Heather M. Throckmorton

The source of microbial C has little impact on soil organic matter stabilisation in forest ecosystems.

The source of microbial C is thought to impact its stability in soil due to variations in cellular biochemistry. It has been hypothesised that a fungal-dominated community stabilises more C than a bacterial-dominated community, in part due to chemical recalcitrance of their non-living biomass, particularly cell wall components and pigments. We compared the turnover of (13)C-labelled (99.9 atom %) temperate and tropical microbial isolates [i.e. fungi, Gram-positive bacteria (including actinobacteria) and Gram-negative bacteria] in temperate (California) and tropical (Puerto Rico) forest soils. While significant differences in (13)C recovery and mean residence times occurred among some microbial additions, similar turnover rates were observed, and in general, results do not support the view that microbial biochemistry affects soil C maintenance. Different effects by microbial necromass additions in California and Puerto Rico suggest that ecosystem-specific effects may be as important to microbial C stabilisation as its macromolecular composition and recalcitrance.

Microtopographic and depth controls on active layer chemistry in Arctic polygonal ground

Polygonal ground is a signature characteristic of Arctic lowlands, and carbon release from permafrost thaw can alter feedbacks to Arctic ecosystems and climate. This study describes the first comprehensive spatial examination of active layer biogeochemistry that extends across high- and low-centered, ice wedge polygons, their features, and with depth. Water chemistry measurements of 54 analytes were made on surface and active layer pore waters collected near Barrow, Alaska, USA. Significant differences were observed between high- and low-centered polygons suggesting that polygon types may be useful for landscape-scale geochemical classification. However, differences were found for polygon features (centers and troughs) for analytes that were not significant for polygon type, suggesting that finer-scale features affect biogeochemistry differently from polygon types. Depth variations were also significant, demonstrating important multidimensional aspects of polygonal ground biogeochemistry. These results have major implications for understanding how polygonal ground ecosystems function, and how they may respond to future change.

The soil matrix increases microbial C stabilization in temperate and tropical forest soils

Microbial biomass represents a substantial source of labile C contributing to soil organic matter (SOM) maintenance. Microbial residues may associate with the soil matrix through a variety of mechanisms, reducing its bioavailability and increasing its persistence in soil. Our objective was to examine soil matrix effects on the stability of non-living microbial C inputs in two contrasting forest ecosystems by following microbial residues (Fungi, Actinobacteria, Gram-positive bacteria (Gm +), Gram-negative bacteria (Gm −)) into SOM fractions in a temperate forest in California (CA) and a tropical forest in Puerto Rico (PR) for 3 and 2 years, respectively. We isolated 3 SOM fractions: (i) free light fraction (FLF), (ii) occluded light fraction (OLF), and (iii) dense fraction (DF). Additionally, we characterized SOM fraction chemistry to infer quality and source of native fraction SOM. Our results showed greater stabilization as mineral-associated microbial C (i.e., as DF and OLF), compared with loose detrital C (i.e., FLF). There was no microbial group effect (i.e., differences in fraction C recovery among different microbial cell types). Our findings suggest that mineral association is more important for stabilizing non-living microbial C in soil than the cellular structure of the initial source of microbial inputs, with site specific edaphic factors as the major controllers of the amount of microbial residues stabilized.

Pathways and transformations of dissolved methane and dissolved inorganic carbon in Arctic tundra watersheds: Evidence from analysis of stable isotopes

Arctic soils contain a large pool of terrestrial C and are of interest due to their potential for releasing significant carbon dioxide (CO2) and methane (CH4) to the atmosphere. Due to substantial landscape heterogeneity, predicting ecosystem-scale CH4 and CO2 production is challenging. This study assessed dissolved inorganic carbon (DIC = Σ (total) dissolved CO2) and CH4 in watershed drainages in Barrow, Alaska as critical convergent zones of regional geochemistry, substrates, and nutrients. In July and September of 2013, surface waters and saturated subsurface pore waters were collected from 17 drainages. Based on simultaneous DIC and CH4 cycling, we synthesized isotopic and geochemical methods to develop a subsurface CH4 and DIC balance by estimating mechanisms of CH4 and DIC production and transport pathways and oxidation of subsurface CH4. We observed a shift from acetoclastic (July) toward hydrogenotropic (September) methanogenesis at sites located toward the end of major freshwater drainages, adjacent to salty estuarine waters, suggesting an interesting landscape-scale effect on CH4 production mechanism. The majority of subsurface CH4 was transported upward by plant-mediated transport and ebullition, predominantly bypassing the potential for CH4 oxidation. Thus, surprisingly, CH4 oxidation only consumed approximately 2.51 ± 0.82% (July) and 0.79 ± 0.79% (September) of CH4 produced at the frost table, contributing to <0.1% of DIC production. DIC was primarily produced from respiration, with iron and organic matter serving as likely e- acceptors. This work highlights the importance of spatial and temporal variability of CH4 production at the watershed scale and suggests broad scale investigations are required to build better regional or pan-Arctic representations of CH4 and CO2 production.

Isotopic identification of soil and permafrost nitrate sources in an Arctic tundra ecosystem

The nitrate (NO3−) dual isotope approach was applied to snowmelt, tundra active layer pore waters, and underlying permafrost in Barrow, Alaska, USA, to distinguish between NO3− derived from atmospheric deposition versus that derived from microbial nitrification. Snowmelt had an atmospheric NO3− signal with δ15N averaging −4.8 ± 1.0‰ (standard error of the mean) and δ18O averaging 70.2 ± 1.7‰. In active layer pore waters, NO3− primarily occurred at concentrations suitable for isotopic analysis in the relatively dry and oxic centers of high-centered polygons. The average δ15N and δ18O of NO3− from high-centered polygons were 0.5 ± 1.1‰ and −4.1 ± 0.6‰, respectively. When compared to the δ15N of reduced nitrogen (N) sources, and the δ18O of soil pore waters, it was evident that NO3− in high-centered polygons was primarily from microbial nitrification. Permafrost NO3− had δ15N ranging from approximately −6‰ to 10‰, similar to atmospheric and microbial NO3−, and highly variable δ18O ranging from approximately −2‰ to 38‰. Permafrost ice wedges contained a significant atmospheric component of NO3−, while permafrost textural ice contained a greater proportion of microbially derived NO3−. Large-scale permafrost thaw in this environment would release NO3− with a δ18O signature intermediate to that of atmospheric and microbial NO3. Consequently, while atmospheric and microbial sources can be readily distinguished by the NO3− dual isotope technique in tundra environments, attribution of NO3− from thawing permafrost will not be straightforward. The NO3− isotopic signature, however, appears useful in identifying NO3− sources in extant permafrost ice.