Nitrogen Availability From Manure in Years Following a One-Time Application

Manure from the semiarid West’s dairy industries is a rich nutrient source, but its use for crops can be problematic because soil N availability from manure may vary substantially depending on the year of application. Experimental plots established in Idaho on a Portneuf silt loam (coarse silty, mixed, superactive, mesic Durinodic Xeric Haplocalcid) included six manure treatments and two non-manure treatments with four replicates. The six manure treatments included combinations of two manure rates, Man-1x (277 lbs total N/ac) and Man-3x (866 lbs total N/ac) applied in the fall either 1, 2, or 3 years previously. The two non-manure treatments were urea fertilizer applied per soil test (Fert) and a control with no amendment. We measured net N mineralization (0-12 in) in the plots using buried bags in 2006, 2007, and 2009 for a sprinkler-irrigated barley, sugarbeet, and dry bean crop, respectively. This resulted in i) two years of net N mineralization data for each manure rate applied 1, 2, or 3 years prior to measurement; and ii) one year of data for each manure rate applied 4 or 5 years previous to the measurement year. A 5-year decay series for each of the two manure rates was derived from functions fitted to the net N mineralization data, expressed as a fraction of total manure-N applied. The decay series (y1-y5) for the manure-1x treatment was 0.23, 0.12, 0.10, 0.09, and 0.08 while that for the manure-3x rate was 0.20, 0.08, 0.05, 0.04, and 0.03. Soil at the 12-to-24-in depth contributed up to 28% of the total N mineralized in the 0-to-60-cm soil layer of manure-amended soils in the 3rd year after application, with lesser amounts contributed in earlier years due to immobilization. The efficacy of N mineralization processes decreased as the manure application increased, thus using a single decay series to predict N availability across a range of manure application rates could lead to substantial estimation errors