Carbon dynamics as a function of land use in eastern Kansas : implication for carbon storage in mollisols.

Abstract

Soils are recognized as an important source and/or sink for atmospheric CO2. The role of soil in the carbon cycle is currently a topic of renewed focus, as causes of possible global climate change and the role of greenhouse gasses continue to be debated. In either capacity, source or sink, we have limited understanding of the effects of soil erosion and land use practices designed to enhance soil carbon assimilation at a variety of spatial scales. Because eastern Kansas contains abundant carbon-rich soils (Mollisols) and holds a relatively short agricultural land use history (150+/-years), it presents an ideal environment for study of land use effects on soil carbon dynamics and erosion recovery. The site identified for this study (near Gridley, in Coffey County, Kansas) contains untilled soil with native prairie vegetation adjacent to an area that had been farmed with traditional agricultural tillage methods in the past. The land previously farmed (from the late 1880's) had been seriously eroded, abandoned from row crop agriculture, and reseeded with native grasses and forbs. This portion of the site has remained unfarmed since the late 1940's. This study investigated the changes in soil carbon dynamics present at the site as a result of changes in land use practices. Sixteen soil cores were described along two transects that traversed the previously tilled and untilIed portions of the study area to characterize spatial changes in soil thickness and horizonation. Ninety-two sample sites provided 197 systematic and random soil samples. These samples were analyzed by the Kansas State University Soil Testing Laboratory for soil organic carbon, total carbon and soil nitrogen data Surface bulk density data were gathered from the native and the eroded soil using the core method. Topographic data were determined with transit and stadia rod in seven transects spaced every ten meters. Bedrock data were determined from soil characterization core information. All data, including recorded GPS information, were entered into spreadsheet form for use in map making and graphing, as well as for statistical analysis. Comparison of soil thickness data lead to the conclusion that erosion, accelerated by the activities associated with traditional farming methods, physically removed the original A horizon and the upper part of the original AB horizon from the northern (eroded) part of the study site. Concentrations of nitrogen, total carbon, and total organic carbon were found to be significantly higher (n = 0.05) in samples from the upper 40 cm of the untilled soil than in samples from the upper 40 cm of the eroded soil. Concentrations of the same three components in samples taken from the 41-80 cm interval at each site yielded significantly higher concentrations oftotal organic carbon in the native soil. There were, however, no statistically significant differences in the nitrogen nor in the total carbon concentration between the two land uses. Comparisons were made of the 0-40 em intervals of the eroded section with the 41-80 em intervals of the untilled soil's samples to determine whether or not a statistically significant difference exists for the same three soil elements. The results indicated that the percent of the three elements from the surface horizons of the eroded soils were significantly different (higher) from the native soils. If erosional processes removed the original surface horizon from the northern portion of the study site, then the present surface there is actually the sub-surface (Bt) horizon of the original soil, and the concentrations of nitrogen, total carbon, and total organic carbon would be similar. Similarities or differences could be determined by comparing the surface horizon of the previously eroded area with the sub-surface (Bt) horizon of the untilled soil. It is concluded that while traditional agriculture is responsible for many soil degrading impacts, it may have actually contributed to sub-surface soil quality by mixing surface soil with underlying horizons. It is also possible that erosion may not have removed all organic matter originally present prior to tilling activities. Additionally, it is concluded that the soil is showing clear signs of recovery since having been left unplowed for the past 50+ years.