The effect of fertilization levels and genetic deployment on the isotopic signature, constituents, and chemistry of soil organic carbon in managed loblolly pine (Pinus taeda L.) forests

Soil organic carbon (SOC) mass and its constituents, chemistry, and isotopic signatures (Δ¹⁴C, δ¹³C) were examined for two different loblolly pine (Pinus taeda L.) research installations located in north-central Florida. Both studies were designed as split-plots with the whole plots as different levels of fertilization and herbicide application (cultural intensity), and full-sib families of loblolly pine were the splits. The cultural intensities and the families of loblolly pine were different at each site and so each site was analyzed separately. The plantations were aged 9 or 10 years at the time of soil sampling. At both sites, the overall mass of SOC to a depth of 0–30 cm was unresponsive to the level of family growth or cultural intensity and did not show a trend with aboveground biomass. The SOC pool was further separated into live roots and wood; and density fractionation was used to separate the SOC sample into a light fraction (LF) (<1.7 g cm⁻³) and heavy fraction (HF) with the LF dissected further for charcoal and dead roots. Higher fertilization levels generally depressed fine root (<1 mm) biomass, but whether the effect was significant varied with family and soil horizon. The HF was a relatively small component (<5%) of SOC in these sandy textured soils, but at one of the two sites, the HF was significantly increased with more intensive silviculture and for the faster growing family. The Δ¹⁴C value of the LF-SOC for one slow growing family under low culture (136 ± 11‰) differed from the faster growing low culture plot, and its relationship to the atmospheric Δ¹⁴C record suggested that the LF-SOC likely originated prior to stand establishment. The LF chemistry was determined with solid-state ¹³C nuclear magnetic resonance (NMR) and cultural intensity did not significantly affect SOC chemistry. However, the family effect was significant for carbohydrates at one site, and for lignin and lipids at the other site. Overall, these results suggest that tree genetics in managed forests can influence SOC chemistry and that the relatively small fractions of SOC can change with management intensity; however, the effect of cultural intensity is minimal for the largest components of SOC and there is no clear relationship between SOC dynamics and aboveground production under the management regimes, and stand ages, examined with these two research installations.