Authors:	Säurich, Annelie
Title:	Influences on mineralisation rates of organic soils under agriculture
Online publication date:	1-Apr-2022
Year of first publication:	2022
Language:	english
Abstract:	Even though peatlands only cover a minor fraction of the global terrestrial surface, drained organic soils are major contributors to worldwide greenhouse gas (GHG) emissions. Due to alterations of hydrological and biogeochemical processes caused by drainage and land-use these soils are now hotspots of GHG emissions from agricultural soils. Caused by both drainage-induced mineralisation of soil organic matter (SOM) and anthropogenic modifications of the soil profile, like peat-sand mixing, large areas of former peatlands underwent a secondary pedogenetic transformation. A large share of these organic soils have strongly degraded topsoils with low soil organic carbon (SOC) contents at the boundary between mineral and organic soil, with regard to definition. Previous research has shown that increasing anthropogenic disturbance enhances GHG emissions from organic soils, but it remains sparsely studied which indicators influence their vulnerability. Therefore, the aim of this thesis was to investigate the sensitivity of drained organic soils under agricultural use to mineralisation and to determine hydrological and biogeochemical parameters that influence their GHG emissions. Thereby the focus was on low carbon (C) organic soils and peat-sand mixtures, where applied sand was mixed into the uppermost peat layer. For this purpose, two laboratory experiments were conducted using the valuable selection of samples of the first German Agricultural Soil Inventory. The first part of the thesis was directed at the evaluation of SOM vulnerability of disturbed organic soils using a broad range of samples, including C-rich and low C organic soils as well as C-rich mineral soils. The second and third part of the thesis elucidated the influence of hydrological and biogeochemical soil properties as well as chemical composition of the peat substrate on mineralisation rates by comparing peat-sand mixtures and strongly degraded organic soils of bog and fen peat. In the first experiment, the potential carbon dioxide (CO2) production rates of 62 soil samples (76–526 g kg−1 SOC) were measured under aerobic conditions. Bog and fen peat samples were grouped according to their degree of disturbance using mapped soil information. The results indicated that SOM vulnerability increased with increasing degree of anthropogenic disturbance and concurrently decreasing SOC content. Additionally, the variability of CO2 production rates increased with increasing disturbance. Even though a clear influence of a single soil property was not found, mineralisation rates tended to be increased with narrow C:N ratios, high pH-values and high contents of plant-available phosphorus (P). In the second experiment, topsoil and subsoil columns from ten different organic soils under grassland use were incubated for six months. Previously to launch, all columns were water-saturated. During the incubation, the columns were drained stepwise and concentrations of CO2, nitrous oxide (N2O) and methane (CH4) were measured continuously. The chemical composition of all samples was analysed using 13C nuclear magnetic resonance (NMR) spectroscopy and wet chemical extractions for fibre analysis. CO2 fluxes and water-filled pore space (WFPS) showed a parabolic dependence, with maximum CO2 fluxes at 84% WFPS. Thereby magnitudes of topsoil fluxes clearly outreached subsoil fluxes. The highly variable N2O fluxes reached maximum values at WFPS over 80%. This implies that – under equilibrium conditions – highest GHG emissions in the field would occur at a water table of 20 to 60 cm below the surface. Systematic influences of sand addition or peat type on GHG emissions were not discovered. However, slight differences in CO2 fluxes were found between the well-preserved subsoils. High densities of plant-available P and potassium (K) correlated with maximum values of CO2 and N2O fluxes. The chemical composition of bog and fen peat was similar for the topsoils, while the subsoil samples among themselves were differentiable by their botanical origin. Overall subsoils had higher shares of carbohydrates and aromatic compounds than topsoils. Differences in decomposition between topsoil and subsoil alone, evident from the age of the samples, fell short in explaining these higher amounts in subsoils. No reliable indicators describing the influence of peat quality on decomposition were detected. All in all, this thesis improved the understanding of SOM decomposition in organic soils under agricultural use by leaning on a broad sample basis and including low C organic soils. The results showed that especially peat-derived soils with low SOC contents had potentially high GHG emissions. The variability of GHG emissions and the SOM vulnerability for decomposition increased with increasing degree of anthropogenic disturbance, suggesting that peat-sand mixtures do not seem like a mitigation option for GHG emissions. The nutrient status of N, P and K, probably heavily influenced by fertilization, seemed to be the most important parameter for peat mineralisation, overshadowing the influence of peat type, chemical composition and sand addition. Furthermore, the findings of this thesis imply that the chemical composition of topsoil peat is a consequence of decomposition instead of a source behind higher mineralisation rates compared with subsoil peat. Thus, a decrease in peat quality might not necessarily slow down future decomposition in drained organic soils.
DDC:	500 Naturwissenschaften 500 Natural sciences and mathematics 550 Geowissenschaften 550 Earth sciences
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 09 Chemie, Pharmazie u. Geowissensch.
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-6784
URN:	urn:nbn:de:hebis:77-openscience-5ee51186-3d43-44c2-bdf3-f5a7be3390ef7
Version:	Original work
Publication type:	Dissertation
License:	CC BY
Information on rights of use:	https://creativecommons.org/licenses/by/4.0/
Extent:	XIII, 110 Seiten (Illustrationen,Diagramme)
Appears in collections:	JGU-Publikationen