Document: JAS-3-59-80

The divergent effects of understory species assemblages on nitrogen cycling in longleaf pine (Pinus palustris) savannas.

WEST, J.B.* and L.A.DONOVAN

University of Georgia, Athens, GA 30602, USA ¹

Abstract:
In savanna ecosystems species assemblages are likely to affect nutrient dynamics through differences in root dynamics, nutrient uptake and nutrient use. We studied fine root and nitrogen dynamics in common understory patch types of resource-limited longleaf pine (Pinus palustris) savannas on fall-line sandhills in the southeastern US. A strong soil moisture gradient occurs from xeric ridges to wetlands in these sandhills. Midway along this gradient the understory vegetation exists in three distinct patch types. These are dominated by either wiregrass (Aristida stricta), a mix of bluestems (Andropogon spp. and Schizachyrium scoparium), or dwarf huckleberry (Gaylussacia dumosa). We compared the belowground dynamics of these patch types in order to understand the degree to which understory species composition affects ecosystem function. We selected three replicate stands (similar overstory, soil type and fire history), each patch type within each stand (100 m² plots), and installed three minirhizotron tubes within each plot. Net N mineralization at 5 cm was estimated using in situ buried bags. Leaching losses of nitrate and ammonium at 100 cm were estimated using ion exchange membranes. There were significant differences among patch types for total root length density, leaching losses of nitrate, and net nitrification. Leaching losses of ammonium and net ammonification were not significantly different among patches. Total root length density followed this trend: bluestemg cm^-2 month^-1) was on average over twice that from wiregrass and four times that from huckleberry. Greater leaching from bluestem patches relative to others results in a significant change in the nitrogen cycle, specifically either increased nitrate losses from the system or availability for more deeply rooted plants. These divergent root length densities and their apparent effects on the nitrogen cycle suggest that species may not conform to predictable functional groups (e.g., C4 bunchgrasses).

Keywords: nitrogen cycling, species effects, nitrogen leaching, nitrogen mineralization, fine roots

This abstract is being presented at: 3:30 PM in session:
Poster Session #15: Nutrient Cycling.