Response of CH<Subscript>4</Subscript> oxidation and methanotrophic diversity to NH<Subscript>4</Subscript><Superscript>+</Superscript> and CH<Subscript>4</Subscript> mixing ratios

Methane oxidising activity and community structure of 11, specifically targeted, methanotrophic species have been examined in an arable soil. Soils were sampled from three different field plots, receiving no fertilisation (C), compost (G) and mineral fertiliser (M), respectively. Incubation experiments were carried out with and without pre-incubation at elevated CH₄ mixing ratios (100 ml CH₄ l⁻¹) and with and without ammonium (100 mg N kg⁻¹) pre-incubation. Four months after fertilisation, plots C, G and M did not show significant differences in physicochemical properties and CH₄ oxidising activity. The total number of methanotrophs (determined as the sum the 11 specifically targeted methanotrophs) in the fresh soils was 17.0×10⁶, 13.7×10⁶ and 15.5×10⁶ cells g⁻¹ for treatment C, G and M, respectively. This corresponded to 0.11 to 0.32% of the total bacterial number. The CH₄ oxidising activity increased 10⁵-fold (20–26 mg CH₄ g⁻¹ h⁻¹), the total number of methanotrophs doubled (28–76×10⁶ cells g⁻¹) and the methanotrophic diversity markedly increased in treatments with a pre-incubation at elevated CH₄ concentrations. In all soils and treatments, type II methanotrophs (62–91%) outnumbered type I methanotrophs (9–38%). Methylocystis and Methylosinus species were always most abundant. After pre-incubation with ammonium, CH₄ oxidation was completely inhibited; however, no change in the methanotrophic community structure could be detected.     

Reducing CH<Subscript>4</Subscript> and CO<Subscript>2</Subscript> emissions from waterlogged paddy soil with biochar

Purpose  

A potential means to diminish increasing levels of CO₂ in the atmosphere is the use of pyrolysis to convert biomass into biochar, which stabilizes the carbon (C) that is then applied to soil. Before biochar can be used on a large scale, especially in agricultural soils, its effects on the soil system need to be assessed. This is especially important in rice paddy soils that release large amounts of greenhouse gases to the atmosphere.     

Changes in CH<Subscript>4</Subscript> production during different stages of litter decomposition under inundation and N addition

Purpose

Our aim was to examine linkages between mass loss, chemical transformation and CH₄ production during decomposition of leaf litters submerged under water. We hypothesised that (i) labile leaf litters would fuel a rapid, high rate of methane (CH₄) production and that recalcitrant litters would fuel long-lasting but lower emissions, (ii) leaf litters experiencing a greater alteration to chemical properties would stimulate increased CH₄ production and (iii) nitrogen (N) addition would increase CH₄ emissions.

Materials and methods

Litters from six plant species were collected from a riparian ecosystem adjacent to Wyaralong Dam, located in Queensland, Australia, i.e., Lophostemon confertus, Cynodon dactylon, Heteropogon contortus, Chamaecrista rotundifolia, Chrysocephalum apiculatum and Imperata cylindrica. We evaluated the rate of mass loss and CH₄ emissions for 122 days of incubation in inundated microcosms with and without N addition. We quantified the chemical changes in the decomposing litters with ¹³ C-cross polarization and magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectrum.

Results and discussion

The inundation treatment of plant litters significantly affected decomposition rates. All litters decomposed in either inundated or aerobic microcosms were quite distinct with regard to the NMR spectra of their initial litters. N addition altered the NMR spectra under both inundation and aerobic conditions. The N treatment only marginally influenced the decomposition rates of I. cylindrica and C. apiculatum litters. The diurnal patterns of CH₄ production in the H. contortus, C. rotundifolia and C. apiculatum litters under inundation incubation could be expressed as one-humped curves, with the peak value dependent on litter species and N treatment. N addition stimulated CH₄ emission by C. rotundifolia and C. apiculatum litters and inhibited CH₄ emission from microcosms containing the litters of the three gramineous species, i.e., I. cylindrica, C. dactylon and H. contortus.

Conclusions

Our results provide evidence that labile leaf litters could fuel a rapid, high rate of CH₄ production and that recalcitrant litters fuelled a lower CH₄ emission. We did not find that leaf litters with altered chemical properties stimulated increased CH₄ production. We also found that N addition was able to increase CH₄ emissions, but this effect was dependent on the litter species.

