The Effects of 40% Throughfall Reduction on Soil Respiration in a Longleaf Pine Plantation

Abstract

Longleaf pine (Pinus palustris) forests can serve as disturbance-resistant, long-term carbon sinks, but net ecosystem productivity of longleaf pine forests has been shown to be sensitive to drought. Soil respiration (Rs) is often the largest component of ecosystem respiration and can be sensitive to drought. The primary objective of this study is to explore the influence of drought, imposed by a 40% reduction in throughfall (TR40), relative to ambient throughfall treatment (TR0) on soil respiration (Rs), heterotrophic respiration (Rh) and fine root biomass. The study was conducted in a longleaf pine plantation located in the Chattahoochee Fall Line Region in Georgia. During a naturally occurring severe drought in 2016, soil moisture and Rs declined to near zero in both treatments, and TR treatment had no effect on Rs, soil moisture, or soil temperature. In 2017, TR40 reduced Rs from 13.8% to 21.6% on six of the 12 measurement dates. Annual Rs ranged from 9.96 Mg C ha-1 yr-1 in the TR0 treatment to 8.70 Mg C ha-1 yr-1 in the TR40 treatment, and Q10 was reduced from 1.92 to 1.67 by TR40. Soil temperature and soil moisture explained the majority of the variation in Rs in 2016 (68% in TR0 and 68% in TR40) and 2017 (54% in TR0 and 42% in TR40). Surrounding tree basal area and distance to nearest tree each accounted for up to 6% of the variation in Rs. Throughfall reduction treatment did not significantly reduce fine root biomass or shift the spatial distribution of fine root biomass in response to TR trays. Heterotrophic respiration accounted for the majority of soil respiration (72.4-89.4%). Throughfall reduction altered the contribution of Rh to Rs in August 2017, suggesting that autotrophic respiration (Ra) was decreased. Results suggest that Ra may be more sensitive to moderate drought in longleaf pine forests but both Ra and Rh are sensitive to soil moisture below the wilting point.