Temporal evolution of bacteria fungi proportions in forest soils in spring

Funded under the Call for Emergence 2016, the SPRING project took place over 6 months (2017).
Project leader: Laure BARTHES (ESE Laboratory: Ecology, Systematics, Evolution).      Other people involved from the laboratory: Gaëlle VINCENT (IE) ; Baptiste Laffont (M2 trainee).             

Partner in BASC: ECOSYS laboratory (Functional ecology and ecotoxicology of agroecosystems): Claire CHENU (Pr) ; Valérie POUTEAU (Tech R).

Tree and forest growth is limited by a range of nutrients and more particularly by nitrogen in temperate environments¹. This limitation could be enhanced by global changes due to the fertilization effect of CO2: increased growth due to increased CO2 would increase plant demand for mineral nitrogen and then its sequestration in plant biomass and soil organic matter, leading to a progressive limitation of soil nitrogen availability². Global changes may also increase mineral N availability in the soil through increased N deposition³ and enhanced mineralization⁴. Understanding the nitrogen cycle in forest ecosystems and its impact on the carbon balance therefore requires a good understanding of the temporal dynamics of the different nitrogen pools in the tree-soil-microbes system.

Soil ¹⁵N labeling experiments have demonstrated a preferred use of nitrogen from the tree's internal reserves for new leaf formation in the spring associated with high microbial immobilization⁵. The question then arises as to whether there is a limitation of soil nitrogen supply at this time by microbial immobilization and to identify the microbial players responsible for this limitation. The fatty acids contained in bacterial and fungal walls, the PLFA (Phopho Lipid Fatty Acid) are specific to large microbial groups and their determination in soil provides a picture of the composition of the living microbial biomass⁶. The objective of the SPRING project is to identify the major microbial groups responsible for nitrogen immobilization in forest soils in spring using the PLFA technique.

The question addressed is important to better understand the current and future functioning of forests and their ability to mitigate or enhance the impact of global changes because of the links between microbial N immobilization, N uptake by trees, tree growth and their function as carbon sinks.
 

==> The project leader explains the project and its RESULTS in VIDEO  (LabEx BASC scientific days, February, 2021)

Results

The bacterial pool is the majority pool of the microbial biomass (Figure 1). The variations of the nitrogen stock in the microbial biomass are linked to the variations of the bacterial pool (Figure 1: Evolution of the proportion of the different PLFA in the microbial biomass of a forest soil over time. Barbeau Forest. T0 = March 1, 2016)

Collaboration between ESE, which quantified microbial immobilization, and ECOSYS, which provided the project with its expertise in identifying microbial groups, made it possible to assess the relative contribution of bacteria and fungi to microbial nitrogen immobilization in the spring. The BASC funding allowed the initiation of this collaboration, which is currently being pursued through the writing of papers resulting from the experimental work of the Master 2 internship and a new thesis project related to agroforestry funded by C-LAND.

References

1. Rennenberg and Dannenmann 2015

2. Luo et al. 2004

3. Galloway et al. ? 2003

4. Zack et al. ? 1993

5. Bazot et al., 2016; Maxwell et al., 2020

6. Kaur et al., 2005