Ethylene (C2H4), which is a potent gaseous plant hormone, has often been found to accumulate in anoxic soils where pathways of anaerobic C2H4 oxidation are so far unknown and other C2H4 ...transformation processes are uncommon. The present study shows that ethylene was reduced almost stoichiometrically (89–92%) to ethane (C2H6) in peat-soil microcosms incubated under methanogenic conditions. Methanogenesis started after a prolonged anoxic lag-phase (>29 weeks) where added ethylene prevailed despite the availability of nitrate (NO3−) as an alternative electron acceptor. Methanogenesis, as well as ethylene reduction to ethane, was inhibited by 90% at 1% oxygen. Likewise, methanogenesis and ethane formation was gradually inhibited (to a similar extent) by increasing ethylene concentrations above 0.2%; this inhibition eventually reached 90–95% at 2.2–4.5% C2H4. The present results extend the known settings for ethylene reduction to ethane, which has been occasionally reported for aquatic sediments. The importance of ethylene turn-over in anoxic peat-soils should be substantiated by studies of the abundance and activity of the relevant microorganisms in native peatlands.
► Demonstrates a stoichiometric reduction of ethylene to ethane in peat-soil. ► Identifies ethylene reduction as a novel process in peat-soil. ► Shows that ethylene and oxygen are inhibitory to methanogenesis and ethane formation. ► Substantiates the involvement of (certain) methanogens in ethylene reduction. ► Discusses anaerobic oxidation versus anaerobic reduction of ethylene.
Comprehensive climate change mitigation necessitates soil carbon (C) storage in cultivated terrestrial ecosystems. Deep-rooted perennial crops may help to turn agricultural soils into efficient C ...sinks, especially in deeper soil layers. Here, we compared C allocation and potential stabilization to 150 cm depth from two functionally distinct deep-rooted perennials, i.e., lucerne (Medicago sativa L.) and intermediate wheatgrass (kernza; Thinopyrum intermedium), representing legume and non-legume crops, respectively. Belowground C input and stabilization was decoupled from nitrogen (N) fertilizer rate in kernza (100 and 200 kg mineral N ha
), with no direct link between increasing mineral N fertilization, rhizodeposited C, and microbial C stabilization. Further, both crops displayed a high ability to bring C to deeper soil layers and remarkably, the N
-fixing lucerne showed greater potential to induce microbial C stabilization than the non-legume kernza. Lucerne stimulated greater microbial biomass and abundance of N cycling genes in rhizosphere soil, likely linked to greater amino acid rhizodeposition, hence underlining the importance of coupled C and N for microbial C stabilization efficiency. Inclusion of legumes in perennial cropping systems is not only key for improved productivity at low fertilizer N inputs, but also appears critical for enhancing soil C stabilization, in particular in N limited deep subsoils.
Eutrophication of natural water bodies is moderated by transformation of nitrate (NO₃⁻) in riparian wetlands, which serve as filters of infiltrating drain water from upland agricultural areas. The ...present study comprised field observations, laboratory experiments and metagenomic studies to describe NO₃⁻ removing transformation pathways and interactions with the cycling of iron (Fe) in a temperate riparian wetland soil profile down to 1 m depth. Water samples from piezometers showed a distinct plume of riparian wetland soil profile down to 1 m depth. Water samples from piezometers showed a distinct plume of NO₃⁻ in the subsurface soil where agricultural drain water was infiltrating. However, within a distance of few meters in the water flow direction, NO₃⁻ was depleted from the percolating water. Sampling and analyses of soil from the active zone of the biogeochemical NO₃⁻ removal showed that denitrifying enzyme activity was ~ tenfold higher in the upper 0–25 cm than in the lower 25–100 cm. Yet, net transformation of NO₃⁻ was substantial also at 25–100 cm when assayed with relatively undisturbed soil samples and by ¹⁵N tracer techniques in soil slurries. Transformation pathways of dissimilatory 3 was substantial also at 25–100 cm when assayed with relatively undisturbed soil samples and by 15 N tracer techniques in soil slurries. Transformation pathways of dissimilatory nitrate reduction to ammonium and anaerobic ammonium oxidation were identified, but were quantitatively minor as compared to denitrification. Heterotrophic denitrification and denitrification mediated by oxidation of ferrous iron, Fe(II), were identified as important processes in the wetland soil. The latter was substantiated by geochemical observations, by rates of NO₃⁻ depletion in slurry incubations with added FeCl 2, and by identification of microorganisms with known capacity of NO₃⁻ reduction coupled to Fe (II) oxidation (Acidovorax sp.). The transformation pathway of iron-mediated NO₃⁻ reduction could involve biotic and abiotic reactions, and N₂O, which is a potent greenhouse gas, was a major product of the process. It remains to be seen under field conditions if N₂O emission hotspots are linked to specific sites of dynamic NO₃⁻ reduction coupled to Fe(II) oxidation. process. It remains to be seen under field conditions if N 2 O emission hotspots are linked to specific sites of dynamic NO-3 reduction coupled to Fe (II) oxidation.
