1.1 Background
Soil contains the largest terrestrial reservoir of organic C, i.e., approximately 1,500 Pg (Schlesinger and Andrews, 2000), or similar in quantity to that in the atmosphere (875 Pg) and plant biomass (560 Pg) combined. Historical losses of soil organic C (SOC) have been dramatic following the cultivation of pristine land uses for agricultural production (Mann, 1986; Davidson and Ackerman, 1993). Loss of SOC has primarily occurred in the enriched surface soil as a result of erosion of exposed soil and oxidation by soil microorganisms from the long fallow periods without crop growth, low C return to soil, and frequent disturbance with tillage (McGregor et al., 1975; Lowrance and Williams, 1988).
The advent of conservation agricultural systems toward the end of the 20th century provided some relief to the general decline of SOC around the world (Langdale et al., 1979; Endale et al., 2000). Conservation agricultural systems focus on several key elements important for managing SOC, i.e., minimal disturbance with tillage, maximum cover with cover cropping and surface residue management, and optimizing biodiversity with crop rotations, animal manure inputs, and livestock grazing when appropriate.
Minimizing soil disturbance with no tillage (NT) and variants of reduced tillage promotes stabilization of soil aggregation and accumulation of SOC nearest the surface (Franzluebbers et al., 1999; Jarecki and Lal, 2003). Conservation tillage systems were originally developed to slow the insidious threat of erosion (Langdale et al., 1979) and to conserve soil water (Blevins et al., 1983). However, with time, additional advantages became evident, including significant surface SOC storage, rejuvenation of internal nutrient cycling, and promotion of soil biological activity and diversity (Doran, 1987; Blevins et al., 1998). Worldwide adoption of conservation tillage systems occurred with machinery development, effective herbicides, and knowledge of labor and fuel savings that could be harnessed (Kassam et al., 2019).
Cover cropping provides significant biomass C input to soil, which elevates opportunities for a portion of this C to be retained in soil. Cover cropping combined with NT allows significant drawdown of soil water during wet periods to promote a more favorable environment for soil biological communities, as well as preservation of soil water near the soil surface during dry periods following termination of cover crop growth by avoiding transpiration with the residue mulch cover (Dabney, 1998). Increasing and maintaining SOC is one of the most common targets of research conducted with cover crops (Poeplau and Don, 2015). The effect of cover crops on SOC content and a diversity of ecosystem services has recently been evaluated with good associations (Schipanski et al., 2014).
Diverse crop rotations can alter the quantity and quality of C inputs to soil, which can influence the resident soil microflora and fauna responsible for the processing of organic matter leading to SOC accumulation. Legumes that produce abundant biomass without the need for synthetic N fertilizer provide both organic C and N substrates to promote SOC accumulation (Zotarelli et al., 2012). Rotation of grain crops with short- and long-term pastures allows SOC to accumulate to greater levels than simple crop rotations with cash crops only (Franzluebbers et al., 2014).
Conservation agricultural systems have generally been shown to improve SOC in the traditional plow layer, i.e., 0–30 cm depth (Balesdent et al., 2000; Virto et al., 2012), and more typically in the surface 0–10 cm depth (Franzluebbers, 2010). From a meta-analysis of tillage comparison studies around the world, Angers and Eriksen-Hamel (2008) found significantly greater SOC in the upper 20 cm of soil across studies under NT than under full-inversion tillage, but lower SOC concentration at depths of 21–35 cm below the surface. Overall, SOC concentration was significantly greater under NT than under full-inversion tillage, but the magnitude of change declined with increase in depth of sampling. Statistical detection of SOC difference between tillage systems is oftentimes challenging with much reduced SOC concentration and inherently greater coefficient of variation with deeper depths (Franzluebbers, 2010; Syswerda et al., 2011).
Soil organic C is a key indicator of agricultural productivity, and it controls many other ecosystem services important to ecological stability and environmental sustainability. How tillage management influences SOC has been a topic of interest for decades, particularly as it relates to location of sequestered C within the soil profile. A review of SOC storage with adoption of NT or reduced tillage systems compared with conventional tillage was performed here to ascertain the likelihood of where in the soil profile that changes in SOC might occur, and if there were unique edaphic and environmental circumstances that led to such changes. This review builds upon previous efforts using a depth distribution approach to SOC sequestration, which can help isolate unique soil formation factors and avoid bias due to assumed initial conditions, especially among chronosequence studies, to which assumed similar soil conditions may not always be appropriate (Franzluebbers, 2021).
Literature was searched for reports of long-term tillage management systems having measurements of SOC in multiple depth increments within the soil profile to at least 40 cm depth and extending to ~100 cm depth if available. The objectives were to (1) assess available evidence for SOC...
