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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. |
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