ABSTRACT
Concentration of soil organic carbon (SOC) under different cropping systems depends mainly on the types of tillage and residue management practices. Haas et al. (1957) reported an average decrease of 42% in the SOC concentration in the 15-cm depth after 30 to 40 years of dry land management at experiment stations in the Great Plains of the United States. Jenny (1933) proposed that SOC concentration would reach a new equilibrium level after 50 years of cultivation. Other studies have reported similar levels of decline in the concentration of SOC in the surface layer as
8.1 Introduction .................................................................................................. 189 8.2 Data and Methods ......................................................................................... 190
8.2.1 Study Area ........................................................................................ 190 8.2.2 Data Collection ................................................................................. 191 8.2.3 Data Analysis .................................................................................... 193
8.3 Results and Discussions ................................................................................ 193 8.3.1 Change in SOC Concentration from 1980 to 1999 ........................... 193 8.3.2 Change in SOC Concentration from 1999 to 2006 .......................... 195 8.3.3 Change in SOC Concentration as a Result of Changes in the
Cropping System ............................................................................... 195 8.3.4 Results of Changes in the Cropping System and the Increased
Use of Fertilizer over Time ............................................................... 197 8.4 Conclusions ................................................................................................... 198 Acknowledgments .................................................................................................. 199 Abbreviations ......................................................................................................... 199 References .............................................................................................................. 199
a result of cultivation (Reeder et al. 1998). Some studies have reported that increase in SOC concentration is correlated with the amount of crop residues that have been returned to the topsoil (Hendrix et al. 1998; Peterson et al. 1998; Janzen et al. 2004).
