Are we losing it? Exploring subsoil organic carbon dynamics in a warming world on the molecular level

Soils store around 1500-2400Gt carbon, half of which is stored in subsoils – here defined as soil below 20cm depth. The carbon in soils accounts for two to three times the amount stored in the atmosphere, making soils a highly relevant carbon pool in the global carbon cycle. Soils are expected to experience unprecedented warming throughout the whole profiles with only a slight delay to the surface air temperature. As temperature affects any (bio)chemical reaction, fluxes of carbon between soils and the atmosphere will change. If warming stimulates microbial activity, inducing losses of carbon from soils to the atmosphere and plant inputs of carbon to soils can’t compensate for this loss, atmospheric carbon concentrations will increase. Recent experiments warming the subsoil indicate that the temperature sensitivity of microbial respiration is similar throughout whole soil profiles. Thus, also the large subsoil organic carbon pool is vulnerable to decomposition under global warming. However, we know very little about processes governing subsoil carbon dynamics under warming, which might be markedly different compared to topsoils. This limits predictions about the magnitude and duration of potential releases of subsoil organic carbon to the atmosphere. The magnitude and duration of microbial respiration might be controlled by carbon availability and/or adaptation of the microorganisms to the new environmental conditions. How these processes balance out in subsoil is largely unknown. Furthermore, it is unclear whether microorganisms will feed on any molecular organic carbon compounds in soils or whether specific compounds could be more resistant to degradation under warming. To address these knowledge gaps, this thesis makes use of a novel experimental setup at Blodgett Experimental Forest, CA, USA, which continuously warms soils to 1m depth, considering vertical and seasonal gradients in temperature. The aim of this thesis was to explore subsoil carbon dynamics in a warming world on the molecular level, with a focus on the microbial response and the vulnerability of various molecular compounds to warming induced degradation. In this thesis I highlight that subsoil carbon dynamics might respond to warming more quickly as compared to topsoils. Microbial abundance decreased by -28±9% in warmed subsoil, but was not affected in topsoil. Lower microbial abundance in subsoil was accompanied by changes in microbial physiology and community composition. Lower microbial abundance did not cause a decrease in soil respiration, which was continually elevated. Thus, the smaller microbial community might have adapted to the lower carbon concentrations and quality, using the carbon less efficiently and releasing proportionally more carbon to the atmosphere. Microorganisms did not only degrade simple molecular structures, but also complex polymers such as hydrolysable lipids (-28±3%), lignin phenols (-17±0%) and pyrogenic carbon (-37±8%) were lost from warmed subsoils. These results show that plant polymers and pyrogenic carbon are equally vulnerable to degradation like bulk soil organic carbon in the subsoils studied in this thesis. On the one hand, this could be because complex plant polymers are often not stabilized on mineral surfaces and thus accessible to microbial degradation. On the other hand, degradation of polymers might be especially stimulated by warming because the multiple enzymatic steps needed for the degradation are facilitated by higher temperatures. These results propose that there might not be specific ‘heat-proof compounds’ in (sub)soil but that persistence of soil organic carbon under global warming depends on multiple biogeochemical factors. In conclusion, the novel in situ whole soil warming experiment gives insight for the first time into the response of subsoil microorganisms to warming and how subsoil organic carbon quality changes with warming-induced degradation. The results improve our understanding of subsoil carbon dynamics under global warming and show that subsoil organic carbon might be more vulnerable to decomposition under warming compared to topsoil organic carbon. Furthermore, complex plant and pyrogenic polymers were lost on the same order of magnitude as bulk soil organic carbon in subsoils. This finding improves our understanding on the long-term fate of compounds which are considered for carbon sequestration purposes. Finally, the thesis opens up exciting avenues for research on subsoil carbon dynamics under global change, concerning the long-term response of subsoils to warming, dynamics in subsoils of other ecosystems and interactions of warming with other global change drivers

