Net Nitrogen Mineralization from Past Years' Manure and Fertilizer Applications

The availability of soil N changes in years following a manure application. Experimental plots were established in a southern Idaho calcareous silt loam soil to measure these changes and aid N management in manure‐amended soils of the semiarid West. The study's six manure treatments included combinations of two manure rates, Man‐1× (0.31 Mg total N ha −1 ) and Man‐3× (0.97 Mg total N ha −1 ) applied in the fall either 1, 2, or 3 yr previously. Two non‐manure treatments were urea fertilizer applied per soil test (Fert) and a control with no amendment. We measured net N mineralization (0–30 cm) in the plots using buried bags in 2006, 2007, and 2009 for sprinkler‐irrigated crops. This resulted in (i) 2 yr of net N mineralization data for each manure rate applied 1, 2, or 3 yr before measurement, and (ii) 1 yr of data for each manure rate applied 4 or 5 yr previous to the measurement year. A 5‐yr 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 (Year 1–Year 5) for the Man‐1× treatment was 0.23, 0.12, 0.10, 0.09, and 0.08, while that for the Man‐3× rate was 0.20, 0.08, 0.05, 0.04, and 0.03. Soil at the 30‐ to 60‐cm depth contributed up to 28% of the total N mineralized in the 0‐ to 60‐cm soil layer of manure‐amended soils in the third 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.