Estimates of Gross Transformation Rates of Dairy Manure N Using ¹⁵N Pool Dilution

Authors

• T. S. Griffin, USDA-ARS New England Plant Soil and Water LaboratoryFollow

Document Type

Article

Date of this Version

2007

Comments

Published in Communications in Soil Science and Plant Analysis, 38: 1451–1465, 2007. DOI: 10.1080/00103620701378409

Abstract

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 ¹⁵N 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 °C with optimal soil water content. The dilution of ¹⁵ammonium (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
¹⁵nitrate (NO^-₃) 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.