The southeastern landscape is composed of agricultural and forest systems that can store carbon (C) in standing biomass and soil. Research is needed to quantify the effects of elevated atmospheric carbon dioxide (CO2) on terrestrial C dynamics including CO2 release back to the atmosphere and soil sequestration. Longleaf pine savannahs are an ecologically and economically important, yet understudied, component of the southeastern landscape. We investigated the effects of ambient and elevated CO2 on soil CO2 efflux in a young longleaf pine system using a continuous monitoring system. A significant increase (26.5%) in soil CO2 efflux across 90 days was observed under elevated CO2; this occurred for all weekly and daily averages except for two days when soil temperature was the lowest. Soil CO2 efflux was positively correlated with soil temperature with a trend towards increased efflux response to temperature under elevated CO2. Efflux was negatively correlated with soil moisture and was best represented using a quadratic relationship. Soil CO2 efflux was not correlated with root biomass. Our data indicate that, while elevated CO2 will increase feedback of CO2 to the atmosphere via soil efflux, terrestrial ecosystems will remain potential sinks for atmospheric CO2 due to greater biomass production and increased soil C sequestration. 1. Introduction The rural southeastern landscape is dominated by three vegetation types (crops, forests, and pastures), all of which have the ability to store atmospheric carbon (C) as standing biomass (including plant roots) or in soil. One particularly important ecosystem is longleaf pine savannahs. Prior to European settlement, the coastal plains of the southeastern United States were dominated by nearly pure stands of longleaf pine (Pinus palustris Mill.) with a diverse understory plant community; some longleaf ecosystems have the highest reported values for species richness, including many threatened and endangered species, in the temperate Western Hemisphere [1]. This system now occupies only 2% of its former range [2], a loss comparable to or exceeding that of most endangered communities throughout the world including the North American tallgrass prairie, the moist tropical coastal forest of Brazil, and the dry forests along the Pacific coast of Central America [3]. Longleaf pine forests in the southeast currently occupy sites at the more xeric end of the moisture continuum and are often found on soils with low N availability. In fact, it is not unusual to find disjunct longleaf pine communities in the rural farm
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