Browsing by Subject "ammonium"

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  • Soronen, Päivi (Helsingin yliopisto, 2019)
    Nitrogen (N) availability often limits plant growth in the boreal forest ecosystem. There has been a lack of reliable method to study soil N supply as in traditionally used potassium chloride (KCl) extraction sampling and sample preparation disturb soil structure and stimulate N mineralization, leading to the overestimation of inorganic N forms ammonium (NH4+) and nitrate (NO3-) and underestimation of organic N forms such as amino acids. Diffusion-based microdialysis technique for the sampling of soil diffusive N fluxes gives an opportunity to study soil N supply at a scale that is relevant for plant N uptake, as microdialysis probe has a membrane that reminds the plant fine root in its scale and also, to some extent, in its function. During sampling, the movement of water inside the microdialysis probe induces diffusive flux of solutes across the membrane surface along the concentration gradient. The aim of this study was to test the performance of microdialysis technique at different soil moisture content levels and its capability to monitor temporal changes in diffusive N fluxes in laboratory experiments (ex situ). Soil fine-scale N dynamics were further studied by comparing the diffusive N fluxes in the field (in situ) in boreal forest soil to multiple factors that are thought to affect forest soil N availability. In this study, soil diffusive NH4+, NO3- and amino acid N fluxes were sampled ex situ from sieved soils taken from three different sites – clear-cut, spruce stand (MT spruce) and pine stand (VT pine) in Lapinjärvi, Finland in November 2017. In ex situ microdialysis experiments, the diffusive N fluxes were observed at three different soil moisture content levels and after N addition. In situ microdialysis sampling was run at the logging residue experiment of the Lapinjärvi clear-cut site and at the MT spruce site in June 2018 and at the pine logging residue experiment in Kiikala, Finland in September 2018. The results from the in situ microdialysis were compared with soil moisture content, pH, C-to-N ratio and temperature as well as with the net N mineralization and net nitrification rates, microbial biomass C and N contents and the concentrations of volatile monoterpenes and condensed tannins, factors that are assumed to affect N availability in forest soil. Nitrogen fluxes sampled ex situ showed that the total amino acid flux in the soil taken from the clear-cut site was only half of that in the MT spruce soil whereas NO3- flux was two times higher at the clear-cut site than at the MT spruce site. MT spruce soil with a moisture content of 60 % water-holding capacity (WHC) had significantly higher NH4+ flux than the same soil in its field moisture content (44 % WHC). Nitrogen pulse was detected in all soil samples as increased NH4+ flux after the N addition, followed by a subsequent decrease near to the initial level. In situ microdialysis sampling showed that the total amino acid fluxes were 5–15 nmol N cm-2 h-1 and they dominated the total diffusive N fluxes in Lapinjärvi and Kiikala. On average, the smallest share of the total free amino acids (54 %) was observed at the control plots of the logging residue experiment in Lapinjärvi. No correlation between the KCl-extractable NH4+-N concentration and the diffusive NH4+ flux was found, but instead the KCl-extractable NH4+-N concentration showed a significant positive correlation with the diffusive fluxes of both total free amino acid N and nitrate. Moreover, the diffusive NH4+ flux correlated positively with the net N mineralization rate. In general, ex situ microdialysis sampling showed 2–10 times higher amino acid fluxes and 10–20 times higher ammonium fluxes than the in situ microdialysis that reflects the effect of sampling, sample storage and preparation. The effect of soil moisture on the diffusive N fluxes could be further studied in laboratory experiments and in situ. The results of this study showed that the diffusive fluxes of different N forms are decoupled from the bulk soil concentrations. Moreover, microdialysis could be possibly used to quantify the transformation processes of N compounds in soil. These results increase the evidence that microdialysis has potential to detect temporal changes in N fluxes and possibly give new information about the ongoing processes at soil microsites.
  • Ruuska, Anna-Riitta (Helsingin yliopisto, 2018)
    Industry, transportation and agriculture release nitrogen oxides (NOx) and ammonia (NH3) into the atmosphere. These compounds deposit to the earth’s surface for example via rain as nitrate (NO3-) or ammonium (NH4+). It has been shown that nitrogen deposition has decreased the share of sphagnum mosses among mire vegetation. Mires are important carbon storages and sinks, and estimates show that in mires over half of the carbon is stored in peat originated from sphagna. Therefore it is important to study the phenomenon that has an impact on the function of mires and also mechanisms that effect sphagnum via nitrogen deposition. The aim of this thesis was to study the effects of nitrogen deposition on sphagnum’s hyaline and chlorophyllous cell morphology. The function of hyaline cell is to store water and maintain optimal water content for sphagnum’s efficient growth. Therefore the relation between the water content of sphagnum and the changes in cell morphology was studied. Changes in cell morphology may decrease sphagnum’s drought tolerance and lead to the disappearance of sphagnum. Samples of sphagnum (Sphagnum capillifolium (Ehrh.) Hedw.) were collected from ombrotrophic Mer Bleue bog which is located in Ottawa, Canada. By the May 2016, plots in Mer Bleue had been fertilized with different forms (NH4+, NO3-, NH4NO3) and amounts (1,6, 3,2 and 6,4 g N m-2 v-1) of nitrogen between 4-16 years. Potassium phosphate (KH2PO4) had been added into some plots. Total number of different treatments was 11, each of which had three replicates. In this study, sample preparates of sphagnum were microscoped and photographed and cell areas (μm2) were determined. Cell material used for statistical analysis was in total 1138 of which half were hyaline cells and half chlorophyllous cells. In addition, this study contained data from sphagnum’s water and nutrient contents and water table measurements. The effects of different treatments were detected mainly as an increase of hyaline cell areas. Out of different forms of nitrogen, only NH4NO3 with amount of 6,4 g N m-2 increased hyaline cell area, and the effects of NH4+ and NO3- did not differ from control. KH2PO4 treatment did grow the area of hyaline cell both alone and when added together with NH4NO3. Between different treatments only minor changes in water content of sphagnum were observed. Even though the results of this study did not refer to impaired drought tolerance of sphagnum, morphological changes observed may hinder sphagnum’s vitality in some other way. This matter needs further research.