Aims Mechanisms of subsoil carbon sequestration from deep-rooted plants are elusive, but may contribute to climate change mitigation. This study addressed the role of root chemistry on carbon ...mineralization and microbiology in a temperate agricultural subsoil (60 and 300 cm depth) compared to topsoil (20 cm depth). Methods Roots from different plant species were chemically characterized and root-induced CO2 production was measured in controlled soil incubations (20 weeks). Total carbon losses, β-glucosidase activity, carbon substrate utilization, and bacterial gene copy numbers were determined. After 20 weeks, resultant carbon mineralization responses to mineral nitrogen (N) were tested. Results Root-induced carbon losses were significantly lower in subsoils (32–41%) than in topsoil (58%). Carbon losses varied according to root chemistry and were mainly linked to root N concentration for subsoils and to lignin and hemicellulose concentration for topsoil. Increases in β-glucosidase activity and bacterial numbers in subsoils were also linked to root N concentration. Added mineral N preferentially stimulated CO2 production from roots with low concentrations of N, lignin and hemicellulose. Conclusions The results were compatible with a concept of N availability and chemically recalcitrant root compounds interacting to control subsoil carbon decomposition. Implications for carbon sequestration from deep-rooted plants are discussed.
Biochar added to agricultural soils may sequester carbon and improve physico-chemical conditions for crop growth, due to effects such as increased water and nutrient retention in the root zone. The ...effects of biochar on soil microbiological properties are less certain. We addressed the effects of wood-based biochar on soil respiration, water contents, potential ammonia oxidation (PAO), arylsulfatase activity (ASA), and crop yields at two temperate sandy loam soils under realistic field conditions. In situ soil respiration, PAO, and ASA were not significantly different in quadruplicate field plots with or without biochar (20 Mg ha
−1
); however, in the same plots, volumetric water contents increased by 7.5 % due to biochar (
P
= 0.007). Crop yields (oat) were not significantly different in the first year after biochar application, but in the second year, total yields of spring barley increased by 11 % (
P
< 0.001), though the increase in grain yield was not significant. Field plots with cumulative biochar rates of up to 100 Mg ha
−1
, applied during two consecutive years, substantiated that biochar was not inhibitory to PAO and ASA as reference plots consistently showed lowest activities. For PAO, it was found that soil pH, rather than biochar rates, was a driving environmental variable. For ASA, the methodological approach was challenged by product sorption, but results did not suggest that biochar significantly stimulated the enzyme activity. Crop yields of maize in field experiments with 10–100 Mg biochar ha
−1
were unaffected by biochar except for a negative effect of the highest annual rates of 50 Mg ha
−1
in the first year after application. In conclusion, the present wood-based biochar poorly affected the measured microbial processes and generally resulted in similar crop yields in reference and biochar-amended soil plots.
