Seasonal controls override forest harvesting effects on the composition of dissolved organic matter mobilized from boreal forest soil organic horizons
Authors: Bowering Keri L. , Edwards Kate A. , Ziegler Susan E. 
Published: June 2023

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 they 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 nor 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 specific ultraviolet absorbance and lower
molecular weight of chromophoric DOM (CDOM; higher spectral slope ratio)
were 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, its
lower molecular weight and rapid delivery 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. These observed seasonal dynamics in O horizon
DOM suggest the predicted decreases in winter and spring snowfall and
increasing autumn and winter rainfall with climate warming in this region
will enhance mobilization of DOM that is more reactive to mineral
interactions in deeper soil, but also more biological and photoreactive in
the aquatic environment. Understanding the downstream consequences of this
mobilized DOM in response to these shifts in precipitation timing and form
can improve our ability to predict and manage forest C balance but requires
understanding the response of landscape hydrology to these changing
precipitation regimes.