Ph.D. on cycling, uptake and sizes of N pools

This Ph.D. project is mainly linked to WP4 and will focus on the effects of warming, increased CO₂ and precipitation changes on ecosystem nitrogen cycling and pool sizes and on microbial and plant acquisition of nitrogen in organic (amino acids) and inorganic form (nitrate and ammonium).

Methods used will include in situ tracing of nitrogen and carbon uptake from the soil into plants and micro organisms by using stable isotopes (¹⁵N and ¹³C) and in situ estimation of mineralization and immobilization by the 'buried bag' method.

Ph.D.-student Louise C. Andresen (employed at University of Copenhagen, Institute of Biology) started this project May 1st-2005 in collaboration with COGCI (Copenhagen Global Change Initiative).

Project description

Nitrogen cycling is presumed to change when soil temperature and ambient CO₂ level increases and precipitation alters. On a temperate heath nitrogen is often limiting for plant production and important as substrate for soil micro organisms. A changed nitrogen cycling will hence affect plant and micro organism community and lead to altered carbon uptake and decomposition of soil organic matter.

Nutrient release during decomposition of soil organic matter is investigated with the 'buried bag' method also in a version with a plant in the bag. This reveals mineralization of inorganic and organic compounds and microbial and plant immobilization.

In heath soil the dissolved nitrogen is to a large extent on organic form. It is now widely accepted that plants are able to take up amino acids as intact molecules, hereby also acquiring carbon from the soil. Soil micro organisms are often carbon limited and also use amino acids as substrates. We investigate this competition for nitrogen sources quantitatively with amino acid labelling experiments using stable isotopes (¹⁵N and ¹³C). Abundance of amino acids and organic acids are analyzed in cooperation with Lund University.

Changes in the microbial community may have impact on mineralization and plant N uptake. Quantification of mycorrhizal colonization of plant roots, bulk soil microbial biomass, chitin, ergosterol and phospholipid acid composition may reveal changes in micro organism function.