Woodchip bioreactors are increasingly used to remove nitrate (NO
3
–
) from agricultural drainage water in order to protect aquatic ecosystems from excess nitrogen. Nitrate removal in woodchip ...bioreactors is based on microbial processes, but the microbiomes and their role in bioreactor efficiency are generally poorly characterized. Using metagenomic analyses, we characterized the microbiomes from 3 full-scale bioreactors in Denmark, which had been operating for 4–7 years. The microbiomes were dominated by
Proteobacteria
and especially the genus
Pseudomonas
, which is consistent with heterotrophic denitrification as the main pathway of NO
3
–
reduction. This was supported by functional gene analyses, showing the presence of the full suite of denitrification genes from NO
3
–
reductases to nitrous oxide reductases. Genes encoding for dissimilatory NO
3
–
reduction to ammonium were found only in minor proportions. In addition to NO
3
–
reducers, the bioreactors harbored distinct functional groups, such as lignocellulose degrading fungi and bacteria, dissimilatory sulfate reducers and methanogens. Further, all bioreactors harbored genera of heterotrophic iron reducers and anaerobic iron oxidizers (
Acidovorax
) indicating a potential for iron-mediated denitrification. Ecological indices of species diversity showed high similarity between the bioreactors and between the different positions along the flow path, indicating that the woodchip resource niche was important in shaping the microbiome. This trait may be favorable for the development of common microbiological strategies to increase the NO
3
–
removal from agricultural drainage water.
We investigated the effects of trace metal additions on microbial nitrogen (N) and carbon (C) cycling using freshwater wetland sediment microcosms amended with micromolar concentrations of copper ...(Cu), molybdenum (Mo), iron (Fe), and all combinations thereof. In addition to monitoring inorganic N transformations (NO
3
–
, NO
2
–
, N
2
O, NH
4
+
) and carbon mineralization (CO
2
, CH
4
), we tracked changes in functional gene abundance associated with denitrification (
nirS
,
nirK
,
nosZ
), dissimilatory nitrate reduction to ammonium (DNRA;
nrfA
), and methanogenesis (
mcrA
). With regards to N cycling, greater availability of Cu led to more complete denitrification (i.e., less N
2
O accumulation) and a higher abundance of the
nirK
and
nosZ
genes, which encode for Cu-dependent reductases. In contrast, we found sparse biochemical evidence of DNRA activity and no consistent effect of the trace metal additions on
nrfA
gene abundance. With regards to C mineralization, CO
2
production was unaffected, but the amendments stimulated net CH
4
production and Mo additions led to increased
mcrA
gene abundance. These findings demonstrate that trace metal effects on sediment microbial physiology can impact community-level function. We observed direct and indirect effects on both N and C biogeochemistry that resulted in increased production of greenhouse gasses, which may have been mediated through the documented changes in microbial community composition and shifts in functional group abundance. Overall, this work supports a more nuanced consideration of metal effects on environmental microbial communities that recognizes the key role that metal limitation plays in microbial physiology.
Direct field emissions of nitrous oxide (N2O) may determine whether biodiesel from oilseed rape (Brassica napus L.) fulfills the EU requirement of at least 50% reduction of greenhouse gas emissions ...as compared to fossil diesel. However, only few studies have documented fertilizer N emission factors (EF) and mitigation options for N2O emissions from oilseed rape cropping systems. We conducted a field experiment with three N levels (0, 171, and 217 kg/ha), where the N fertilizer was applied as ammonium sulfate nitrate with or without the nitrification inhibitor 3,4‐dimethylpyrazole phosphate (DMPP). N2O fluxes were measured using static chambers technique and soil samples were analyzed for water and mineral N content during a monitoring period of 368 days. The DMPP treatments showed a significantly increased level of ammonium (NH4+) for up to 18 weeks after spring fertilization as compared to the treatments without DMPP. However, this difference did not result in a corresponding decrease in NO3- soil content, and no differences in cumulative N2O emissions were found between any fertilized treatments with or without DMPP (mean, 1.26 kg N2O‐N ha−1 year−1). More field experiments are needed to clarify whether DMPP‐coated mineral fertilizers could mitigate N2O emissions under different weather conditions, for example, under conditions where fertilization events concurred with rainfall events increasing water‐filled pore space to the assumed 60% threshold for denitrification. Emission factors for mineral N fertilizer were 0.28%–0.36% with a mean of 0.32% across the fertilized treatments. These data concur with recent European studies suggesting that the EF for mineral N fertilizers in oilseed rape cropping systems may typically be lower than the default IPCC value of 1%. Further studies are needed to consolidate an EF for oilseed rape under temperate conditions, which will be determining for the sustainability of Northern European oilseed rape cultivation for biodiesel.
