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

Soil nitrogen and organic carbon differences between no-till and conve ntional tillage systems are often dramatic and well documented, but th ese 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 s outhern High Plains as affected by stubble mulch and no-till tillage s ystems from 1988 to 1992. The cropping system was a sorghum (Sorghum b icolor (L.) Moench)-sorghum-fallow-wheat (Triticum aestivum L.) rotati on conducted on a Pullman sandy clay loam (fine, mixed, thermic Torrer tic Paleustoll) under dryland conditions near Clovis, NM. Tillage trea tments were main plots and N fertilization treatments were subplots. A veraged 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 noticeabl e during sorghum planting and development. No-till unfertilized treatm ents often showed N deficiency symptoms during development. Nitrogen f ertilization was more important than tillage system in determining sor ghum and wheat yields in wet years. In dry years, N-fertilized stubble mulch treatments had the lowest yields. Since conversion to either st ubble mulch or no-till in 1987, organic C levels under both systems ha ve continually increased.