Into the Deep: Effects of Agricultural Management on Deep Soil Carbon Dioxide

Abstract:

Sustainable agricultural methods must be identified and practiced in order to ensure food security for a growing global population while also mitigating the effects of climate change. Organic agriculture commonly supports more surface soil organic carbon (SOC) than conventional agriculture. However, it is unclear whether organically managed soil profiles act as a net sink or source of carbon dioxide (CO₂) a major greenhouse gas. Little is known about the role of soil CO₂ in subsoil carbon sequestration or CO₂ emissions to the atmosphere from organically managed soils. In a replicated study on Palouse silt loam soils we found that CO₂ concentrations were greater in organically managed soils than in conventionally managed soils at depths down to 1.5m throughout the year, particularly during the growing season. In the organically managed system, the CO₂ concentration averaged across all depths was over 23,000 ppm, and just under 7,000ppm in the conventionally managed systems. The greater concentrations in the organic system were primarily the result of greater CO₂ production at 60cm. CO₂ as gas and dissolved into soil water comprises only 0.22% of total C stocks in the organic system, and less than half that in the conventional system, but its agency within the soil C cycle may be critical. Elevated CO₂ in the soil depresses microbial oxidation of SOC, preserving soil SOC stocks. Elevated CO₂ in the subsoil also drives greater rates of chemical weathering, which is a long-term pathway for CO₂ sequestration in the hydrosphere and lithosphere. These results demonstrate that organic agriculture may remove additional C from the atmosphere via enhanced deep soil respiration, and chemical weathering processes, which leads to greater total rates of carbon sequestration than conventionally estimated.