Nitrate removal and environmental side-effects controlled by hydraulic residence time in woodchip bioreactors treating cold agricultural drainage water

Denitrifying woodchip bioreactors (WBRs) remove nitrate (NO 3 - ) from agricultural drainage water at field-scale, but their efficacy at cold temperatures remains uncertain. This study shows how hydraulic residence time (HRT) controls NO 3 - removal and environmental side-effects of WBRs at low water temperature under pilot-scale conditions with controlled operation of nine WBRs (94 dm3). Hydraulic properties were assessed by a bromide tracer test, and NO 3 - removal, emissions of nitrous oxide (N2O) and methane (CH4), and losses of dissolved organic carbon (DOC) were measured at HRTs of 5-30 h. Inlet NO 3 - concentrations were increasingly reduced at higher HRTs. The relationship between HRT and the efficiency (%) of NO 3 - removal was linear (R a d j 2  = 0.94), while the relationship between HRT and NO 3 - reduction rates (NRR) was logistic (R a d j 2  = 0.88). Gaseous emissions of N2O were equally low at HRTs of 10-30 h, but higher at 5 h (P < 0.05). Methane fluxes were small, but with consistent emissions at HRTs of 20-30 h and uptake at 5-15 h. HRT had limited effect on effluent DOC concentrations, but strong effect on mass losses that were five-fold higher (320 mg L-1) at the HRT of 5 h than at 30 h. In summary, at cold temperatures HRTs of ≤ 20 h resulted in suboptimal NRR, accelerating DOC losses, and increased risk of N2O losses at least below a threshold HRT of 5-10 h. HRTs of 20-30 h gave maximal NRR, smallest losses of DOC and N2O, but an increased risk of CH4 emissions.

Soil greenhouse gas emissions from drained and rewetted agricultural bare peat mesocosms are linked to geochemistry

In view of climate considerations regarding the management of peatlands, there is a need to assess whether rewetting can mitigate greenhouse gas (GHG) emissions, and notably how site-specific soil-geochemistry will influence differences in emission magnitudes. However, there are inconsistent results regarding the correlation of soil properties with heterotrophic respiration (Rh) of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from bare peat. In this study, we determined 1) soil-, and site-specific geochemical components as drivers for emissions from Rh on five Danish fens and bogs, and 2) emission magnitudes under drained and rewetted conditions. For this, a mesocosm experiment was performed under equal exposure to climatic conditions and water table depths controlled to either -40 cm, or -5 cm. For the drained soils, we found that annual cumulative emissions, accounting for all three gases, were dominated by CO2, contributing with, on average, 99 % to a varying global warming potential (GWP) of 12.2-16.9 t CO2eq ha-1 yr-1. Rewetting lowered annual cumulative emissions from Rh by 3.2-5.1 t CO2eq ha-1 yr-1 for fens and bogs, respectively, despite a high variability of site-specific CH4 emissions, contributing with 0.3-3.4 t CO2 ha-1 yr-1 to the GWP. Overall, analyses using generalized additive models (GAM) showed that emission magnitudes were well explained by geochemical variables. Under drained conditions, significant soil-specific predictor variables for CO2 flux magnitudes were pH, phosphorus (P), and the soil substrate's relative water holding capacity (WHC). When rewetted, CO2 and CH4 emissions from Rh were affected by pH, WHC, as well as contents of P, total carbon and nitrogen. In conclusion, our results found the highest GHG reduction on fen peatlands, further highlighting that peat nutrient status and acidity, and the potential availability of alternative electron acceptors, might be used as proxies for prioritising peatland areas for GHG mitigation efforts by rewetting.

