Aki Lindén

Precipitation and net ecosystem exchange are the most important drivers of DOC flux in upland boreal catchments

According to recent studies, dissolved organic carbon (DOC) concentrations in rivers throughout the boreal zone are increasing. However, the mechanistic explanation of this phenomenon is not yet well known. We studied how the short and long-term changes in precipitation, soil temperature, soil water content, and net ecosystem exchange (NEE) are reflected to DOC concentrations and runoff DOC fluxes in two small forested upland catchments in Southern Finland. We used continuous eddy covariance measurements above the forest and runoff flow measurements from the catchment areas conducted over a 15 year long time period to study the correlation between NEE, gross photosynthetic production, total ecosystem respiration, litter production, and runoff DOC. In addition, we looked for the most important environmental variables in explaining the interannual changes in runoff DOC by using multiple linear regression. Finally, we studied the temporal connection between runoff DOC concentrations, precipitation, soil water content, and NEE by using wavelet coherence analysis technique. Our results indicate that the DOC concentrations have increased over the last 15 years. The DOC flux was to a large extent determined by the amount of precipitation, but the previous years NEE and litter production had also a small but significant effect on runoff DOC fluxes.

Contrasting effects of increased carbon input on boreal SOM decomposition with and without presence of living root system of Pinus sylvestris L.

AimsWe studied how the availability of carbon affects the decomposition of soil organic matter (SOM) in the presence of living root system compared to bare SOM treatments without roots.MethodsWe measured the effect of living Pinus sylvestris L. root systems on SOM C and N budgets, on the age of carbon in CO2 efflux, and on SOM protease enzyme activity. We examined the possible explanatory factors for increased SOM mineralization such as plant biomass, photosynthesis, microbial C, and protease enzyme activity in the soil.ResultsThe age of carbon respired during the decomposition of soil organic matter was significantly older in the presence of living root systems than in treatments lacking roots. If plants were present, glucose added directly in the SOM accelerated the C and N loss from the bulk material and decreased the rate of photosynthesis.ConclusionsWe conclude that the priming effect of SOM decomposition was affected by the presence of living root system. Our results stress the importance of including the plant–soil interactions in the SOM decomposition models used in climate change studies.

Effects of prolonged drought stress on Scots pine seedling carbon allocation

As the number of drought occurrences has been predicted to increase with increasing temperatures, it is believed that boreal forests will become particularly vulnerable to decreased growth and increased tree mortality caused by the hydraulic failure, carbon starvation and vulnerability to pests following these. Although drought-affected trees are known to have stunted growth, as well as increased allocation of carbon to roots, still not enough is known about the ways in which trees can acclimate to drought. We studied how drought stress affects belowground and aboveground carbon dynamics, as well as nitrogen uptake, in Scots pine (Pinus sylvestris L.) seedlings exposed to prolonged drought. Overall 40 Scots pine seedlings were divided into control and drought treatments over two growing seasons. Seedlings were pulse-labelled with 13CO2 and litter bags containing 15N-labelled root biomass, and these were used to follow nutrient uptake of trees. We determined photosynthesis, biomass distribution, root and rhizosphere respiration, water potential, leaf osmolalities and carbon and nitrogen assimilation patterns in both treatments. The photosynthetic rate of the drought-induced seedlings did not decrease compared to the control group, the maximum leaf specific photosynthetic rate being 0.058 and 0.045 µmol g-1 s-1 for the drought and control treatments, respectively. The effects of drought were, however, observed as lower water potentials, increased osmolalities as well as decreased growth and greater fine root-to-shoot ratio in the drought-treated seedlings. We also observed improved uptake of labelled nitrogen from soil to needles in the drought-treated seedlings. The results indicate acclimation of seedlings to long-term drought by aiming to retain sufficient water uptake with adequate allocation to roots and root-associated mycorrhizal fungi. The plants seem to control water potential with osmolysis, for which sufficient photosynthetic capability is needed.

Studying the impact of living roots on the decomposition of soil organic matter in two different forestry-drained peatlands

Background and aimsForestry drainage is the main management practice of peatlands in Finland. The influence of drainage and management on carbon (C) fluxes may vary, e.g., depending on the original peatland type. We have studied C fluxes in two forestry-drained peatlands with different nutrient status.MethodsOur hypothesis that the differences in the C balance between these two sites can be attributed to differences in soil respiration rates, and in particular to the priming effect, was tested with laboratory microcosm flux measurements and 14C isotopic partitioning method. A two-pool mixing-model based on the natural difference in the respired 14CO2 between the peat and plants was employed.ResultsWe found no statistically significant priming effect in either nutrient-poor or nutrient-rich soil, respectively.ConclusionsAs no differences in priming effect were found, we can conclude that the nutrient status of the sites does not affect the priming effect in the peat soils studied here, thus our results suggest that organic soils do not support priming to the same extent as mineral soils.

