Dirty Carbon: Relationship of Farming Practices and Soil Organic Carbon Content
Abstract
Soil degradation stands as a premier global issue that threatens humanity’s food supply, exacerbates deforestation, quickens desertification, and lowers crop yields. Soil organic carbon (SOC) is known to increase crop yields by returning nutrients to the soil, making these nutrients more bioavailable to plants, and increasing water retention rates—feature that reduces the per-acre cost of growing crops. This study examines the influence of a variety of conventional and alternative farming practices on SOC content by aggregating previously conducted studies that compare farming practices across a variety of soil types. The farming practices examined are as follows: conventional and reduced tillage, cover cropping and polycropping, and manure and crop residue application. Initial findings indicate that decreasing fallow periods and plant mass removal increases SOC carbon in the top 10cm of soil, but only the deep plowing used in conventional tillage transfers this increased SOC into the lower layers of the soil. Therefore, the emerging trend of no-till farming may warrant reconsideration, as occasional conventional tilling could help rebuild the deep organic layers seen in undisturbed prairie lands. Finally, the most beneficial methods for increasing SOC in the Treasure Valley’s aridisol soils are presented.
Executive Summary
Introduction. Both the threat of climate change and a rising global population stand as evidence for the need to increase carbon sequestration in soils. Soil organic carbon (SOC) is a key component of soil fertility, but it is depleted by traditional farming practices. This study synthesizes knowledge about the role of SOC in soil health for the purpose of informing stakeholders in soil fertility, and to show the importance of making informed decisions about farming practices such as tillage, amendments, and cropping intensity that influence SOC content. The Life of Carbon in the Soil. SOC is formed as microbes and other soil biota decompose plant matter. SOC improves soil properties such as porosity, density, and aggregation while feeding soil organisms. Conventional agriculture depletes SOC within a matter of decades. Methods. 22 studies including 6 meta-analyses were examined to determine the impact of conventional tillage versus no-tillage, organic versus inorganic amendments, and monoculture versus crop rotations and cover cropping. Tillage: Conventional Deep Plowing and No-Till. Research generally agrees that no-till increases SOC in the top 10cm of soil while the SOC content of the whole soil profile is not significantly different in no-till versus conventional till systems. However, the impact of tillage on SOC is complicated by climate, soil type, crop type, and other factors. Amendments: Fertilizer, Residue, and Manure. Organic amendments, especially cattle manure, increase SOC and provide nutrients to crops, resulting in equivalent or increased crop yields relative to the use of inorganic fertilizer alone. Cropping Intensification. Relative to monocultures, both rotation and cover cropping increases SOC content. Cover cropping in particular is a powerful method to increase SOC and improve soil texture both in no-till and conventional till systems. Recommendations for Treasure Valley Farmers. Cover cropping is the most useful sustainable farming practice for farmers in the semi-arid Treasure Valley. It decreases soil erosion, can benefit water retention, and reduces the need for expensive inorganic fertilizers. Conclusions. SOC is necessary for the continued fertility of farmland. Sustainable practices such as organic amendment application and cover cropping must be identified and optimized for particular climate and soil types such that farmland can continue to produce enough food for a rising global population.