Browsing by Subject "Brachiaria"

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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.
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    Developing indicators and characterizing direct and residual effects of biological nitrification inhibition (BNI) by the tropical forage grass Brachiaria humidicola
    (2018) Karwat, Hannes; Cadisch, Georg
    Nitrogen (N) losses from agroecosystems harm the environment via increased nitrate (NO3-) amounts in water-bodies and nitrous oxide (N2O) emissions to the atmosphere. Bacteria and archaea oxidize ammonium (NH4+) to NO3- under aerobic conditions. Furthermore, under mainly anaerobic conditions, microbial denitrification reduces NO3- to gaseous N forms. The tropical forage grass Brachiaria humidicola (Rendle) Schweick (Bh) has been shown to reduce soil microbial nitrification via root derived substances. Therefore, biological nitrification inhibition (BNI) by Bh might contribute to reduction of N losses from agroecosystems. The present doctoral thesis aimed at assessing the potential of the actual BNI by Bh, as well as the residual BNI effect with new developed methodologies. The overall research was based on the following major objectives: (1) characterization of the residual BNI effect by Bh on recovery of N by subsequent cropped maize (Zea mays L.) under different N fertilization rates; (2) investigate if low enzymatic nitrate reductase activity (NRA) in leaves of Bh is linked to reduced NO3- nutrition by effective BNI; (3) identify a possible link between plant delta 15N of Bh and the BNI effect of different Bh genotypes on nitrification, plant N uptake and NO3- leaching losses. The overall objective was to use and test new methodologies with a minimum of disturbance of the plant-soil system, to characterize BNI of different Bh genotypes in greenhouse and field studies. The first research study focused on the investigation of a potential residual BNI effect of a converted long-term Bh pasture on subsequent maize cropping, where a long-term maize monocrop field served as control. The residual BNI effect was characterized in terms of enhanced maize grain yield, total N uptake and 15N (labeled) fertilizer recovery. Furthermore, the impact of residual BNI effect on soil N dynamics was investigated. The residual BNI effect was confirmed for the first maize crop season after pasture conversion on the basis of lower nitrification in incubation soil, higher total N uptake and higher maize grain yields. However, the residual BNI effect did not result in higher 15N fertilizer uptake or reduced 15N fertilizer losses, nor in reduced N20 emissions. Applied N was strongly immobilized due to long-term root turnover effects, while a significant residual BNI effect from Bh prevented re-mineralized N from rapid nitrification resulting in improved maize performance. A significant residual Bh BNI effect was evident for less than one year only. In the second research study it was the aim to verify the potential of nitrate reductase activity (NRA) as a proxy for the detection of in vivo performance of BNI by selected Bh accessions and genotypes grown under contrasting fertilization regimes. NRA was detected in Bh leaves rather than in roots, regardless of NO3- availability. Leaf NRA correlated with NO3- contents in soils and stem sap of contrasting Bh genotypes substantiating its use as a proxy of in vivo performance of BNI. The leaf NRA assay facilitated a rapid screening of contrasting Bh genotypes for their differences in in vivo performance of BNI under field and greenhouse conditions; but inconsistency of the BNI potential by selected Bh genotypes was observed. The third research study emphasized to link the natural abundance of delta 15N in Bh plants with reduced NO3- losses and enhanced N uptake due to BNI. Increased leached NO3- was positively correlated to rising delta 15N in Bh grass, whereas the correlation between plant N uptake and plant delta 15N was inverse. Long-term field cultivation of Bh decreased nitrification in incubated soil, whereas delta 15N of Bh declined and plant N% rose over time. Delta 15N of Bh correlated positively with assessed nitrification rates in incubated soil. It was concluded that decreasing delta 15N of Bh over time reflects the long-term effect of BNI linked to lower NO3- formation and reduced NO3- leaching, and that generally higher BNI activity of Bh is indicated by lower delta 15N plant values. Within the framework of this thesis, a residual BNI effect by Bh on maize cropping could be confirmed for one season due to the combined methodological approaches of soil incubation and 15N recovery. The development of the NRA assay for sampled Bh leaves was validated as a rapid and reliable method linked to the actual soil nitrification after NH4+ fertilizer supply. Consequently, the assay could be used for both greenhouse and field studies as BNI proxy. The gathered data from the third study indicated that decreasing delta 15N of Bh over time reflects the long-term effect of BNI linked to lower NO3- formation and reduced NO3- leaching, and that generally higher BNI activity of Bh is indicated by lower delta 15N plant values. Consequently, it was suggested that delta 15N of Bh could serve as an indicator of cumulative NO3- losses. Overall, this doctoral thesis suggests the depressing effect on nitrification by Bh might be a combined effect by BNI and fostered N immobilization. Furthermore, BNI by Bh might be altered by different factors such as soil type, plant age and root morphology of the genotypes. Finally, future studies should consider that Bh genotypes express their respective BNI potential differently under contrasting conditions.