COS 78-4
Abiotic drivers, not woody legume encroachment, predict nitrogen trace gas flux from a semi-arid subtropical savanna

Wednesday, August 12, 2015: 2:30 PM
303, Baltimore Convention Center
Fiona M. Soper, Ecology and Evolutionary Biology, Cornell University, Ithaca, NY
Peter M. Groffman, Cary Institute of Ecosystem Studies, Millbrook, NY
Thomas W. Boutton, Department of Ecosystem Science and Management, Texas A&M University, College Station, TX
Jed P. Sparks, Ecology and Evolutionary Biology, Cornell University, Ithaca, NY

Many savanna and grassland ecosystems are experiencing widespread encroachment by woody plants that is often associated with large increases in soil N. This increased soil N and concomitant increases in rates of soil N cycling might be expected to drive increased N gas loss in these ecosystems. We investigated the effects of both encroachment by N-fixing tree Prosopis glandulosa and abiotic conditions (temperature, rainfall) on total reactive N gas flux (NH3 , NOy, NO, N2O) from a south Texas savanna. To differentiate between the effects of Prosopis encroachment and soil type on N trace gas flux, we compared several adjacent vegetation types. These included unencroached upland remnant grasslands, upland woody clusters, and Prosopis-dominated patches that extended from sandy uplands through drainage woodlands into low-lying, clay-rich playa. We measured fluxes under ambient conditions and performed manipulative wetting experiments in the field, simulating the addition of a large (15.3 mm) rainfall event in winter (January) and summer (May, August). We also performed repeated-wetting additions during the summer months to characterize the effect of temporal rainfall dynamics. Ambient and wetting-response fluxes, vegetation cover and rainfall records were used to generate annual reactive N flux estimates for this ecosystem. 


Contrary to previous studies, encroachment alone did not significantly increase N gas fluxes when comparing upland grasslands to adjacent Prosopis groves, though 2-3 fold higher emissions were observed from Prosopis playa. Large seasonal variation in response to precipitation was driven by a positive relationship between NO, NH3 and NOy fluxes and temperature. Rainfall events increased emissions by 3-22 fold in summer, but had no effect in winter. Watering experiments showed reduced N flux rates after secondary soil wettings, independent of soil moisture, indicating the importance of temporal wetting dynamics. Duration of the antecedent dry period was also a significant predictor of N flux in a linear mixed effects model. Though NO was the most abundant compound emitted, other infrequently-measured compounds NH3 and non-NO forms of NOy comprised 12-18% of the soil N flux under summer conditions. Combining flux measures, vegetation distribution and rainfall records, we calculated total ecosystem fluxes of 0.61-0.74 kg N ha-1 yr-1 for the period 2003-2014. We conclude that woody legume encroachment does not significantly increase N gas flux from savanna uplands, but rather that temperature and wetting dynamics are stronger drivers and significant given the likelihood of altered climate patterns.