SOIL-NITROGEN STATUS AS AFFECTED BY TILLAGE, CROPS, AND CROP SEQUENCES

Conservation tillage practices, including no-till (NT), reduce soil erosion and increase precipitation storage efficiency, but may decrease available soil N. We conducted studies at two sites to determine the comparative effects of NT and stubble mulch (SM) on the N supplying capacity of Pullman clay loam (fine, mixed, thermic Torrertic Paleustolls) cropped to continuous wheat (Triticum aestivum L.) (CW), continuous grain sorghum [Sorghum bicolor (L.) Moench] (CS), wheat-sorghum-fallow (WSF), and wheat-fallow (WF) sequences. AT one site, accumulation of NO3-N in the surface 1.2 m (in kg ha-1) was CW NT - 20, CW SM - 37, CS NT - 28; CS SM - 24, WSF NT - 34, WSF SM - 52, WF NT - 57, and WF SM - 60. Tillage significantly affected N accumulation only on the WSF sequence. Nitrate -N moved deeper into the profile under NT than under SM, indicating that differences in the root zone may have resulted from differential leaching rather than from differential nitrification. Yields under no-till and stubble much were similar except on continuous grain sorghum where nitrogen deficiency was encountered and stubble mulch outyielded no till.