Effect of Mineral and Organic Soil Constituents on Microbial Mineralization of Organic Compounds in a Natural Soil

This research addressed the effect of mineral and organic soil constituents on the fate of organic compounds in soils. Specifically, it sought to determine how the associations between organic chemicals and different soil constituents affect their subsequent biodegradation in soil. Four ¹⁴C-labeled surfactants were aseptically adsorbed to montmorillonite, kaolinite, illite, sand, and humic acids. These complexes were mixed with a woodlot soil, and ¹⁴CO₂ production was measured over time. The mineralization data were fitted to various production models by nonlinear regression, and a mixed (3/2)-order model was found to most accurately describe the mineralization patterns. Different mineralization patterns were observed as a function of the chemical and soil constituents. Surfactants that had been preadsorbed to sand or kaolinite usually showed similar mineralization kinetics to the control treatments, in which the surfactants were added to the soil as an aqueous solution. Surfactants that had been bound to illite or montmorillonite were typically degraded to lesser extents than the other forms, while surfactant-humic acid complexes were degraded more slowly than the other forms. The desorption coefficients (K_d) of the soil constituent-bound surfactants were negatively correlated with the initial rates of degradation (k₁) and estimates of ¹⁴CO₂ yield (P_o) as well as actual total yields of ¹⁴CO₂. However, there was no relationship between K_d and second-stage zero-order rates of mineralization (k_o). Microbial community characteristics (biomass and activity) were not correlated with any of the mineralization kinetic parameters. Overall, this study showed that environmental form had a profound effect on the ultimate fate of biodegradable chemicals in soil. This form is defined by the physicochemical characteristics of the chemical, the composition and mineralogy of the soil, and the mode of entry of the chemical into the soil environment.     

Immobilization and mineralization of dissolved free amino acids by stream-bed biofilms

1. Radiolabelled (¹⁴C) amino acids were used to investigate the influence of sediment size as well as dissolved free amino acid (DFAA) concentration and composition on immobilization and mineralization of DFAAs by biofilms from a first-order stream. 2. Over time (240 min), biofilms on stony substrata immobilized a DFAA mixture more effectively than those on sandy substrata, however proportional mineralization of immobilized DFAAs was higher for sandy substrata (36 v 20%). 3. Using stony substrata, the DFAA mixture was immobilized more rapidly than glycine alone at ‘near-natural’ amino acid concentrations (c. 37 μgl^?1), as well as enriched concentrations (1 and 100 mg 1-^?1). Instantaneous rates of glycine immobilization and mineralization were not saturated at glycine enrichments of up to 980 mgl^?1. 4. With both the amino acid mixture and glycine alone, proportional mineralization of the immobilized amino acids increased on enrichment to Img 1-^?1 (DFAA mixture: from 25 to 37%; glycine alone: from 50 to 54%), but then fell on further enrichment to 100mgl^?1 (DFAA mixture: 11%; glycine alone: 7%). 5. Results are discussed in terms of the potential trophic utility of immobilized DFAAs as well as the apparent roles of biotic and abiotic immobilization mechanisms. Immobilization and mineralization responses to variables investigated in this study give an insight into potential variability of carbon immobilization and retention in stream-bed sediments. This is fundamental to an understanding of how DOC may become available to higher trophic levels.     

Photochemical and Microbial Degradation of 2,4,5-Trichloroaniline in a Freshwater Lake

Surface waters from a eutrophic lake in northern Georgia were incubated with ¹⁴C-labeled 2,4,5-trichloroaniline to study the disappearance of the parent compound and production of ¹⁴CO₂. There was no degradation of the compound in the dark. Under 12 h of sunlight and 12 h of darkness, 28% of the trichloroaniline was degraded in both poisoned and untreated samples. Mineralization after 24 h in poisoned and untreated lake water was 5.5 and 6.8%, respectively. Thus, 81% of the mineralization was attributable to photochemical processes, and 19% was attributable to microbial processes. Most biological mineralization was due to microbes of bacterial size (<1.0 μm). Approximately 90% of the trichloroaniline bioaccumulated was associated with organisms larger than 1.0 μm, e.g., algae. When algae were removed by filtration, the amount of trichloroaniline mineralized increased to 9.4%, compared with 6.8% in the presence of algae. The excretion of organic compounds by algae may have inhibited bacterial mineralization of photoproducts.     

