Factors affecting the molecular structure and mean residence time of occluded organics in a lithosequence of soils under ponderosa pine

Abstract

Occluded, or intra-aggregate, soil organic matter (SOM) comprises a significant portion of the total C pool in forest soils and often has very long mean residence times (MRTs). However, occluded C characteristics vary widely among soils and the genesis and composition of the occluded organic matter pool are not well understood. This work sought to define the major controls on the composition and MRT of occluded SOM in western U.S. conifer forest soils with specific focus on the influence of soil mineral assemblage and aggregate stability. We sampled soils from a lithosequence of four parent materials (rhyolite, granite, basalt, and dolostone) under Pinus ponderosa. Three pedons were excavated to the depth of refusal at each site and sampled by genetic horizon. After density separation at 1.8 g cm-3 into free/light, occluded and mineral fractions, the chemical nature and mean residence time of organics in each fraction were compared. SOM chemistry was explored through the use of stable isotope analyses, 13 C NMR, and pyrolysis GC/MS. Soil charcoal content estimates were based on 13 C NMR analyses. Estimates of SOM MRT were based on steady-state modelling of SOM radiocarbon abundance measurements. Across all soils, the occluded fraction was 0.5-5 times enriched in charcoal in comparison to the bulk soil and had a substantially longer MRT than either the mineral fraction or the free/light fraction. These results suggest that charcoal from periodic burning is the primary source of occluded organics in these soils, and that the structural properties of charcoal promote its aggregation and long-term preservation. Surprisingly, aggregate stability, as measured through ultrasonic dispersion, was not correlated with occluded SOM abundance or MRT, perhaps raising questions of how well laboratory measurements of aggregate stability capture the dynamics of aggregate turnover under field conditions. Examination of the molecular

Soil genesis and mineralogy across a volcanic lithosequence

“…These data suggest that parent material composition is not a driving factor regulating carbon sequestration in this lithosequence. This is in agreement with the findings of Heckman et al (2014), who found no significant differences in soil organic carbon or occluded soil organic matter attributable to variations in lithology, concluding that climate, fire frequency and preferential occlusion of charcoal are more substantial factors in carbon dynamics than initial parent material composition.…”

Variation in the Molecular Structure and Radiocarbon Abundance of Mineral-Associated Organic Matter across a Lithosequence of Forest Soils

“…Surface area was measured under N 2 and modeled using the BET equation [37]. Soils were density separated into free/light (F/L), occluded (OCC) and heavy fractions (HF) using a sodium polytungstate (SPT) solution adjusted to a density of 1.8 g cm −3 [33,38]. In brief, soils were mixed with SPT and centrifuged to separate low density organics (F/L) from the rest of the soil.…”

Soil and vegetation carbon turnover times from tropical to boreal forests

“…Soil is the major C pool in terrestrial ecosystems and has a longer turnover time than vegetation (Schmidt et al., ). A central topic in previous studies on soil organic C (SOC) dynamics is SOC turnover time (τ soil ) and its determinants (Heckman et al., ; Koven et al., ; Schimel et al., ). It has been documented that at a small spatial scale, soil C turnover is mainly dependent on soil temperature and moisture (Craine, Fierer, & McLauchlan, ; Davidson & Janssens, ; Thomsen, Schjønning, Jensen, Kirstensen, & Christensen, ), soil chemical properties (Schindlbacher et al., ; Xu, He, & Yu, ; Xu, Shi, et al., ), C quality (Chen, Liang, et al., ) or soil microbial community (Chen, Li, Lan, Hu, & Bai, ; Cleveland, Nemergut, Schmidt, & Townsend, ), while at national and global scales, latitude, altitude and associated climatic variables are suggested to be responsible for the variability of τ soil (Chen, Huang, Zou, & Shi, ).…”