The long-term impact of low-intensity surface fires on litter decomposition and enzyme activities in boreal coniferous forests

In boreal forest ecosystems fire, fungi and bacteria, and their interactions, have a pronounced effect on soil carbon dynamics. In this study we measured enzymatic activities, litter decomposition rates, carbon stocks and fungal and microbial biomasses in a boreal subarctic coniferous forest on a four age classes of non-stand replacing fire chronosequence (2, 42, 60 and 152 years after the fire). The results show that microbial activity recovered slowly after fire and the decomposition of new litter was affected by the disturbance. The percent mass loss of Scots pine litter increased with time from the last fire. Slow litter decomposition during the first post-fire years accelerates soil organic matter accumulation that is essential for the recovery of soil biological activities. Fire reduced the enzymatic activity across all the enzyme types measured. Carbon-degrading, chitin-degrading and phosphorus-dissolving enzymes showed different responses with the time elapsed since the fire disturbance. Microbial and enzymatic activity took decades before recovering to the levels observed in old forest stands. Our study demonstrates that slower post-fire litter decomposition has a pronounced impact on the recovery of soil organic matter following forest fires in northern boreal coniferous forests.

The effect of roots and easily available carbon on the decomposition of soil organic matter fractions in boreal forest soil

Summary The priming effect induced by carbon (C) that is easily available to microbes has been shown to increase the mineralization of soil organic matter (SOM) that is resistant to decomposition, but the combined effects of easily available carbon and the living root system have rarely been studied. The aim of this research was to study the decomposition of SOM fractions of different solubility in water and their 13C, 14C and 15N abundance with and without the presence of a living root system and easily available carbohydrate in the form of glucose. The SOM collected from the organic horizon of a boreal forest soil in H yytiälä, southern F inland (61°51′N, 24°17′E), and exposed to laboratory incubations with and without the presence of P inus sylvestris L. seedlings and glucose, was separated into three chemical fractions with accelerated solvent (ASE) and pressurized hot water extractions (PHWE). Changes in the natural abundance of 13C, 14C and 15N, spectral properties assessed by F ourier transform infrared spectroscopy (FTIR) and the C and N pools of SOM fractions were studied after incubation for 6 months. The extractions separated SOM into fractions with distinctive isotopic composition. The most easily soluble SOM fraction showed the largest abundance of 15N and 14C, and the living root system induced changes in the abundance of 15N and FTIR spectra. Our research suggests that plant roots may induce SOM degradation and N uptake from soluble SOM fractions, but 13C, 14C, 15N or FTIR alone cannot be used to describe the recalcitrance of SOM and its accessibility to microorganisms. It is better to use several methods in parallel to study the decomposability of SOM. HighlightsWe studied the effects of tree roots and addition of glucose on the solubility of soil organic matter (SOM).SOM can be separated into pools with distinctive isotopic concentrations with pressurized hot water.Roots may induce organic matter degradation and N uptake from soluble SOM fractions.Larger 14C abundance in soluble SOM indicated it contained older C than the recently assimilated C.

Corrigendum to: The long-term impact of low-intensity surface fires on litter decomposition and enzyme activities in boreal coniferous forests

In boreal forest ecosystems fire, fungi and bacteria, and their interactions, have a pronounced effect on soil carbon dynamics. In this study we measured enzymatic activities, litter decomposition rates, carbon stocks and fungal and microbial biomasses in a boreal subarctic coniferous forest on a four age classes of non-stand replacing fire chronosequence (2, 42, 60 and 152 years after the fire). The results show that microbial activity recovered slowly after fire and the decomposition of new litter was affected by the disturbance. The percent mass loss of Scots pine litter increased with time from the last fire. Slow litter decomposition during the first post-fire years accelerates soil organic matter accumulation that is essential for the recovery of soil biological activities. Fire reduced the enzymatic activity across all the enzyme types measured. Carbon-degrading, chitin-degrading and phosphorus-dissolving enzymes showed different responses with the time elapsed since the fire disturbance. Microbial and enzymatic activity took decades before recovering to the levels observed in old forest stands. Our study demonstrates that slower post-fire litter decomposition has a pronounced impact on the recovery of soil organic matter following forest fires in northern boreal coniferous forests.