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DOC and N₂O Dynamics in Upland and Peatland Forest Soils after Clear-Cutting and Soil Preparation

Päivi Saari, Sanna Saarnio, Jussi V. K. Kukkonen, Jarkko Akkanen, Jaakko Heinonen, Veli Saari and Jukka Alm
Biogeochemistry
Vol. 94, No. 3 (Jul., 2009), pp. 217-231
Published by: Springer
Stable URL: http://www.jstor.org/stable/20519880
Page Count: 15
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DOC and N₂O Dynamics in Upland and Peatland Forest Soils after Clear-Cutting and Soil Preparation
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Abstract

Forest clear-cutting followed by soil preparation means disturbance for soil microorganisms and disruption of N and C cycles. We measured fluxes of N₂O and dissolved organic carbon (DOC) in upland soil (podzol) and adjacent peat within a clear-cut forest catchment. Both soil types behaved in a similar way, showing net uptake of N₂O in the first year after the clear-cutting, and turning to net release in the second. The N₂O flux dynamics were similar to those of N content in logging residues, as reported from a nearby site. As organic matter is used in the food web of the decomposers, we attempted to explain the dynamics of N₂O uptake and release by measuring the concurrent dynamics of the low molecular weight (LMW) fraction and the aromaticity of DOC in a soil solution. The labile and most readily available LMW fractions of DOC were nearly absent in the year following the clear-cutting, but rose after two years. The more refractory high molecular weight (HMW) fraction of DOC decreased two years after the clear-cutting. The first year's net uptake of N₂O could be accounted for by the growth of decomposer biomass in the logging residues and detritus from the degenerating ground vegetation, resulting in immobilization of nitrogen. Simultaneously, the labile, LMW fraction of DOC became almost completely exhausted. The low availability of the LMW fraction could retard the growth and cause the accumulated decomposer biomass to collapse. During the following winter and summer the fraction of LMW clearly increased, followed by increased N₂O emissions. The presence of LMW DOC fractions, not the concentration of DOC, seems to be an important controller for N₂O liberation after a major disturbance such as clear-cutting and site preparation. The complex connection between DOC characteristics, nitrification or denitrification merits further studies.

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