Browsing by Person "Souri, Mohammad Kazem"

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Characterisation of natural and synthetic nitrification inhibitors and their potential use in tomato cultivation
(2008) Souri, Mohammad Kazem; Römheld, Volker
Summary Besides commercial NIs, many chemicals could also inhibit nitrification. In our study (Chapter 3) regarding efficiency of chloride compared to 3,4-Dimethylpyrazole phosphate (DMPP), it was found that chloride at applied concentration of 30.5 mg per 100g dry soil, could effectively inhibit nitrification. Despite a lag period of 3 weeks in detectable net nitrification, inhibitory effect of chloride continued to persist even after 7 weeks of soil incubation compared to control. Nevertheless, DMPP particularly with higher concentration (2 % of N-NH4+ instead of 1%) stabilized ammonium more strongly than Cl-1. The extent of nitrification inhibition after 5 and 7 week of incubation was in order of: (2 % of N-NH4+) DMPP > (1 % of N-NH4+) DMPP> NH4Cl > KCl > control. The residue ammonium in the soil as well as the produced nitrate concentrations in samples showed a significant NI activity of chloride in both forms NH4Cl and KCl. Nitrification-induced pH decrease, however, showed a better correlation with measured nitrate than ammonium in this experiment. In a second series of experiments undertaken to identify whether the reported NI release by Brachiaria humidicola accession 26159 is an active or passive phenomenon, root exudates of plants grown under various treatments, have been collected in distilled water or in 1 mM NH4Cl. Under various pre-culture conditions such as N form (NH4+ versus NO3-), N concentrations (1, 2, 4 mM), light intensities (180, 240, 350 µmol m-2 s-1), plant age (3-weeks old versus 7-weeks old) and collecting periods (24 versus 6 h), there was no significant NI activity when root exudates were collected in distilled water. However, NI activity was detectable in root washings when the plants were exposed to extended collection times (24 h) in combination with NH4+ supply, but not after short term collection (6 h) or with NO3- in the collection solution. This observation is consistent with the results of Subbarao et al., (2006, 2007), but it also strongly suggests that the observed release of NI compounds was rather a consequence of membrane damage (passive phenomena) due to inadequate collection conditions, than mediated by controlled exudation from undamaged roots. It has been assumed that supplying only ammonium (1 mM) in distilled water as root washing medium over extended time periods (24 h) could lead to rapid ammonium uptake and medium acidification associated with the risk of Ca2+ desorption, which is an important element required for membrane stabilisation and integrity. To test the hypothesis that NI compounds are released from damaged plant cells of Brachiaria, the NI potential of fresh root and shoot homogenates was measured after soil incorporation and incubation. Surprisingly, NI potential was detected in shoot but not in root homogenates. The NI effect of soil-incorporated shoot tissues lasted for at least 8 d, while root tissue even stimulated nitrification with increasing incubation time. This NI effect was independent of the N form. However, the variability of data increased with NO3- form, higher light intensity or higher N concentrations during plant pre-culture. Independent of N forms, further extraction and characterisation of NI compounds in shoot tissue of Brachiaria plants revealed a particularly high activity in the ethanol-soluble fraction, both in plants with NH4+ and NO3- pre-culture. In a third experiment, the role of Ca2+ ions on improvement of tomato growth under ammonium nutrition was investigated. In this experiment root damage, probably by membrane damage and cytosolic sensitivity were hypothesised to be the main cause of toxicity symptoms of NH4+ on tomato plants. At application of 2 mM N as NH4+, plant biomass, number of lateral shoots, and transpiration were strongly inhibited and an increased Ca2+ application into the nutrient solution counteracted these observed negative effects. Transpiration or water consumption was found to be a good indicator of plant performance under NH4+ nutrition. Plants grown under nitrate nutrition had the highest transpiration rates, as well as the best growth characteristics. There was a positive correlation between nitrate concentrations and transpiration rates. On the other hand, plants grown in ammonium (as control, or 3 and 6 split applications of NH4+ during 4 days) showed severe toxicity symptoms including growth inhibition and leaf abscission. However, when ammonium was applied together with 10 mM Ca2+ (as CaSO4), or in a buffered solution of pH 6.6 with CaCO3 (pH or/and Ca2+ effect), transpiration and other growth factors (e.g. root and shoot dry matter, number of lateral shoots), as well as the nutrients especially N concentrations in the biomass were significantly improved. In other words, shoot and particularly root growth inhibited when NH4+ treated plants (control and split applications) did not received CaSO4 or CaCO3. Micro molar concentrations of NH4+ in 6 split applications also could not prevent ammonium toxicity symptoms.