Maja Siegenthaler


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Siegenthaler

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Maja

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0000-0001-8764-2767

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Publications 1 - 4 of 4
  • Dual isotopic (³³P and ¹⁸O) tracing and solution ³¹P NMR spectroscopy to reveal organic phosphorus synthesis in organic soil horizons
    Item type: Journal Article
    Siegenthaler, Maja; McLaren, Timothy Ian; Frossard, Emmanuel; et al. (2024)
    Soil Biology and Biochemistry
    Soil microorganisms can do both, mineralize and synthesize organic and condensed phosphate (P) species. Whereas P mineralization has been extensively studied, few studies have assessed the biological synthesis of organic P species, which can potentially accumulate in soil. The goal of this study was to investigate biotic and abiotic P transformations, particularly the synthesis of organic P species, upon water-soluble P addition in the organic (O) horizons of two beech forest sites with contrasting P availability. The two O horizons (low-P and high-P) were subjected to four different nutrient addition treatments (Control without addition, CN, P, and CNP additions) in an incubation experiment of up to 104 days. We combined isotopic tracing (³³P-labelled P addition and ¹⁸O-enriched soil water) into sequentially extracted P pools with the characterization of organic P species (solution ³¹P nuclear magnetic resonance (NMR) spectroscopy) and soil respiration measurements. The P availability of the two O horizons shaped the microbial response to the nutrient additions. In the low-P O horizon, P addition stimulated microbial activity together with the increase of organic (phosphodiesters and phosphonates) and condensed (polyphosphates) P species, most likely from microbial origin. In the high-P O horizon, microbes were unaffected by the added P and abiotic processes controlled its fate. CN addition had no effect on P fate in the high-P O horizon but reduced the transformation of added P into organic P and increased soil-derived P in the resin P pool in the low-P O horizon. The ¹⁸O isotopic values in phosphate of the resin P pool suggest that the released P was biologically cycled. Our study confirms with a unique multi-analytical approach the microbial synthesis of phosphodiesters, phosphonates, and polyphosphates upon inorganic P addition under low P availability.
  • Effects of phosphorus and nitrogen inputs on phosphorus cycling in organic soil horizons of beech forests
    Item type: Doctoral Thesis
    Siegenthaler, Maja (2022)
    High atmospheric nitrogen (N) deposition together with climatic changes have been suggested to drive European temperate forests towards phosphorus (P) limitation. In forest ecosystems, trees and microorganisms take up phosphate from the soil solution, which represents a small proportion of total P in soil. To meet P demands, the soil solution must be replenished with P from less labile soil P pools by abiotic (desorption and dissolution) and biotic (solubilization and mineralization) processes. Soil microorganisms contribute to inorganic P solubilization through the release of organic acids and to organic P mineralization through the release of enzymes (e.g. phosphatases). Besides, they synthesize organic P forms. Upon microbial cell death, the microbially synthesized P species return to the soil and can potentially get mineralized or stabilized. Understanding the role of microorganisms in soil P cycling is thus crucial to assess the resilience of forest ecosystems to the increasing imbalance between N and P concentrations. The objective of this thesis was to study the effects of P and N additions on P cycling in the organic (O) soil horizon of temperate beech (Fagus sylvatica L.) forests developed on silicate bedrock. Although not extensively studied, the O horizon is crucial for microbial processes and beech nutrition, particularly under low P availability. Inputs of P and N were hypothesized to affect microbial activity, microbial community composition and the processes microorganisms modulate. Consequently, the proportions of inorganic and organic P pools and forms were expected to change. The effects of P and N inputs were assumed to depend on the initial nutrient status of the soils, which is defined by the underlying parent material. To gain a holistic view, several experimental (field and incubation experiments) and methodological approaches (chemical extractions, spectroscopy, isotopic tracing, enzyme assays, and molecular fingerprinting) were combined. In chapter 1, bacterial (16S rRNA), fungal (ITS), and alkaline (phoD) and acid (acpA) phosphatase harbouring bacterial community responses to changes in soil N and P concentrations in the O horizon were investigated. A field experiment with water-soluble N and P additions was conducted at two beech forest sites with contrasting P stocks (low-P site Lüss (LUE) and high-P site Bad-Brückenau (BBR)). In BBR, microbial community structure changed in response to the increase in resin P induced by the P addition. In LUE, microbial community structure changed in response to decreasing P concentrations in several soil P pools induced by the N addition. Furthermore, the increased importance of strategies to access organic P forms in sites with low P supply from minerals was reflected by a higher relative abundance of some dominant phoD harbouring taxa in LUE than in BBR. In chapter 2, the fate of the 33P-labelled P fertilizer was traced into soil P pools and the impact of enzyme-mediated P cycling processes was determined by the level of incorporation of 18O from 18O-enriched water into phosphate in the field. During a dry summer, a field experiment with water-soluble N and P