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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 10 Jan 2019

Research article | 10 Jan 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Dissolved organic carbon mobilized from organic horizons of mature and harvested black spruce plots in a mesic boreal region

Keri Bowering1, Kate A. Edwards2, Xinbiao Zhu2, and Susan E. Ziegler1 Keri Bowering et al.
  • 1Earth Sciences, Memorial University of Newfoundland, St. John's, A1C 5S7, Canada
  • 2Canadian Forest Service, Natural Resources Canada, Ottawa, K1A 0E4, Canada

Abstract. Boreal forests are subject to a wide range of temporally and spatially variable environmental conditions driven by seasonal and regional climate variations, in addition to disturbances such as forest harvesting and climate change. Among the various ecological mechanisms affected by disturbance, is the transport rate of dissolved organic carbon (DOC) from surface soil organic (O) horizons to deeper mineral SOC pools and the adjacent aquatic systems. Here, we examine the transport of DOC from surface O horizons across a boreal forest landscape using passive pan lysimeters in order to identify controls and hot moments of DOC mobilization from this key C source. To do so, we specifically addressed (1) how DOC fluxes from O horizons vary on a weekly to seasonal basis in both forest and harvested plots, and (2) how soil temperature, soil moisture and water inputs relate to DOC fluxes in these plots over time. The total annual DOC flux from O horizons was greater in the warmer harvested plots than in the forest plots (54 g C m−2 vs 38 g C m−2 respectively; p = 0.008), despite smaller aboveground C inputs and smaller SOC stocks in the harvested plots. Water input, measured as rain, throughfall and/or snowmelt depending on season, was positively correlated to temporal variations in soil water and DOC fluxes. Soil temperature was positively correlated to temporal variations of DOC concentration ([DOC]) of soil water and negatively correlated with water fluxes, but no relationship existed between soil temperature and DOC fluxes. Soil moisture was negatively correlated to temporal variations in [DOC] in the harvested plots only.

The relationship between water input to soil and DOC fluxes was seasonally dependent in both plot types. In summer, a water limitation on DOC flux existed where weekly periods of no flux alternated with periods of large fluxes, suggesting that increased water fluxes over this period would result in proportional increases in DOC fluxes. In contrast, a flushing of O horizons occurred during increasing water inputs and decreasing soil temperatures in autumn, prior to snowpack development. Soils of both plot types remained snow-covered all winter, which protected soils from frost and limited winter soil water fluxes. The largest water input and soil water fluxes occurred during spring snowmelt, but did not result in the largest fluxes of DOC, suggesting a production limitation on DOC fluxes over both the wet autumn and snowmelt periods.

While future increases in annual precipitation could lead to increased DOC fluxes, the response may be dependent on the intra-annual distribution of this increase. Increased water input during the already wet autumn, for instance, may not lead to increased fluxes if the DOC pool is not replenished. Potential reductions in snow cover, however, leading to a reduction in soil insulation and increased occurrence of soil frost in addition to increases in winter-time water fluxes, could be an important mechanism of increased DOC production and fluxes from O horizons in winter.

Keri Bowering et al.
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Keri Bowering et al.
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Short summary
We examined the effects of season and tree harvesting on the flow of water and the organic carbon (OC) it carries from boreal forest soils. We found that more OC was lost from the harvested forest because more precipitation reached the soil surface, but that during periods of flushing in autumn and snowmelt a limit on the amount of water-extractable OC is reached. These results have implications for how we predict the effect of changing precipitation regimes on the loss of carbon in forests.
We examined the effects of season and tree harvesting on the flow of water and the organic...