Limits to soil carbon stability; Deep, ancient soil carbon decomposition stimulated by new labile organic inputs

Authors: BERNAL, BLANCA - Smithsonian Environmental Research Center; MCKINLEY, DUNCAN - Us Forest Service (FS); HUNGATE, BRUCE - Northern Arizona University; White, Paul; MOZDZER, THOMAS - Bryn Mawr College; MEGONIGAL, J - Smithsonian Environmental Research Center

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/2016
Publication Date: 4/12/2016
Publication URL: http://handle.nal.usda.gov/10113/62365
Citation: Bernal, B., Mckinley, D.C., Hungate, B.A., White Jr, P.M., Mozdzer, T.J., Megonigal, J.P. 2016. Limits to soil carbon stability; Deep, ancient soil carbon decomposition stimulated by new labile organic inputs. Soil Biology and Biochemistry. 98:85-94.

Interpretive Summary: The soil contains about one-third of the total land-based carbon on earth. Releasing this carbon into the atmosphere could be a augmentor of climate change. Deeper pools of carbon, below 1 m, are thought to age slower and have a longer residence time. The objective of the study was to examine these deep carbon pools present in soil down to 3 m. Soil samples were evaluated in the laboratory alone or with additions of chemical typically found in plant roots. Carbon and nitrogen tracers were used to examine organic matter cycling in these deeper soils. These tracers induced changes to the soil microorganisms present in the soil, including increasing their enzyme production. This large increase in enzyme abundance (13X greater) could lead to a large amount of organic carbon becoming carbon dioxide in the atmosphere, should these soils be exposed to surface conditions. However, the carbon and nitrogen tracers added did not generate identical results. This could be an indication of very specific conditions needed for carbon cycling to be activated by additional substrate (e.g., tracers). Overall, these tracers demonstrated that the carbon in these deep soil horizons could be very labile if exposed to increased nutrients or temperate and moist surface conditions.

Technical Abstract: Soil carbon (C) pools store about one-third of the total terrestrial organic carbon. Deep soil C pools (below 1 m) are thought to be stable due to their low biodegradability, but little is known about soil microbial processes and carbon dynamics below the soil surface, or how global change might affect these pools. We examined the dynamics of deep C pools and their response to changes in substrate availability throughout the profile of a coarse textured spodosol (0–0.1, 1–1.3, and 2.7–3 m). We incubated these soils with substrate amendments similar to those commonly found in the rhizosphere (glucose, alanine, plant leaf litter), crossed with an inorganic nitrogen (N) treatment. CO2 experiments at this site found increased belowground biomass under elevated CO2 at depth, which could introduce more C throughout the soil profile, particularly in deep soils. The organic substrates were isotopically labeled (13C), allowing us to determine the source of mineralized C and assess the potential priming effect caused by substrate addition. Enzyme activity increased as much as 13 times in the deeper horizons after the addition of the organic substrates, even though the deepest horizon had no detectable microbial biomass and microbial phospholipid fatty acids before the experiment. The deep horizon yielded the largest priming response under alanine, indicating that deep soil C pools likely host dormant microbial communities that become active in response to input of organic substrates. Inorganic nitrogen amendments significantly decreased the priming effect of the organic substrates suggesting that decomposition was not N limited. However, alanine (i.e., labile C and organic N) yielded the highest priming effect at every soil depth, indicating the importance of differentiating between the effect of organic and inorganic N on decomposition. Differences in soil C mineralization induced by the substrates increased with depth, suggesting that portions of the soil profile can respond differently to organic inputs. Our findings indicate that the deep soil C pools might be more vulnerable to climate change factors than previously thought, which could potentially influence net CO2 exchange between the land and the atmosphere.