The impact of Lithology and Climate on litter decomposition and soil carbon dynamics in Mediterranean forests

Abstract

Forest soils are a major component of the global carbon cycle, influencing atmospheric CO2 concentration and thus, global warming. They are integral to nutrient cycling, carbon sequestration, water regulation and biodiversity. Lithology is one of the main state factors that influences ecosystem function and structure, from landscape evolution to microbial dynamics and phyllosilicate mineralogy, yet its role in forest carbon cycling and response to climate change is underexplored. Hence, this thesis examines lithology's impact on forest soil carbon cycling and its response to varying precipitation.
To achieve these aims this research utilised three mountain ranges located in Andalucía, Spain where maritime pine (Pinus pinaster) and Spanish fir (Abies pinsapo) forests grow on three distinctive lithological substrates: calcareous, metapelite and peridotite along a precipitation gradient ranging more than 500 mm yr-1. This unique natural set up constitutes a perfect experimental laboratory to explore the potential role of lithology on the response of Mediterranean pine forests to climate change (project LITHOFOR funded by the Spanish Science and Innovation Ministry).
First, we established two plant litter decomposition experiments (chapter 3), one utilising all sites along the precipitation gradient and a reciprocal transplant experiment utilising a subset sites (centre of the precipitation gradient; calcareous and peridotite lithologies). After 1.5 years of decomposition, needle mass loss ranged from 15 to 45% with differences being attributable to variations in hemicellulose content. At intermediate levels of precipitation mass loss was 25% higher on calcareous soils and demonstrated a pronounced home-field advantage. Furthermore, decreased precipitation reduced litter mass loss only on calcareous soils (35%). The lack of response in other lithologies was attributable to the influence of low pH and heavy metals on microbial physiology.
Next, we wanted to determine whether lithology derived changes in soil physicochemistry affected soil carbon contents, mineralisation, and temperature sensitivity (Q10) along with their response to historically reduced precipitation. The primary impacts of lithology manifested through alterations in microbial stoichiometry, functionality, and accessibility to feedstocks causing strong differences in rates of carbon mineralisation ranging from 0.15 and 1.24 (µM h-1 g dry soil-1). Reduced precipitation dictated long-term carbon storage with particulate and minerally associated organic matter contents reducing by 38 and 49% from west to east. Conversely, lithology dictated short-term processes due to significant increases in microbial biomass. Rates of carbon mineralisation were 64% higher on calcareous substrates, decreasing by 44% with reduced precipitation metapelite and peridotite lithologies showed no response. Interestingly, temperature sensitivity was not impacted by lithology or reduced precipitation suggesting that physiological constraints imposed by moisture limitation in the summer dry season predominated.
Finally, we examined whether the influence of lithology on carbon storage and release was stronger in deeper soil horizons when water was not limiting across two forests with dominant species, maritime pine and Spanish fir. In deeper, mineral soils, the influence of lithology on carbon mineralisation was stronger (eta2 = 72%). Rates were 0.28 µM g-1 h-1 CO2, being 87% lower than the organic horizon. As seen in chapter 4, calcareous soils mineralised the most CO2 (2.0 µM g-1 h-1 CO2) being 2.5-21.6 times higher than other lithologies, again, due to increased bioavailability of nutrients. In contrast to chapter 4, Q10 was strongly influenced by lithology ranging from 1.0 – 5.8. Across lithologies, depths and species, microbial carbon use efficiency explained 38-92% in Q10. At lower Q10 values the increased carbon investment in cellular maintenance seems to act as a compensatory mechanism to offset temperature sensitivity.
Overall, this dissertation provides new evidence and insights into the modulating role of lithology on soil carbon which is essential to understand the fate of global carbon stocks under a changing climate. It highlights that Mediterranean forests grown on more productive lithologies (calcareous) emit more CO2 and are far more sensitive to changes in temperature and precipitation. Based on the temperature sensitivity of carbon mineralisation (Q10), calcareous soils will emit 1.5-2.5 times more CO2 based on the projected 3-5°C temperature increase for the Mediterranean region, being 42-73% higher than metapelite and peridotite substrates. Future research should extend this inquiry to ascertain if the lithology-dependent physicochemical influences on carbon cycling observed here apply other ecosystems and climate zones.

Date of Award	11 Nov 2024
Original language	English
Awarding Institution	Bangor University
Sponsors	Envision DTP & Spanish Ministry of Science and Innovation
Supervisor	Andy Smith (Supervisor), Lars Markesteijn (Supervisor) & Ana Rey (Supervisor)