Inferred bioavailability of pyrogenic organic matter compared to natural organic matter from global sediments and surface waters

Wildfires are increasing in severity and extent, creating many negative consequences for aquatic ecosystems. Pyrogenic materials generated by wildfires are transported across terrestrial landscapes into inland waters, where ~10% of organic matter pools may be comprised of black carbon (BC), a major component of pyrogenic organic matter (PyOM). Yet, the heterogeneity of PyOM from various fuels and burn conditions complicates efforts to understand its bioavailability. We used a substrate-explicit model to predict the energy content, metabolic efficiency, and rate of aerobic decomposition of representative PyOM compounds. This enabled us to systematically evaluate a full spectrum of PyOM chemistries that is unfeasible with laboratory experiments. The model relies on the elemental stoichiometry, allowing comparison of known PyOM chemistry to formula assignments of natural organic matter (NOM) from a recent high resolution mass spectrometry assessment of global aquatic NOM. Overall, we found the range of predicted bioavailability of PyOM was similar to NOM. Phenolic and BC molecules had lower metabolic efficiency than other PyOM and NOM compounds, and BC metabolism was less negatively impacted by oxygen limitation. In total, our work supports the recent paradigm shift regarding PyOM bioavailability, highlighting its potential role in global C emissions as the prevalence of wildfires increases.

Inferred bioavailability of pyrogenic organic matter in comparison to natural organic matter from global sediments and surface waters

<p>Wildfires are increasing in frequency, severity, and area burned in response to pervasive hotter and drier conditions, creating a multitude of negative consequences for aquatic ecosystems. Pyrogenic materials generated by wildfires are transported across terrestrial landscapes into inland waters, where approximately 10% of organic matter pools is comprised of black carbon. While recent work suggests pyrogenic organic matter (PyOM) is more bioavailable than indicated by traditional paradigms, the heterogeneity of PyOM pools generated through various feedstocks and combustion scenarios complicates our efforts to understand its bioavailability. Here, we use a mathematical model to predict the energy content, metabolic efficiency, and rate of aerobic decomposition of representative PyOM compounds. We compare these metrics to model outputs derived from measured natural organic matter in global surface waters and sediments to assess differences in bioavailability. We find that PyOM generally has a similar range of bioavailabilities to that of natural systems. However, phenols and black carbon (defined as highly condensed molecules with high solubility) have lower metabolic efficiency than most representative PyOM classes and natural organic matter pools, denoted by higher lambda and lower carbon use efficiency. Rates of aerobic metabolism of phenols and black carbon are also less negatively impacted by oxygen limitation than any other group. Overall, our work suggests that PyOM may be more bioavailable than previously thought and could be an unrecognized contributor to global C emissions as the prevalence of wildfires increases.</p>

Microbial community shifts reflect losses of native soil carbon with pyrogenic and fresh organic matter additions and are greatest in low-carbon soils

Soil organic carbon (SOC) plays an important role in regulating global climate change, carbon and nutrient cycling in soils, and soil moisture. Organic matter (OM) additions to soils can affect the rate at which SOC is mineralized by microbes, with potentially important effects on SOC stocks. Understanding how pyrogenic organic matter (PyOM) affects the cycling of native SOC (nSOC) and the soil microbes responsible for these effects is important for fire-affected ecosystems as well as for biochar-amended systems. We used an incubation trial with five different soils from National Ecological Observatory Network sites across the US and 13C-labelled 350°C corn stover PyOM and fresh corn stover OM to trace nSOC-derived CO2 emissions with and without PyOM and OM amendments. We used high-throughput sequencing of rRNA genes to characterize bacterial, archaeal, and fungal communities and their response to PyOM and OM in soils that were previously stored at -80°C. We found that the effects of amendments on nSOC-derived CO2 reflected the unamended soil C status, where relative increases in C mineralization were greatest in low-C soils. OM additions produced much greater effects on nSOC-CO2 emissions than PyOM additions. Furthermore, the magnitude of microbial community composition change mirrored the magnitude of increases in nSOC-CO2, indicating a specific subset of microbes were likely responsible for the observed changes in nSOC mineralization. However, PyOM responders differed across soils and did not necessarily reflect a common “charosphere”. Overall, this study suggests that soils that already have low SOC may be particularly vulnerable to short-term increases in SOC loss with OM or PyOM additions. Importance Soil organic matter (SOM) has an important role in global climate change, carbon and nutrient cycling in soils, and soil moisture dynamics. Understanding the processes that affect SOM stocks is important for managing these functions. Recently, understanding how fire-affected organic matter (or “pyrogenic” organic matter (PyOM)) affects existing SOM stocks has become increasingly important, both due to changing fire regimes, and to interest in “biochar” – pyrogenic organic matter that is produced intentionally for carbon management or as an agricultural soil amendment. We found that soils with less SOM were more prone to increased losses with PyOM (and fresh organic matter) additions, and that soil microbial communities changed more in soils that also had greater SOM losses with PyOM additions. This suggests that soils that already have low SOM content may be particularly vulnerable to short-term increases in SOM loss, and that a subset of the soil microbial community is likely responsible for these effects.