     

Effect of soil water status and mulching on N<Subscript>2</Subscript>O and CH<Subscript>4</Subscript> emission from lowland rice field in China

Cultivation of rice in unsaturated soils covered with mulch is receiving more attention in China because of increasingly serious water shortage; however, greenhouse gas emission from this cultivation system is still poorly understood. A field experiment was conducted in 2001 to compare nitrous oxide (N₂O) and methane (CH₄) emission from rice cultivated in unsaturated soil covered with plastic or straw mulch and the traditional waterlogged production system. Trace gas fluxes from the soil were measured weekly throughout the entire growth period using a closed chamber method. Nitrous oxide emissions from unsaturated rice fields were large and varied considerably during the rice season. They were significantly affected by N fertilizer application rate. In contrast, N₂O emission from the waterlogged system was very low with a maximum of 0.28 mg N₂O m^–2 h^–1. However, CH₄ emission from the waterlogged system was significantly higher than from the unsaturated system, with a maximum emission rate of 5.01 mg CH₄ m^–2 h^–1. Our results suggested that unsaturated rice cultivation with straw mulch reduce greenhouse gas emissions.     

Elevated CO<Subscript>2</Subscript> alters the abundance but not the structure of diazotrophic community in the rhizosphere of soybean grown in a Mollisol

The effect of elevated CO₂ (eCO₂) on rhizospheric diazotrophic community in cropland has little been studied, although eCO₂ facilitates nodulation and N₂ fixation in legumes. In this study, four soybean cultivars (Xiaohuangjin, Suinong 8, Suinong 14, and Heinong 45) were grown in Mollisols for 65 days under ambient CO₂ (aCO₂) (390 ppm) or eCO₂ (550 ppm). Quantitative PCR and Illumina MiSeq sequencing targeting the nifH gene that reflects the composition of diazotrophic community were determined. Elevated CO₂ significantly increased the abundance of nifH gene copies in the rhizospheres of the Suinong 8 and Heinong 45 cultivars, but not in the Suinong 14 and Xiaohuangjin cultivars. The nifH abundance correlated negatively with nodule density (p?≤?0.01) but positively with nodule size (p?≤?0.01). Elevated CO₂ did not significantly alter the composition of diazotrophic community, nor shift dominant bacterial operational taxonomic units (OTUs). These results indicated that eCO₂ stimulated the growth but did not alter the community composition of diazotrophs in the rhizosphere of soybean, which depended on cultivar and might contribute to nodulation responses to eCO₂.     

Stimulation of methane oxidation by CH<Subscript>4</Subscript>-emitting rose chafer larvae in well-aerated grassland soil

In this study, the impact of rose chafer (Cetonia aurata L.) larvae on net and gross methane (CH₄) fluxes in soil from an old permanent grassland site (Giessen, Germany) was investigated. Previous studies at this site suggested the existence of Scarabaeidae larvae-induced “CH₄-emitting hot spots” within the soil profile which may subsequently lead to increased CH₄ oxidation. The net (soil + larvae) and gross (soil and larvae separated) CH₄ fluxes were studied in a 3-month laboratory incubation. Addition of larvae changed the soil from a net sink (?330 ± 11 ng CH₄ kg^?1 h^?1) to a net source (637 ± 205 ng CH₄ kg^?1 h^?1). Supply of plant litter to the soil + larvae incubation jars tended to increase CH₄ emissions which was not significant due to large variability. After 11–13 weeks of incubation, the net soil CH₄ oxidation was significantly stimulated by 13–21% in the treatments containing larvae when these were taken out. Analysis of archaeal 16S rRNA genes revealed that the majority of the obtained clones were closely related to uncultured methanogens from guts of insects and other animals. Other sequences were relative to cultivated species of Methanobrevibacter, Methanoculleus, and Methanosarcina. Hence, Scarabaeidae larvae in soils (i) may represent an underestimated source of CH₄ emissions in aerobic upland soils, (ii) may stimulate gross CH₄ consumption in their direct soil environment, and, thus, (iii) contribute to the spatial heterogeneity often observed in the field with closed-chamber measurements. Long-term CH₄-flux balances may be wrongly assessed when “exceptional” net CH₄ flux rates (due to larvae hot spots) are excluded from data sets.     

