Browsing by Subject "aminohapot"

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.

Tiivistelmä  Referat  Abstract Lettuce (Lactuca sativa L.) may accumulate high amounts of nitrate in the hydroponic NFT-cultivation system. Accumulation is known to increase under low light conditions, and thus high nitrate content causes problems in year-round lettuce production in Finland. Nitrate is classified as a harmful food additive, and the regulation (194/97) of the European Commission requires the Member States to monitor the nitrate content of commercial lettuce. Hence, to control the nitrate content, farmers have requested an efficient method that does not shorten the shelf-life or negatively affect the quality of lettuce. In the earlier studies, root-applied glycinebetaine (GB) was shown to reduce lettuce nitrate content and increase the contents of amino acids and minerals. The present study aimed to investigate, whether glycinebetaine can serve as a precise method to control lettuce nitrate content in commercial-scale greenhouse production. Glycinebetaine 10 mM was applied into the nutrient solution twice at three days intervals, and lettuce was exposed to treatment during the last six growing days before the harvest. Plant samples were collected every second day for 14 days after the first GB application, and samples from nutrient solution were collected simultaneously. The reduction of GB from the nutrient solution was monitored. The fresh weight of the plants was weighed during the harvest, and contents of nitrate, dry matter, minerals, amino acids, and GB in the plant samples were analyzed. Root-applied GB reduced lettuce nitrate content by over 29 % in comparison to control plants. The total contents of hydrolyzed- and free amino acids were increased. The total content of essential amino acids was increased up to 14 % following the GB treatment. Also, the contents of minerals in the lettuce leaves were altered. The potassium (K+) content in lettuce was reduced by over 40 % after GB application. Lettuce accumulated the applied GB and leaf GB content was 7 mg kg-1 fresh lettuce at the highest. Root-applied GB reduced the fresh weight of the harvested lettuce, but all plants reached the commercial size (>100 g) during the typical commercial growing period of 41–43 days. The result of the present study proved that GB is a practical method to control nitrate content and the quality of lettuce on commercial-scale lettuce production. Glycinebetaine was detectable in the nutrient solution for three days after application and thus, continuously maintained lower nitrate content in plants requires continuous GB application in the commercial lettuce production. The reduction of the nitrate content was significant six days after the first GB application, which indicates that a six-day treatment period is required.