Vertical mobility of pyrogenic organic matter in soils: a column experiment

Pyrogenic organic matter (PyOM) is a major and persistent component of soil organic matter, but its mobility and cycling in soils is largely unknown. We conducted a column experiment with a topsoil and subsoil of a sand and a sandy loam to study the mobility of highly 13C labeled ryegrass PyOM (>2.8 at. %), applied as a layer on a 7 cm long soil column, under saturated conditions. Further, we used fresh and oxidized PyOM (accelerated aging with H2O2) to identify changes in its migration through the soil with aging and associated surface oxidation. Due to the isotopic signature, we were able to trace the PyOM carbon (PyOM-C) in the soil columns, including density fractions, its effect on native soil organic carbon (nSOC) and its total export in percolates sequentially sampled after 1000–18 000 L m−2. In total, 4 %–11 % of the added PyOM-C was mobilized and <1 % leached from the columns. The majority of PyOM-C was mobilized with the first flush of 1000 L m−2 (51 %–84 % of exported PyOM-C), but its export was ongoing for the sandy soil and the loamy subsoil. Oxidized PyOM showed a 2–7 times higher mobility than fresh PyOM. In addition, 2-fold higher quantities of oxidized PyOM-C were leached from the sandy soil compared to the loamy soil. Besides the higher mobility of oxidized PyOM, its retention in both soils increased due to an increased reactivity of the oxidized PyOM surfaces and enhanced the interaction with the soil mineral phase. Density fractionation of the upper 0–2.3 cm, below the PyOM application layer, revealed that up to 40 % of the migrated PyOM was associated with the mineral phase in the loamy soil, highlighting the importance of mineral interaction for the long-term fate of PyOM in soils. The nSOC export from the sandy soil significantly increased by 48 %–270 % with addition of PyOM compared to the control, while no effect was found for the loamy soil after the whole percolation. Due to its high sorption affinity towards the soil mineral phase, PyOM can mobilize mineral-associated soil organic matter in coarse-textured soils, where organo-mineral interactions are limited, while finer-textured soils have the ability to re-adsorb the mobilized soil organic matter. Our results show that the vertical mobility of PyOM in soils is limited to a small fraction. Aging (oxidation) increases this fraction but also increases the PyOM surface reactivity and thus its long-term retention in soils. Moreover, the migration of PyOM affects the cycling of nSOC in coarse soils and thus influences the carbon cycle of fire-affected soils.

Microbial community shifts reflect losses of native soil carbon with pyrogenic and fresh organic matter additions and are greatest in low-carbon soils

Soil organic carbon (SOC) plays an important role in regulating global climate change, carbon and nutrient cycling in soils, and soil moisture. Organic matter (OM) additions to soils can affect the rate at which SOC is mineralized by microbes, with potentially important effects on SOC stocks. Understanding how pyrogenic organic matter (PyOM) affects the cycling of native SOC (nSOC) and the soil microbes responsible for these effects is important for fire-affected ecosystems as well as for biochar-amended systems. We used an incubation trial with five different soils from National Ecological Observatory Network sites across the US and 13C-labelled 350°C corn stover PyOM and fresh corn stover OM to trace nSOC-derived CO2 emissions with and without PyOM and OM amendments. We used high-throughput sequencing of rRNA genes to characterize bacterial, archaeal, and fungal communities and their response to PyOM and OM. We found that the effects of amendments on nSOC-derived CO2 reflected the unamended soil C status, where amendments increased C mineralization the most in low-C soils. OM additions produced much greater effects on nSOC-CO2 emissions than PyOM additions. Furthermore, the magnitude of microbial community composition change mirrored the magnitude of increases in nSOC-CO2, indicating a specific subset of microbes were likely responsible for the observed changes in nSOC mineralization. However, PyOM responders differed across soils and did not necessarily reflect a common “charosphere”. Overall, this study suggests that soils that already have low SOC may be particularly vulnerable to short-term increases in SOC loss with OM or PyOM additions.ImportanceSoil organic matter (SOM) has an important role in global climate change, carbon and nutrient cycling in soils, and soil moisture dynamics. Understanding the processes that affect SOM stocks is important for managing these functions. Recently, understanding how fire-affected, or “pyrogenic” organic matter (PyOM) affects existing SOM stocks has become increasingly important, both due to changing fire regimes, and to interest in “biochar” – pyrogenic organic matter that is produced intentionally for carbon management or as an agricultural soil amendment. We found that soils with less SOM were more prone to increased losses with PyOM (and fresh organic matter) additions, and that soil microbial communities changed more in soils that also had greater SOM losses with PyOM additions. This suggests that soils that already have low SOM content may be particularly vulnerable to short-term increases in SOM loss, and that a subset of the soil microbial community is likely responsible for these effects.