生源要素有效性及生物因子对湿地土壤碳矿化的影响

湿地土壤是全球碳存储的重要场所,湿地生态系统的碳循环过程对全球变化有重要指示作用。土壤碳矿化是湿地生态系统碳循环的重要环节,对于认知湿地生态系统生物地球化学循环过程具有重要的意义。综述了生源要素及生物因素对湿地土壤碳矿化的内在作用机制。土壤活性有机碳库通过调节土壤能源物质和微生物活性影响土壤碳库的有效性,是表征土壤碳矿化的敏感指标。湿地其它养分如N、P、S等元素的有效性也是影响土壤碳矿化的关键要素。电子受体(NO₃^-、SO₄^2-、Fe³⁺、Mn⁴⁺等)对湿地土壤碳矿化和有机碳转变的影响主要通过电子受体的还原过程完成,在厌氧分解过程中,湿地土壤利用难溶性电子受体可能是土壤C矿化的更重要途径。动物、植物、微生物群落和区系等则是土壤碳矿化的主要驱动因子。土壤动物区系在有机态养分矿化为无机态养分的过程有着独特的功能,能显著增加土壤碳矿化。土壤微生物的活性,决定着土壤中有机碎屑的降解速率,是土壤有机碳分解周转的主要诱导因素。湿地植物则通过影响根系、微生物呼吸底物的供应以及对小气候和土壤因子的调节而影响土壤有机质的分解。湿地生源要素和生物因子还极易与土壤理化性质如温度、水分、pH值和质地等环境因素形成交互和制约,共同影响土壤碳矿化。最后,提出了进一步研究生源要素和生物因素与湿地土壤碳矿化关系需要解决的一些重要问题。     

Degradation of Humic Acids by the Litter-Decomposing Basidiomycete Collybia dryophila

The basidiomycete Collybia dryophila K209, which colonizes forest soil, was found to decompose a natural humic acid isolated from pine-forest litter (LHA) and a synthetic ¹⁴C-labeled humic acid (¹⁴C-HA) prepared from [U-¹⁴C]catechol in liquid culture. Degradation resulted in the formation of polar, lower-molecular-mass fulvic acid (FA) and carbon dioxide. HA decomposition was considerably enhanced in the presence of Mn²⁺ (200 μM), leading to 75% conversion of LHA and 50% mineralization of ¹⁴C-HA (compared to 60% and 20%, respectively, in the absence of Mn²⁺). There was a strong indication that manganese peroxidase (MnP), the production of which was noticeably increased in Mn²⁺-supplemented cultures, was responsible for this effect. The enzyme was produced as a single protein with a pI of 4.7 and a molecular mass of 44 kDa. During solid-state cultivation, C. dryophila released substantial amounts of water-soluble FA (predominantly of 0.9 kDa molecular mass) from insoluble litter material. The results indicate that basidiomycetes such as C. dryophila which colonize forest litter and soil are involved in humus turnover by their recycling of high-molecular-mass humic substances. Extracellular MnP seems to be a key enzyme in the conversion process.     

The growth of wheat plants in humic acid solutions under axenic conditions

Summary A technique is described for growing wheat plants in nutrient solutions containing C¹⁴-labelled humic acid under axenic conditions. The general appearance of axenic plants was indistinguishable from plants grown in association with microbes. C¹⁴-labelled humic acid enhanced the growth of both roots and shoots showing that by-products of microbial degradation of humic acid are unnecessary for this enhanced plant growth. Thus humic acid had a direct effect on the growth processes. The C¹⁴-labelled humic acid was taken up by the roots and virtually none was transported to the shoot. Only some 30 to 40 per cent of the incorporated radioactivity was associated with the root cell walls and thus more than 60 per cent was in the cytoplasm and may have influenced the biochemical processes involved in the regulation of plant growth.     

