Effects of repeated fire and environmental variation on the efflux of CO2 from soils of a subtropical woodland savanna

Background/Question/Methods

Subtropical longleaf pine (Pinus palustris) savannas are important ecosystems, in terms of their biodiversity (> 40 species per m²) as well as ecosystem services, such as carbon sequestration. The structure and function of these ecosystems are maintained with frequent low intensity fires. However, the role that cyclic fire plays in soil respiration is relatively unknown. Soil respiration is a major component in the global carbon cycle and greatly dependent on air temperature and soil moisture. Approximately half of the soil respiration is attributed to metabolic activity in plants, supporting root growth.

We collected soil respiration measurements in longleaf pine savannas in southwestern Georgia, USA, over a four-year period, to increase our understanding of carbon dynamics in these ecosystems. The study included monthly observations of soil respiration (F_soil), air temperature (T_air) and soil moisture (VWC), over three fire cycles, at three sites with differing water holding capacities from 2009 to 2013. We used mixed model approaches to answer the following research questions: 1) How does F_soil contribute to the net ecosystem exchange (NEE) of longleaf pine savannas? 2) How does VWC affect F_soil in longleaf pine ecosystems? 3) Does seasonal fire alter F_soil rates at the three sites?

Results/Conclusions

Our results show that F_soil accounts for approximately 60% of NEE at all sites. All sites vary significantly in terms of VWC, ranging on average from 7% at the xeric site to 11% at the mesic site, with higher variability at the mesic and lower variation at the xeric site. Our mixed model indicated that F_soil was significantly different by site and year, and varied with VWC and T_air (α = 0.05). We found that lower VWC resulted in higher F_soil, and that F_soil was higher following drought years (2012). However, lower VWC at the xeric site did not cause higher F_soil rates, in contrast to the mesic site. Whereas T_air was positively correlated with F_soil, VWC showed negative correlation with F_soil. We did not find significant effects of seasonal burn on F_soil. We conclude that F_soil largely depends on the prevailing weather conditions and soil texture. Hence, drier soil conditions as one consequence of global climate change may lead to increased F_soil in these ecosystems and therefore alter NEE.