The susceptibility of soils to priming: the impact of litter addition, nitrogen fertilisation, and heat shock

Abstract

Climate change is expected to alter soil nutrient availability and microbial community functionality. Inputs of labile carbon (C) from root exudates can stimulate the decomposition of soil organic matter (SOM) in a phenomenon known as “priming”; driven by microbial demand for limiting resources. Climate-driven changes in resource availability may therefore influence soil susceptibility to priming, with implications for long-term soil C storage.
This thesis investigated how climate change-associated shifts in resource availability – via increased litter inputs, nitrogen (N) addition, and heat-shock treatments – influenced microbial activity and subsequent SOM priming in response to glucose addition, in temperate beech forest soils. Two overarching hypotheses were tested: (H1) The climate-simulation treatments will have an effect on the functioning of the microbial community, (H2) The susceptibility of soils to priming will vary depending on the climate-simulation treatments. Soils were first subjected to the respective treatments, followed by 13C-labelled glucose to simulate root exudation, to assess the effect on “priming” of SOM decomposition. Microbial respiration (glucose- and SOM-derived CO2), growth (via isotope incorporation), and carbon use efficiency (CUE) were measured to understand the mechanisms behind the priming dynamics.
All climate-treatments altered microbial functioning and influenced soil priming susceptibility. Glucose addition consistently reduced CUE, indicating a microbial shift from growth towards respiration. Preferential substrate use was observed in control, litter, and N-amended soils after initial glucose addition, suppressing SOM mineralisation, but this reversed over time. Strong priming in the litter-amended soils was likely driven by elevated microbial nutrient demand from the high C:nutrient imbalance of the added litter and greater microbial biomass, promoting SOM via mechanisms like selective N-mining. N addition increased microbial CUE but reduced overall growth and respiration, suggesting more efficient but reduced resource use. Here, priming was also enhanced, likely via stoichiometric decomposition due to the increased N availability. Heat-shock reduced microbial biomass and bacterial growth but not fungal, shifting the community composition. Respiration increased despite the lower microbial biomass, while CUE declined, indicating a stress response. In contrast to the other treatments, heat-shock reduced the susceptibility of soils to priming.
Overall, these findings highlight how microbial responses to changing resource regimes can impact priming susceptibility and microbial C-use strategies, with implications for soil C dynamics under future climate scenarios.

Popular Abstract

Forests do more than just grow trees – they play a vital role in capturing and storing carbon from the atmosphere, helping to slow climate change. A lot of this carbon is locked away underground in the soil, but as the climate warms changes in temperature and nutrient availability could disrupt this balance, potentially causing soils to release more carbon, as carbon dioxide, back into the atmosphere.
This project explored how certain climate-related changes – more leaf litter from increased plant growth, extra nitrogen in the soil due to more active microbes, and sudden heatwaves – affect the soil's “priming” behaviour. This refers to how microbes in the soil respond to ‘fresh food sources’ (like sugars from plant roots), and whether this encourages them to break down older, stored carbon in the soil, potentially releasing it as carbon dioxide.
To investigate this, soil was collected from a beech forest in southern Sweden and exposed in the lab to three different treatments: added leaf litter, added nitrogen, or a short-term heatwave. After one week, sugar was added to the treated soils to mimic the ‘fresh food sources’ from plant roots. Over the following week, how active the microbes became, how much they grew (an indicator of carbon storage), and how much carbon they released as gas was measured.
The results showed that these climate-related changes do affect how much carbon is stored in the soils. More leaf litter led to more microbial growth and a greater release of stored soil carbon to the air (as carbon dioxide). Extra nitrogen made microbes use carbon more efficiently, but still led to more carbon being released to the atmosphere, while heatwaves reduced microbial growth overall, and led to less stored carbon being lost. Interestingly, the addition of sugar generally reduced how well carbon was stored in the soil. This suggests that, as climate change boosts plant root growth, beech forest soils may become less effective at storing carbon.
In short, not all climate changes affect soil in the same way. Some make soils more likely to lose stored carbon, while others may actually reduce this risk, at least for a while. These findings help us better understand how future climate conditions could influence the ability of beech forest soils to store carbon, and ultimately their role in regulating the Earth's climate.

• author: Lunn, Sophie Charlotte
• supervisor: Lettice Hicks
• organization: Dept of Physical Geography and Ecosystem Science
• course: NGEM01 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Physical Geography, Ecosystem analysis, Priming Effect, Soil Carbon Cycling, Temperate Beech Forest
• publication/series: Student thesis series INES
• report number: 721
• language: English
• id: 9205820
• date added to LUP: 2025-06-27 11:40:34
• date last changed: 2025-06-27 11:40:34