Mechanisms and pathways of microbial and plant mediated C and N assimilation –
Work package 4.5

Research during the last decade have shown that plants in various ecosystem types are able to access organic N, which was typically ignored in earlier studies of the N cycle (Lipson and Näsholm, 2001). Consequently, this pathway need to be included in studies of N transformation, and plant uptake of organic N and the short- and long-term partitioning of N between plants and microbes will therefore be quantified.

In spring of the third year, and prior to soil sampling, we will do an in situ labeling of the soil solution by injections with universally ¹³C,¹⁵N-labeled glycine in three 20Í20 cm subplots per treatment plot. Glycine is an amino acid that can be absorbed either directly by plants or absorbed after microbial mineralization to inorganic N and CO₂. This labeling technique allows nitrogen and, in the short term, carbon to be followed through the soil, plant and microbial pools with high resolution and will give complementary information on the timing and pathways of the nutrient mobilizations measured by the modified buried bag technique (WP4.3).

We will sample one of the three subplots for isotopic analyses of plant and microbes 6 hours after injection to determine to which extent intact glycine is absorbed. In addition to the measurements of the short-term organic N uptake, we will follow the longer-term, winter and summer, absorption of the added ¹⁵N by plants and microbes. We do this because we expect that the treatments will displace the timing of plant and microbial activity in the system. Thus, sampling of the two remaining subplots will take place after ½ year and after one year, respectively, to determine the longer-term partitioning of the added N in the glycine between ecosystem pools. The combined samplings after ½ year (autumn) and one year (spring) will provide information on the effect of winter processes on nutrient partitioning in the treatments.

The analyzed material will include total soil, litter, microbial biomass, plant roots and aboveground biomass of the different plant functional types (graminoids, herbs, deciduous dwarf shrubs, evergreen dwarf shrubs, mosses). These substrates will be analyzed for C and N concentrations and their ¹³C and ¹⁵N contents by IRMS. Samplings and analyses will furthermore provide important information on treatment responses in plant and microbial biomass and N pool sizes in the third year (spring and autumn) and the fourth year (spring). Additionally, the short-term fate of glycine-C will be assessed by inclusion of ¹³C analysis of CO₂ evolved 6 hours, one week and three weeks after labeling.