Wet–dry cycles impact DOM retention in subsurface soils

Abstract

Abstract. Transport and reactivity of carbon in the critical zone are highly controlled by reactions of dissolved organic matter (DOM) with subsurface soils, including adsorption, transformation and exchange. These reactions are dependent on frequent wet-dry cycles common to the unsaturated zone, particularly in semi-arid regions. To test for an effect of wetdry cycles on DOM interaction and stabilization in subsoils, samples were collected from subsurface (Bw) horizons of an Entisol and an Alfisol from the Catalina-Jemez Critical Zone Observatory and sequentially reacted (four batch steps) with DOM extracted from the corresponding soil litter layers. Between each reaction step, soils either were allowed to air dry ("wet-dry" treatment) before introduction of the following DOM solution or were maintained under constant wetness ("continually wet" treatment). Microbial degradation was the dominant mechanism of DOM loss from solution for the Entisol subsoil, which had higher initial organic C content, whereas sorptive retention predominated in the lower C Alfisol subsoil. For a given soil, bulk dissolved organic C losses from solution were similar across treatments. However, a combination of Fourier transform infrared (FTIR) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopic analyses revealed that wet-dry treatments enhanced the interactions between carboxyl functional groups and soil particle surfaces. Scanning transmission X-ray microscopy (STXM) data suggested that cation bridging by Ca 2+ was the primary mechanism for carboxyl association with soil surfaces. STXM data also showed that spatial fractionation of adsorbed OM on soil organo-mineral surfaces was diminished relative to what might be inferred from previously published observations pertaining to DOM fractionation on reaction with specimen mineral phases. This study provides direct evidence of the role of wet-dry cycles in affecting sorption reactions of DOM to a complex soil matrix. In the soil environment, where wet-dry cycles occur at different frequencies from site to site and along the soil profile, different interactions between DOM and soil surfaces are expected and need to be considered for the overall assessment of carbon dynamics.

Shifting mineral and redox controls on carbon cycling in seasonally flooded mineral soils

“…Lower C content in lowland as compared to adjacent upland subsurface soils (Table 3) was likely a consequence of differences in root biomass, a difference that can be attributed to restricted root growth under oxygen limitations (Tokarz and Urban, 2015). With roots recognized as primary C inputs belowground, especially in the subsoil (Rasse et al, 2005), the lack of root-derived C may explain the low C content in deeper lowland horizons. With limited C inputs at depth, microbial oxygen consumption resulting from heterotrophic respiration may not be sufficient to cause prolonged oxygen limitations (Keiluweit et al, 2016).…”

“…Conversely, we expected the upland positions to contain smaller and chemically more oxidized OM as a result of consistently largely aerobic conditions. While prior studies have primarily focused on total C in surface soils (Hall and Silver, 2015), subsurface soils (Olshansky et al, 2018), or DOM (Rouwane et al, 2018), this work represents the first examination of the depth-resolved chemical characteristics of C composition across uplandlowland transitions. Analysis of the composition of solidphase and water-extractable C supported our predictions of a greater abundance of lignin-rich, higher-molecular-weight, chemically reduced OM in the lowland positions but only in the surface horizons (Figs.…”

“…Plant roots contribute to soil C stocks through rhizodeposition (exudates, secretions, dead border cells, and mucilage), dead root residues (Jones et al, 2009), and root-associated microbes (Bradford et al, 2013). Roots are the main contributors to C stocks in upland soils (Rasse et al, 2005), but the impacts of roots on soil C stocks in wetlands are less clear. Water saturation directly inhibits root growth due to the associated low dissolved oxygen concentrations (Day and Megonigal, 1993;Tokarz and Urban, 2015).…”

Leveraging Environmental Research and Observation Networks to Advance Soil Carbon Science

“…For example, clay content has been used as a proxy for SOM stabilization in models for years, yet recent work led by CZO investigators underscores the limits of this approach and the need to better understand and represent integrated soil geochemical controls (Rasmussen et al, ). CZO researchers are poised to lead in this area (Heckman et al, ; Olshansky et al, ), and novel data sets from advanced near‐surface geophysical methods and new analytical techniques, combined with unifying conceptual frameworks surrounding hydrologic effects on soil C retention and loss, represent potentially transformative avenues of inquiry. At the landscape scale, SOM process understanding remains limited (O'Rourke et al, ).…”

Shifting Mineral and Redox Controls on Carbon Cycling in Seasonally Flooded Soils

“…Conversely, we expected the upland positions to contain smaller and chemically more oxidized OM as a result of consistently largely aerobic conditions. While prior studies have primarily focused on total C in surface soils (Hall and Silver, 2015), subsurface soils (Olshansky et al, 2018), or DOM (Rouwane et al, 2018), this work represents the first examination of the depth-resolved chemical characteristics of C composition across upland-to-lowland transitions. Analysis of the composition of solid-phase and water-extractable C supported our predictions of a greater abundance of higher-molecular weight, chemically-reduced OM in the lowland positions, but only in the topsoil (Fig.…”