Winter Ecosystem Respiration and Sources of CO2 From the High Arctic Tundra of Svalbard: Response to a Deeper Snow Experiment

Currently, there is a lack of understanding on how the magnitude and sources of carbon (C) emissions from High Arctic tundra are impacted by changing snow cover duration and depth during winter. Here we investigated this issue in a graminoid tundra snow fence experiment on shale-derived gelisols in Svalbard from the end of the growing season and throughout the winter. To characterize emissions, we measured ecosystem respiration (Reco) along with its radiocarbon (14C) content. We assessed the composition of soil organic matter (SOM) by measuring its bulk-C and nitrogen (N), 14C content, and n-alkane composition. Our findings reveal that greater snow depth increased soil temperatures and winter Reco (25 mg C m−2 d−1 under deeper snow compared to 13 mg C m−2 d−1 in ambient conditions). At the end of the growing season, Reco was dominated by plant respiration and microbial decomposition of C fixed within the past 60 years (Δ14C = 62 ± 8‰). During winter, emissions were significantly older (Δ14C = −64 ± 14‰), and likely sourced from microorganisms decomposing aged SOM formed during the Holocene mixed with biotic or abiotic mineralization of the carbonaceous, fossil parent material. Our findings imply that snow cover duration and depth is a key control on soil temperatures and thus the magnitude of Reco in winter. We also show that in shallow Arctic soils, mineralization of carbonaceous parent materials can contribute significant proportions of fossil C to Reco. Therefore, permafrost-C inventories informing C emission projections must carefully distinguish between more vulnerable SOM from recently fixed biomass and more recalcitrant ancient sedimentary C sources

Charcoal does not change the decomposition rate of mixed litters in a mineral cambisol: a controlled conditions study

It has been recently shown that the presence of charcoal might promote humus decomposition in the soil. We investigated the decomposition rate of charcoal and litters of different biochemical quality mixed together in a soil incubation under controlled conditions. Despite the large range of organic substrate quality used in this study, we did not find any difference in the decomposition between the average of two individual substrates decomposing separately and the same substrates mixed together. We concluded that charcoal does not always promote other organic matter decomposition and that its particular effect might depend on various factors, for example, soil properties

Effects of increasing fire frequency on black carbon and organic matter in Podzols of Siberian Scots pine forests.

Fires in boreal forests frequently convert organic matter in the organic layer to black carbon, but we know little of how changing fire frequency alters the amount, composition and distribution of black carbon and organic matter within soils, or affects podzolization. We compared black carbon and organic matter (organic carbon and nitrogen) in soils of three Siberian Scots pine forests with frequent, moderately frequent and infrequent fires. Black carbon did not significantly contribute to the storage of organic matter, most likely because it is consumed by intense fires. We found 99% of black carbon in the organic layer; maximum stocks were 72 g m-2. Less intense fires consumed only parts of the organic layer and converted some organic matter to black carbon (> 5 g m-2), whereas more intense fires consumed almost the entire organic layer. In the upper 0.25 m of the mineral soil, black carbon stocks were 0.1 g m-2 in the infrequent fire regime. After fire, organic carbon and nitrogen in the organic layer accumulated with an estimated rate of 14.4 g C m-2 year-1 or 0.241 g N m -2 year-1. Maximum stocks 140 years after fire were 2190 g organic C m-2 and 40 g N m-2, with no differences among fire regimes. With increasing fire frequency, stocks of organic carbon increased from 600 to 1100 g m-2 (0-0.25 m). Stocks of nitrogen in the mineral soil were similar among the regimes (0.04 g m-2). We found that greater intensities of fire reduce amounts of organic matter in the organic layer but that the greater frequencies may slightly increase amounts in the mineral soil. © 2004 British Society of Soil Science

Effects of increasing fire frequency on black carbon and organic matter in Podzols of Siberian Scots pine forests.

Fires in boreal forests frequently convert organic matter in the organic layer to black carbon, but we know little of how changing fire frequency alters the amount, composition and distribution of black carbon and organic matter within soils, or affects podzolization. We compared black carbon and organic matter (organic carbon and nitrogen) in soils of three Siberian Scots pine forests with frequent, moderately frequent and infrequent fires. Black carbon did not significantly contribute to the storage of organic matter, most likely because it is consumed by intense fires. We found 99% of black carbon in the organic layer; maximum stocks were 72 g m-2. Less intense fires consumed only parts of the organic layer and converted some organic matter to black carbon (> 5 g m-2), whereas more intense fires consumed almost the entire organic layer. In the upper 0.25 m of the mineral soil, black carbon stocks were 0.1 g m-2 in the infrequent fire regime. After fire, organic carbon and nitrogen in the organic layer accumulated with an estimated rate of 14.4 g C m-2 year-1 or 0.241 g N m -2 year-1. Maximum stocks 140 years after fire were 2190 g organic C m-2 and 40 g N m-2, with no differences among fire regimes. With increasing fire frequency, stocks of organic carbon increased from 600 to 1100 g m-2 (0-0.25 m). Stocks of nitrogen in the mineral soil were similar among the regimes (0.04 g m-2). We found that greater intensities of fire reduce amounts of organic matter in the organic layer but that the greater frequencies may slightly increase amounts in the mineral soil. © 2004 British Society of Soil Science

Rates and radiocarbon content of summer ecosystem respiration in response to long-term deeper snow in the High Arctic of NW Greenland

The amount and timing of snow cover control the cycling of carbon (C), water, and energy in arctic ecosystems. The implications of changing snow cover for regional C budgets, biogeochemistry, hydrology, and albedo due to climate change are rudimentary, especially for the High Arctic. In a polar semidesert of NW Greenland, we used a ~10-year old snow manipulation experiment to quantify how deeper snow affects magnitude, seasonality, and 14C content of summer C emissions. We monitored ecosystem respiration (Reco), soil CO2, and their 14C contents over three summers in vegetated and bare areas. Additional snowpack, elevated soil water content (SWC), and temperature throughout the growing season in vegetated, but not in bare, areas. Daily Reco was positively correlated to temperature, but negatively correlated to SWC; consequently, we found no effect of increased snow on daily flux. Cumulative summertime Reco was not related to annual snowfall, but to water year precipitation (winter snow plus summer rain). Experimentally increased snowpack shortened the growing season length and reduced summertime Reco up to 40%. Soil CO2 was older under increased snow. However, we found no effect of snow depth on the R eco age because older C emissions were masked by younger CO 2 produced from the litter layer or plant respiration. In the High Arctic, anticipated changes in precipitation regime associated with warming are a key uncertainty for understanding future C cycling. In polar semideserts, water year precipitation is an important driver of summertime Reco. Permafrost C is vulnerable to changes in snowpack, with a deeper snowpack-promoting decomposition of older soil C. ©2014. American Geophysical Union. All Rights Reserved