Title: Topography and canopy cover influence soil organic carbon composition and distribution across a forested hillslope in the discontinuous permafrost zone

Abstract

Abstract Topography and canopy cover influence ground temperature in warming permafrost landscapes, yet soil temperature heterogeneity introduced by mesotopographic slope positions, microtopographic differences in vegetation cover, and the subsequent impact of contrasting temperature conditions on soil organic carbon (SOC) dynamics are understudied. Buffering of permafrost‐affected soils against warming air temperatures in boreal forests can reflect surface soil characteristics (e.g., thickness of organic material) as well as the degree and type of canopy cover (e.g., open cover vs. closed cover). Both landscape and soil properties interact to determine meso‐ and microscale heterogeneity of ground warming. We sampled a hillslope catena transect in a discontinuous permafrost zone near Fairbanks, Alaska, to test the small‐scale (1 to 3 m) impacts of slope position and cover type on soil organic matter composition. Mineral active layer samples were collected from backslope, low backslope, and footslope positions at depths spanning 19 to 60 cm. We examined soil mineralogical composition, soil moisture, total carbon and nitrogen content, and organic mat thickness in conjunction with an assessment of SOC composition using Fourier‐transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). Soils in the footslope position had a higher relative contribution of lignin‐like compounds, whereas backslope soils had more aliphatic and condensed aromaticmore » compounds as determined using FT‐ICR‐MS. The effect of open versus closed tree canopy cover varied with the slope position. On the backslope, we found higher oxidation of molecules under open cover than closed cover, indicating an effect of warmer soil temperature on decomposition. Little to no effect of the canopy was observed in soils at the footslope position, which we attributed, in part, to the strong impact of soil moisture content in SOC dynamics in the water‐gathering footslope position. The thin organic mat under open cover on the backslope position may have contributed to differences in soil temperature and thus SOC oxidation under open and closed canopies. Here, the thinner organic mat did not appear to buffer the underlying soil against warm season air temperatures and thus increased SOC decomposition as indicated by the higher oxidation of SOC molecules and a lower contribution of simple molecules under open cover than the closed canopy sites. Our findings suggest that the role of canopy cover in SOC dynamics varies as a function of landscape position and soil properties, namely, organic mat thickness and soil moisture. Condition‐specific heterogeneity of SOC composition under open and closed canopy cover highlights the protective effect of canopy cover for soils on backslope positions.« less

Authors:

Rooney, Erin C.  ^[1];

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Bailey, Vanessa L. ^[2]; Patel, Kaizad F. ^[2]; Kholodov, Alexander ^[3]; Golightly, Holly ^[4]; Lybrand, Rebecca A. ^[5]

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+ Show Author Affiliations

1. Department of Crop and Soil Science Oregon State University Corvallis Oregon USA, Biological Sciences Division Pacific Northwest National Laboratory Richland Washington USA, Department of Earth and Planetary Sciences University of Tennessee‐Knoxville Knoxville Tennessee USA
2. Biological Sciences Division Pacific Northwest National Laboratory Richland Washington USA
3. Geophysical Institute University of Alaska Fairbanks Fairbanks Alaska USA
4. Department of Crop and Soil Science Oregon State University Corvallis Oregon USA
5. Department of Crop and Soil Science Oregon State University Corvallis Oregon USA, Department of Land, Air, and Water Resources University of California‐Davis Davis California USA

Publication Date:

Tue Jul 11 00:00:00 EDT 2023

Research Org.:

Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)

Sponsoring Org.:

USDOE; USDOE Office of Science (SC), Biological and Environmental Research (BER). Earth & Environmental Systems Science (EESS); USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); National Science Foundation (NSF)

OSTI Identifier:

1989274

Alternate Identifier(s):

OSTI ID: 1991435; OSTI ID: 2000725

Report Number(s):

PNNL-SA-177257
Journal ID: ISSN 1045-6740

Grant/Contract Number:  

AC05-76RL01830; 2138937

Resource Type:

Published Article

Journal Name:

Permafrost and Periglacial Processes

Additional Journal Information:

Journal Name: Permafrost and Periglacial Processes Journal Volume: 34 Journal Issue: 3; Journal ID: ISSN 1045-6740

Publisher:

Wiley Blackwell (John Wiley & Sons)

Country of Publication:

United Kingdom

Language:

English

Subject:

54 ENVIRONMENTAL SCIENCES; canopy; carbon; forest; hillslope; permafrost; soil; thaw