoto (Björkö and Replot; 63°20’N, 21°15’E). The transect represented a spatial continuum at right angles to the coastline as a result of ongoing post-glacial isostatic rebound (8 – 9 mm yr-1). The transect comprised four forest sites: alder/rowan [70-year-old Alnus incana (L.) Moench/Sorbus aucuparia L.], birch (mainly 80-year-old Betula pubescens Ehrh.), birch/spruce [75-year old B. pubescens Ehrh. and B. pendula Roth./Picea abies (L.) Karst.] and spruce I (95-year-old P. abies). In order to extend the age sequence, a fifth forest site (spruce II; 130-year old P. abies) was chosen 12.2 km to the south of the transect. Hypothesizing that a reduction in the availability of nutrients (especially nitrogen) during forest succession contributes to the poor condition of aged spruce crowns, I focused attention on the changes occurring in carbon (C)- and nitrogen (N)-related microbial activities (net and gross N mineralisation, microbial biomass N, BASAL and SIR) in the organic layer along the successional transect. Phospholipid fatty acid (PLFA) analysis was used to detect concurrent changes in the microbial community structure. The soil C/N ratio along the primary successional transect increased from 16 to 37, and the pH(H2O) decreased from 5.1 to 4.0. Net N mineralisation decreased substantially. The young alder/rowan site was the only site to show net nitrification. BASAL and SIR remained mainly stable although, during the most favourable temperature and moisture conditions in the field, they tended to increase along the transect from the alder/rowan site to spruce I, and decreased again in spruce II. Microbial biomass N, measured once during the most favourable conditions in the field, also increased along the transect from the alder/rowan site to spruce I. Concurrently, gross N mineralisation showed a tentative increasing trend along the transect, although the differences between the sites were non-significant. The lower net N mineralisation in the spruce sites compared to the alder/rowan site was thus due to higher microbial immobilisation of N, rather than to a lower gross N mineralisation. It may also further be hypothesized that, in late successional spruce sites, a higher proportion of the N in the microbial pool will be further transformed to the more stabile N pool, i.e. to humic substances, resulting in a decreasing net N mineralisation along the transect. As shown by NMS (non metric multidimensional scaling) ordination of the PLFA data, the microbial community structure showed clear differences along the transect and was closely related to the C/N ratio and pH of the organic layer. The transect study provided evidence of distinctive changes in organic matter quality and decreasing availability of mineral N during forest succession. Low N availability may contribute to the poor crown condition and growth of the aged Norway spruce stands on the land-uplift coast in western Finland.

Yksikkö: Parkanon tutkimusasema
Hankkeet: Metsäekosysteemin vaste ympäristötekijöihin, Metsätalouden kestävä kehitys Pohjanlahden rannikolla, Länsi-Suomen metsien terveydentila

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