The sustainability of oilseed rape‐based biodiesel is challenged by soil nitrous oxide (N2O) emissions related to the cultivation step. However, uncertainty remains regarding the magnitude and potential mitigation of N2O field emissions. This field experiment measured the N2O emissions through 1 year while also testing a nitrification inhibitor (NI) as possible N2O mitigation agent. We found the N2O emissions to be lower than generally expected, but did not find an effect of the NI. This means that the general sustainability of oilseed rape‐based biodiesel is better than believed; however, no further reductions due to specific management actions could be confirmed.
Quantifying in-house emissions of methane (CH4) from liquid manure (slurry) is difficult due to high background emissions from enteric processes, yet of great importance for correct estimation of CH4 ...emissions from manure management and effects of treatment technologies such as anaerobic digestion. In this study CH4 production rates were determined in 20 pig slurry and 11 cattle slurry samples collected beneath slatted floors on six representative farms; rates were determined within 24 h at temperatures close to the temperature in slurry pits at the time of collection. Methane production rates in pig and cattle slurry differed significantly at 0.030 and 0.011 kg CH4 kg-1 VS (volatile solids). Current estimates of CH4 emissions from pig and cattle manure management correspond to 0.032 and 0.015 kg CH4 kg-1, respectively, indicating that slurry pits under animal confinements are a significant source. Fractions of degradable volatile solids (VSd, kg kg-1 VS) were estimated using an aerobic biodegradability assay and total organic C analyses. The VSd in pig and cattle slurry averaged 0.51 and 0.33 kg kg-1 VS, and it was estimated that on average 43 and 28% of VSd in fresh excreta from pigs and cattle, respectively, had been lost at the time of sampling. An empirical model of CH4 emissions from slurry was reparameterised based on experimental results. A sensitivity analysis indicated that predicted CH4 emissions were highly sensitive to uncertainties in the value of lnA of the Arrhenius equation, but much less sensitive to uncertainties in VSd or slurry temperature. A model application indicated that losses of carbon in VS as CO2 may be much greater than losses as CH4. Implications of these results for the correct estimation of CH4 emissions from manure management, and for the mitigation potential of treatments such as anaerobic digestion, are discussed.
BACKGROUND AND AIMS: Combination of rewetting and wetland crop cultivation (paludiculture) is pursued as a wider carbon dioxide (CO₂) mitigation option in drained peatland. However, information on ...the overall greenhouse gas (GHG) balance for paludiculture is lacking. We investigated the GHG balance of peatlands grown with reed canary grass (RCG) and rewetted to various extents. METHODS: Gas fluxes of CO₂, methane (CH₄) and nitrous oxide (N₂O) were measured with a static chamber technique for 10 months from mesocosms sown with RCG and manipulated to ground water levels (GWL) of 0, −10, −20, −30 and −40 cm below the soil surface. Gross primary production (GPP) was estimated from the above ground biomass yield. RESULTS: The mean dry biomass yield across all water table treatments was 6 Mg ha⁻¹with no significant differences between the treatments. Raising the GWL to the surface decreased both the net ecosystem exchange (NEE) of CO₂and N₂O emissions whereas CH₄emissions increased. Total cumulative GHG emissions (for 10 months) corresponded to 0.08, 0.13, 0.61, 0.68 and 0.98 kg CO₂equivalents m⁻²from the GWL treatments at 0, −10, −20, −30 and −40 cm below the soil surface, respectively. CONCLUSIONS: The results showed that a reduction in total GHG emission can be achieved without losing the productivity of newly established RCG when GWL is maintained close to the surface. Further studies should address the practical constrains and long-term productivity of RCG cultivation in rewetted peatlands.
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