Author
 |
Griffin, Timothy |
|
Submitted to: Communications in Soil Science and Plant Analysis
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/12/2006 Publication Date: 6/11/2007 Citation: Griffin, T.S. 2007. Estimation of gross transformation rates of dairy manure N using 15N pool dilution. Communications in Soil Science and Plant Analysis. 38: 1451-1465 Interpretive Summary: Estimates of dairy manure nitrogen (N) availability are commonly developed by measuring net changes in soil inorganic N concentration over time, but this does not provide information on the cycling of manure N in the soil. Estimates of gross N transformation rates, are needed to better understand cycling of manure N in soil. To do this, a sandy loam soil was amended with four different dairy manures at 25oC and optimal soil water level. At two points in time (7-9 d and 56-58 d after manure application), sets of each soil/manure combination received either ammonium (NH4) or nitrate (NO3) that contained 15N. By measuring how each 15N source is diluted over a 48 hour period, we can measure changes in N. Gross mineralization (the change from organic to mineral form) in the unamended soil was similar at 7-9d and 56-58 d intervals and was significantly increased by the application of manures. For both amended and unamended soil, mineralization measured in this way was much greater (i.e. three- to nine-fold) than if the only change in soil N was measured mineralization. This illustrates the degree to which manure N can be cycled in soil, even when nitrate is not accumulating. Manure application also increased immobilization at both intervals. At the early interval, both reactions were directly related to the manure C input, demonstrating the linkage between substrate C availability and N utilization my soil microbes. This research clearly shows that the application of dairy manures stimulates gross N transformation rates in the soil, increasing our understanding of manure N cycling. Technical Abstract: Estimates of gross N transformation rates, including mineralization (m), immobilization (i), and nitrification (n) are needed to better understand cycling of manure N in soil, which can be accomplished using 15N pool dilution methods. The objectives of this research were to 1) estimate m, i, and n for four freeze-dried dairy manures, and 2) compare net and gross mineralization and nitrification rates. Sandy loam soil was amended with four different freeze-dried dairy manures in glass beakers and incubated at 25oC and optimal soil water level. At two sampling intervals (7-9 d and 56-58 d after manure application), triplicate sets of each soil/manure combination received 15NH4, 15NO3, or no 15N. For each 15N source, one vessel was extracted in 2 M KCl immediately and the other after 48 hr. The 15NH4 and 15NO3 in the extracts were sequentially diffused onto acidified filter paper disks. The dilution of 15NH4 enrichment during the 48 hr interval was used to estimate m, while the dilution of 15NO3 was used to estimate n. Gross immobilization was estimated as gross minus net mineralization. Gross mineralization in the unamended soil was similar at 7-9d and 56-58 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. Manure application also increased immobilization (i) at both intervals. 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 my soil microbes. This research clearly shows that the application of dairy manures stimulates gross N transformation rates in the soil, increasing our understanding of manure N cycling. |