Rapid loss of complex polymers and pyrogenic carbon in subsoils under whole-soil warming

Subsoils contain more than half of soil organic carbon (SOC) and are expected to experience rapid warming in the coming decades. Yet our understanding of the stability of this vast carbon pool under global warming is uncertain. In particular, the fate of complex molecular structures (polymers) remains debated. Here we show that 4.5 years of whole-soil warming (+4 °C) resulted in less polymeric SOC (sum of specific polymers contributing to SOC) in the warmed subsoil (20–90 cm) relative to control, with no detectable change in topsoil. Warming stimulated the subsoil loss of lignin phenols (−17 ± 0%) derived from woody plant biomass, hydrolysable lipids cutin and suberin, derived from leaf and woody plant biomass (−28 ± 3%), and pyrogenic carbon (−37 ± 8%) produced during incomplete combustion. Given that these compounds have been proposed for long-term carbon sequestration, it is notable that they were rapidly lost in warmed soils. We conclude that complex polymeric carbon in subsoil is vulnerable to decomposition and propose that molecular structure alone may not protect compounds from degradation under future warming

Whole-soil warming decreases abundance and modifies the community structure of microorganisms in the subsoil but not in surface soil

The microbial community composition in subsoils remains understudied, and it is largely unknown whether subsoil microorganisms show a similar response to global warming as microorganisms at the soil surface do. Since microorganisms are the key drivers of soil organic carbon decomposition, this knowledge gap causes uncertainty in the predictions of future carbon cycling in the subsoil carbon pool (> 50 % of the soil organic carbon stocks are below 30 cm soil depth). In the Blodgett Forest field warming experiment (California, USA) we investigated how +4 ∘C warming in the whole-soil profile to 100 cm soil depth for 4.5 years has affected the abundance and community structure of microorganisms. We used proxies for bulk microbial biomass carbon (MBC) and functional microbial groups based on lipid biomarkers, such as phospholipid fatty acids (PLFAs) and branched glycerol dialkyl glycerol tetraethers (brGDGTs). With depth, the microbial biomass decreased and the community composition changed. Our results show that the concentration of PLFAs decreased with warming in the subsoil (below 30 cm) by 28 % but was not affected in the topsoil. Phospholipid fatty acid concentrations changed in concert with soil organic carbon. The microbial community response to warming was depth dependent. The relative abundance of Actinobacteria increased in warmed subsoil, and Gram+ bacteria in subsoils adapted their cell membrane structure to warming-induced stress, as indicated by the ratio of anteiso to iso branched PLFAs. Our results show for the first time that subsoil microorganisms can be more affected by warming compared to topsoil microorganisms. These microbial responses could be explained by the observed decrease in subsoil organic carbon concentrations in the warmed plots. A decrease in microbial abundance in warmed subsoils might reduce the magnitude of the respiration response over time. The shift in the subsoil microbial community towards more Actinobacteria might disproportionately enhance the degradation of previously stable subsoil carbon, as this group is able to metabolize complex carbon sources

Methylation procedures affect PLFA results more than selected extraction parameters

Microorganisms are key players in organic matter and nutrient cycles of terrestrial ecosystems. The analysis of microbial membrane lipids, phospholipid fatty acids (PLFAs) has strongly improved our understanding of how microbial processes contribute to these cycles. The analysis has proven to yield robust results, but adaptations of analytical parameters to laboratory needs might lead to pitfalls and impede comparability of PLFA results between different studies. Here, we show how a set of four analytical parameters (freeze-drying vs. field moist, amount of sample extracted, age of solvent mixture, and methylation methods) influence the quantitative and qualitative results of PLFA analysis. Freeze-drying vs. field moist samples and the amount of sample extracted had only minor effects on PLFA concentrations and recovery of the microbial community structure. Nevertheless, these parameters are important to consider, especially if treatment effects in an experiment are expected to be low. The use of a four weeks old extraction solution resulted in 12% lower PLFA concentrations as well as significant differences in the relative abundance of functional microbial groups. This suggests that extraction solution should be prepared on the day of extraction or that the different components of the extraction solution should be added sequentially to the sample. Most importantly, the choice of the methylation method led to differences in both, PLFA concentrations (35%) and the relative abundance of functional microbial groups, making comparisons between studies difficult. Our study provides a valuable ranking of parameters that need to be considered during PLFA method implementation in a laboratory and also highlights the fact that comparability of studies using different methylation methods might be limited

