Nitrogen speciation and transformations in fire-derived organic matter

A potential slow-release fertilizer based on biogas residue biochar: Nutrient release patterns and synergistic mechanism for improving soil fertility

The large yield of anaerobic digestates and the suboptimal efficacy of nutrient slow-release severely limit its practical application. To address these issues, a new biochar based fertilizer (MAP@BRC) was developed using biogas residue biochar (BRC) to recover nitrogen and phosphorus from biogas slurry. The nutrient release patterns of MAP@BRC and mechanisms for enhancing soil fertility were studied, and it demonstrated excellent performance, with 59% total nitrogen and 50% total phosphorus nutrient release rates within 28 days. This was attributed to the coupling of the mechanism involving the dissolution of struvite skeletons and the release of biochar pores. Pot experiments showed that crop yield and water productivity were doubled in the MAP@BRC group compared with unfertilized planting. The application of MAP@BRC also improved soil nutrient levels, reduced soil acidification, increased microbial populations, and decreased soil heavy metal pollution risk. The key factors that contributed to the improvement in soil fertility by MAP@BRC were an increase in available nitrogen and the optimization of pH levels in the soil. Overall, MAP@BRC is a safe, slow-release fertilizer that exhibits biochar-fertilizer interactions and synergistic effects. This slow-release fertilizer was prepared by treating a phosphorus-rich biogas slurry with a nitrogen-rich biogas slurry, and it simultaneously addresses problems associated with livestock waste treatment and provides a promising strategy to promote zero-waste agriculture.

Fire Impacts on the Soil Metabolome and Organic Matter Biodegradability

Global wildfire activity has increased since the 1970s and is projected to intensify throughout the 21st century. Wildfires change the composition and biodegradability of soil organic matter (SOM) which contains nutrients that fuel microbial metabolism. Though persistent forms of SOM often increase postfire, the response of more biodegradable SOM remains unclear. Here we simulated severe wildfires through a controlled "pyrocosm" approach to identify biodegradable sources of SOM and characterize the soil metabolome immediately postfire. Using microbial amplicon (16S/ITS) sequencing and gas chromatography-mass spectrometry, heterotrophic microbes (Actinobacteria, Firmicutes, and Protobacteria) and specific metabolites (glycine, protocatechuate, citric cycle intermediates) were enriched in burned soils, indicating that burned soils contain a variety of substrates that support microbial metabolism. Molecular formulas assigned by 21 T Fourier transform ion cyclotron resonance mass spectrometry showed that SOM in burned soil was lower in molecular weight and featured 20 to 43% more nitrogen-containing molecular formulas than unburned soil. We also measured higher water extractable organic carbon concentrations and higher CO2 efflux in burned soils. The observed enrichment of biodegradable SOM and microbial heterotrophs demonstrates the resilience of these soils to severe burning, providing important implications for postfire soil microbial and plant recolonization and ecosystem recovery.

Soil microbiome feedbacks during disturbance-driven forest ecosystem conversion

Disturbances cause rapid changes to forests, with different disturbance types and severities creating unique ecosystem trajectories that can impact the underlying soil microbiome. Pile burning-the combustion of logging residue on the forest floor-is a common fuel reduction practice that can have impacts on forest soils analogous to those following high-severity wildfire. Further, pile burning following clear-cut harvesting can create persistent openings dominated by nonwoody plants surrounded by dense regenerating conifer forest. A paired 60-year chronosequence of burn scar openings and surrounding regenerating forest after clear-cut harvesting provides a unique opportunity to assess whether belowground microbial processes mirror aboveground vegetation during disturbance-induced ecosystem shifts. Soil ectomycorrhizal fungal diversity was reduced the first decade after pile burning, which could explain poor tree seedling establishment and subsequent persistence of herbaceous species within the openings. Fine-scale changes in the soil microbiome mirrored aboveground shifts in vegetation, with short-term changes to microbial carbon cycling functions resembling a postfire microbiome (e.g. enrichment of aromatic degradation genes) and respiration in burn scars decoupled from substrate quantity and quality. Broadly, however, soil microbiome composition and function within burn scar soils converged with that of the surrounding regenerating forest six decades after the disturbances, indicating potential microbial resilience that was disconnected from aboveground vegetation shifts. This work begins to unravel the belowground microbial processes that underlie disturbance-induced ecosystem changes, which are increasing in frequency tied to climate change.