Preparation of Highly Efficient CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/Zn<Subscript>2</Subscript>SnO<Subscript>4</Subscript> Composite Photocatalyst for the Degradation of Rhodamine B Dye from Aqueous Solution

CoFe₂O₄/Zn₂SnO₄ composite was synthesized using a simple two-step process and applied as a novel-efficient photocatalyst for the rhodamine B degradation from aqueous solution. Characterization techniques such as X-ray diffraction (XRD), N₂ adsorption-desorption isotherms, scanning electron microscopy (SEM), EDS analysis, and diffuse reflectance spectroscopy were employed in order to investigate the physical and chemical properties of composite. Higher values of the specific surface area, pore volume and diameter, and a smaller band-gap energy promoted a greater catalytic activity of CoFe₂O₄/Zn₂SnO₄ composite when compared to Zn₂SnO₄. A rapid decolorization of dye solution was observed at 40 min of reaction using the CoFe₂O₄/Zn₂SnO₄ catalyst, being 2.5 times faster than the Zn₂SnO₄ alone. Therefore, the CoFe₂O₄/Zn₂SnO₄ composite shows extraordinarily high photocatalytic activity toward the degradation of rhodamine B dye from aqueous solution.     

N<Subscript>2</Subscript>O emission from conventional and minimum-tilled soils

In this study, we investigated N₂O emissions from two fields under minimum tillage, cropped with maize (MT maize) and summer oats (MT oats), and a conventionally tilled field cropped with maize (CT maize). Nitrous oxide losses from the MT maize and MT oats fields (5.27 and 3.64 kg N₂O-N ha⁻¹, respectively) were significantly higher than those from the CT maize field (0.27 kg N₂O-N ha⁻¹) over a period of 1 year. The lower moisture content in CT maize (43% water-filled pore space [WFPS] compared to 60–65%) probably caused the difference in total N₂O emissions. Denitrification was found to be the major source of N₂O loss. Emission factors calculated from the MT field data were high (0.04) compared to the CT field (0.001). All data were simulated with the denitrification decomposition model (DNDC). For the CT field, N₂O and N₂O + N₂ emissions were largely overestimated. For the MT fields, there was a better agreement with the total N₂O and N₂O + N₂ emissions, although the N₂O emissions from the MT maize field were underestimated. The simulated N₂O emissions were particularly influenced by fertilization, but several other measured N₂O emission peaks associated with other management practices at higher WFPS were not captured by the model. Several mismatches between simulated and measured \textNH₄⁺ {\text{NH}}_4^ + , \textNO₃^- {\text{NO}}_3^ - and WFPS for all fields were observed. These mismatches together with the insensitivity of the DNDC model for increased N₂O emissions at the management practices different from fertilizer application explain the limited similarity between the simulated and measured N₂O emissions pattern from the MT fields.     

A stable CH<Subscript>4</Subscript> sink responding to extreme precipitation events in a fenced semiarid steppe

Purpose

Climate models predict that amplification of the hydrological cycle results in more extreme (more intensive but less frequent) precipitation events (EPEs) that have larger effects on ecosystem functioning than mean precipitation conditions. Semiarid grassland ecosystems are considered important CH₄ sinks whose functioning is greatly affected by variations in precipitation patterns. An experiment was performed to assess the effects of extreme precipitation events on the functioning of a fenced semiarid steppe grassland on the Inner Mongolian Plateau of China.

Materials and methods

Extreme precipitation events (282 mm over 20 consecutive days) during the middle (Pm) and late (Ps) growing periods of 2014 were simulated to assess the effects of extreme precipitation events on the CH₄ uptake of the ecosystem.

Results and discussion

The extreme precipitation events had no significant effect on the CH₄ uptake rate during the growing season but did result in 62 and 45% reductions in the CH₄ uptake rate during the Pm and Ps events, respectively. There were legacy effects on suppression of the CH₄ uptake rate for approximately 40 and 35 days after the events in the Pm and Ps plots, respectively, but the suppression disappeared rapidly during the late season as a result of faster water loss. No significant differences in cumulative CH₄ uptake were detected between the treatment and the control plots over the growing season as a whole, which demonstrates that the ecosystem functions as a CH₄ sink. The average CH₄ uptake rates were found to be strongly regulated by changes in the soil water content.

