ABSTRACT
Over the last several decades, growers in California’s Central Valley, as in many other agricultural areas throughout the world, have become increasingly aware of the importance of soil organic matter (SOM) in relation to sustained soil quality or function (West Side On-Farm Demonstration Project Participant Survey, 1999; Romig et al., 1995; Magdoff, 1992). Soil carbon (C), which typically constitutes about one half of SOM, is closely linked to many desirable soil physical, chemical, and biological properties that are associated with enhanced soil productivity and quality (Reicosky, 1996b; Ismail et al., 1994). More recently, awareness has also increased concerning the negative impact that tillage has on SOM storage (Reicosky, 1995; Jackson, 1998). While moderate tillage may provide more favorable soil conditions for crop growth and development and weed control over the short term (Carter, 1998), intensive tillage of agricultural soils has historically led to substantial losses of soil C that range from 30 to 50%. Conventional tillage practices disrupt soil aggregates exposing more organic matter to microbial degradation and oxidation (Reicosky, 1996a), and are one of the primary causes of tilth deterioration over the long term (Karlen et al., 1990). Micro- and macrochannels within the soil, created by natural processes, such as decaying roots and worms, may also be destroyed by tillage (Carter, 1998). Deep tillage, as is customarily done as a routine soil preparation operation, is also costly and requires high energy and increased subsequent effort to prepare seedbeds. A recent survey by Jackson (1998), documenting a 40% decline in SOM since intensive tillage practices began in the Salinas Valley, confirms the conclusion drawn from other long-term crop rotation studies (e.g., the Morrow Plots at the University of Illinois; the Sanborn Field Plots in Columbia, MO; and the Columbia Plateau Plots near Pendleton, OR) that intensive tillage typically leads to decreased soil C via gaseous CO2 emissions in virtually all crop production systems (reviewed by Reicosky, 1996b). There is mounting evidence as well as concern that this C source has been a significant component of the historic increase in atmospheric CO2 (Wilson, 1978; Post et al., 1990), and the potentially associated greenhouse effect (Lal et al., 1998) that is attracting intense attention worldwide.