Nitrous oxide fluxes from long-term limed soils following P and glucose addition: Nonlinear response to liming rates and interaction from added P

Liming of acidic soils to regulate pH for crop growth may decrease emissions of nitrous oxide (N₂O) due to direct effects of pH on the synthesis of N₂O reductases by denitrifying bacteria. However, liming also changes general pH-dependent soil properties, including availability of phosphorus (P), with a feedback on N₂O fluxes that remains largely unknown. Here we used a mesocosm approach to study the combined role of liming and P in regulating N₂O fluxes from denitrification in an arable coarse sandy soil where N₂O emissions under field condition coincided with rainfall events and irrigation, which facilitated anoxia. Soils from three long-term liming treatments (0, 4, and 12 Mg ha^-1) with resulting pH(CaCl₂) of 3.6, 4.7 and 6.3 were incubated at original bulk density first at 60% water filled pore space (WFPS) and successively at 75% WFPS with added nitrate, inorganic P (0 and 10 μg P g^-1 soil) and glucose as labile carbon. N₂O fluxes were measured during 28 days and were supplemented with measurements of CO₂ fluxes, microbial biomass, potential denitrification, and acid phosphatase activity. The results showed a nonlinear response of N₂O fluxes to liming rates, with highest fluxes at the intermediate liming level (4 Mg ha^-1). Furthermore, inorganic P stimulated N₂O fluxes only at the intermediate liming level. Assays of potential denitrification indicated that the N₂O/(N₂O + N₂) product ratio decreased consistently with increasing liming rates, but total N₂O fluxes responded nonlinearly likely due to combined effects on N₂O/(N₂O + N₂) product ratios and total denitrification rates. The results suggest that liming and P addition interact on microbial properties and N₂O emissions from acidic arable soils and may not follow linear trends. This makes it uncertain to predict and model the resulting net effect, which may depend on the actual pH range and P availability from the unlimed to the limed treatments.

Soil N2O emission from organic and conventional cotton farming in Northern Tanzania

The effort to increase the sustainable supply of food and fibre is challenged by the potential for increased greenhouse gas (GHG) emissions from farming systems with intensified production systems. This study aimed at quantifying soil N₂O emissions from smallholder organic and conventional cotton production practices in a semi-arid area, Meatu, Northern Tanzania. Field experiments were conducted to quantify N₂O emissions under (i) current practices with organic (3 Mg ha^-1 farmyard manure (FYM)) and conventional (30 kg mineral N ha^-1) cultivation; (ii) a high input practice with organic (5 Mg ha^-1 FYM) and conventional (60 kg mineral N ha^-1) cultivation; and (iii) an integrated practice with organic (3 Mg FYM + legume intercropping) and conventional (30 kg N + 3 Mg ha^-1 FYM) cultivation. In both organic and conventional farming, control treatments with no fertilizer application were included. The study was performed over two growing seasons, where season 1 was rather wet and season 2 was rather dry. Static chambers were used for in-situ measurement of N₂O emission from soil. The current organic and conventional cotton farming practices did not differ (P > 0.05) in cumulative area-scaled and yield-scaled N₂O emissions. High input conventional cotton showed higher area scaled N₂O emissions than organic cotton during the wetter season, but not during the drier season. The inorganic fertilizer + FYM combination did not differ (P > 0.05) in area- and yield-scaled N₂O emissions from conventional practice. Intercropping cotton and legumes did not affect (P > 0.05) N₂O emission compared to 3 Mg FYM ha^-1. The emission factors for both conventional and organic systems were generally above 1% in the dry season 2, but below 1% in the wetter season 1. The use of organic and inorganic fertilizers at rates up to 60 kg N ha^-1, FYM-inorganic fertilizer combination, and cotton-legume intercropping increased yields, while N₂O emissions stayed low, in particular with use of mineral fertilizers.

Microbiome Structure and Function in Woodchip Bioreactors for Nitrate Removal in Agricultural Drainage Water

Woodchip bioreactors are increasingly used to remove nitrate (NO₃^–) 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 NO₃^– reduction. This was supported by functional gene analyses, showing the presence of the full suite of denitrification genes from NO₃^– reductases to nitrous oxide reductases. Genes encoding for dissimilatory NO₃^– reduction to ammonium were found only in minor proportions. In addition to NO₃^– 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 NO₃^– removal from agricultural drainage water.

