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SOIL & WATER MANAGEMENT & CONSERVATION

Cropping Systems Effects on Improving Soil Carbon Stocks of Exposed Subsoil

Dep. of Agronomy, Iowa State Univ., Ames, IA 50010-1010

^* Corresponding author (malkaisi{at}iastate.edu).

Removal of topsoil from glacial-till-derived soils exposes unproductive subsoil that is low in soil organic carbon (SOC) and nutrient availability. The overall objective of this study was to evaluate the long-term impacts of a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation and a managed switchgrass (Panicum virgatum L.) system in improving soil C stocks of exposed subsoil. The experimental layout of this study was a randomized complete block design with four replications. Field soil CO₂ emissions, potential crop residue total C input, microbial biomass C, soil C fractions, soil incubation CO₂ emission, and soil bulk density were measured from switchgrass burned annually (SA), switchgrass burned every 5 yr (S5), and corn–soybean rotation (CS) cropping systems in 2003 and 2004. During both years of the study, the SA cropping system had the greatest cumulative soil CO₂–C emissions, followed by the S5 and CS cropping systems, respectively. The S5 cropping system produced 3.47 and 2.33 Mg ha^–1 more aboveground biomass than soybean and corn, respectively. Switchgrass had 14 Mg ha^–1 greater root biomass than corn or soybean. As a result, potential C input from the S5 switchgrass treatment was 6.08 and 6.71 Mg ha^–1 greater than corn and soybean, respectively. Microbial biomass C was 200 0reater in the switchgrass cropping systems (S5 and SA) than in the corn–soybean rotation. The switchgrass system is an effective strategy for improving exposed subsoil C fractions and providing greater potential C input through a more extensive root system than the corn–soybean rotation.

Abbreviations: CS, corn–soybean rotation • DOY, day of the year • MBC, microbial biomass carbon • POM-C, particulate organic matter carbon • S5, switchgrass burned every five years • SA, switchgrass burned annually • SOC, soil organic carbon • TC, total carbon • TN, total nitrogen