Metal-humic complexes and plant micronutrient uptake: a study based on different plant species cultivated in diverse soil types

There are several studies in the literature dealing with the effect of metal-humic complexes on plant metal uptake, but none of them correlate the physicochemical properties of the complexes with agronomic results. Our study covers both aspects under various experimental conditions. A humic extract (SHE) obtained from a sapric peat was selected for preparing the metal–humic complexes used in plant experiments. Fe–, Zn– and Cu–humic complexes with a reaction stoichiometry of 2:0.25 (humic:metal, w/w) were chosen after studying their stability and solubility with respect to pH (6–9) and the humic:metal reaction stoichiometry. Wheat and alfalfa plants were greenhouse cultured in pots containing one of three model soils: an acid, sandy soil and two alkaline, calcareous soils. Treatments were: control (no additions), SHE (53 mg kg^–1 of SHE), and metal (Cu, Zn and Fe)–SHE complexes (2.5 and 5 mg kg^–1 of metal rate and a SHE concentration to make 53 mg kg ^–1). Cu- and Zn–humic complexes significantly (p0.05) increased the plant uptake and the DTPA-extractable soil fraction of complexed micronutrients in most plant–soil systems. However, these effects were associated with significant increases (p0.05) of shoot and root dry weight only in alfalfa plants. In wheat, significant increases of root and shoot dry matter were only observed in the Cu–humic treated plants growing in the acid soil, where Cu deficiency was more intense. The Fe–humic complex did not increase Fe plant assimilation in any plant–soil system, but SHE increased Fe-uptake and/or DTPA-extractable soil Fe in the wheat–calcareous soil systems. These results, taken together with those obtained from the study of the pH- and SHE:metal ratio-dependent SHE complex solubility and stability, highlight the importance of the humic:Fe complex stoichiometry on iron bioavailability as a result of its influence on complex solubility.     

Isolation of Soil Bacteria Adapted To Degrade Humic Acid-Sorbed Phenanthrene

The goal of these studies was to determine how sorption by humic acids affected the bioavailability of polynuclear aromatic hydrocarbons (PAHs) to PAH-degrading microbes. Micellar solutions of humic acid were used as sorbents, and phenanthrene was used as a model PAH. Enrichments from PAH-contaminated soils established with nonsorbed phenanthrene yielded a total of 25 different isolates representing a diversity of bacterial phylotypes. In contrast, only three strains of Burkholderia spp. and one strain each of Delftia sp. and Sphingomonas sp. were isolated from enrichments with humic acid-sorbed phenanthrene (HASP). Using [¹⁴C]phenanthrene as a radiotracer, we verified that only HASP isolates were capable of mineralizing HASP, a phenotype hence termed “competence.” Competence was an all-or-nothing phenotype: noncompetent strains showed no detectable phenanthrene mineralization in HASP cultures, but levels of phenanthrene mineralization effected by competent strains in HASP and NSP cultures were not significantly different. Levels and rates of phenanthrene mineralization exceeded those predicted to be supported solely by the metabolism of phenanthrene in the aqueous phase of HASP cultures. Thus, competent strains were able to directly access phenanthrene sorbed by the humic acids and did not rely on desorption for substrate uptake. To the best of our knowledge, this is the first report of (i) a selective interaction between aerobic bacteria and humic acid molecules and (ii) differential bioavailability to bacteria of PAHs sorbed to a natural biogeopolymer.     

Method for measuring microbial degradation and mineralization of14C-labeled chitin obtained from the blue crab,Callinectes sapidus

A method for measuring microbial degradation and mineralization of radiolabeled native chitin is described.¹⁴C-labeled chitin was synthesized in vivo by injecting shed blue crabs (Callinectes sapidus) with N-acetyl-D-[1-¹⁴C]-glucosamine and allowing for its incorporation into the exoskeleton. The cuticle had a total organic carbon content of 0.48 mg C mg^–1 with a specific radioactivity of 6,356 CPM mg^–1. Glucosamine, i.e., chitin content as determined colorimetrically, was 22% (w/w). Microbial degradation and mineralization rates were assessed in batch culture using¹⁴C-chitin as substrate and York River water as inoculum. Replicate flasks were sampled daily for enumeration of chitinoclastic bacteria and the radiolabel recovered as particulate¹⁴C-chitin or¹⁴CO₂. The amount of¹⁴CO₂ generated was directly proportional to the loss of particulate¹⁴C-chitin, with 96% of the added label recovered as the sum of both phases. The maximum rate of mineralization was 207 mg day^–1 g^–1 seeded¹⁴C-chitin at 20°C. Highest chitinoclastic bacterial counts corresponded to the period of maximum rate of chitinolysis. It is suggested that the rate of chitin mineralization is limited by exoenzymatic depolymerization and not by chitin concentration.VIMS Contribution no. 1215.     

