Carbon and Nitrogen Conservation in Dryland Tillage and Cropping Systems

Soil C and N greatly influence long‐term sustainability of agricultural systems. We hypothesized that cropping and tillage differentially influence dryland soil C and N characteristics in the Southern High Plains. A Pullman clay loam (fine, mixed, thermic Torrertic Paleustol) cropped to wheat ( Triticum aestivum L.)–sorghum [ Sorghum bicolor (L.) Moench]–fallow (WSF), continuous wheat (CW) and continuous sorghum (CS) under no‐tillage (NT), and stubble mulch (SM) was sampled at three depths to determine soil C and N characteristics. For CW, CS, and WSF phases (F WSF , S WSF , W WSF ), soil organic C (SOC) averaged 10.6 to 13.1 kg m −3 and was greatest for CW. Carbon mineralization (C MIN ) at 0 to 20 mm was 30 to 40% greater for CW and F WSF than for CS, S WSF , or W WSF Cropping system by depth influenced soil organic N (SON) with greatest SON at 0 to 20 mm in CW (1.5 kg m −3 ). At 0 to 20 mm for SM and NT, SOC was 9.9 and 12.5 kg m −3 , soil microbial biomass C (SMBC) was 0.80 and 1.1 kg m −3 , and soil microbial biomass N (SMBN) was 0.14 and 0.11 kg m −3 Also at 0 to 20 mm, NT had 60% greater C MIN , 11% more SMBC as a portion SOC, and 25% more SON compared to SM. Summed for 0 to 80 mm, NT had more SOC (0.98 vs 0.85 kg m −2 ) and SON (0.10 vs 0.9 kg m −2 ) than SM, and CW had greater or equal C and N activity as other systems. Negative correlations between yield and SOC, SMBC, C MIN , SON, and SMBN indicate N removal in grain negatively affects active and labile C and N pools. Under dryland conditions, C and N conservation is greater with NT and with winter wheat because of less soil disturbance and shorter fallow.