PARENT SESSION
Oral Session # 98: Biogeochemistry V: Forests; Nitrogen.
Presiding: RL Sanford
Friday, August 8. 8:30 AM to 12:00 PM, SITCC Meeting Room 204.
Evaluation of the ammonia diffusion method for determining soil gross N mineralization rates by 15N dilution.
Jin, Virginia*,1, 2, Romanek, Christopher1, 3, Sharitz, Rebecca1, 2, 1 Savannah River Ecology Laboratory, Aiken, SC2 Department of Plant Biology, Athens, GA3 Department of Geology, Athens, GA
ABSTRACT- The ammonia diffusion method is commonly used for the stable isotope dilution technique to determine soil gross N mineralization rate. We evaluated the ammonia diffusion method developed by Brooks et al. 1989 using laboratory incubations of stable isotope-labeled field soils extracted with 2M KCl. Temperate hardwood (H) soils and sandhills (S) soils were sampled in the field, brought back to the laboratory, and labeled with ( 15NH4)2SO4 (
15N 537‰) to increase soil N by approximately 2 mg N kg-1 dry soil. Control incubations were also conducted using 2M KCl spiked with 300 
g NH4+-N. Initial samples were extracted and diffused immediately after addition of label, and labeled samples were incubated for 48 hours at room temperature then extracted and diffused. Triplicate samples of extracts and diffused N from controls and each soil type for initial and incubated soils were taken daily over a five-day period to characterize changes in extract NH4+-N concentrations and isotope values of diffused N. After the diffusion period, percent recovery of both total N and 15N were highest in controls (88-103% and 97-115%, respectively), then H soils (64-70% and 54-61%, respectively), then S soils (31-53% and 26-57%, respectively). Isotopic values for diffused N in the controls were similar for initial and incubated soils; however, 15N values of diffused N were higher for initial soils than incubated soils in both H and S diffusions. Diffused NH4+ 15N values from S soils were higher than H soils for both initial and incubated soils on each day of the diffusion period. Overall recovery of N label using the diffusion method appears positively correlated to the initial NH4+-N concentration of the diffusing solution. Our findings suggest that more accurate calculations of gross N mineralization rates can be made if the initial N of labeled soil extracts for diffusion approximate 300 g NH4+-N.
Key words: ammonia diffusion, stable isotope dilution, gross nitrogen mineralization