emit root exudates in the form of organic acids to locally lower soil pH and mobilize el-ements. Some grasses can exude chelate compounds (phytosiderophores) which mo-bilize Fe (Römheld, 1991). Also, the Fabaceae plants possess the ability to fix nitrogen from the air with the assistance of rhizobia bacteria in the root nodules. The various mechanisms by different plant species can also lead to different element concentra-tions in the plant tissue. This aspect is adressed by the investigation of 12 different variants in this study. The cultivation system can modify the uptake too. For example, the plant concentrations may be modified when the plants are intercropped with an-other plant species. Other external factors include humidity and temperature. When there is water shortage, element mobility and transport to the plants is hindered.
1.6 Structure of the Thesis
In Chapter 1 the background and framework of this Thesis are given. An overview of biogas production and trace element requirements are presented. Also, a short sum-mary about the main factors influencing element uptake by plants is provided. Chap-ter 2 is theMaterials and Methodssection. Here, general techniques utilized frequently throughout this Thesis are described. The sample locations at which either field ex-periments were conducted, or which soil was taken for pot exex-periments are shown.
The analytical workflow is presented here from the collection of raw data from the analysis device to the fully evaluated concentrations based on dry weight (in mg/kg DM). Finally, the field experiment settings for the main field trials are shown.
Chapter 3 gives recommendations for determining of trace elements with ICP-OES and ICP-MS, with a particular focus on Co. The fit of external reference standards are given in diagrams. Recommendations are given, especially for plant tissue which in most cases have very low concentration. German DIN regulations for ICP-OES measurement are discussed throughout.
Chapter 4 shows how the measured element concentrations are influenced by adher-ing soil or dust particles. Plants, which are grown in the open air and are subjected to a complete digestion will always be affected by adhering particles. The element data of the plants will be corrected with mathematical functions. Three approaches are pre-sented in this Chapter, in which the part attributed to the soil/ dust is calculated and subtracted from the measured element concentration. For this comprehensive study a large number of samples were considered collected between 2012 and 2016 –at total 1040 plant samples– comprising different species and families. It is shown to what extend the element concentrations change after applying the correction method over a broad range of elements (about 45 elements). Most affected were non-nutrient ele-ments and major eleele-ments in the soil (Al, Ti, Sc, REEs, Hf, Ta, Nb, Zr). Major nutrients and most of the trace nutrients (Fe, Mn, Cu) were not changed. Co is affected, and cor-rected by about 20 %. The second part of this Chapter shows how a very short cutting height can lead to elevated element concentrations in plant tissue and how they are then corrected.
Chapter 5 is a case study on how elevated soil total concentration affects the concen-tration levels in the plants. The plants were grown in a soil close to the village of Bühren (Dransfeld) and is derived from basalt weathering. Basalts are naturally en-riched in Co, Cr, Fe, Mg, Mn, and Ni. A small scale field trial and pot experiments were performed. A detailed study on the plants showed elevated concentrations in Co, Cr, Ni, Mg, and Zn for some plant species compared to the plants grown on the
8 Chapter 1. General Introduction main field trials. Total soil element concentrations from several soil sample locations in this study were given.
Chapter 6 deals with the soil to plant transfer. In this Chapter, all measured elements were considered, with transfer factors per element and a multivariate analysis on a large group of elements. Two transfer factors are introduced, a transfer factor of plant concentrations to soil total concentrations and a factor of plant concentrations to soil extraction data with ammonium nitrate. The results show distinctive patterns, proba-bly resulting from different uptake mechanisms of plant species. The Fabaceae plants always display greater transfer rates. Maize and Poaceae plants (rye, triticale) always had low transfer factors. Biplots, the graphical representations of the results of a Prin-cipal Component Analysis, show groups formed by the plant species. The soils (main field trials) do not show this prominent distinguishing feature in the biplots.
The element concentration in plants and the intercroppings from the main field trials are discussed in Chapter 7. The results of the main nutrient elements (K, P, S, Mg and Ca) and of the trace elements Co, Ni, Mn, Mo, Fe, Cu and Zn are presented as plant concentrations. In this part, the concentrations are different for most elements for the different plant species. Ryegrass samples show the most variable concentrations for the elements S, Mg, Ni, Mo, Mn and Fe. The greatest trace element concentrations were detected in plants of faba bean (winter and summer variety), hairy vetch or ama-ranth. The intercroppings on amaranth/maize, rye/vetch and faba bean/triticale are presented with their yield shares and concentrations. Only faba bean/triticale reached equal shares of DM yield. In the other two intercropping systems, the poaceae plant (maize and rye) equals to more than 90% of DM yield. The resulting concentrations in the intercropping systems were calculated based on yield share. Only for faba bean/triticale elevated concentrations were detected, because of the higher concen-tration in the faba bean.
The soil extraction method is discussed in detail with respect to bioavailability in Chapter 8. The extraction method with ammonium nitrate is well suited to charac-terize different soils concerning their readily available pool of metals. The extraction results of the soils are shown, revealing that with a low pH in the soil Cd, Co, Ni, Mn and La are enriched in the soil extraction solution. Figures outlining the plant con-centration vs. total soil concon-centration, or vs. soil extraction are displayed to detect possible correlations. Correlation trends can be observed for amaranth, faba bean and ryegrass for some of the trace elements (Co, Ni, Mn). For Mo, correlation trends were not detected because of variable element concentrations in samples from the main field trials (in particular ryegrass), or no recovery of Mo in the soil extractions.
In Chapter 9 element concentrations and DM yield are considered together. A cal-culation of total trace element delivery to biogas plants with the harvest, or to say it differently, extraction from the field, was performed. It is shown, how the figures develop for the two main field trials in 2015 and 2016. The total element deliveries are calculated for the whole year, including maize as a follow-up crop. To deliver a high amount of Co ing /(ha a), the best choice is either summer faba bean in sole crop-ping, or winter faba bean-maize, or intercropping of faba bean /triticale-maize. For a high Mo, Mn and Ni harvest, ryegrass is a good option. This Chapter finally shows how addition of alternative energy crops to maize as input substrates can change trace element concentrations in biogas fermenters. Since this Chapter is a submit-ted manuscript some details are repeasubmit-ted from the Introduction. It was placed here, because it closes the circle towards biogas production.
1.6. Structure of the Thesis 9 Chapter 10 is a short summary of the results and gives an outlook.
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