Conclusions

The results suggest that the CH₄ uptake budget of this fenced steppe grassland can be maintained even in the face of consecutive extreme precipitation events, regardless of the timing of the events. Nevertheless, long-term experiments are needed to detect the thresholds for CH₄ uptake budget changes, in case of an increasing occurrence of EPEs in the future.

     

Abundance of transcripts of functional gene reflects the inverse relationship between CH<Subscript>4</Subscript> and N<Subscript>2</Subscript>O emissions during mid-season drainage in acidic paddy soil

Agricultural management significantly affects methane (CH₄) and nitrous oxide (N₂O) emissions from paddy fields. However, little is known about the underlying microbiological mechanism. Field experiment was conducted to investigate the effect of the water regime and straw incorporation on CH₄ and N₂O emissions and soil properties. Quantitative PCR was applied to measure the abundance of soil methanogens, methane-oxidising bacteria, nitrifiers, and denitrifiers according to DNA and mRNA expression levels of microbial genes, including mcrA, pmoA, amoA, and nirK/nirS/nosZ. Field trials showed that the CH₄ and N₂O flux rates were negatively correlated with each other, and N₂O emissions were far lower than CH₄ emissions. Drainage and straw incorporation affected functional gene abundance through altered soil environment. The present (DNA-level) gene abundances of amoA, nosZ, and mcrA were higher with straw incorporation than those without straw incorporation, and they were positively correlated with high concentrations of soil exchangeable NH₄⁺ and dissolved organic carbon. The active (mRNA-level) gene abundance of mcrA was lower in the drainage treatment than in continuous flooding, which was negatively correlated with soil redox potential (Eh). The CH₄ flux rate was significantly and positively correlated with active mcrA abundance but negatively correlated with Eh. The N₂O flux rate was significantly and positively correlated with present and active nirS abundance and positively correlated with soil Eh. Thus, we demonstrated that active gene abundance, such as of mcrA for CH₄ and nirS for N₂O, reflects the contradictory relationship between CH₄ and N₂O emissions regulated by soil Eh in acidic paddy soils.     

In situ quantification of CH<Subscript>4</Subscript> bubbling events from a peat soil using a new infrared laser spectrometer

Purpose  

CH₄ emissions from peatlands are space and time dependent. The variety of efflux routes contributes to these variabilities. CH₄ bubbling remains difficult to investigate since it occurs on a timescale of seconds. The aims of this study were to use for the first time the recently built infrared high-resolution spectrometer, SPectrometre Infra-Rouge In situ Troposphérique to (1) measure in situ CH₄ fluxes in natural and artificial peatland plot and (2) observe online bubbling events with quantification of CH₄ emission fluxes corresponding to this very sudden degassing event.     

Long-term animal impact modifies potential production of N<Subscript>2</Subscript>O from pasture soil

At cattle overwintering areas, inputs of nutrients in animal excrements create conditions favourable for intensive microbial activity in soil. During nitrogen transformations, significant amounts of N₂O are released, which makes overwintering areas important sources of N₂O emission. In previous studies, however, increasing intensity of long-term cattle impact did not always increase emissions of N₂O from the soil: in some cases, N₂O emissions from the soil were lower at the most impacted area than at the moderately impacted one. Thus, the relationships between the level of long-term animal impact and potential production of N₂O from soil by denitrification were investigated in field and laboratory experiments. Field measurements indicated that the production of N₂O after glucose and nitrate amendments was greater in severely and moderately impacted locations than in an unimpacted location, while differences between the severely and moderately impacted locations were not significant. In laboratory experiments, the potential production of N₂O (measured as anaerobic production of N₂O after addition of glucose and nitrate) was highest in the moderately impacted soil. Surprisingly, potential N₂O production was lower in the most impacted than in the moderately impacted soil, and the net N₂O production in the highly impacted soil was further decreased by a significant reduction of N₂O to N₂. The expected stimulating effect of an increasing ratio of glucose C to nitrate N on the reduction of N₂O to N₂ during denitrification was not confirmed. The results show that cattle increase the denitrification potential of the soil but suggest that the denitrification potential does not increase indefinitely with increasing cattle impact.     