Assess the formation of disinfection by-products from pyrogenic dissolved organic matter (pyDOM): impact of wildfire on the water quality of forest watershed

Wildfires can release pyrogenic dissolved organic matter (pyDOM) into the forest watershed, which may pose challenges for water treatment operations downstream due to the formation of disinfection by-products (DBPs). In this study, we systematically assessed the physio-chemical properties of pyDOM (e.g., electron-donating and -accepting capacities; EDC and EAC) and their contributions to DBP formation under different disinfection scenarios using (1) ten lab samples produced from various feedstocks and pyrolysis temperatures, and (2) pre- and post-fire field samples with different burning severities. A comprehensive suite of DBPs-four trihalomethanes (THMs), nine haloacetic acids (HAAs), and seven N-nitrosamines-were included. The formations of THM and HAA showed an up to 5.7- and 8.9-fold decrease as the pyrolysis temperature increased, while the formation of N-nitrosamines exhibited an up to 6.6-fold increase for the laboratory-derived pyDOM. These results were supported by field pyDOM samples, where the post-fire samples consistently showed a higher level of N-nitrosamine formation (i.e., up to 5.3-fold), but lower THMs and HAAs compared to the pre-fire samples. To mimic environmental reducing conditions, two field samples were further reduced electrochemically and compared with Suwannee River natural organic matter (SRNOM) to evaluate their DBP formation. We found increased DBP formation in pyDOM samples following electrochemical reduction but not for SRNOM, which showed increased N-nitrosamines but decreased THMs and HAAs post-electrochemical reduction. Furthermore, this study reported for the first time the formation of two previously overlooked N-nitrosamines (i.e., nitrosodiethylamine (NDEA), N-nitrosodi-n-propylamine (NDPA)) in both laboratory and field pyDOM samples, raising concerns for drinking water safety given their higher toxicity as compared to the regulated counterparts. Results from this study provide new insights for DBP mitigation during post-fire recovery, which are particularly relevant to communities that rely on forest watersheds as their drinking water sources.

Experimentally determined traits shape bacterial community composition one and five years following wildfire

Wildfires represent major ecological disturbances, burning 2-3% of Earth's terrestrial area each year with sometimes drastic effects above- and belowground. Soil bacteria offer an ideal, yet understudied system within which to explore fundamental principles of fire ecology. To understand how wildfires restructure soil bacterial communities and alter their functioning, we sought to translate aboveground fire ecology to belowground systems by determining which microbial traits are important post-fire and whether changes in bacterial communities affect carbon cycling. We employed an uncommon approach to assigning bacterial traits, by first running three laboratory experiments to directly determine which microbes survive fires, grow quickly post-fire and/or thrive in the post-fire environment, while tracking CO2 emissions. We then quantified the abundance of taxa assigned to each trait in a large field dataset of soils one and five years after wildfires in the boreal forest of northern Canada. We found that fast-growing bacteria rapidly dominate post-fire soils but return to pre-burn relative abundances by five years post-fire. Although both fire survival and affinity for the post-fire environment were statistically significant predictors of post-fire community composition, neither are particularly influential. Our results from the incubation trials indicate that soil carbon fluxes post-wildfire are not likely limited by microbial communities, suggesting strong functional resilience. From these findings, we offer a traits-based framework of bacterial responses to wildfire.

Nitrogen availability in biochar-based fertilizers depending on activation treatment and nitrogen source

Different activation and N-doping treatments were used to produce biochar-based fertilizers (BBFs) with increased N concentration and slow N release. Pristine biochars were produced by pyrolysis of olive tree pruning feedstock at low and high temperatures (400 and 800 °C). These biochars were activated either by ultrasonication, or oxidation with hydrogen peroxide (H₂O₂) or nitric acid (HNO₃) to increase their N retention potential. Subsequently biochars were enriched with N with either urea or ammonium sulfate. The activation of low-temperature biochars with HNO₃ was the most effective treatment leading to new surface carboxylic groups that facilitated the later enrichment with N. When treated with urea, BBFs reached 7.0 N%, whereas the H₂O₂ activation only allowed an increase up to 2.0 N%. The use of urea as the external N source was the most efficient for incorporating N. Urea treated biochars had a water-soluble fraction that represented up to 14.5 % of the total N. The hydrolyzable N fraction, composed by amides and simple N heterocycles originated by the N-doping treatments, and nitro groups generated from HNO₃ activation, represented up to 60 % of the total N. This study relates the N chemical forms in the new BBFs to potential N availability in soil. The presence of water-soluble, hydrolyzable and non-hydrolyzable N implied that these BBFs may supply N that would be progressively available for plants, acting as slow-release fertilizers.

