Organo-mineral interactions promote greater soil organic carbon storage under aspen in semi-arid montane forests from Utah

Question: Forest species influence soil organic carbon (SOC) storage through litter input and microclimate, which in interaction with soil texture and mineralogy may lead to differences in SOC stabilization and chemistry. The decline of quacking aspen (Populus tremuloides) and expansion of conifers in the western United States due to natural succession, management practices and climate change could affect SOC dynamics. The objectives of this study were to: (i) assess the effects of overstory composition on SOC storage and stability across the aspen-conifer ecotone, and (ii) characterize the chemical composition of SOC with Fourier transform infrared spectroscopy. Methods: We sampled mineral soil (0-15 cm) across the natural gradient of aspen and mixed conifers stands (Abies lasiocarpa and Pseudotsuga menziesii) in semi-arid montane forests from Utah. SOC was divided into light fraction (LF), mineral-associated SOC in the silt and clay fraction (MoM), and a dense subfraction > 53 µm (SMoM) using wet sieving and electrostatic attraction. SOC decomposability and solubility was derived from long term laboratory incubations and hot water extractions. Mineral matrix samples were obtained by removal of organic matter with NaOCl (6%) at room temperature. We applied Fourier transform infrared spectroscopy to analyze the chemistry of organic matter (OM) (i.e., bulk soil spectra – mineral matrix spectra), LF, and MoM. Results: Vegetation cover did not affect SOC storage (47.0 ± 16.5 Mg C ha−1), SOC decomposability (cumulative CO2-C release of 93.2 ± 65.4 g C g−1 C), or SOC solubility (9.8 ± 7.2 mg C g−1 C), but MoM content increased with presence of aspen [pure aspen (31.2 ± 15.1 Mg C ha-1) > mixed (25.7 ± 8.8 Mg C ha−1) > conifer (22.8 ± 9.0 Mg C ha−1)]. Silt+clay (%) had a positive effect on MoM content (r = 0.64, p < 0.0001), and was negatively correlated to decomposable SOC per gram of C (r = -0.48, p = 0.001) or soluble SOC (r = -0.59, p < 0.0001), indicating that organo-mineral complexes reduced biological availability of SOC. Differences in chemistry among vegetation classes were patent in the LF, with greater proportion of polysaccharides and C-O groups (e.g., esters, phenols, carboxylate) for aspen and mixed LF and greater proportion of aliphatic C for mixed and conifer LF. The same patterns remained in MoM, although the effect of vegetation was statistically significant only for aliphatic C. Conclusions: Our results suggest that aspen dominance favors SOC storage as MoM, although the influence of vegetation may be surpassed by texture in sites with relatively high content of silt and clay (i.e., > 70 %). Management efforts towards the conservation and regeneration of aspen may promote long-term C sequestration in sites with silt + clay content around 40 - 70 %. Greater storage of MoM under aspen may be caused by chemical protection of relatively simple molecules resulting from litter breakdown, fine root turnover, or rhizodeposition, rather than the preservation of recalcitrant compounds (i.e., aliphatic C)