Influences of Hillslope Biogeochemistry on Anaerobic Soil Organic Matter Decomposition in a Tundra Watershed
Abstract
We investigated rates and controls on greenhouse gas (CO2 and CH4) production in two contrasting water-saturated tundra soils within a watershed near Nome, Alaska. Three years of field sample analyses have shown that soil from a fen-like area at the base of the hillslope had higher pH and higher porewater ion concentrations than soil collected from a bog-like peat plateau at the top of the hillslope. The influence of these contrasting geochemical environments on CO2 and CH4 production was tested in microcosms by incubating both the organic- and mineral-layer soils anaerobically for 55 days. NH4Cl was added to half of the microcosms to test the effects of N limitation on microbial greenhouse gas production. We found that total CO2 and CH4 production were higher in the soils from the bottom of the hillslope. Dissolved organic C (DOC) was also higher in these soils, and fermentation of this C pool resulted in an increasing supply of low-molecular weight organic acids (e.g., acetate and propionate) throughout the incubations. Higher availability of labile DOC, in addition to higher pH, likely contributed to the more rapid CO2 and CH4 production at the bottom of the hillslope. Our results also indicate that inorganic N concentrations were lower and soil C decomposition was more N-limited in the peat plateau soils than the toeslope soils, which exhibited net N mineralization while the peat plateau soils had net N immobilization. N addition increased CO2 production in the peat plateau soils, but not the toeslope soils, consistent with greater N limitation. Microbial community compositions changed substantially in incubations of organic soils with added N, compared to smaller changes in mineral soils. Our results suggest that the movement of water, ions, and nutrients down the tundra hillslope can increase the rate of anaerobic soil organic matter decomposition by (1) increasing the pH of soil porewater; (2) providing bioavailable DOC and fermentation products such as acetate; and (3) relieving microbial N limitation through nutrient runoff. We suggest that the soil geochemistry as mediated by landscape position could be an important predictor of the fate of soil organic matter liberated from thawing permafrost.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B23I2532P
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0475 Permafrost;
- cryosphere;
- and high-latitude processes;
- BIOGEOSCIENCES;
- 0708 Thermokarst;
- CRYOSPHERE;
- 0793 Biogeochemistry;
- CRYOSPHERE;
- 1823 Frozen ground;
- HYDROLOGY;
- 1865 Soils;
- HYDROLOGY