Composted dairy manure (CDM) is among the management practices used in transitioning from a conventional to an organic agricultural system. The objectives of this study are to evaluate the impact of several organic nitrogen (N) sources on: (i) soil organic C (SOC) and soil total N (STN) content; (ii) soil C and N distribution among soil fractions; and (iii) N mineralization. This study was initiated in 2007 on a recently renovated alfalfa (Medicago sativa
L.) field located at the Agricultural Research, Development and Education Center near Fort Collins, Colorado. The soil type is a Fort Collins loam (fine-loamy, mixed, superactive, mesic Aridic Haplustalfs). Alfalfa and sainfoin (Onobrychis viciifolia
Scop.) were interseeded with the grass mixtures as organic N sources. Three grass treatments were established with and without alfalfa or sainfoin. The CDM was also applied to the grass and to grass-alfalfa mixture at a rate of 22.4 Mg ha−1
in 2008 and at rates of 0, 11.2, and 22.4 Mg ha−1
in 2009. Soil samples were collected from the 0–5 cm and 5–10 cm depths in the fall of 2008 and 2009. Throughout the study period, SOC and STN were significantly influenced by depth, but not by treatment combinations. Averaged across the treatments, SOC was greater by 13.7% in 2008 and 24.2% in 2009 at 0–5 than the 5–10 cm depth. Similarly, STN was significantly higher by approximately 9.4% at 0–5 cm in 2008 and 18.7% in 2009 compared with the 5–10 cm depth. The C and N parameters studied and their distributions among various fractions (mineralizable, slow, and resistant) were influenced by the C and N contents of the added CDM. The low C and N contents of the CDM added in the second year of the study did not contribute to soil C and N build-up. The results generated from this study supported our hypothesis because the quality of CDM addition highly influenced C and N distribution among different fractions. Overall, for a transitioning system, CDM should to be added based on the manure-N content to ensure an adequate amount of N addition. To fully evaluate treatment benefits, a longer study period would be required to allow for system adjustment.