|Baumhardt, Roland - Louis|
|SCANLON, BRIDGET - University Of Texas|
|BELL, JOURDAN - Texas A&M Agrilife|
|DAVIS, RONALD - Retired ARS Employee|
|IBRAGIMOV, NAZIRBAY - Uzbekistan Cotton Research Institute|
|JONES, ORDIE - Retired ARS Employee|
|REEDY, ROBERT - University Of Texas|
Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/2015
Publication Date: 12/11/2015
Citation: Schwartz, R.C., Baumhardt, R.L., Scanlon, B.R., Bell, J.M., Davis, R.G., Ibragimov, N., Jones, O.R., Reedy, R.C. 2015. Long-term changes in soil organic carbon and nitrogen under semiarid tillage and cropping practices. Soil Science Society of America Journal. 79:1771-1781.
Interpretive Summary: Organic carbon and total nitrogen are important constituents of a healthy soil for crop production; however how soil and crop management optimize these parameters are not fully understood. Scientists from USDA-ARS, Texas A&M AgriLife Extension Service, University of Texas at Austin, and Cotton Research Institute, Kibray District Taskent (Uzbekistan) evaluated long-term (30 - 86 yrs.) soil organic carbon (SOC) and total soil nitrogen (TSN) changes under dryland wheat-fallow and wheat-sorghum-fallow rotations on a clay loam soil in Bushland, Texas. SOC declined by 41% after 86 years of cultivation with half of the estimated change occurring during the first 20 years. Synthetic fertilizer N applications will need to be gradually increased with time to supplement native N sources. These results are of interest to farmers, crop consultants and extension agents who grow dryland wheat and sorghum or advise such growers.
Technical Abstract: Understanding long-term changes in soil organic carbon (SOC) and total soil nitrogen (TSN) is important for evaluating C fluxes and optimizing N management. We evaluated long-term SOC and TSN changes under dryland rotations for historical stubble-mulch (HSM) and graded terrace (GT) plots on a clay loam soil in Bushland, Texas. Adjacent grassland with no history of cultivation was assumed representative of pre-cultivation SOC and TSN levels. Stored SOC in the surface 0.30 m of HSM declined by 41% after 86 years of cultivation, with half of the estimated changes occurring during the first 20 years. In the HSM plots under a winter wheat (Triticum aestivum L.) –fallow rotation, SOC and TSN (0.0 to 0.152 m) significantly increased with decreasing tillage intensities (P < 0.05) in 1977 for treatments imposed in 1941. On GT plots under a winter wheat–sorghum (Sorghum bicolor (L.) Moench)–fallow rotation, SOC and TSN storage under no tillage were not significantly different from stubble-mulch tillage (P=0.396 and P=0.261, respectively) 30 years after treatments were imposed. Calculated export of N in grain from 1927 to 2013 (1.6 Mg ha-1) accounted for 80 percent of the difference between TSN in grassland and GT plots with no history of N fertilization. From 1927 to 1960, TSN decline exceeded N in exported grain by 1.2 Mg ha-1, and may explain present day accumulation of NO3–N at 1 to 6 m in the unsaturated zone. Since 1966, crops have likely been assimilating NO3–N located deeper in the soil profile to supplement N requirements. Eventually, accessible NO3–N sources will decline with time due to assimilation by the crops and continued leaching below the root zone.