Molecular transformation of organic nitrogen in Antarctic penguin guano-affected soil

Organic nitrogen (ON) is an important participant in the Earth's N cycle. Previous studies have shown that penguin feces add an abundance of nutrients including N to the soil, significantly changing the eco-environment in ice-free areas in Antarctica. To explore the molecular transformation of ON in penguin guano-affected soil, we collected guano-free weathered soil, modern guano-affected soil from penguin colonies, ancient guano-affected soil from abandoned penguin colonies, and penguin feces from the Ross Sea region, Antarctica, and Fourier transform ion cyclotron mass spectrometry (FT-ICR MS) was used to investigate the chemical composition of water-extractable ON. By comparing the molecular compositions of ON among different samples, we found that the number of ON compounds (>4,000) in weathered soil is minimal, while carboxylic-rich alicyclic-like molecules (CRAM-like) are dominant. Penguin feces adds ON into the soil with > 10,000 CHON, CHONS and CHN compounds, including CRAM-like, lipid-like, aliphatic/ peptide-like molecules and amines in the guano-affected soil. After the input of penguin feces, macromolecules continue to degrade, and other ON compounds tend to be oxidized into relatively stable CRAM-like molecules, this is an important transformation process of ON in guano-affected soils. We conclude the roles of various forms of ON in the N cycle are complex and diverse. Combined with previous studies, ON eventually turns into inorganic N and is lost from the soil. The lost N ultimately returns to the ocean and the food web, thus completing the N cycle. Our study preliminarily reveals the molecular transformation of ON in penguin guano-affected soil and is important for understanding the N cycle in Antarctica.

Enhanced Speciation of Pyrogenic Organic Matter from Wildfires Enabled by 21 T FT-ICR Mass Spectrometry

Wildfires affect soils through the formation of pyrogenic organic matter (pyOM) (e.g., char and soot). While many studies examine the connection between pyOM persistence and carbon (C) composition, nitrogen (N) transformation in wildfire-impacted systems remains poorly understood. Thermal reactions in wildfires transform biomass into a highly complex, polyfunctional, and polydisperse organic mixture that challenges most mass analyzers. High-field Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is the only mass analyzer that achieves resolving powers sufficient to separate species that differ in mass by the mass of an electron across a wide molecular weight range (m/z 150-1500). We report enhanced speciation of organic N by positive-ion electrospray ionization (ESI) that leverages ultrahigh resolving power (m/Δm_50% = 1 800 000 at m/z 400) and mass accuracy (<10-100 ppb) achieved by FT-ICR MS at 21 T. Isobaric overlaps, roughly the mass of an electron (M_e^- = 548 μDa), are resolved across a wide molecular weight range and are more prevalent in positive ESI than negative ESI. The custom-built 21 T FT-ICR MS instrument identifies previously unresolved mass differences in C_cH_hN_nO_oS_s formulas and assigns more than 30 000 peaks in a pyOM sample. This is the first molecular catalogue of pyOM by positive-ion ESI 21 T FT-ICR MS and presents a method to provide new insight into terrestrial cycling of organic carbon and nitrogen in wildfire impacted ecosystems.

Photochemical reactivity of nitrogen-doped biochars under simulated sunlight irradiation: Generation of singlet oxygen

This study explored the photochemical activity of nitrogen-doped biochars (NCMs) by investigating their role in the degradation of sulfamethazine under simulated sunlight irradiation. NCMs with different doping amounts were prepared from corn straw and urea. Results showed that nitrogen doping can notably enhance the photodegradation of SMT rather than raw char. NCMs are of photochemical activity under visible light, which was confirmed by monochromatic light experiments. Quenching experiments, ESR, pH effect, and the influence of O₂ were carried out to explore the involved oxidation mechanism in this system. Results showed that ¹O₂ was the main reactive oxygen species. ¹O₂ was produced from O₂ by both energy transfer and electron transfer. DFT calculations showed that pyridinic N doping can decrease the energy of intersystem crossing and thus benefit the generation of ¹O₂ by triplet-triplet energy transfer. Results underscore the explicit importance of nitrogen element in photochemical reactivity of chars under simulated light irradiation even when the nitrogen content is low. It is a meaningful reminder for us to pay more attention to the assessment of the fate and transport of contaminants in the soil where it is rich in NCMs as well as the potential use of NCMs for pollutants remediation, since visible light is very abundant near the earth's surface.

