Effects of Organic Agriculture on the Soil Carbon Stock

Abstract

The soil carbon (C) stock is comprised of the soil inorganic carbon (SIC) and the soil organic carbon (SOC) stock. A site-specific steady state equilibrium soil C stock evolves under natural conditions depending on the balance between soil C inputs (plant residues) and losses (decomposition, erosion, leaching). The SIC stock is perceived as being less dynamic than the SOC stock with uncertain effects of organic agriculture (OA) on SIC sequestration rate, and not the focus of agricultural soil and land-use management. In contrast, the SOC stock receives increasing attention due to its importance for the global climate and soil health. However, increases in the SOC stock may also alter the greenhouse gas (GHG) balance and this must be addressed in the assessment of soil C sequestration practices to mitigate climate change. The historical loss of SOC due to the conversion of natural ecosystems to agroecosystems provides an opportunity to use soil and land-use management practices to partially replenish lost SOC stocks. Topsoil (0–15 cm depth) SOC stocks have been shown to increase under OA management by 1.98–3.50 Mg C ha⁻¹ compared to nonorganic management. But the addition of exogenous C (e.g., with manure) for this improvement and SOC sequestration for climate change adaptation and mitigation may be important. Compared to nonorganic management, topsoil SOC sequestration rates did either not differ or were 0.29–0.45 Mg C ha⁻¹ year⁻¹ higher under OA, respectively. However, assessments of SOC sequestration and stocks for the entire rooted soil profile are scanty but needed to fully address long-term effects of agricultural management on SOC. Lower primary soil C inputs due to lower OA yields and higher losses by tillage compared to conventional no-tillage (NT) system may result in lower steady state equilibrium SOC stocks in OA systems. There is some evidence that root C allocation is higher under OA than that under nonorganic management. More agricultural soils will be managed in the future by OA driven by increasing consumer demand. The net effects of increased soil and land-use management for OA on the global soil C stocks must be critically assessed also in relation to long-term field experiments to support the design of climate-smart and climate resilient agroecosystems. Therefore, the objectives of this chapter are to describe in detail what processes and practices result in changes in SIC and SOC stocks and sequestration in soils under OA management.