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