Temperature Sensitivity and Composition of Nitrate-Reducing Microbiomes from a Full-Scale Woodchip Bioreactor Treating Agricultural Drainage Water

Denitrifying woodchip bioreactors (WBR), which aim to reduce nitrate (NO3-) pollution from agricultural drainage water, are less efficient when cold temperatures slow down the microbial transformation processes. Conducting bioaugmentation could potentially increase the NO3- removal efficiency during these specific periods. First, it is necessary to investigate denitrifying microbial populations in these facilities and understand their temperature responses. We hypothesized that seasonal changes and subsequent adaptations of microbial populations would allow for enrichment of cold-adapted denitrifying bacterial populations with potential use for bioaugmentation. Woodchip material was sampled from an operating WBR during spring, fall, and winter and used for enrichments of denitrifiers that were characterized by studies of metagenomics and temperature dependence of NO3- depletion. The successful enrichment of psychrotolerant denitrifiers was supported by the differences in temperature response, with the apparent domination of the phylum Proteobacteria and the genus Pseudomonas. The enrichments were found to have different microbiomes' composition and they mainly differed with native woodchip microbiomes by a lower abundance of the genus Flavobacterium. Overall, the performance and composition of the enriched denitrifying population from the WBR microbiome indicated a potential for efficient NO3- removal at cold temperatures that could be stimulated by the addition of selected cold-adapted denitrifying bacteria.

Trace Metal Availability Affects Greenhouse Gas Emissions and Microbial Functional Group Abundance in Freshwater Wetland Sediments

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 (NO₃ ^-, NO₂ ^-, N₂O, NH₄ ⁺) and carbon mineralization (CO₂, CH₄), 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 N₂O 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, CO₂ production was unaffected, but the amendments stimulated net CH₄ 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.

Long-term nitrogen loading alleviates phosphorus limitation in terrestrial ecosystems

Increased human-derived nitrogen (N) deposition to terrestrial ecosystems has resulted in widespread phosphorus (P) limitation of net primary productivity. However, it remains unclear if and how N-induced P limitation varies over time. Soil extracellular phosphatases catalyze the hydrolysis of P from soil organic matter, an important adaptive mechanism for ecosystems to cope with N-induced P limitation. Here we show, using a meta-analysis of 140 studies and 668 observations worldwide, that N stimulation of soil phosphatase activity diminishes over time. Whereas short-term N loading (≤5 years) significantly increased soil phosphatase activity by 28%, long-term N loading had no significant effect. Nitrogen loading did not affect soil available P and total P content in either short- or long-term studies. Together, these results suggest that N-induced P limitation in ecosystems is alleviated in the long-term through the initial stimulation of soil phosphatase activity, thereby securing P supply to support plant growth. Our results suggest that increases in terrestrial carbon uptake due to ongoing anthropogenic N loading may be greater than previously thought.

Nitrous oxide emissions from oilseed rape cultivation were unaffected by flash pyrolysis biochar of different type, rate and field ageing

Nitrous oxide (N₂O) emission from winter oilseed rape (WOSR) cultivation may compromise the sustainability of oilseed rape biodiesel. Typically, greenhouse gas budgets of WOSR cultivation assume an N₂O emission factor (EF) of 1% of the N added in fertilizer and crop residues. Management options to reduce direct soil emissions of N₂O include the application of biochar, but efficacy and mechanisms of N₂O suppression are elusive. We measured N₂O emissions in a WOSR field trial on a sandy loam soil in Denmark over 402 days in 2017-2018, comparing biochar applications from two feedstocks (wheat straw and pig manure fibers), two application rates (1.5 and 15 Mg ha^-1) and field ageing of up to three years. Further, a controlled incubation experiment was performed to examine the effect of biochar dose and ageing on N₂O production and consumption by denitrification. Biochar treatments had no significant effects on cumulative N₂O emissions (1.71-2.78 kg N ha^-1 yr^-1). Likewise, no significant effects were found on crop yield, yield-scaled N₂O emission, soil mineral N content, gravimetric soil moisture or pH. The fertilizer induced EF was 0.51% which is well below the IPCC Tier 1 EF of 1%. High doses of fresh, but not field-aged biochar suppressed N₂O production under anoxic conditions ex situ, suggesting that biochar with sufficient liming capacity could mitigate N₂O emissions from denitrification also under field conditions. Yet, rates of up to 15 Mg ha^-1 flash pyrolysis biochar in the current in situ study, which comprised a pronounced summer drought, showed no significant N₂O mitigation. This highlights the need for selecting dedicated biochars and doses and test them in multi-year studies to conclude on their N₂O mitigating effect. Yet, in relation to sustainability of WOSR cultivation for biodiesel, the current study suggests that C sequestration by biochar is not compromised by increased N₂O emissions.

