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Abstract Maintaining or improving soil organic matter has high priority in agriculture because of its beneficial effect on soil physical, chemical, and biological properties. Soil organic N and C were measured 44 yr after establishment of a long‐term experiment to evaluate tillage and fertilizer effects in a winter wheat ( Triticum aestivum L.)‐fallow rotation on a coarse‐silty mixed mesic Typic Haploxeroll. Main treatments consisted of three primary tillage systems, one conventional (moldboard plow) and two stubble mulch (offset disc, subsurface sweeps). Subplots consisted of six N treatments, 493, 728, 986, 1221, 1714, and 2207 kg N ha −1 applied over 44 yr. Organic N and C in the top 75 mm of soil were 26 and 32% higher, respectively, in the two stubble mulch systems than in conventional tillage, and equal below 75 mm. Stubble mulch plots contained 245 kg more N ha −1 than conventionally tilled plots, representing the conservation of 5.7 kg N ha −1 yr −1 . Nitrogen fertilization increased soil N linearly in all tillage treatments, with 18% of the applied N incorporated into the soil organic fraction. Applied N also increased soil C linearly on plots with previous S application. Soil C was higher on plots with no previous S than on comparable plots with previous S, however, which suggests an S deficiency that altered S, but not N, transformations in soil. Identical N fertilization effects on soil organic N and C in both stubble mulch and conventional tillage suggests that N transformations were the same in both systems.
Rasmussen et al. (Fri,) studied this question.