Crop Residue Influences on Soil Carbon and Nitrogen in a Wheat‐Fallow System

Abstract Seven crop residue treatments were initiated in 1931 to measure long‐term residue management effects on soil organic matter in a wheat‐fallow cropping system on Pacific Northwest semiarid soils. There was evidence at this time of substantial organic matter (OM) loss during the first 50 years of wheat cultivation in the Great Plains. Organic carbon (C) and total (N) were measured at approximately 11‐year intervals over a 45‐year period to determine residue effects on the rate of change in soil OM content. Only the addition of 22.4 metric tons of manure/ha to straw residue before incorporation prevented a decline in soil N and C. The addition of 45 or 90 kg fertilizer N or of 2.2 metric tons of pea vines/ha to straw residue before incorporation reduced N and C loss when compared to straw only incorporation. Burning of straw in the fall following wheat harvest accelerated the loss of N but not C. Burning of straw in the spring just prior to tillage had no effect on N or C loss. Changes in N and C were primarily confined to the top 20 cm of soil. Soil C/N ratios in 1976 differed between treatments proportional to the rate of N loss; they were highest in burn or straw only treatments and lowest in the manure treatment. In all treatments, changes in soil N were best described by a linear function of time; slope within the linear function depended upon residue treatment. This linear function of time over a 45‐year period following approximately 50 years of previous cultivation suggests that 100 or more years may be required before N levels become stationary. Residual effects confirm that the new stationary level will depend on past crop residue management practices. Changes in soil C correlated highly with the amount of organic C supplied by each treatment, regardless of the different kinds of residue applied. Thus, changes in soil organic matter levels were controlled primarily by the amount of organic C supplied in crop residue. Regression equations indicate that approximately 5 metric tons of mature crop residue ha −1 year −1 are needed to maintain soil organic matter at its present level when cropped in wheat‐fallow rotation in this climatic zone.