Browsing by Subject "Free air CO2 enrichment"

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Analyse von Wachstum und Qualität von Weizen unter ansteigender CO2 Konzentration als Folge des Klimawandels
(2019) Dier, Markus; Zörb, Christian
The atmospheric CO2 concentration is expected to increase to 500–620 ppm in the future. Such an elevated atmospheric CO2 concentration (e[CO2]) increases grain yield, but can decrease tissue N concentrations by about 9% in wheat. This could endanger global food security. Moreover, in previous studies, a decrease of grain N concentration by e[CO2] has closely been associated with that of gluten proteins, indicating a decreased baking quality under e[CO2]. The mechanisms by which e[CO2] decreases N concentration are still unclear and FACE studies investigating CO2 x N interactions on the formation of grain yield and the quality of winter wheat are scarce. The first main objective was the analysis of a decreased N concentration in the grain by e[CO2] in winter wheat based on a two-year FACE experiment with widely differing N levels (35 to 320 kg N ha-1) and different N forms (NO3- and NH4+). The focus was on key processes of grain N acquisition that are leaf NO3- assimilation, N remobilization and post-anthesis N uptake. The hypotheses were: e[CO2] inhibits leaf NO3- assimilation, e[CO2] decreases N remobilization (Nrem) by decreased N concentrations at anthesis and e[CO2] decreases post-anthesis N uptake (Nabs) by inhibition of leaf NO3- assimilation or acceleration of senescence. The second main objective was the simultaneous analysis of the e[CO2] effect on the grain proteome and baking quality with the hypothesis that e[CO2] reduces gluten proteins and thereby baking quality. e[CO2] increased grain yield in all N levels by 10% to 17% mainly through enhanced grain number per m2 ground area. This was due to increased radiation use efficiency (chapter 2). These increases were smaller under N deficiency compared with high N supply. The reasons were a reduction of photosynthesis capacity by e[CO2] and a sink limitation concerning grain yield due to N deficiency during ear growth. The indication for the reduction of photosynthesis capacity was a decrease of leaf N concentration under e[CO2] regardless of green leaf area index under N deficiency. An indication for sink limitation of grain yield was the decrease of harvest index by e[CO2] because of a strong and small stimulation of stem and ear growth, respectively by e[CO2]. Grain N yield was increased by e[CO2] under all N levels (chapter 3). There was a strong linear relation between grain N yield and grain number that was unaffected by e[CO2]. In contrast with the hypotheses of an decreased Nrem and Nabs under e[CO2], e[CO2] resulted in an increase of Nrem, Nrem efficiency and Nabs, causing the increase of grain N yield. Nevertheless, e[CO2] slightly decreased grain N concentration (by 1 to 6%), whereby the smallest effect of 1% was found under N deficiency. This decrease was primarily related to a growth dilution effect due to an increased individual grain weight under e[CO2]. A further reason was a stronger increase of grain number than an increase of vegetative N yield at anthesis by e[CO2] and thereby a decrease of the ratio between the N source and the N sink. Indication for an e[CO2] induced inhibition of leaf NO3- assimilation was not found as e[CO2] did not result in a decreased activity of leaf nitrate reductase under all N levels at both cool (17 °C) and warm (28 °C) temperatures (chapter 4). Furthermore, the e[CO2] induced stimulation of growth and N acquisition was not stronger under NH4+ compared with NO3- based N-fertilization. Reduction of grain protein concentration by e[CO2] was associated with reduced albumin/globulin and gluten concentrations under all N levels (chapter 5). Under optimal N supply, the grain protein composition was changed by e[CO2] with altogether 19 decreased and 17 increased protein spots. 15 out of the 16 identified decreased proteins were globulins, whereas specific gluten proteins were not found to be affected by e[CO2]. Correspondingly, baking quality remained unaffected under e[CO2] under all N conditions. In conclusion, grain N yields were increased by e[CO2] due to an increase of Nrem and Nabs with grain number being the driving force. Grain N concentrations were slightly reduced under e[CO2] with a growth dilution effect and a changed source to sink ratio as the underlying mechanisms. The reduction of the grain N concentration by e[CO2] was not specifically associated with a reduction of gluten proteins.