Rapid loss of complex polymers and pyrogenic carbon in subsoils under whole-soil warming

Subsoils contain more than half of soil organic carbon (SOC) and are expected to experience rapid warming in the coming decades. Yet our understanding of the stability of this vast carbon pool under global warming is uncertain. In particular, the fate of complex molecular structures (polymers) remains debated. Here we show that 4.5 years of whole-soil warming (+4 °C) resulted in less polymeric SOC (sum of specific polymers contributing to SOC) in the warmed subsoil (20-90 cm) relative to control, with no detectable change in topsoil. Warming stimulated the subsoil loss of lignin phenols (-17 ± 0%) derived from woody plant biomass, hydrolysable lipids cutin and suberin, derived from leaf and woody plant biomass (-28 ± 3%), and pyrogenic carbon (-37 ± 8%) produced during incomplete combustion. Given that these compounds have been proposed for long-term carbon sequestration, it is notable that they were rapidly lost in warmed soils. We conclude that complex polymeric carbon in subsoil is vulnerable to decomposition and propose that molecular structure alone may not protect compounds from degradation under future warming

Warming promotes loss of subsoil carbon through accelerated degradation of plant-derived organic matter

Increasing global temperatures have the potential to stimulate decomposition and alter the composition of soil organic matter (SOM). However, questions remain about the extent to which SOM quality and quantity along the soil profile may change under future warming. In this study we assessed how +4 °C whole-soil warming affected the quantity and quality of SOM down to 90 cm depth in a mixed-coniferous temperate forest using biomarker analyses. Our findings indicate that 4.5 years of soil warming led to divergent responses in subsoils (>20 cm) as compared to surface soils. Warming enhanced the accumulation of plant-derived n-alkanes over the whole soil profile. In the subsoil, this was at the expense of plant- and microorganism-derived fatty acids, and the relative abundance of SOM molecular components shifted from less microbially transformed to more transformed organic matter. Fine root mass declined by 24.0 ± 7.5% with warming over the whole soil profile, accompanied by reduced plant-derived inputs and accelerated decomposition of aromatic compounds and plant-derived fatty acids in the subsoils. Our study suggests that warming accelerated microbial decomposition of plant-derived inputs, leaving behind more degraded organic matter. The non-uniform, and depth dependent SOM composition and warming response implies that subsoil carbon cycling is as sensitive and complex as in surface soils

Warming and elevated CO2 promote incorporation of plant-derived lipids into soil organic matter in a spruce-dominated ombrotrophic bog

