NITROGEN AND CARBON DYNAMICS IN NO-TILL AND STUBBLE MULCH TILLAGE SYSTEMS

Soil nitrogen and organic carbon differences between no-till and conventional tillage systems are often dramatic and well documented, but these differences between no-till and stubble mulch tillage systems are more subtle. Our objective was to evaluate changes in soil inorganic N, organic N, organic C, surface soil moisture and grain yield on the southern High Plains as affected by stubble mulch and no-till tillage systems from 1988 to 1992. The cropping system was a sorghum (Sorghum bicolor (L.) Moench)-sorghum-fallow-wheat (Triticum aestivum L.) rotation conducted on a Pullman sandy clay loam (fine, mixed, thermic Torrertic Paleustoll) under dryland conditions near Clovis, NM. Tillage treatments were main plots and N fertilization treatments were subplots. Averaged over 5 yr and 37 sampling dates, the no-till treatment had 2.0 mg kg-1 less inorganic N, 40 mg kg-1 more organic N, and 617 mg kg-1 more organic C than the stubble mulch treatment. Although differences between tillage systems were generally small, they were most noticeable during sorghum planting and development. No-till unfertilized treatments often showed N deficiency symptoms during development. Nitrogen fertilization was more important than tillage system in determining sorghum and wheat yields in wet years. In dry years, N-fertilized stubble mulch treatments had the lowest yields. Since conversion to either stubble mulch or no-till in 1987, organic C levels under both systems have continually increased.