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DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Carbon and Nitrogen Conservation in Dryland Tillage and Cropping Systems

^a USDA-ARS, J.P. Campbell, Sr., Natural Resources Conservation Center, 1420 Experiment Station Rd., Watkinsville, GA, 30677-2373 USA
^b USDA-ARS, Conservation and Production Research Lab., Bushland, TX USA

hschomberg{at}ag.gov

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.

Abbreviations: C_MIN, potentially mineralizable C • CS, continuous sorghum • CW, continuous wheat • F_WSF, fallow phase of WSF • N_MIN, potentially mineralizable N • NT, no-tillage • SM, stubble mulch tillage • SMBC, soil microbial biomass carbon • SMBC/SOC, SMBC portion of SOC • SMBN, soil microbial biomass nitrogen • SMBN/SON, SMBN portion of SON • SOC, soil organic carbon • SON, soil organic nitrogen • SPMAN, specific N mineralization activity of SMBN • SPRAC, specific respiratory activity of SMBC • S_WSF, sorghum phase of WSF • WF, wheat-fallow • WSF, wheat-sorghum-fallow • W_WSF, wheat phase of WSF

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