| | Description: | Soil organic carbon (SOC) sequestration is important to climate change and cropland agriculture. Crops naturally use the greenhouse gas, carbon dioxide (CO2), from the atmosphere; the greater the crop productivity, the greater the amount of CO2 used. Agronomic practices that enhance sequestration of crop biomass in soil as SOC also enhance removal of CO2 from the atmosphere, and improve and sustain soil fertility. To effectively reduce the concentration of CO2 in the atmosphere and mitigate climate change, sequestration of SOC must be long term, defined as decades or longer. This report presents a review and synthesis of scientific understanding of SOC sequestration, based on the history and genesis of soils and vegetation in Illinois, and the response of SOC and crops to agronomic practices. Recommendations for future cropland SOC research are made. The scientific literature is reviewed in light of the Illinois conditions affecting the five interactive soil-forming factors that are widely recognized (biology, parent material, climate, topography, and time). The literature also shows that human activity can be considered a sixth soil-forming factor. Native American land-use practices of whole ecosystem manipulation were important in governing soil formation and SOC contents in Illinois, as were the land-use practices of the settlers who displaced them. An important finding of this work is that to reduce the atmospheric CO2 content and sustain cropland agriculture, SOC must be sequestered throughout the soil profile. The modern literature reports SOC increases when tillage is changed from conventional to conservation tillage practices. However, SOC measurements are surficial, usually no more than the top 30 cm, with most of the C being sequestered in the top 15 cm. The unstated assumption in the modern literature is that surficial SOC changes represent all the SOC changes in the soil profile. This work shows that the SOC losses in the deeper soil layers may overwhelm surficial SOC increases. In order to assert that C is being sequestered in the soil, the whole-soil profile must be considered. It is recommended that future research into SOC sequestration be conducted from a whole-plant/whole-soil perspective in a soil genesis context using the following strategies. Mine the Literature. Most of the literature needed to provide the requisite whole-plant/whole-soil perspective and soil genesis context is scattered and not organized, summarized, or synthesized in the current SOC sequestration literature. The evolution of SOC sequestration research has been a narrowing of perspective away from the more holistic whole-plant/whole-soil perspective of the foundational agronomic literature to the perspective of the near-surface soil layer. This vast foundational literature needs to be located, restored, and incorporated with the current literature on crop rhizosphere and C and nutrient cycles throughout the whole-soil profile, soil genesis, soil fertility, subsoil amelioration, and other literatures to be organized, summarized, and synthesized into the SOC sequestration literature. Long-term Whole Plant/Whole Soil Monitoring and Assessment. Assessment of the effects of agronomic practices on SOC must be expanded to include the whole-soil profile. Improved estimates of presettlement soil SOC contents are needed to better assess SOC loss and SOC sequestration potential of Illinois' prairie and forest soils. The magnitude and swiftness with which natural factors govern SOC contents need to be better identified and quantified while incorporating a more comprehensive definition of soil aging along with consideration of presettlement and postsettlement anthropogenic landscape management practices as soil-forming factors. SOC Sequestration Research. Finally, research on how agronomic practices can increase SOC throughout the soil profile needs to be conducted from a whole-plant/whole-soil perspective in a soil genesis context. This report indicates that the optimal way to sequester SOC is to convert land back to native prairie, burn frequently, add fertilizers, and remove anthropogenic surface and subsurface drainage. Such an approach is not practical. Constraints on optimizing cropland SOC sequestration include: 1) the need to maintain good soil drainage in Illinois soils for timely spring planting that allows for growth of long-season corn hybrids and soybean varieties; and 2) maintaining soil-nutrient levels that do not result in water-quality issues. Within these constraints, the authors hypothesize that SOC sequestration can best be done by 1) developing balanced soil-fertility programs and other agronomic practices that restore soil nutrients to levels optimum for plant growth, promote movement of plant nutrients throughout the root zone using organic and/or inorganic carriers, and promote deep rooting of plants with minimal mechanical disturbance of the soil by tillage; and 2) developing chemical pest control programs that minimize the effects of pesticides on soil bacteria, and microfauna and macrofauna, thus promoting conversion of biomass to SOC, pedoturbation and net movement of SOC through the soil profile, and creation of soil structure and aggregation that optimize biomass production and conversion to stabilized SOC. Research on the development of these practices must include evaluation of nutrient movement into ground and surface waters. Losses of SOC have occurred on the order of the century time scale. SOC sequestration and the measure of its success (permanence of SOC sequestration) are also necessarily measured on the order of the century time scale. Therefore, long-term (20- to 30-year) agronomic SOC sequestration research at both the farm and individual plot level needs to be designed and conducted for hypothesis and model testing, as well as evaluation of the permanence of SOC in the surface and whole-soil profile. Even longer term research needs to be designed and conducted for hypothesis refinement and for monitoring. | |