Optimal biochar amendment rate reduced the yield-scaled N2O emissions from Ultisols in an intensive vegetable field in South China

Nitrous oxide (N₂O) emissions, vegetable yields, and soil microbial properties were studied in response to different rates of rice-straw biochar applied to an intensive vegetable soil (Ultisol) in South China. The study was conducted over a one-year period as a block-designed field experiment (n = 3) with two successive crops and five harvests in total. Biochar was applied at rates of 0, 10, 20, 30 and 40 Mg ha^-1 and splits of nitrogen (N) fertilizer were added in the form of urea (1010 kg N in total). References without biochar and N fertilization were included. Biochar significantly decreased the cumulative annual N₂O emissions by 34-67%, which concurred with decreased denitrification enzyme activity and increased nosZ gene abundance in the vegetable soil. The absolute N₂O mitigation increased with increasing flux rates, which were positively correlated to soil temperature and water-filled pore space. Conversely, weak increases of N₂O emissions were recurrently induced by biochar when the soil temperature was lower than 20 °C and the absolute fluxes were low. A significant 17-29% increase in vegetable yield was induced by biochar, which also ameliorated soil fertility by increasing the soil carbon content and the cation exchange capacity. Overall, biochar significantly decreased the yield-scaled N₂O emissions by 44-71% with the lowest yield-scaled N₂O emissions for the intermediate biochar application rate of 20 Mg ha^-1. Higher biochar application rates failed to further decrease the yield-scaled N₂O emissions, but rather caused weak increases. Based on the present results, a biochar application rate of 20 Mg ha^-1 combined with N fertilization seemed to be recommendable to achieve highest vegetable yield with lowest N₂O emissions in intensive vegetable production in South China.

Methane fluxes from a rewetted agricultural fen during two initial years of paludiculture

Rewetting agricultural peatland abates carbon dioxide (CO₂) emission, but the resulting waterlogged anaerobic soil condition may create hotspots of methane (CH₄) emissions. In this study, we measured CH₄ emissions from side-by-side replicated plots in an agricultural fen cultivated with reed canary grass under a control and two experimental rewetting (i.e., paludiculture) conditions as either continuously flooded to soil surface or semi-flooded where water from the flooded plots intruded from sub-surface. Fluxes were measured for two successive years at 1-2 week intervals (total 59 measurement dates) using static chambers. Annual emissions were estimated by trapezoidal linear interpolation of the measured fluxes between the measurement dates. Two-year time-weighted average ground water tables (GWT) in the flooded, semi-flooded and control plots were 1, 3 and 9 cm below soil surface, respectively. The annual average emissions from flooded plots were 82 and 116 g CH₄ m^-2 yr^-1 in Year 1 and 2, respectively, which were significantly higher than the emissions from semi-flooded plots (35 and 69 g CH₄ m^-2 yr^-1 in Year 1 and 2, respectively) and from control plots (3 and 9 g CH₄ m^-2 yr^-1 in Year 1 and 2, respectively). Overall, the results showed that the GWT in paludiculture should be maintained few cm below soil surface during high temperature periods to prevent risks of high CH₄ emissions.

Soil carbon loss with warming: New evidence from carbon-degrading enzymes

Climate warming affects soil carbon (C) dynamics, with possible serious consequences for soil C stocks and atmospheric CO2 concentrations. However, the mechanisms underlying changes in soil C storage are not well understood, hampering long-term predictions of climate C-feedbacks. The activity of the extracellular enzymes ligninase and cellulase can be used to track changes in the predominant C sources of soil microbes and can thus provide mechanistic insights into soil C loss pathways. Here we show, using meta-analysis, that reductions in soil C stocks with warming are associated with increased ratios of ligninase to cellulase activity. Furthermore, whereas long-term (≥5 years) warming reduced the soil recalcitrant C pool by 14%, short-term warming had no significant effect. Together, these results suggest that warming stimulates microbial utilization of recalcitrant C pools, possibly exacerbating long-term climate-C feedbacks.

