Abstract
Forest harvesting induces a step change in the climatic variables (temperature and moisture), that control carbon dioxide (CO 2) production arising from soil organic matter decomposition within soils. Efforts to examine these vertically complex relationships in situ within soil profiles are lacking. In this study we examined how the climatic controls on CO 2 production change within vertically distinct layers of the soil profile in intact and clearcut forest soils of a humid temperate forest system of Atlantic Canada. We measured mineral soil temperature (0, 5, 10, 20, 50 and 100 cm depth) and moisture (0–15 cm and 30–60 cm depth), along with CO 2 surface efflux and subsurface concentrations (0, 2.5, 5, 10, 20, 35, 50, 75 and 100 cm depth) in 1 m deep soil pits at 4 sites represented by two forest-clearcut pairs over a complete annual cycle. We examined relationships between surface efflux at each site, and soil heat, moisture, and mineral soil CO 2 production. Following clearcut harvesting we observed increases in temperature through depth (1–2°C annually; often in excess of 4°C in summer and spring), alongside increases in soil moisture (30%). We observed a systematic breakdown in the expected exponential relationship between CO 2 production and heat with mineral soil depth, consistent with an increase in the role moisture plays in constraining CO 2 production. These findings should be considered in efforts to model and characterize mineral soil organic matter decomposition in harvested forest soils.
Highlights
Forest soil organic matter (SOM) represents an important global carbon (C) reservoir [1, 2]
The calculated soil heat anomalies (Fig 2), demonstrate the greater range in soil heat associated with clear-cutting of the Lakevale sites, the same ranges are not observed at the Pomquet sites
Examinations of soil organic matter decomposition processes in managed temperate forest soil profiles that are conducted in situ and consider the variability in the controlling relationships through depth are largely undocumented in the literature
Summary
Forest soil organic matter (SOM) represents an important global carbon (C) reservoir [1, 2]. Recent studies have documented mineral SOM profile losses in the decades following clearcut harvesting in temperate forests of north eastern North America [17,18,19], with isotopic evidence pointing to increased decomposition rates following harvesting [17, 20], within the organomineral fraction [21]. These mineral SOM pools had been assumed to represent a stable fraction that would persist over the timescales of a complete forest harvest cycle. These recent shifts in our understanding of mineral SOM stability suggest a greater potential than previously realized for SOM destabilization, and points to the need to evaluate deep mineral SOM decomposition rates following this disturbance
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