Estimates of gross transformation rates of dairy manure N using N-15 pool dilution

Most measurements of dairy manure nitrogen ( N) availability depend on net changes in soil inorganic N concentration over time, which overlooks the cycling of manure N in the soil. Gross transformations of manure N, including mineralization ( m), immobilization ( i), and nitrification ( n), can be quantified using 15 N pool dilution methods. This research measures gross m, n, and i resulting from application of four freeze- dried dairy manures that had distinctly different patterns of N availability. A sandy loam soil ( coarse- loamy, mixed, frigid Typic Haplorthod) was amended with four different freeze- dried dairy manures and incubated at 25 degrees C with optimal soil water content. The dilution of 15 ammonium ( NH4(+)) during a 48- h interval ( 7 - 9 d and 56 - 58 d after manure application) was used to estimate m, whereas the dilution of 15 nitrate ( NO3-) was used to estimate n. Gross immobilization was calculated as gross minus net mineralization. Gross mineralization in the unamended soil was similar at 7- to 9- d and 56- to 58- d intervals and was significantly increased by the application of manures. For both amended and unamended soil, m was much greater ( i. e., three- to nine- fold) than estimated net mineralization, illustrating the degree to which manure N can be cycled in soil. At the early interval, both m and i were directly related to the manure C input, demonstrating the linkage between substrate C availability and N utilization by soil microbes. This research clearly shows that the application of dairy manures stimulates gross N transformation rates in the soil, improving our understanding of the impact of manure application on soil N cycling.