Abstract
The forest floor of boreal forest stores large amounts of organic C that may react to a warming climate and increased N deposition. It is therefore crucial to assess the impact of these factors on the temperature sensitivity of this C pool to help predict future soil CO2 emissions from boreal forest soils to the atmosphere. In this study, soil warming (+2-4°C) and canopy N addition (CNA; +0.30-0.35 kg·N·ha-1·yr-1) were replicated along a topographic gradient (upper, back and lower slope) in a boreal forest in Quebec, Canada. After nine years of treatment, the forest floor was collected in each plot, and its organic C composition was characterized through solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Forest floor samples were incubated at four temperatures (16, 24, 32 and 40°C) and respiration rates (RR) measured to assess the temperature sensitivity of forest floor RR (Q10 = e10k) and basal RR (B). Both soil warming and CNA had no significant effect on forest floor chemistry (e.g., C, N, Ca and Mg content, amount of soil organic matter, pH, chemical functional groups). The NMR analyses did not show evidence of significant changes in the forest floor organic C quality. Nonetheless, a significant effect of soil warming on both the Q10 of RR and B was observed. On average, B was 72% lower and Q10 45% higher in the warmed, versus the control plots. This result implies that forest floor respiration will more strongly react to changes in soil temperature in a future warmer climate. CNA had no significant effect on the measured soil and respiration parameters, and no interaction effects with warming. In contrast, slope position had a significant effect on forest floor organic C quality. Upper slope plots had higher soil alkyl C:O-alkyl C ratios and lower B values than those in the lower slope, across all different treatments. This result likely resulted from a relative decrease in the labile C fraction in the upper slope, characterized by lower moisture levels. Our results point towards higher temperature sensitivity of RR under warmer conditions, accompanied by an overall down-regulation of RR at low temperatures (lower B). Since soil C quantity and quality were unaffected by the nine years of warming, the observed patterns could result from microbial adaptations to warming.
Highlights
As with many biological processes, the soil respiration rate (RR) is strongly influenced by temperature [1,2]
The increase in forest floor RR with incubation temperature was well described by a first-order exponential model (RR = BekT) for all treatments (C, W+, N+ and with no N addition (W+)N+) with or without the RR values obtained from the incubation at 40 ̊C (Fig 2)
Samples collected from the back and lower slopes were mostly associated with high B values, C:N ratios and the percentage of aromatic compounds
Summary
As with many biological processes, the soil respiration rate (RR) is strongly influenced by temperature [1,2]. Climate change is expected to increase soil temperatures by 2–4 ̊C in northeastern North America by the end of the century and extend the snow-free period by one month [16]. Whether this warming will result in net C losses from the soil depends on the temperature sensitivity of biochemical processes controlling C inputs (i.e., mainly photosynthesis) to and outputs (i.e., mainly soil respiration) from the soil organic C (SOC) pool. In situ soil warming experiments have produced conflicting results, with studies reporting a reduction [11,12] or an increase [6,7] in the Q10 of soil RR, while others report no significant effect [8,20,21]
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