Abstract
Soil respiration is a major component of the global carbon budget and Mediterranean ecosystems have usually been studied in locations with shallow soils, mild temperatures, and a prolonged dry season. This study investigates seasonal soil respiration rates and underlying mechanisms under wetter, warmer, and more fertile conditions in a Mediterranean cork oak forest of Northern Tunisia (Africa), acknowledged as one of the most productive forests in the Mediterranean basin. We applied a soil respiration model based on soil temperature and relative water content and investigated how ecosystem functioning under these favorable conditions affected soil carbon storage through carbon inputs to the soil litter. Annual soil respiration rates varied between 1774 gC m−2 year−1 and 2227 gC m−2 year−1, which is on the highest range of observations under Mediterranean climate conditions. We attributed this high soil carbon flux as a response to favorable temperatures and soil water content, but this could be sustained only by a small carbon allocation to roots (root/shoot ratio = 0.31–0.41) leading to a large allocation to leaves with a multiannual leaf production, enhanced annual twig elongation (11.5–28.5 cm) with a reduced leaf life span (<1 year) maintaining a low LAI (1.68–1.88) and generating a high litterfall (386–636 gC m−2 year−1). Thus, the favorable climatic and edaphic conditions experienced by these Mediterranean cork oak forests drove high soil respiration fluxes which balanced the high carbon assimilation leading to a relatively small overall contribution (10.96–14.79 kgC m−2) to soil carbon storage.
Highlights
Soil respiration (Rs) is the dominant flux of total ecosystem respiration [1,2,3,4] globally releasing10 times as much CO2 to the atmosphere as the combustion of fossil fuels [5]
The site offers a deep and fertile soil, a high soil water content, and a high mean annual precipitation associated with hot and dry summers, overall leading to extremely favorable conditions in the range of Mediterranean climates covered by the global dataset assemblage [56] (Figure 8)
This allowed us to investigate how carbon fluxes adjust to favorable conditions for a usually water-limited ecosystem
Summary
Soil respiration (Rs) is the dominant flux of total ecosystem respiration [1,2,3,4] globally releasing10 times as much CO2 to the atmosphere as the combustion of fossil fuels [5]. Given the projected decrease in precipitation and increase in temperatures for the Mediterranean basin [7,8,9], soil respiration has a potential role either to amplify global warming due to its sensitivity to environmental conditions [10,11,12], or to mitigate climate change due to enhanced soil carbon sequestration and reduced CO2 efflux [13,14]. One main assumption is the stimulation of organic matter decomposition increasing the loss of organic carbon stored in the soils towards the atmosphere under global warming [15,16,17,18,19] or changes in the precipitation pattern [1,4]. Other biotic and abiotic factors, ranging from soil chemistry and physics to stand structure, have been reported to influence soil respiration, as soil organic matter quantity and quality [20,21], soil acidity, and site fertility [1,22,23]
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