Mineralization of synthetic humic substances by manganese peroxidase from the white-rot fungus Nematoloma frowardii

A manganese peroxidase preparation from the white-rot fungus Nematoloma frowardii was found to be capable of releasing up to 17% ¹⁴CO₂ from ¹⁴C-labelled synthetic humic substances. The latter were prepared from [U-¹⁴C]catechol by spontaneous oxidative polymerization or laccase-catalysed polymerization. The ex-tent of humic substance mineralization was considerably enhanced in the presence of the thiol mediator glutathione (up to 50%). Besides the evolution of ¹⁴CO₂, the treatment of humic substances with Mn peroxidase resulted in the formation of lower-molecular-mass products. Analysis of residual radioactivity by gel-permeation chromatography demonstrated that the predominant molecular masses of the initial humic substances ranged between 2 kDa and 6 kDa; after treatment with Mn peroxidase, they were reduced to 0.5–2 kDa. The extracellular depolymerization and mineralization of humic substances by the Mn peroxidase system may play an important role in humus turnover of habitats that are rich in basidiomycetous fungi. Received: 25 September 1997 / Received revision: 12 January 1998 / Accepted: 13 January 1998     

The transformation of nitrogen and carbon in the soil during humification of straw labelled with N15

Summary Laboratory tests were made on the effect of NaNO₃ or (NH₄)₂CO₃ on the dynamics of humification in the soil of oat straw tagged with N¹⁵. The mixture was incubated for 112 days, at constant temperature and moisture conditions. It was found that NH₄-N accelerated the straw humifications more than NO₃-N. Humification started directly after the straw was introduced into the soil. N¹⁵ derived from straw consituted a part of the forming humic compounds. Already after 14 days of incubation, the N¹⁵ of straw was found in all fractions of humic compounds. Mineralization accompanied humification. The added inorganic-N accelerated not only straw humification, but also the mineralization of forming humic compounds. This is why the added inorganic-N had no influence on the content of humic compounds. The decisive factor in the increase of humic and fulvic acids in the soil, was the straw. The inorganic N added to the straw, had no influence on the quality of humic acids formed in the soil.     

Linking microbial Sphagnum degradation and acetate mineralization in acidic peat bogs: from global insights to a genome-centric case study

Ombrotrophic bogs accumulate large stores of soil carbon that eventually decompose to carbon dioxide and methane. Carbon accumulates because Sphagnum mosses slow microbial carbon decomposition processes, leading to the production of labile intermediate compounds. Acetate is a major product of Sphagnum degradation, yet rates of hydrogenotrophic methanogenesis far exceed rates of aceticlastic methanogenesis, suggesting that alternative acetate mineralization processes exist. Two possible explanations are aerobic respiration and anaerobic respiration via humic acids as electron acceptors. While these processes have been widely observed, microbial community interactions linking Sphagnum degradation and acetate mineralization remain cryptic. In this work, we use ordination and network analysis of functional genes from 110 globally distributed peatland metagenomes to identify conserved metabolic pathways in Sphagnum bogs. We then use metagenome-assembled genomes (MAGs) from McLean Bog, a Sphagnum bog in New York State, as a local case study to reconstruct pathways of Sphagnum degradation and acetate mineralization. We describe metabolically flexible Acidobacteriota MAGs that contain all genes to completely degrade Sphagnum cell wall sugars under both aerobic and anaerobic conditions. Finally, we propose a hypothetical model of acetate oxidation driven by changes in peat redox potential that explain how bogs may circumvent aceticlastic methanogenesis through aerobic and humics-driven respiration.Subject terms: Microbial ecology, Metagenomics, Soil microbiology, Biogeochemistry, Microbial ecology     

Enhancing pyrene mineralization in contaminated soil by the addition of humic acids or composted contaminated soil

The addition of composted PAH-contaminated soil to PAH-contaminated soil spiked with 14C-labeled pyrene resulted in rapid mineralization of pyrene (more than 57% after 21 days compared with 3.4% in un-amended soil). The addition of the humic acid fraction of the composted soil also increased the mineralization potential of the soil significantly, but to a lesser extent (37.5% mineralization after 106 days compared with 20.6% in unamended soil). Increasing the humic acid concentration increased mineralization up to a maximum of more than three times the unamended rate, after which the rate of pyrene mineralization decreased, possibly due to inhibitory pH or concentrations of salts. The amendment of PAH-contaminated soil with materials containing humic acids or humic acid extracts is suggested as a method of bioremediation.     

