PARENT SESSION
Poster Session 30: Soil Ecology.

Wednesday, August 4 Presentations from 5:00 PM to 6:30 PM, Exhibit Hall A 1.

Altered soil nitrogen cycling following Juniperus virginiana encroachment into tallgrass prairie.

McKinley, Duncan^*,1, Blair, John¹, Johnson, Loretta¹, ¹ Kansas State University, Manhattan

ABSTRACT- Expansion in the cover of woody plants in grasslands and savannas (afforestation) is a worldwide phenomenon. Recently, Juniperus virginiana cover has dramatically increased in areas previously dominated by tallgrass prairie. Four paired sites of native tallgrass prairie and adjacent J. virginiana, forest were selected to assess differences in nitrogen cycling. Net mineralization was assessed using soil cores incubated in situ for 30-day intervals. Potential gross nitrogen mineralization, nitrification, and consumption rates were determined using ¹⁵N isotope-dilution. Field net mineralization assays indicated similar seasonal patterns in forests and prairies (p>0.05). There were also no significant differences in potential gross mineralization rates in soils from prairie (2.85 g N/g soil/day) and forest sites (2.91 g N/g soil/day)(F=0.13, p=0.915). However, gross nitrification rates were approximately three times greater in prairie soils (1.36 g N/g soil /day) than in forest soils (0.43 g N/g soil/day) (F=6.91, p=0.039). Rates of gross NO₃^- consumption in prairie soils (1.56 g N/ g soil/day) and forest (0.56 g N/g soil/day) were also significantly different (F=13.14, p=0.011), and both rates were slightly higher than gross production, suggesting net immobilization. Lower rates of gross nitrification in forest soils may have resulted from reduced NH₄⁺ availability or possibly differences in nitrifier population size or activity. These differences in nitrification rates suggest that J. virginiana forests may conserve more N by mitigating potential loss of N from leaching or denitrification.

Key words: nitrogen cycling, juniperus, encroachment