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Resilience of Soil Stoichiometry in subartic soils under Temperature-Induced Soil Carbon Losses: Where does the N go?

Periodic Reporting for period 1 - StoiCa (Resilience of Soil Stoichiometry in subartic soils under Temperature-Induced Soil Carbon Losses: Where does the N go?)

Période du rapport: 2017-09-01 au 2019-08-31

Global warming can trigger enormous releases of carbon (C) from soils, with positive feedbacks to climate change. Northern high latitude soils can constitute a major contributor to this positive feedback loop. Climate change predictions are however still largely uncertain, partly due the lack of accurate representation of vegetation and soil microbial feedbacks and C and nitrogen (N) interactions. Warming enhances microbial mineralization of soil organic matter (SOM) (i.e. soil C outputs) to a higher degree than vegetation productivity (i.e. soil C inputs), resulting in large C losses from northern soils. Pioneer results point to proportional N losses in response to warming, which may be the key to this phenomenon.

StoiCa combined the expertise of a multidisciplinary group of leading researchers on ecosystem stoichiometry and stable N isotopic methods with the existence of unique and established research sites in geothermal systems in Iceland to reveal the fate of N lost in response to warming, and uncover the mechanisms behind observed soil C losses. This project directly addresses two cross-cutting priorities of sustainable development and climate action from the H2020 Work Programme and it also represented an exceptional opportunity for the candidate to acquire essential skills that positioned her as a highly competitive European researcher in climate change biogeochemistry.

StoiCa`s findings suggest a strong control of microbial physiology and C:N stoichiometric needs on the retention of soil N and on the resilience of soil C stocks from high-latitudes to warming, particularly during periods of vegetation dormancy and low C inputs. In mineral soils, soil microbes are predominantly C limited (Soong et al. 2019), microbial N mineralization predominates to N immobilization and microbes rather use DON as a C source, releasing mineral N as a byproduct. This has crucial implications for the responses of mineral soils of high latitudes to warming, where most of the N release is in asynchrony with plant N uptake. Warming may therefore cause an opening of the N cycle in these ecosystems, diminishing their capacity to sustain levels of vegetation biomass and productivity with consequent cascade effects on plant C inputs to the soil (Marañón-Jimenez et al. in prep). These results may imply that high latitude mineral soils are much more vulnerable to warming induces C losses than has been predicted by ecosystem models and that their contribution to positive climate change feedbacks may have been underestimated.
Work performed:

In order to determine the warming impacts on the balance between microbial N mineralization and plant N acquisition and get insights on the warming alterations of the N cycle a seasonal 15N labeling experiment, together with a simultaneous soil sampling were performed in situ at the geothermal gradients in Iceland, at four seasonal sampling campaigns in summer 2017, autumn 2017, early spring 2018 and spring 2018.

At each of these seasonal campaigns, a solution of 15N labeled aminoacids (15N-Aas) was injected homogeneously along the upper 7 cm of soil along the geothermal gradient. Soil and vegetation samples were carefully collected two weeks after 15N labeling, and the core was separated into belowground biomass (roots), aboveground biomass (shoots) and fresh sieved and homogenized soil. Samples were then shipped to CREAF (Spain), where they were further processed prior to analyses of total C, total N and 15N isotopic content in the microbial biomass and bulk soil at the Terrestrial Ecology Laboratories (TER, University of Vienna).

A soil sampling campaign was performed simultaneously to each 15N labeling campaign. Each seasonal sampling campaign in Iceland was therefore followed by secondments of 1-3 weeks duration at TER, were soil N transformation rates were determined in the soil samples. Specifically, Isotope Pool Dilution (IPD) techniques allowed to determine the rates of protein depolymerization, gross N mineralization and immobilization and gross nitrification. Total C, total N, 13C and 15N content of the soil samples and microbial biomass were also determined in soil.

Results achieved:

Warmer temperatures provoked persistent increases in the respiration rates of microbes from subarctic Icelandic soils (Marañón-Jiménez et al. 2018), which provoked the predominant depletion of the most physically accessible C (Marañón-Jiménez et al. 2019). Soil microbes met the increasing respiratory demands under conditions of low C accessibility at the expenses of a reduction of the standing biomass in warmer soils (Marañón-Jiménez et al. 2019). Moreover, soil warming impaired the build up of microbial biomass that usually happens at high latitude soils during winter. This together with strict microbial C:N stoichiometric demands also constrained the capacity of microbes to retain N (Marañón-Jiménez et al. 2019), and increased the vulnerability of soil to N losses, particularly during winter when vegetation N uptake is lowest (Marañón-Jiménez et al. in prep).

Dissemination and outreach:

The results of the project have been disseminated in 10 international scientific conferences. Of these, the fellow has convened a specific session on carbon and nutrient interactions in terrestrial ecosystems at the American Geosciences Union (AGU) in 2018 and 2019. Dr. Marañón-Jimenez has also presented her work in 7 oral presentations, of which she was invited speaker in 6. In addition, she has contributed with scientific posters in 3 international conferences.

The fellow has also disseminated her research to the general public by different means, such as the publication of personal interviews for the CREAF blog, for radio podcasts and for a secondary school about the role of women on science. The main results of her research have been also published in >20 paper and online newspapers. The fellow has also participated in the European Researcher´s Night, in education programmes in public schools (MAGNET project), in the elaboration of the ClimaTickTock pedagogical board game together with other Marie-Curie fellows, in scientific round tables with the general public (scientific coffees) and as invited speaker in a outreach conference to raise awareness about the climate change problems.
StoiCa`s findings suggest a strong control of microbial physiology and C:N stoichiometric needs on the retention of soil N and on the resilience of soil C stocks from high-latitudes to warming, particularly during periods of vegetation dormancy and low C inputs. In mineral soils, soil microbes are predominantly C limited (Soong et al. 2019), microbial N mineralization predominates to N immobilization and microbes rather use DON as a C source, releasing mineral N as a byproduct. This has crucial implications for the responses of mineral soils of high latitudes to warming, where most of the N release is in asynchrony with plant N uptake. Warming therefore causes an opening of the N cycle in these ecosystems, diminishing their capacity to sustain levels of vegetation biomass and productivity with consequent cascade effects on plant C inputs to the soil (Marañón-Jimenez et al. in prep). These results may imply that high latitude mineral soils are much more vulnerable to warming induces C losses than has been predicted by ecosystem models and that their contribution to positive climate change feedbacks may have been underestimated.
Hypotheses on the warming effects on vegetation-soil microbial feedbacks