Competition for N in forest soils: Stabilization of organic N in recalcitrant pools. - Department of Soil Science, Life Science Center Weihenstephan.

The partitioning of nitrogen in forest soils is defined by a competitive uptake between plants, mycorrhizal fungi, soil microbes and sequestration of N into soil organic matter (SOM). Central aim of our project is to elucidate the major competitive long-term stabilization mechanisms of organic nitrogen (ON) and to characterize the formation and structure of ON.
Presently, different N stabilization mechanisms such as N stabilization through formation of organo-mineral associations, tannin-protein complexes and abiotic N immobilization are discussed. We assume that the formation of recalcitrant N pools will result in an altered N partitioning in soils and therefore will have a decisive impact on the long-term N budget of forest stands. Consequently it will affect the N availability for plant growth. In this context, the project team intends to specify the function of the above-mentioned N stabilization mechanisms in order to be able to elucidate the impact of N immobilization on N partitioning on a long-term scale.
For the project realization, a combined approach of field and laboratory investigations is scheduled. Solid-state ¹⁵N and ¹³C NMR spectroscopy will provide information on the binding forms of organic N, MALDI-TOF-MS on tannins in tannin-protein complexes and chemolytic techniques will allow to quantify hydrolyzable peptids in bulks soils and organo-mineral complexes.
This approach will allow to differentiate and to quantify the stabilization of organic N versus the organic N present in labile forms. This will provide information on the competition for N between plants and microorganisms taking into account the formation and slow turnover of recalcitrant organic N pools in soil.