Search

Soil organic matter (SOM) is comprised of a diverse array of reactive carbon molecules, including hydrophilic and hydrophobic compounds, that impact rates of SOM formation and persistence. Despite clear importance to ecosystem science, little is known about broad-scale controls on SOM diversity and variability in soil. Here, we show that microbial decomposition drives significant variability in the molecular richness and diversity of SOM between soil horizons and across a continental-scale gradient in climate and ecosystem type (arid shrubs, coniferous, deciduous, and mixed forests, grasslands, and tundra sedges). The molecular dissimilarity of SOM was strongly influenced by ecosystem type (hydrophilic compounds: 17%, P < 0.001; hydrophobic compounds: 10% P < 0.001) and soil horizon (hydrophilic compounds: 17%, P < 0.001; hydrophobic compounds: 21%, P < 0.001), as assessed using metabolomic analysis of hydrophilic and hydrophobic metabolites. While the proportion of shared molecular features was significantly higher in the litter layer than subsoil C horizons across ecosystems (12 times and 4 times higher for hydrophilic and hydrophobic compounds, respectively), the proportion of site-specific molecular features nearly doubled from the litter layer to the subsoil horizon, suggesting greater differentiation of compounds after microbial decomposition within each ecosystem. Together, these results suggest that microbial decomposition of plant litter leads to a decrease in SOM α-molecular diversity, yet an increase in β-molecular diversity across ecosystems. The degree of microbial degradation, determined by the position in the soil profile, exerts a greater control on SOM molecular diversity than environmental factors, such as soil texture, moisture, and ecosystem type.

Microorganisms help govern soil organic carbon (SOC) turnover and accumulation. Whilst it is increasingly clear that microbial necromass is a precursor of SOC formation, the relationship between living microorganisms, necromass turnover and SOC persistence remains elusive. In this study, we used phospholipid fatty acids and amino sugars to quantify living versus dead microbial carbon concentrations and evaluated the utility of each pool as an indicator of SOC persistence across a range of climates (low‐, mid‐ and high‐latitude sites) and ecotypes (old‐growth forests vs. managed croplands). We found that microbial necromass was higher in forest than in cropland soils and was positively correlated with soil moisture, SOC and total nitrogen (TN). However, the flow of microbial biomass into necromass and SOC was decoupled in forest sites, likely because the high soil SOC/TN ratio accelerated necromass turnover and recycling by living microorganisms. In contrast, microbial biomass and necromass pools were tightly coupled in croplands and influenced by multiple environmental and biological factors (e.g., necromass concentrations exhibited greater variability in soils with more bacteria than fungi, and those with more gram‐positive than gram‐negative taxa). Contrasting our expectations, the proportion of microbially‐derived necromass in SOC was decoupled from soil properties and microbial biomass in both ecotypes. Whilst SOC and pH appear to be universal drivers of necromass cycling, feedbacks between living biomass, necromass and SOC are shaped by local factors. Our results contribute to ecological theory by highlighting the environmental and biological factors underpinning SOC formation and turnover that can be used to inform land‐management practices that optimize below‐ground carbon sequestration. [ABSTRACT FROM AUTHOR]

Tasmanian eucalypt forests are among the most carbon‐dense in the world, but projected climate change could destabilize this critical carbon sink. While the impact of abiotic factors on forest ecosystem carbon dynamics have received considerable attention, biotic factors such as the input of animal scat are less understood. Tasmanian devils (Sarcophilus harrisii)—an osteophageous scavenger that can ingest and solubilize nutrients locked in bone material—may subsidize plant and microbial productivity by concentrating bioavailable nutrients (e.g., nitrogen and phosphorus) in scat latrines. However, dramatic declines in devil population densities, driven by the spread of a transmissible cancer, may have underappreciated consequences for soil organic carbon (SOC) storage and forest productivity by altering nutrient cycling. Here, we fuse experimental data and modeling to quantify and predict future changes to forest productivity and SOC under various climate and scat‐quality futures. We find that devil scat significantly increases concentrations of nitrogen, ammonium, phosphorus, and phosphate in the soil and shifts soil microbial communities toward those dominated by r‐selected (e.g., fast‐growing) phyla. Further, under expected increases in temperature and changes in precipitation, devil scat inputs are projected to increase above‐ and below‐ground net primary productivity and microbial biomass carbon through 2100. In contrast, when devil scat is replaced by lower‐quality scat (e.g., from non‐osteophageous scavengers and herbivores), forest carbon pools are likely to increase more slowly, or in some cases, decline. Together, our results suggest often overlooked biotic factors will interact with climate change to drive current and future carbon pool dynamics in Tasmanian forests. [ABSTRACT FROM AUTHOR]

The vulnerability of vast carbon (C) stocks stored in Arctic soils to rapid climate warming is widely recognized. But, climate warming is also increasing plant productivity, which could either ameliorate or enhance soil C loss. The potential for new C inputs to balance losses will depend on how efficiently plant-derived C is incorporated into microbial products, the precursor of soil organic matter (SOM) formation, versus converted to CO2 and released to the atmosphere. Additionally, new plant litter and root exudates could induce a priming effect, enhancing the turnover and loss of older SOM stocks. To test whether enhanced plant productivity in the Arctic promotes new soil C formation, or increases SOM losses, we amended mineral soils collected from northern Alaska with isotopically enriched (13C and 15N) root products collected from two dominant Arctic plants: B. nana, a dwarf shrub with relatively high-quality litter, and E. vaginatum, a tussock-forming sedge with lower quality litter. We also amended soils with isotopically enriched model compounds—including glucose (no N), glycine (low C:N), and a mixture of glucose and glycine (intermediate C:N)—to test whether substrate stoichiometry controls the fate of SOM stocks. We hypothesized that (1) substrate chemistry controls the efficiency of microbial metabolism and (2) C:N ratios are sufficient in describing the fate of SOM stocks. To test these hypotheses, we used stable isotope tracing techniques to quantify the fraction of each substrate that was converted to CO2, assimilated into microbial biomass, or transformed into necromass after 10 or 30 days of incubation. We quantified the proportion of substrate associated with soil fractions (particulate, sand, or silt/clay) to estimate turnover rates of substrate remaining in mineral soils. Finally, we used ultrahigh resolution FT-ICR mass spectrometry to elucidate the chemical diversity of shrub versus shrub-derived root products, and to assess shifts in the molecular composition of metabolites following microbial metabolism during incubation.We found that model compounds induced priming responses, resulting in significant turnover of native SOM stocks. Glycine was metabolized more efficiently than glucose, resulting in a significant transformation of the metabolite profile. Specifically, higher quality compounds were consumed (high O/C), microbial derived metabolites were produced (including proteins, amino sugars, and lipids), and lower quality compounds accumulated (lignins and tannins). As microbe-derived products are more easily stabilized within the mineral matrix, the addition of higher quality substrates may contribute to SOM formation, even as high-latitude regions warm. However, while these results suggest variable C:N ratios induce different microbial responses, we found no evidence of priming when soils were amended with chemically diverse, natural root exudates. This was particularly evident in soils amended with sedge-derived root products, which increased microbial growth and SOM formation relative to soils amended with less diverse, tannin-rich shrub-derived root products. Together, our results suggest model compounds may overestimate SOM losses, and C:N ratios alone may be insufficient in predicting the fate of SOM stocks. [ABSTRACT FROM AUTHOR]