Properties of poultry litter humic acid fractions and their metal-complexes

Summary Molecularly homogenous fractions of humic acid extracted from poultry litter were characterized by elemental and functional group analysis, molecular weight determination, U.V. and infrared spectroscopy. The divalent and trivalent metal complexes prepared from different fractions of humic acid were characterized by infrared spectroscopy. The molecular weight of molecularly homogenous fractions of poultry litter humic acid ranged from 2545 to 40219. High amounts of functional groups in low molecular weight of humic acid fraction has been indicated by infrared spectra and by chemical analysis. The presence of chromophores C=C and C=O and auxochromes C−OH, C−NH were indicated by infrared and U.V. spectra of these humic acid fractions. Stable complex formation of Fe³⁺, Cu²⁺, and Zn²⁺ with −OH, −NH₂ and −COOH ligands of humic acid fractions involved electrovalent and coordinate-covalent bonds. Intensity of absorption bands of molecularly homogenous fractions of humic acid in I.R. spectra is differing depending upon the functional groups content of humic acid fractions. Journal paper No. 5. Department of Soil Science, R.A.U., T.C.A., Pusa-Dholi Campus, Dholi-843121, Muzaffarpur, Bihar, India.     

Characterisation and reactivity continuum of dissolved organic matter in forested headwater catchments of Andean Patagonia

相似文献   

Low molecular weight humic substances stimulate H+-ATPase activity of plasma membrane vesicles isolated from oat (Avena sativa L.) roots

The effect of <5 KDa (low molecular weight, LMW) and >5 KDa (high molecular weight, HMW) humic fractions on transport activities of isolated plasma membrane vesicles was studied. The K⁺-stimulated component of the ATP-hydrolyzing activity was considerably increased by LMW humic substances at concentrations ranging from 0.075 mg org CL^-1 to 1 mg org CL^-1. The stimulation was still evident when the detergent Brij-35 was added in the assay mixture, indicating a direct effect of LMW humic substances on plasma membrane ATPase activity. The LMW humic fraction stimulated ATP-dependent intravesicular H⁺-accumulation with a pattern similar to that recorded for ATP hydrolysis. LMW humic substances induced also an increase in passive membrane permeability to protons, as revealed by following the dissipation of an artificially imposed pH gradient. Membrane permeability to anions, as measured by the anion-dependent active proton accumulation was affected by LMW humic substances. In the presence of NO₃ ^- these molecules clearly enhanced proton transport, while Cl^--dependent activity was almost unaffected, thus suggesting a specific action of LMW humic fraction on transmembrane NO₃ ^- fluxes. On the other hand, HMW humic substances decreased the passive permeability to protons and reduced the anion-dependent intravesicular H⁺-accumulation. The results suggest that the stimulatory effect of soil humic substances on plant nutrition and growth might be, at least in part, explained on the basis of both direct action of LMW humic molecules on plasma membrane H⁺-ATPase and specific modification of cell membrane permeability.     

Stability of Ca2+-, Cd2+-, Cu2+-[illite-humic] complexes and pH influence

Decomposition of fresh plant residues in soil is expected to produce humic fractions varying in molecular size. It was hypothesized that metal adsorption by soil, to some degree, will depend on humic acid content and molecular size. The latter is expected to vary in number and type of functional groups. In this study, illite-humic complexes were used to evaluate Ca²⁺, Cd²⁺, and Cu²⁺ adsorption and how this adsorption was affected by humic acids, differing in molecular size, under various pH values. Potentiometric titration using ion-selective electrodes with a stop-and-go procedure was employed to evaluate metal-[illite-humic] complex formation. The results showed that illite-humic complexes exhibited at least two types of metal-ion adsorption sites (low and high affinity) and molecular size of humic fractions had a large potential influence on total metal adsorption but a relatively smaller influence on metal-complex stability. Relative strength of metal-ion-[illite-humic] complexes followed the order of Cu²⁺>Cd²⁺>Ca²⁺ and were affected by pH, especially for low metal-ion affinity sites. Magnitude of metal-[illite-humic] stability constants, depending on molecular size of humic fraction and pH, varied on a log-scale from 3.52 to 4.21 for Ca²⁺, 4.38 to 5.18 for Cd²⁺and from 5.23 to 5.83 for Cu²⁺. There was an approximate 5-fold difference in these stability constants between the three different sizes of humic fractions. The larger the humic fraction, the lower the metal-[illite-humic] stability constant. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of concentration and environmental form of tetradecenyl succinic acid on its mineralization in soil