Biochar potentially mitigates greenhouse gas emissions from cultivation of oilseed rape for biodiesel

Winter oilseed rape (WOSR) is the main crop for biodiesel in the EU, where legislation demands at least 50% savings in greenhouse gas (GHG) emissions as compared to fossil diesel. Thus industrial sectors search for optimized management systems to lower GHG emissions from oilseed rape cultivation. Recently, pyrolysis of biomass with subsequent soil amendment of biochar has shown potentials for GHG mitigation in terms of carbon (C) sequestration, avoidance of fossil based electricity, and mitigation of soil nitrous oxide (N₂O) emissions. Here we analyzed three WOSR scenarios in terms of their global warming impact using a life cycle assessment approach. The first was a reference scenario with average Danish WOSR cultivation where straw residues were incorporated to the soil. The others were biochar scenarios in which the oilseed rape straw was pyrolysed to biochar at two process temperatures (400 and 800 °C) and returned to the field. The concept of avoided atmospheric CO₂ load was applied for calculation of C sequestration factors for biochar, which resulted in larger mitigation effects than derived from calculations of just the remaining C in soil. In total, GHG emissions were reduced by 73 to 83% in the two biochar scenarios as compared to the reference scenario, mainly due to increased C sequestration. The climate benefits were higher for pyrolysis of oilseed rape straw at 800 than at 400 °C. The results demonstrated that biochar has a potential to improve the life cycle GHG emissions of oilseed rape biodiesel, and highlighted the importance of consolidated key assumptions, such as biochar stability in soil and the CO₂ load of marginal grid electricity.

The influence of hydrolysis and derivatization on the determination of amino acid content and isotopic ratios in dual-labeled (13 C, 15 N) white clover

RATIONALE

The cycling of peptide- and protein-bound amino acids (AAs) is important for studying the rate-limiting steps in soil nitrogen (N) turnover. A strong tool is stable C and N isotopes used in combination with compound-specific isotope analysis (CSIA), where a prerequisite for analysis is appropriate methods for peptide and protein hydrolysis and appropriate methods for derivatization of AAs for analysis by gas chromatography (GC).

METHODS

We examined the efficiency of a standard acidic hydrolysis (6 M HCl, 20 h at 110°C) and a fast acidic hydrolysis (6 M HCl, 70 min at 150°C) on the recovery of AAs from a protein standard (bovine serum albumin). The best methods were used on dual-labeled (¹³ C and ¹⁵ N) clover shoot and root juice, divided into four molecular weight (Mw) size fractions. We used NAIP (N-acetyl isopropyl esterification) derivatization for GC/combustion-isotope ratio mass spectrometry (C-IRMS) analysis of AA standards.

RESULTS

The NAIP derivatization gave very low limits of detection (LODs) (< 2 pmol) and limits of quantification (LOQs) ranging from 0.55 to 4.89 pmol. Comparing the concentrations of individual AAs in hydrolyzed versus unhydrolyzed clover juice samples of the low Mw size fraction (<1 kDa) showed a significant decline in concentration (p <0.03) for seven AAs after hydrolysis. Despite the decline in AA concentration, we found a linear connection between the obtained atomic fraction (¹³ C/total carbon and ¹⁵ N/total nitrogen) for individual AAs of hydrolyzed versus unhydrolyzed samples.

CONCLUSIONS

The methodology distinguished differences in atomic fractions across AAs, in individual AAs in Mw size fractions, and between shoot and root samples of experimentally labeled white clover. Specifically, the method separated L-glutamate (Glu) and glutamine (Gln). Thus, for a broader use in plant and soil ecology, we present an optimized methodology for GC/C-IRMS analysis of AAs from organic nitrogen samples enriched with ¹³ C and ¹⁵ N - AA stable isotope probing (SIP).