Pathways of C and N turnover in soil under elevated atmospheric CO2
(2008) Dorodnikov, Maxim; Fangmeier, Andreas
In the present thesis the C and N transformations in soil as influenced by indirect effect of elevated atmospheric CO2, soil physical structure and land use change were studied in four laboratory experiments using stable-C and N isotopes, as well as soil microbiological techniques. To test the interrelations between chemical and biological characteristics of soil organic matter (SOM) as affected by land use change and elevated atmospheric CO2 an approach for SOM partitioning based on its thermal stability was chosen. In the first experiment C isotopic composition of soils subjected to C3-C4 vegetation change (grassland to Miscanthus x gigantheus, respectively) was used for the estimation of C turnover in SOM pools. In the 2nd (Free Air CO2 Enrichment ? FACE ? Hohenheim) and 3rd (FACE Braunschweig) experiments CO2 applied for FACE was strongly depleted in 13C and thus provided an opportunity to study C turnover in SOM based on its δ13C value. Simultaneous use of 15N labeled fertilizers allowed N turnover to be studied (in the 2nd experiment). We hypothesized that the biological availability of SOM pools expressed as the mean residence time (MRT) of C or N is inversely proportional to their thermal stability. Soil samples were analysed by thermogravimetry coupled with differential scanning calorimetry (TG-DSC). According to differential weight losses between 20 and 1000 °C (dTG) and energy release or consumption (DSC), SOM pools (4 to 5 depending on experiment) with increasing thermal stability were distinguished. Soil samples were heated up to the respective temperature and the remaining soil was analyzed for δ13C and δ15N by IRMS. For all three experiments the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C and N. A possible explanation for the inability of thermal oxidation for isolating SOM pools of contrasting turnover times is that the fractionation of SOM pools according to their thermal stability is close to chemical separation. In turn, it was found that chemical separations of SOM failed to isolate the SOM pools of different turnover time because different biochemical plant components (cellulose, lignin) are decomposed in a wide temperature range. Individual components of plant residues may be directly incorporated into, or even mixed with the thermal stable SOM pools and will so mask low turnover rates of these pools. To evaluate the interactions between availability of SOM for decomposition by soil microbial biomass (biological characteristic) under elevated atmospheric CO2 and protection of SOM due to the occlusion within aggregates of different sizes (physical property, responsible for SOM sequestration) we measured the activity of microbial biomass (indicated by enzyme activities) and growth strategies of soil microorganisms (fast- vs. slow growing organisms) in isolated macro- and microaggregates. The contribution of fast (r-strategists) and slowly growing microorganisms (K-strategists) in microbial communities was estimated by the kinetics of the CO2 emission from bulk soil and aggregates amended with glucose and nutrients (Substrate Induced Growth Respiration method). Although Corg and total Cmic were unaffected by elevated CO2, maximal specific growth rates were significantly higher under elevated than ambient CO2 for bulk soil, small macroaggregates, and microaggregates. Thus, we conclude that elevated atmospheric CO2 stimulated the r-selected microorganisms. Such an increase in r-selected microorganisms could increase C turnover in terrestrial ecosystems in a future elevated atmospheric CO2 environment. The activities of β-glucosidase, phosphatase and sulphatase were unaffected in bulk soil and in aggregate-size classes by elevated CO2, however, significant changes were observed in potential enzyme production after substrate amendment. After adding glucose, enzyme activities under elevated CO2 were 1.2-1.9-fold higher than under ambient CO2. This indicates an increased activity of microorganisms, which leads to accelerated C turnover in soil under elevated CO2. Significantly higher chitinase activity in bulk soil and in large macroaggregates under elevated CO2 revealed an increased contribution of fungi to turnover processes. At the same time, less chitinase activity in microaggregates underlined microaggregate stability and the difficulties for fungi hyphae penetrating them. We conclude that quantitative and qualitative changes of C input by plants into the soil at elevated CO2 affect microbial community functioning, but not its total content. Future studies should therefore focus more on the changes of functions and activities, but less on the pools. In conclusion, elevated CO2 concentrations in the atmosphere along with soil physical structure have a pronounced effect on qualitative but not quantitative changes in C and N transformations in soil under agricultural ecosystem. The physical parameters of soil such as aggregation correlate more with biological availability of SOM than the chemical properties of soil organic materials. The increase of soil microbial activity under elevated CO2 detected especially in soil microaggregates, which are supposed to be responsible for SOM preservation, prejudice sequestration of C in agroecosystems affected by elevated atmospheric CO2.