Effects of biochar addition on N<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> emissions from two paddy soils

Impacts of biochar addition on nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions from paddy soils are not well documented. Here, we have hypothesized that N₂O emissions from paddy soils could be depressed by biochar incorporation during the upland crop season without any effect on CO₂ emissions. Therefore, we have carried out the 60-day aerobic incubation experiment to investigate the influences of rice husk biochar incorporation (50 t ha⁻¹) into two typical paddy soils with or without nitrogen (N) fertilizer on N₂O and CO₂ evolution from soil. Biochar addition significantly decreased N₂O emissions during the 60-day period by 73.1% as an average value while the inhibition ranged from 51.4% to 93.5% (P < 0.05–0.01) in terms of cumulative emissions. Significant interactions were observed between biochar, N fertilizer, and soil type indicating that the effect of biochar addition on N₂O emissions was influenced by soil type. Moreover, biochar addition did not increase CO₂ emissions from both paddy soils (P > 0.05) in terms of cumulative emissions. Therefore, biochar can be added to paddy fields during the upland crop growing season to mitigate N₂O evolution and thus global warming.     

Soil nitrogen mineralization and fate of (<Superscript>15</Superscript>NH<Subscript>4</Subscript>)<Subscript>2</Subscript>SO<Subscript>4</Subscript> in field-incubated soil in a hardwood plantation of subtropical Australia: the effect of mulching

Background, aim, and scope  

Mulching is frequently used to overcome the drought problem in hardwood plantations that are increasingly being established in lower rainfall areas of Queensland, Australia because of increasing land values. In addition to soil water content, soil nitrogen (N) availability is another critical determinant of plantation productivity in these areas. The purpose of this study was to understand how soil mineral N dynamics, in situ N mineralization, and the fate of fertilized N would be affected by mulching during the early establishment of hardwood plantations.     

Changes in the system of chemical bonds in gibbsite under the impact of NH<Subscript>4</Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript> solutions of different concentrations

The participation of anionic aluminum hydroxo complexes in the binding of phosphate anions on the surface of gibbsite has been shown. The succession of changes in the anionic aluminum phosphate complexes under increasing concentration of phosphate solution has been studied. It has been found that aluminum polyphosphate complexes responsible for the intensive dissolution of gibbsite are formed, along with aluminum orthophosphate complexes, at phosphate solution concentrations of 1 and 2 mol P/L. The decisive role of polyphosphate (P–O–P) groups in the ligand structure of anionic complexes in the transformation of gibbsite to a phosphate mineral (ammonium taranakite) has been revealed. The role of hydrogen bonds with the participation of ligand P(O)OH groups in the formation of ammonium taranakite crystals has been discussed.     

Effect of CO<Subscript>2</Subscript> on nitrification and immobilization of NH<Subscript>4</Subscript><Superscript>+</Superscript>-N

A laboratory incubation experiment was conducted to demonstrate that reduced availability of CO₂ may be an important factor limiting nitrification. Soil samples amended with wheat straw (0%, 0.1% and 0.2%) and (¹⁵NH₄)₂SO₄ (200 mg N kg^–1 soil, 2.213 atom% ¹⁵N excess) were incubated at 30±2°C for 20 days with or without the arrangement for trapping CO₂ resulting from the decomposition of organic matter. Nitrification (as determined by the disappearance of NH₄⁺ and accumulation of NO₃^–) was found to be highly sensitive to available CO₂ decreasing significantly when CO₂ was trapped in alkali solution and increasing substantially when the amount of CO₂ in the soil atmosphere increased due to the decomposition of added wheat straw. The co-efficient of correlation between NH₄⁺-N and NO₃^–-N content of soil was highly significant (r =0.99). During incubation, 0.1–78% of the applied NH₄⁺ was recovered as NO₃^– at different incubation intervals. Amendment of soil with wheat straw significantly increased NH₄⁺ immobilization. From 1.6% to 4.5% of the applied N was unaccounted for and was due to N losses. The results of the study suggest that decreased availability of CO₂ will limit the process of nitrification during soil incubations involving trapping of CO₂ (in closed vessels) or its removal from the stream of air passing over the incubated soil (in open-ended systems).     

