Long-Term Changes in Soil Organic Carbon and Nitrogen under Semiarid Tillage and Cropping Practices.

Understanding changes in soil organic C (SOC) and total soil N (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, TX. Compared with adjacent grassland with no history of cultivation, stored SOC in the surface 0.30 m of HSM declined by 41% after 86 yr of cultivation, with half of the estimated changes occurring during the first 20 yr. In the HSM plots under a winter wheat (Triticum aestivum L.)-fallow rotation, SOC and TSN (0.0-0.152 m) were significantly greater with decreasing tillage intensity (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-till were not significantly (P = 0.396 and P = 0.261, respectively) different from stubble- mulch tillage 30 yr after treatments were imposed in 1984. Calculated export of N in wheat and sorghum grain from 1927 to 2013 from GT (1.6 Mg ha^-1) accounted for 80% of the difference between TSN in grassland and GT plots. 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 probably been assimilating NO₃-N located deeper in the soil profile to supplement N requirements. [ABSTRACT FROM AUTHOR]