Seasonal controls override forest harvesting effects on the composition of dissolved organic matter mobilized from boreal forest soil organic horizons
Abstract. Dissolved organic matter (DOM) mobilized from the organic (O) horizons of forest soils is a temporally dynamic flux of carbon (C) and nutrients, and the fate of this DOM in downstream pools is dependent on the rate and pathways of water flow as well as its chemical composition. Here, we present observations of the composition of DOM mobilized weekly to monthly from O horizons in mature forest and adjacent harvested treatment plots. The study site was experimentally harvested, without replanting, 10-years prior to this study. Thus, the treatments differ significantly in terms of forest stand and soil properties, and interact differently with the regional hydrometeorological conditions. This presented an opportunity to investigate the role of forest structure relative to environmental variation on soil DOM mobilization. On an annual basis, fluxes of total dissolved nitrogen (TDN) and dissolved organic nitrogen (DON) were largest from the warmer and thinner O horizons of the harvested (H) treatment compared to the forest (F) treatment, however, neither phosphate or ammonium fluxes differed by treatment type. On a short-term basis in both H and F treatments, all fluxes were positively correlated to water input, and all concentrations were positively correlated to soil temperature and negatively correlated to water input. Soil moisture was negatively correlated to the C : N of DOM. These results suggest common seasonal controls on DOM mobilization regardless of harvesting treatment. Optical characterization of seasonally representative samples additionally supported a stronger control of season over harvesting. The chemical character of DOM mobilized during winter and snowmelt: lower C : N, higher SUVA and lower molecular weight of CDOM (higher spectral slope ratio), was representative of relatively more decomposed DOM, compared to that mobilized in summer and autumn. This shows that the decomposition of soil organic matter underneath a consistently deep snowpack is a key determinant of the composition of DOM mobilized from O horizons during winter and the hydrologically significant snowmelt period regardless of harvesting impact. Despite the higher proportion of aromatic DOM in the snowmelt samples, the rapid delivery of DOM from O to mineral horizons suggests that the snowmelt period is not likely to be a significant period of DOM sequestration by mineral soil. Rather, the higher molecular weight, high C : N DOM mobilized during slow and relatively infrequent delivery during summer and rapid, frequent delivery during autumn are more likely to support periods of mineral soil sequestration and increased export of fresher terrestrial DOM, respectively. Understanding these seasonal dynamics improves our ability to accurately portray the role of O horizon DOM in the downstream fate of carbon. It also helps to understand and anticipate the response of soil organic matter to a rapidly changing climate in northern latitudes and could improve our ability to manage forest carbon balance.
Keri L. Bowering et al.
Keri L. Bowering et al.
Pynn's Brook Experimental Forest Lysimeter Datasets 2103-1014 https://github.com/ArkosicGreywacke/PBEWA/tree/main/Hillslope/Pynn's%20Brook%20Experimental%20Forest%20Lysimeters%202013-2014
Keri L. Bowering et al.
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This study investigates the seasonal variations in dissolved organic matter fluxes and concentrations from the organic layer of boreal forests. Sites that have been logged ten years ago are compared with unharvested controls in a replicated experimental design.
The study design is sound, and the information generated is original and pertinent. DOM originating from the organic layer of boreal forests represent an important and poorly known flux of carbon that may be affected by disturbances and climate change. The factors that control this flux as well as those that control the quality of DOM are poorly known. This study brings new knowledge on how disturbances and climate change may affect DOM originating from boreal forest organic horizons. The results, showing a similar response of DOM to climate for both harvested and control plots as well as an important effect of the season are pertinent to better understanding the C cycling of boreal forests.
To better understand the results, more information is needed concerning the experimental design. Specifically, what is the status of regeneration on the harvested plots? Is it bare soil? What is the status of tree regeneration, 10 years after harvesting? Has the site been recolonized by trees? What is the vegetation height, % cover… This information would be useful to understand the results. It is interesting that the differences in water and DOC fluxes are still present 10 years after harvesting. Another study conducted in somewhat similar conditions found coherent results with this one. However, it was conducted one year after harvest: warmer and wetter conditions in harvested treatment led to greater soil humidity, higher soil temperatures, and greater ammonification and DON production of harvested plots in the fall season (Coulombe et al. 2016). Can the author comment on the reason for a 10-year effect?
l.54: fate of DOM: there is abundant literature on the importance of mineral soil properties on the fate of DON that could be mentioned here.
l.74: recalcitrance of coniferous litter, how about that of bryophytes, are they not present?
l.104: 10 years may not be considered long-term in a rotation that lasts more than 50 years; preferably use 10 years, – also line 535 and elsewhere.
l.381; l. 535 and elsewhere: the use of Sr for slope could be misleading (Strontium), use another notation.
Winter rain/thaw events are increasing. The results of this study suggest that these events may bring more transformed DOM into streams. Is it possible to support this claim by referring to studies in warmer ecosystems, such as coastal Maine or New England?
https://doi.org/10.1139/cjfr-2016-0301 Coulombe et al. 2016. Effect of harvest gap formation and thinning on soil nitrogen cycling at the boreal–temperate interface