Influence of different soil structures on the adsorption and Nitrogen availability (100%)

Challenge

Soil quality, in particular the ratio of soil organic matter (SOM) to clay, influences soil physical and biological parameters, plant growth and finally nutrient availability. Nitrogen is organically bound in the soil and must be mineralised in order to be available to plants. A high SOM:Clay ratio promotes the formation of relatively stable clay humus complexes in which Nitrogen is protected from mineralisation. If the SOM content exceeds the maximum amount which can be bound to the existing clay content, it is to be expected that a larger proportion will be available as more easily mineralisable nitrogen and the nitrogen availability of the soil will increase. In fertilised soils, the increase in Nitrogen availability with a higher SOM content could be due either to the increased Nitrogen mineralisation from the SOM itself or to a lower adsorption of added fertilisers as clay-humus complexes. This question forms the core of the advertised master's thesis.

Methodology

In February 2025 - June 2025, a pot trial with summer wheat will be carried out under controlled conditions as part of a PhD in the EU NutriBudget project. In this trial, 10 soils with different SOM:Clay ratios will be tested at two fertilisation levels (all nutrients including nitrogen vs. all nutrients except nitrogen). The effects of the different soils on yield, nutrient use efficiency, plant and root growth will be investigated. By using isotopically labelled nitrogen (¹⁵N), the fate of the added Nitrogen can be precisely followed. In this Master's thesis, soil samples from the beginning and end of the experiment will be separated into their physical fractions (the stable fraction ‘mineral associated organic matter/MA-OM’ and the rather unstable fraction ‘particulate organic matter/P-OM’) in order to draw conclusions about the stability of the SOM and the fate of the fertiliser Nitrogen depending on the physical soil characteristics. 

The topic is primarily suitable for a Master's thesis. If there is interest in a Bachelor's thesis, the scope could be restricted if necessary.

Contacts

Else Bünemann-König

Wiebke Reymann

Timeline

By arrangement, from May 2025, sampling May/Juni, fractionation from June 2025; evaluation flexible

Literature

• Johannes, A., Matter, A., Schulin, R., Weisskopf, P., Baveye, P. C., & Boivin, P. (2017). Optimal organic carbon values for soil structure quality of arable soils. Does clay content matter? Geoderma, 302, 14– 21. https://doi.org/10.1016/j.geode rma.2017.04.021
• Castellano, M. J., Kaye, J. P., Lin, H., & Schmidt, J. P. (2012). Linking carbon saturation concepts to nitrogen saturation and retention. Ecosystems, 15, 175–187
• Fuchs, M.; Frick, H.; Moinet, G.Y.K.; Mayer, M.; Bünemann, E.K. Residual nitrogen from slurry and mineral fertiliser two years after application: Fractionation and plant availability. Soil Biol. Biochem. 2023, 177, 108908