Determining Nitrogen Mineralization From Composted Dairy Manure and Impact on Irrigated Perennial Grass Mixes.

Along Colorado’s Front Range and Eastern Plains, organic dairy expansion is driving the need for increased production of locally grown high quality organic forages. One of the challenges facing organic farmers who use manure/compost to maintain soil fertility is ensuring adequate nutrient (particularly nitrogen, N) supply to maintain optimum forage yields while avoiding over-application to reduce N loss to the environment. Thus, it is important to estimate the amount of N that can be mineralized from applied manure/compost and the soil during the growing season. This study determined rate of N mineralization from composted dairy manure (CDM) applied annually at 22.4 Mg ha-1 for 2-years (2008 to 2009) to two irrigated perennial grass mixes. An in situ incubation experiment adopting the method of soil cores with ion exchange resin bags at the bottom was conducted on Fort Collins loam in northern Colorado. Untreated control and CDM amended tubes were retrieved at approximately 4-week intervals during the grass growing season (April to September). Ammonium-N and nitrate-N concentrations in the soil and resin of the incubation tubes were analyzed. Net CDM-N mineralization as inorganic N was independent of the grass species. Our first-year data suggest that CDM application to perennial grass mixes resulted in immobilization of soil N. About 12% of organic N added in CDM was mineralized in the second-year (estimated by difference). Although not significant, forage yield and quality (crude protein, NDF, and ADF) generally declined in the second-year. Although conclusive results are still lacking, the observed (i) soil N immobilization following CDM amendment and (ii) low N release in the second-year present a case for a need to consider applying higher CDM rates and/or supplementing CDM application with N-fixing legumes to avoid forage yield reduction due to N deficiency and to improve forage quality.