Temporal and spatial variation in longleaf pine soil respiration and its heterotrophic and autotrophic components

Abstract

Soil respiration (Rs) is the sum efflux of CO2 from soil derived from the metabolic activity of autotrophs and heterotrophs in the litter layer, root-affected soil (rhizosphere), and bulk soil. Soil respiration exhibits a strong influence on the carbon balance of forests; specifically, the heterotrophic respiration (Rh) component can be weighed against all carbon assimilation, or net primary productivity (NPP), to estimate the carbon sink or source status of forested ecosystems. Soil respiration varies both temporally and spatially as environmental and ecological factors influence the individual components of Rs to varying degrees, and these dynamics of Rs have been studied in diverse ecosystems across the globe. However, Rs has been relatively understudied in longleaf pine (Pinus palustris Mill.) forests, which are being restored throughout the southeastern United States for a myriad of ecosystem services, including rare species habitat. This research project was conducted in diverse longleaf pine forests to increase our understanding of longleaf pine Rs dynamics, including the temporal and spatial variation in Rs and the ecological factors affecting said variation and the proportion of total Rs derived from heterotrophs. The intra-annual variation in Rs was measured from January 2012 through January 2013 across four diverse longleaf pine stands varying in age and structure, and the spatial variation of Rs in a 64-year-old longleaf pine forest was measured in July and August 2012. Concurrent measurements of the soil environment (soil temperature and moisture) and ecological factors (e.g. litter mass, distance to and diameter of nearby trees, root biomass) were made to determine how these factors influenced the temporal and spatial variation in Rs. Soil respiration was positively related to soil temperature on an intra-annual basis with a corresponding temperature sensitivity (Q10) of 2.18; however, soil temperature did not influence the spatial variation in Rs in the 64-year-old longleaf pine stand. The value of Q10 was decreased during periods of drought-like soil conditions, and soil moisture also influenced the spatial variation of Rs by homogenizing Rs variability in the wettest areas and decreasing Rs where soil conditions approached saturated levels. Litter mass and nearby trees increased Rs on both a temporal and a spatial basis; however, the influence of these variables on the intra-annual variation in Rs was marginal after first isolating the effect of soil temperature. Understory cover was correlated with the temporal variation in Rs, but confounded with the seasonal influence of soil temperature, and forb cover was the only cover category related to the spatial variation in Rs. Live fine root biomass was negatively related to the intra-annual variation in Rs and positively related to the spatial variation in Rs, and dead root biomass was negatively related to the spatial variation in Rs and not related to the temporal variation in Rs. Finally, Rs was partitioned into its autotrophic and heterotrophic components by means of small root exclusion tubes installed in three 26-year-old longleaf pine forests during the growing season of 2013. The presence of root exclusion tubes, when compared to adjacent control soil, significantly decreased Rs and live root biomass, and increased dead root biomass after 102-104 days of incubation. The corresponding estimates of the proportion of Rh to total Rs were 61 to 82 %, with the lowest ratio estimates after correcting for the initial within-block, pre-treatment variation in Rs and CO2 lost due to root decay. This research provides a comprehensive view of the spatial and temporal variation in Rs and an estimate of the proportion of total Rs resulting from heterotrophic activity in longleaf pine forests located centrally within their native range.