Tillage Effects on Long-Term Changes in Soil Organic Carbon and Nitrogen in the Texas High Plains.

Understanding the long-term changes in soil organic carbon (SOC) and total soil nitrogen (TSN) under reduced tillage is important for evaluating C sequestration potentials, optimizing N management, and sustaining or improving dryland crop yields. We evaluated SOC and TSN loss rates and patterns under differing tillage intensities and dryland rotations for long-term (30 to 84 yrs.) historical stubble-mulch (HSM) and graded terrace (GT) plots on a clay loam soil in Bushland, TX. Adjacent grassland with no history of cultivation was assumed representative of pre-cultivation SOC and TSN levels. An exponential decay equation was fitted to measured SOC and TSN to describe changes in inventories on the HSM plots from 1927 to 2013.  SOC inventories in the surface 0.30 m of HSM plots declined by 41% after 86 years of cultivation with half of the estimated changes occurring during the first 20 years. Tillage in the HSM plots under a winter wheat (Triticum aestivum L.) –fallow rotation significantly influenced SOC and TSN at 0.0 to 0.152 m (P < 0.05) in 1977 for treatments imposed in 1941with one-way disk tillage (DT) < stubble-mulch tillage (ST) < ST delayed until spring. Under a winter wheat–sorghum (Sorghum bicolor (L.) Moench) –fallow rotation, SOC and TSN inventories measured under no tillage (NT) were not significantly different from ST (P=0.396) 30 years after treatments were imposed in 1984. Lack of tillage effect likely results from only marginally (8 percent) greater aboveground biomass retained after harvest for NT compared with ST. Calculated export of N in harvested grain from 1927 to 2013 (1.6 Mg ha^-1) accounted for 80 percent of the difference between TSN in grassland and GT plots. From 1927 to 1960, change in TSN inventories exceeded N in exported grain by 1.2 Mg ha^-1, which likely explains the present day accumulation of NO₃–N at 1 to 6 m in the unsaturated zone. Since 1966, measured TSN inventories at 0.0 to 0.30 m have not appreciably changed likely because crops have been assimilating NO₃–N located deeper in the 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.