Soil Carbon and Nitrogen Fractions in Transitioning System to Organic Management.

Composted dairy manure (CDM) is one of the management practices used in an organic agricultural systems. The study objectives are to evaluate the impact of several organic nitrogen (N) sources on: (i) soil C and N distribution among soil fractions; and (ii) N mineralization. This study was established in 2007 on a renovated alfalfa (Medicago sativa) field at the Agricultural Research, Development and Education Center near Fort Collins, Colorado. Soil type is a Fort Collins loam (fine-loamy, mixed, superactive, mesic Aridic Haplustalfs). Alfalfa and sainfoin (Onobrychis viciifolia) were used as an organic N sources and interseeded with the grass mixtures. In 2008, the CDM was also applied to the grass and to the grass-alfalfa mixture at the rate of 22.4 Mg ha^-1. Whereas, in 2009 the CDM was applied at the rates of 0, 11.2, and 22.4 Mg ha^-1. In the fall of both years, soil were sampled from 0-5 and 5-10 cm depths. Throughout the study period, soil depth influenced SOC and STN. 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 at 0-5 than 5-10 cm depth by an average of 9.4% in 2008 and 18.7% in 2009. The C and N content of the CDM added influenced the C and N distributions among various fractions (mineralizable, slow, and resistant). The low C and N content of the CDM added in the second year of the study did not contribute to soil C and N build up. Our results supported our hypothesis because the quality of CDM addition highly influenced C and N distribution among different fractions. Overall, for a transitioning system and to ensure an adequate amount of N addition, CDM should to be added based on manure-N content. Longer study period would be required for system adjustment before evaluating treatment benefits.