Tetradecenyl succinic acid (TSA) is the major component of a detergent builder (C12-C14 alkenyl succinic acid), which is inherently biodegradable. ¹⁴C-TSA was dosed as a component of sewage sludge into a soil with a history of sludge amendment at final added concentrations of 1.5 and 30 mg (kg soil)^-1. In addition, it was dosed to the soil in an aqueous solution to a final added concentration of 30 mg (kg soil)^-1. Dose and form were found to have a pronouced effect on the mineralization kinetics. When dosed in a realistic form and concentration (i.e. 1.5 mg (kg soil)^-1 as a component of sludge), TSA was mineralized at its highest rate and to its greatest extent, and the mineralization half-life was 2.4 days. When dosed at 30 mg (kg soil)^-1 as a component of sludge, mineralization began immediately, and the half-life was 23 days. In contrast, when dosed at this concentration in aqueous solution, the onset of mineralization was preceded by a 13 day lag period and the mineralization half-life was 69 days. Primary biodegradation and mineralization rates of TSA were very similar. Approximately, half the radioactivity was evolved as ¹⁴CO₂, while the remaining radioactivity became non-extractable, having presumably been incorporated into biomass or natural soil organic matter (humics). This study demonstrated that TSA is effectively removed from sludge-amended soils as a result of biodegradation. Furthermore, it showed the effect that dose form and concentration have on the biodegradation kinetics and the importance of dosing a chemical not only at a relevant concentration but also in the environmental form in which it enters the soil environment.     

Effects of DOM photochemistry on bacterial metabolism and CO2 evasion during falling water in a humic and a whitewater river in the Brazilian Amazon

In the Amazon river system, the source of the large quantity of CO₂ evading from river surfaces remains unidentified. Photochemical transformation of dissolved organic matter (DOM) into dissolved inorganic carbon (DIC) and low molecular weight organic acids (LMWOAs) is a promising candidate. Few studies in the Amazon river system, and river systems in general, have attempted to quantify the contribution of these specific photoproducts to CO₂ evasion. We conducted photochemical degradation and ¹⁴C addition experiments to measure the rate of production and the impact on bacterial metabolism, respectively, in the black water Rio Negro and in the white water Rio Solimões during low water. We found statistically significant production of both photoproducts in the Rio Negro and none in Rio Solimões. We also found that two photochemically produced LMWOAs??acetic and formic acid??may play a significant role in bacterial metabolism in both rivers. Based on our experimental results, we estimate that photochemically produced CO₂, acetic acid and formic acid alone contribute to only 0.5% of the CO₂ evading from the Rio Negro. Due to our experimental set-up, analytical methods and time of sampling, we caution that our estimate is very conservative. More extensive research is needed before drawing conclusions on the contribution of photochemistry to CO₂ evasion from river surfaces of the Amazon basin.     

Photochemical mineralization of synthetic lignin in lake water indicates enhanced turnover of aromatic organic matter under solar radiation

The degradation of ¹⁴C-[ring]-labelled syntheticlignin (¹⁴C-DHP) and dissolved organic carbon(DOC) from lake water were studied simultaneously.¹⁴C-DHP was incubated in humic lake water (colour173 mg Pt l^-1) for 7 d in the dark or under solarradiation. In the dark <0.4% of the introduced¹⁴C-DHP label and 4% of the indigenous DOC weremineralized, indicating that the ¹⁴C-labelledaromatic rings of DHP and the humic DOC weremicrobiologically recalcitrant. Under solar radiation(116 MJ m^-2), 17–21% of the ¹⁴C-labelledcarbons in DHP and 18–23% of the indigenous DOC weremineralized in 7 d. Simultaneously the watersolubility of ¹⁴C-DHP increased. Solar radiationconverted the aromatic cores of synthetic lignin toCO₂ and soluble organic photoproducts. Theresults suggest that solar radiation plays a key rolein the decomposition of natural polyaromatic matter.