The fungal and bacterial biomass (selective inhibition) and the production of CO<Subscript>2</Subscript> and N<Subscript>2</Subscript>O by soddy-podzolic soils of postagrogenic biogeocenoses

In the humus horizon of soddy-podzolic soils of postagrogenic cenoses and primary forests, the contributions of the fungi and bacteria were determined by the selective inhibition of the substrate-induced respiration (SIR) by antibiotics; the basal (microbial) respiration and the net-produced nitrous oxide (N₂O) were also determined. The procedure of the SIR separation using antibiotics (cycloheximide and streptomycin) into the fungal and bacterial components was optimized. It was shown that the fungi: bacteria ratio was 1.58, 2.04, 1.55, 1.39, 2.09, and 1.86 for the cropland, fallow, and different-aged forests (20, 45, 90, and 450 years), respectively. The fungal and bacterial production of CO₂ in the primary forest soil was higher than in the cropland by 6.3 and 11.4 times, respectively. The production of N₂O in the soils of the primary and secondary (90-year-old) forests (3 and 7 ng N-N₂O/g soil per hour, respectively) was 2–13 times lower than in the postagrogenic cenoses, where low values were also found for the microbial biomass carbon (C_mic), its components (the C_mic-bacteria and C_mic-fungi), and the portion of C_mic in the organic carbon of the soil. A conclusion was drawn about the misbalance of the microbial processes in the overgrown cropland accompanied by the increased production of N₂O by the soil during its enrichment with an organic substrate (glucose).     

Short-term uptake of <Superscript>15</Superscript>NH<Subscript>4</Subscript><Superscript>+</Superscript> into soil microbes and seedlings of pine,spruce and birch in potted soils

Short-term competition between soil microbes and seedlings of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.) and silver birch (Betula pendula Roth) for N was assessed in a pot study using (¹⁵NH₄)₂SO₄ as a tracer. Seedlings were grown in organic and mineral soil, collected from a podsol soil; 3.18 mg (¹⁵NH₄)₂SO₄ per pot were injected into the soil, corresponding to 4 µg ¹⁵N g^-1 d.m. (dry matter) mineral soil and 17 µg ¹⁵N g^-1 d.m. organic soil. The amounts of N and ¹⁵N in the seedlings and in microbial biomass derived from fumigation-extraction were measured 48 h after addition of ¹⁵N. In the mineral soil, 19–30% of the added ¹⁵N was found in the plants and 14–20% in the microbial biomass. There were no statistically significant differences between the tree species. In the organic soil, 74% of the added ¹⁵N was recovered in the microbial biomass in birch soil, compared to 26% and 17% in pine and spruce soils, respectively. Correspondingly, about 70% of the ¹⁵N was recovered in pine and spruce seedlings, and only 23% in birch seedlings. In conclusion, plants generally competed more successfully for added ¹⁵NH₄ ⁺ than soil microbes did. An exception was birch growing in organic soil, where the greater amount of available C from birch root exudates perhaps enabled micro-organisms to utilise more N.     

Net and gross mineral N production rates at three levels of forest canopy retention: evidence that NH<Subscript>4</Subscript><Superscript>+</Superscript> and NO<Subscript>3</Subscript><Superscript>−</Superscript> dynamics are uncoupled

Alternative silvicultural systems were introduced in Coastal Western Hemlock forests of British Columbia, Canada, to reduce disturbance incurred by conventional clear-cutting and to maintain the forest influence on soil nutrient cycling. As we hypothesized, in situ pools and net mineralization of NH₄ ⁺ were lower under no and low disturbance (old-growth forest and shelterwood) compared to clear-cuts (high disturbance); in situ pools and net production of NO₃ ⁻ were very low across all treatments. Gross transformation rates of NH₄ ⁺ increased while those of NO₃ ⁻ decreased with increasing disturbance, suggesting that these processes were uncoupled. We conclude that shelterwood harvesting reduces the impact on forest floor NH₄ ⁺ cycling compared to clear-cutting, and that neither low nor high disturbance intensity results in substantial NO₃ ⁻ accumulation, as what occasionally occurs in other ecosystems. We hypothesize that the uncoupling of NH₄ ⁺ and NO₃ ⁻ dynamics may be due to the predominance of heterotrophic nitrification by lignin-degrading fungi that oxidize organic N rather than NH₄ ⁺–N, and whose activities are suppressed at high NH₄ ⁺ concentrations.