Recycling pyrolyzed organic waste from plant nurseries, rice production and shrimp industry as peat substitute in potting substrates

Organic waste from greens of tomato plants, gardening substrate, rice husks and shrimp-derived chitin were pyrolyzed at 400 °C and 500 °C for 3 h, with the aim to elucidate the feasibility of using such products as replacement of peat in soilless gardening substrates. Characterization of the carbonized organic matter (COM) and the gardening substrate indicated that neither the peat nor the COMs provided the recommended levels of nutrients for the cultivation of tomato plants, although improvements could be obtained using COM/substrate mixtures. The toxicity thresholds for Zn were exceeded significantly by the COMs of the tomato greens and high boron levels were found for all the COMs except for those derived from chitin. In a 40-days pot experiment, germination and development of tomato seeds and plants (Solanum lycopersicum L.) were tested on COM/peat mixtures at 30%, 60% and 100% COM substitution rate. The lack of seed germination on the mixtures with COM from tomato greens is best explained with the high salinity of the COM. Best plant growth was obtained with COM from chitin at 60%, most likely because its high N content satisfied best the N-needs of the growing tomato plants without increasing the pH of the growing media. Moreover, an increase of water retention was evidenced for COM/substrate mixtures. Although the use of COM from chitin and rice husks showed promising results, the proposed recycling of organic waste from agriculture or fishery as soilless gardening substrate requires the development of formulations of COM/peat/and added nutrients with ready-to-use characteristics to increase its feasibility.

Ammonia volatilization from composting with oxidized biochar

Animal manure, agricultural residues, and other sources of biomass can be diverted from the waste stream and composted into valuable fertilizer. However, composting often results in substantial N loss through NH₃ gas volatilization. We investigated biochar's capacity to improve NH₃ -N retention during composting of poultry manure and straw. After 7 wk, total N loss from composting with unoxidized biochar was twofold and sixfold higher than N loss from composting with oxidized biochar and without biochar (307, 142, and 51 mg N g^-1 N in the initial compost feedstocks, respectively). When cumulative NH₃ -N loss was calculated relative to CO₂ -C loss to account for differences in microbial activity, NH₃ -N/CO₂ -C loss from compost with oxidized biochar was 55% lower than from compost with unoxidized biochar (82% lower based on mass balance). Oxidized biochar particles removed from compost after 7 wk retained 16.0 mg N g^-1 biochar, compared with only 6.1 mg N g^-1 retained by unoxidized biochar, suggesting that N retention by biochar particles provides a mechanism for reduced NH₃ -N loss. These data show that oxidized biochar enhanced microbial activity, doubled composting rate, and reduced NH₃ -N loss compared with unoxidized biochar and that biochar's physiochemical characteristics modulate its performance in compost. In particular, the presence of oxidized surface functional groups, which can be increased artificially or through environmental weathering, appear to play an important role in key compost processes. This has implications for other natural and managed systems where pyrogenic organic matter may mediate biological activity and nutrient cycles.

Sequential Ammonia and Carbon Dioxide Adsorption on Pyrolyzed Biomass to Recover Waste Stream Nutrients

The amine-rich surfaces of pyrolyzed human solid waste (py-HSW) can be "primed" or "regenerated" with carbon dioxide (CO₂) to enhance their adsorption of ammonia (NH₃) for use as a soil amendment. To better understand the mechanism by which CO₂ exposure facilitates NH₃ adsorption to py-HSW, we artificially enriched a model sorbent, pyrolyzed, oxidized wood (py-ox wood) with amine functional groups through exposure to NH₃. We then exposed these N-enriched materials to CO₂ and then resorbed NH₃. The high heat of CO₂ adsorption (Q _st) on py-HSW, 49 kJ mol^-1, at low surface coverage, 0.4 mmol CO₂ g^-1, showed that the naturally occurring N compounds in py-HSW have a high affinity for CO₂. The Q _st of CO₂ on py-ox wood also increased after exposure to NH₃, reaching 50 kJ mol^-1 at 0.7 mmol CO₂ g^-1, demonstrating that the incorporation of N-rich functional groups by NH₃ adsorption is favorable for CO₂ uptake. Adsorption kinetics of py-ox wood revealed continued, albeit diminishing NH₃ uptake after each CO₂ treatment, averaging 5.9 mmol NH₃ g^-1 for the first NH₃ exposure event and 3.5 and 2.9 mmol NH₃ g^-1 for the second and third; the electrophilic character of CO₂ serves as a Lewis acid, enhancing surface affinity for NH₃ uptake. Furthermore, penetration of ¹⁵NH₃ and ¹³CO₂ measured by NanoSIMS reached over 7 μm deep into both materials, explaining the large NH₃ capture. We expected similar NH₃ uptake in py-HSW sorbed with CO₂ and py-ox wood because both materials, py-HSW and py-ox wood sorbed with NH₃, had similar N contents and similarly high CO₂ uptake. Yet NH₃ sorption in py-HSW was unexpectedly low, apparently from potassium (K) bicarbonate precipitation, reducing interactions between NH₃ and sorbed CO₂; 2-fold greater surface K in py-HSW was detected after exposure to CO₂ and NH₃ than before gas exposure. We show that amine-rich pyrolyzed waste materials have high CO₂ affinity, which facilitates NH₃ uptake. However, high ash contents as found in py-HSW hinder this mechanism.