Staggenborg, Scott (Kansas St. Univ., Dept. of Agronomy, 2004 Throckmorton Hall, Manhattan, KS, 66506; Phone: 785-532-7214; Fax: 785-532-6094; Email: sstaggen@ksu.edu)

Intensifying No-till Cropping Systems Increases Soil Carbon Levels: A Modeling Approach

S.A. Staggenborg *, C.W. Rice, R.G. Nelson, J.R. Williams

Cropping systems research faces numerous challenges, one of which is the time required to conduct field experiments. Soil carbon research is faced with many of the same challenges since changes are subtle and require long-term monitoring. Crop simulation models represent a tool to address these challenges as often the only limit is measured weather data at a given location. The objective of this research was to evaluate the impact of intensifying crop rotations on soil carbon dynamics across Kansas through a crop and soil simulation framework. The two cropping systems, a traditional and an intensified rotation were simulated at three locations in Kansas. Brown, Reno and Greeley counties, in eastern, central and western Kansas respectively, were selected as they represent the range of growing conditions experienced in the state. The dominant crop rotations are corn-soybean in Brown county, continuous wheat in Reno county, and wheat-fallow in Greeley county. Simulated intensified rotations were soybean-corn-wheat-double crop soybean-corn in Brown county, wheat-grain sorghum in Reno county, and wheat-sorghum-fallow in Greeley county. These rotations were simulated using CERES-Maize, CERES-Sorghum, CERES-Wheat and CROPGRO-Soybean in the DSSAT framework from 1960 to 1990 using measured weather data from each site. Grain and dry matter yields were used as input into ROTH-C 26.3, a soil carbon simulation model. Three soils from each county were used in the simulations and were selected based on cropland acres according to the NRCS soil survey. No-till and conventional till systems were simulated by modifying the amount of dry matter returned to each system and adjusting yields for the tillage systems in Greeley county. At all three sites, intensifying the crop rotation increased soil carbon levels compared to the conventional rotation. In Brown county, intensifying the corn-soybean with wheat and double crop soybeans increased ending soil carbon levels 35%. This occurred largely as a result of reducing fallow periods and replacing soybean, a low carbon contributing crop, every other year with wheat, a high carbon contributing crop. In Reno county, the inclusion of grain sorghum in an annual wheat rotation increased final soil carbon only 6% compared with continuous wheat. This is not unexpected as grain sorghum and wheat are similar in contributing carbon into a system, so the only advantage to adding grain sorghum is the elimination of fallow periods, especially during the summer months. In Greeley county, adding grain sorghum to a wheat-fallow cropping system increased final soil carbon levels comparatively by approximately 50%. However, regardless of rotation used, soil carbon levels declined over the 30 year simulation period compared with the initial soil carbon levels. This loss in soil carbon is attributed largely to the extended fallow periods experience in both cropping systems along with the low productivity levels experienced in this semi-arid environment. The largest impact on soil carbon levels after 30 years, from a management perspective was the adoption of no-till systems. In all three environments and across soil types, the simulated no-till system resulted in soil carbon levels more than twice those simulated in the conventionally tilled system. These results illustrate the potential of modeling crop and soil systems to study management impacts on soil carbon and support field work that indicates that intensifying crop rotations, eliminating fallow periods and the adoption of reduced or no-till production practices increase soil carbon levels with the final levels being dictated by the growing conditions.