Continuous cropping or decreasing the frequency of summer fallow (F) in cereal-based dryland rotations may have benefits other than greater water utilization and erosion control. We hypothesized that rotations with no fallow or minimum fallow frequency can produce more biomass and cover than the traditional winter wheat (Triticum aestivum L.)-summer fallow systems (W-F), and ultimately, greater amounts of soil organic matter (SOM). To this end, we evaluated changes in various pools of SOM at the 0- to 5- and 0- to 15-cm depths on a Weld loam (fine, smectitic, mesic aridic Paleustolls) that were caused by (i) decreasing fallow or increasing cropping intensities, (ii) specific rotations of the same length but with different crop sequencing, and (iii) accumulated residue and roots from reduced- or notillage from 1993 to 1997. Total soil organic carbon (SOC) and N for the 0- to 5-cm depth increased by =20% with continuous cropping rotations compared with W-F rotations. Particulate organic matter-carbon (POM-C) doubled, while POM-N, and soluble organic C (OC) increased by one third for the same comparison. At the 0- to 15-cm depth, SOC, POM-C, and POM-N did not differ among systems with fallow, nor among systems with cropping intensities greater than W-F. Thus, significant differences always existed between W-F and continuous cropping. Generally, fallow had a negative influence on SOC accumulation, and continuous cropping a positive influence on surface SOM. Changes in SOC did not correlate with yields in the five-year comparison of this ongoing study.
