Tillage and Long Term Changes in Soil Carbon and Nitrogen Under Dryland Agriculture Production.

Soil organic carbon and total nitrogen were evaluated in a clay loam soil in historical dryland wheat-fallow field plots established in 1941 with tillage treatments maintained through 1977. In 1977, soil samples were collected and sealed in airtight containers, and all plots were thereafter managed identically under stubble-mulch tillage. In 2013, the plots were re-sampled and analyzed along with the 1977 samples for total soil organic carbon (SOC) and total nitrogen (TN). Native grassland with no history of cultivation was also sampled (0 – 0.6 m) to assess SOC and TN levels representative of pre-cultivation conditions. Lastly, an adjacent field managed under stubble-mulch tillage and in a wheat-sorghum-fallow rotation since 1949 was also evaluated for SOC and TN (0 – 0.6 m). The long-term fate of mineralized N in the soil profile was evaluated in this field, which has detailed management records available since 1937, deep borehole inventories of NO₃–N, and no history of fertilizer use. In 1977, total soil organic carbon (SOC) was significantly affected by tillage (P < 0.05) at the three surface depth increments (0 – 0.076; 0.076 – 0.152; 0.152 – 0.229 m) in the historical dryland wheat-fallow plots. Delayed stubble-mulch tillage, where tillage was withheld until the spring after harvest, had consistently greater organic carbon at all depths compared with stubble-mulch tillage and the one-way disk plow. Mean organic carbon contents in 1977 for all tillage treatments and depths (0 – 0.305 m) was 9.5 g kg^-1 and declined to 8.8 g kg^-1 in 2013. Changes in SOC during this 36-year period were not tillage dependent. Reflecting the change to uniform tillage practices in 1977 and thereafter, significant tillage effects on SOC were absent in 2013 although, similar to 1977, carbon levels increased with decreasing tillage intensity (disk < stubble-mulch tillage < delayed stubble-mulch tillage) at all depths. Total N was influenced by tillage only at the surface depth increment (0 – 0.076 m) and, in contrast to SOC, did not significantly decline from 1977 to 2013. Earlier analyses of these plots sampled in 1966 suggest no changes in TN for the past 47 years. Total carbon and nitrogen levels in the adjacent field in the wheat-sorghum-fallow rotation were not significantly different from those measured in 2013 for the historical dryland wheat-fallow field plots. Assuming that the native grassland total N profiles to 0.6 m are representative of pre-cultivation conditions, approximately 1.95 Mg ha^-1 of N was mineralized in these soils since they were first cultivated in 1927. Approximately 1.73 Mg ha^-1 of N was exported from the field during the 86-year period. Borehole inventories of NO₃–N beneath this field (0 – 14 m) show a distinct bulge at 1.5 – 5 m depth. This analysis indicates that approximately 220 kg ha^-1 NO₃–N was leached below  ~1 m during this 86-year period, most of which likely occurred during the first half of the century when mineralization rates were high and grain yields were low. Present-day crops are likely using NO₃–N located deep 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.