More than one third of global soil organic matter (SOM) is stored in peatlands, despite them occupying less than 3% of the land surface. Increasing global temperatures have the potential to stimulate the decomposition of carbon stored in peatlands, contributing to the release of disproportionate amounts of greenhouse gases to the atmosphere but increasing atmospheric CO2 concentrations may stimulate photosynthesis and return C into ecosystems. Key questions remain about the magnitude and rate of these interacting and opposite processes to environmental change drivers. We assessed the impact of a 0–9°C temperature gradient of deep peat warming (4 years of warming; 0-200 cm depth) in ambient or elevated CO2 (2 years of +500 ppm CO2 addition) on the quantity and quality of SOM at the climate change manipulation experiment SPRUCE (Spruce and Peatland Responses Under Changing Environments) in Minnesota USA. We assessed how warming and elevated CO2 affect the degradation of plant and microbial residues as well as the incorporation of these compounds into SOM. Specifically, we combined the analyses of free extractable n-alkanes and fatty acids together with measurements of compound-specific stable carbon isotopes (δ13C). We observed a 6‰ offset in δ13C between bulk SOM and n-alkanes, which were uniformly depleted in δ13C when compared to bulk organic matter. Such an offset between SOM and n-alkanes is common due to biosynthetic isotope fractionation processes and confirms previous findings. After 4 years of deep peat warming, and 2 years of elevated CO2 addition a strong depth-specific response became visible with changes in SOM quantity and quality. In the upper 0-30 cm depth, individual n-alkanes and fatty acid concentrations declined with increasing temperatures with warming treatments, but not below 50 cm depth. In turn, the δ13C values of bulk organic matter and of individual n-alkanes and fatty acids increased in the upper 0-30 cm with increasing temperatures, but not below 50 cm depth. Thus n-alkanes, which typically turnover slower than bulk SOM, underwent a rapid transformation after a relatively short period of simulated warming in the acrotelm. Our results suggest that warming accelerated microbial decomposition of plant-derived lipids, leaving behind more degraded organic matter. The non-uniform, and depth dependent warming response implies that warming will have cascading effects on SOM decomposition in the acrotelm in peatlands. It remains to be seen how fast the catotelm will respond to rising temperatures and atmospheric CO2 concentrations

Warming promotes loss of subsoil carbon through accelerated degradation of plant-derived organic matter

Increasing global temperatures have the potential to stimulate decomposition and alter the composition of soil organic matter (SOM). However, questions remain about the extent to which SOM quality and quantity along the soil profile may change under future warming. In this study we assessed how +4 °C whole-soil warming affected the quantity and quality of SOM down to 90 cm depth in a mixed-coniferous temperate forest using biomarker analyses. Our findings indicate that 4.5 years of soil warming led to divergent responses in subsoils (>20 cm) as compared to surface soils. Warming enhanced the accumulation of plant-derived n-alkanes over the whole soil profile. In the subsoil, this was at the expense of plant- and microorganism-derived fatty acids, and the relative abundance of SOM molecular components shifted from less microbially transformed to more transformed organic matter. Fine root mass declined by 24.0 ± 7.5% with warming over the whole soil profile, accompanied by reduced plant-derived inputs and accelerated decomposition of aromatic compounds and plant-derived fatty acids in the subsoils. Our study suggests that warming accelerated microbial decomposition of plant-derived inputs, leaving behind more degraded organic matter. The non-uniform, and depth dependent SOM composition and warming response implies that subsoil carbon cycling is as sensitive and complex as in surface soils.ISSN:0038-0717ISSN:1879-342

Warming promotes loss of subsoil carbon through accelerated degradation of plant-derived organic matter

Increasing global temperatures have the potential to stimulate decomposition and alter the composition of soil organic matter (SOM). However, questions remain about the extent to which SOM quality and quantity along the soil profile may change under future warming. In this study we assessed how +4 °C whole-soil warming affected the quantity and quality of SOM down to 90 cm depth in a mixed-coniferous temperate forest using biomarker analyses. Our findings indicate that 4.5 years of soil warming led to divergent responses in subsoils (>20 cm) as compared to surface soils. Warming enhanced the accumulation of plant-derived n-alkanes over the whole soil profile. In the subsoil, this was at the expense of plant- and microorganism-derived fatty acids, and the relative abundance of SOM molecular components shifted from less microbially transformed to more transformed organic matter. Fine root mass declined by 24.0 ± 7.5% with warming over the whole soil profile, accompanied by reduced plant-derived inputs and accelerated decomposition of aromatic compounds and plant-derived fatty acids in the subsoils. Our study suggests that warming accelerated microbial decomposition of plant-derived inputs, leaving behind more degraded organic matter. The non-uniform, and depth dependent SOM composition and warming response implies that subsoil carbon cycling is as sensitive and complex as in surface soils