additions was conducted at a guarded beech forest site in Jülich (JUE). Approximately 40% of P fertilizer was recovered in the studied soil layer, the majority being in inorganic P pools independent of N addition. However, the incorporation of 18O into resin P was mainly stimulated by N addition. In chapter 3, the organic soil horizons from two beech forest sites with contrasting P stocks (low-P site LUE and high-P site Vessertal (VES)) were subjected to four nutrient addition treatments (control without addition, P addition, combined carbon (C) and N addition, combined CNP addition) in an incubation experiment of 103 days. Enzyme-mediated P cycling processes were assessed by studying the oxygen (O) incorporation from 18O-enriched water into phosphate. Phosphorus fluxes into sequentially extracted P pools were traced by labelling the added water-soluble P with 33P. Furthermore, the chemical nature of P in NaOH/EDTA extracts was studied with solution 31P NMR spectroscopy. In the LUE O horizon, half of the added P was recovered in the microbial and organic P pools, while 31P NMR revealed increases in the P classes polyphosphates, phosphonates, and phosphodiesters following inorganic P addition. In contrast, in the VES O horizon, added P was mainly recovered in inorganic P pools. Overall, P and N additions were shown to influence the soil microbial communities and the biological processes they modulate, the P fluxes between soil P pools, and the forms of P present in O horizons of temperate beech forests. On the short-term (days) and under unfavourable environmental conditions for microorganisms, abiotic processes dominated the response to inorganic P addition. In contrast, on a longer-term (months), the P stock, as influenced by the underlying parent material, determined the response to P additions. Abiotic processes dominated in high-P and biotic processes in low-P sites. In the O horizons of low-P sites, microorganisms are strongly interacting with soil organic P, as they mineralize and produce organic P forms. These microbial P cycling processes can be negatively affected by N deposition and extreme environmental conditions (e.g. drought or heat waves). In low-P sites, N addition can induce decreases in soil P pool concentrations, which affects microbial communities (Chapter 1), and can reduce microbial organic P synthesis (Chapter 3). Dry conditions can reduce the influence of biotic processes on P fluxes (Chapter 2). High N deposition and extreme environmental conditions thus represent a greater threat to low-P O horizons with high competition for P than to high-P O horizons.
  • Microbial community responses to phosphorus and nitrogen inputs in the organic soil horizons of two contrasting temperate beech forests
    Item type: Journal Article
    Siegenthaler, Maja; Ramoneda, Josep; Frossard, Emmanuel; et al. (2022)
    Applied Soil Ecology
    Due to increasing nitrogen (N) deposition from the atmosphere, temperate forests are progressively becoming phosphorus (P) limited. Trees take up P mainly from the soil solution, which soil microorganisms can replenish by mineralizing organic P through enzymatic hydrolysis (e.g. using phosphatases). We investigated how bacterial (including phosphatase harboring bacteria) and fungal communities in organic horizons with contrasting nutrient status respond within months to changes in soil N and P concentrations. A field experiment with water-soluble N (NH4NO3) and P (KH2PO4) additions was conducted at a high-P and a low-P beech (Fagus sylvatica L.) forest site in Germany. Bacterial (16S rRNA), fungal (ITS), and alkaline (phoD) and acid (acpA) phosphatase harboring community structure was investigated by molecular fingerprinting. Bacterial and fungal community structure was mainly related to available P (resin P) in the high-P site but related to more stable P pools (sequentially extracted NaOH/EDTA P) and total P in the low-P site. The increased importance of strategies to access more stable P forms in the low-P site was reflected by a higher relative abundance of some dominant alkaline phosphatase harboring taxa. In the high-P site, increased available P following P addition was the main influencing factor on community structure. By contrast, in the low-P site predominantly N addition induced differences in the microbial communities, which was linked to decreased P concentrations in several soil pools. Increasing N deposition might thus pose greater impacts on microbial communities in low-P compared to high-P sites, by further increasing the already high competition for P. Our findings illustrate that the soil P status in the organic horizon can shape the structure of microbial communities and their roles in the P cycle.
  • Geochemical Soil Atlas of Switzerland - Distribution of Toxic Elements
    Item type: Journal Article
    Reusser, Jolanda; Siegenthaler, Maja; Winkel, Lenny H.E.; et al. (2023)
    Chimia
    Chemical elements such as copper and molybdenum are essential for animal and human health but may become toxic at elevated concentrations depending on the exposure and intake rate. Other elements such as mercury pose a threat to human health at already low concentrations. The soil acts as the main source of these elements for plant uptake and is thus driving accumulation along the food chain. However, in Switzerland, no nationwide information on elemental distributions in soils has existed up to now. The geochemical soil atlas of Switzerland will fill this gap by presenting the concentration ranges and the spatial distribution of 20 elements in the topsoil. In this summary, we present the methodological approaches and some main findings of the atlas with a focus on toxic elements as well as elements that can be or are toxic at higher concentrations.
Publications 1 - 4 of 4