Availability of manure nitrogen (N) to crops is mitigated by many factors, including manure type and composition. Whereas relationships between dairy diets, milk production, manure N excretion, and urine N losses as ammonia have been documented, very little information exists on how diets impact fecal carbon (C), N content and partitioning, and how these factors impact fecal N mineralization and plant N uptake after application to soil. Feces from 24 to 63 dairy cows fed fourteen typical diets were incubated aerobically in a sandy loam and two silt loam soils, and soil inorganic N (IN) was determined periodically during a 365 d period. Feces from twelve of the fourteen diets were applied to the same soils and oat, sorghum, and sorghum ratoon dry matter (DM) and N uptake were determined over a 155 d period. Feces from cows fed alfalfa silage (AS)-based diets generally lead to higher soil IN levels than soils amended with feces from corn silage (CS)-based diets, especially in soils amended with feces from CS-low protein (LP) diets; feces from AS-based diets increased plant DM and N uptake; after application to a silt loam, feces from high protein (HP) diets resulted in greater soil IN levels than feces from LP diets; and feces from LP diets did not impact soil IN but decreased plant DM and N uptake. C:N ratios of applied feces were found to be significant predictors of plant DM and N uptake. Most observed diets had no impact on milk production but affected the amount, relative N partitioning, and composition of urine and feces. There appears, therefore, to be a range of dietary options that satisfy nutritional requirements of high-producing dairy cows and produce feces having differential effects on soil N mineralization and plant N uptake after application to soil.