Activity of Type I Methanotrophs Dominates under High Methane Concentration: Methanotrophic Activity in Slurry Surface Crusts as Influenced by Methane, Oxygen, and Inorganic Nitrogen

Livestock slurry is a major source of atmospheric methane (CH), but surface crusts harboring methane-oxidizing bacteria (MOB) could mediate against CH emissions. This study examined conditions for CH oxidation by in situ measurements of oxygen (O) and nitrous oxide (NO), as a proxy for inorganic N transformations, in intact crusts using microsensors. This was combined with laboratory incubations of crust material to investigate the effects of O, CH, and inorganic N on CH oxidation, using CH to trace C incorporation into lipids of MOB. Oxygen penetration into the crust was 2 to 14 mm, confining the potential for aerobic CH oxidation to a shallow layer. Nitrous oxide accumulated within or below the zone of O depletion. With 10 ppmv CH there was no O limitation on CH oxidation at O concentrations as low as 2%, whereas CH oxidation at 10 ppmv CH was reduced at ≤5% O. As hypothesized, CH oxidation was in general inhibited by inorganic N, especially NO, and there was an interaction between N inhibition and O limitation at 10 ppmv CH, as indicated by consistently stronger inhibition of CH oxidation by NH and NO at 3% compared with 20% O. Recovery of C in phospholipid fatty acids suggested that both Type I and Type II MOB were active, with Type I dominating high-concentration CH oxidation. Given the structural heterogeneity of crusts, CH oxidation activity likely varies spatially as constrained by the combined effects of CH, O, and inorganic N availability in microsites.

Effects of Biochar on Dispersibility of Colloids in Agricultural Soils

The mobility of water-dispersible colloids (WDCs) in soil may be influenced by soil management practices such as organic soil amendments. Biochar has recently been promoted as a useful soil amendment, and extensive research has been devoted to investigating its effects on soil macroscopic properties and functions. However, there is limited understanding of the effects of biochar application on micro-scale particle dynamics. We conducted a field study to investigate the effects of the application of birch ( spp.) wood biochar on colloid dispersibility with respect to application rate, history, and physicochemical soil properties. Undisturbed soil cores (100 cm) were collected from the topsoil of two agricultural sites in Denmark with soils of sandy loam texture. The two sites received biochar at different application rates (0-100 Mg ha) and were sampled 7 to 19 mo later. The WDC content was determined using an end-over-end shaking method on 100-cm intact soil cores, and the colloid solution was analyzed for electrical conductivity, pH, and zeta potential. The WDC content increased with biochar application rate because of biochar-induced changes in soil chemistry and was strongly and positively correlated with the concentration of exchangeable monovalent cations in the soils. Biochar application increased pH and decreased electrical conductivity and zeta potential in the colloid suspension more in the short term (7 mo) than in the long term (19 mo). Thus, there is potential for biochar to induce short-term changes in soil solution chemistry in agricultural soils, which may influence the mobility of soil colloids.

Estimation of Methane Emissions from Slurry Pits below Pig and Cattle Confinements

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 (VS_d, kg kg^-1 VS) were estimated using an aerobic biodegradability assay and total organic C analyses. The VS_d 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 VS_d 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 VS_d 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.

Temperature response of methane production in liquid manures and co-digestates

Intensification of livestock production makes correct estimation of methanogenesis in liquid manure increasingly important for inventories of CH4 emissions. Such inventories currently rely on fixed methane conversion factors as knowledge gaps remain with respect to detailed temperature responses of CH4 emissions from liquid manure. Here, we describe the temperature response of CH4 production in liquid cattle slurry, pig slurry, and fresh and stored co-digested slurry from a thermophilic biogas plant. Subsamples of slurry were anoxically incubated at 20 temperatures from 5-52°C in a temperature gradient incubator and CH4 production was measured by gas chromatographic analysis of headspace gas after a 17-h incubation period. Methane production potentials at 5-37°C were described by the Arrhenius equation (modelling efficiencies, 79.2-98.1%), and the four materials showed a consistent activation energy (Ea) which averaged 81.0kJmol(-1) (95% confidence interval, 74.9-87.1kJmol(-1)) corresponding to a temperature sensitivity (Q10) of 3.4. In contrast, the frequency factor (A) differed among the slurry materials (30.1<ln A<33.3; mean, 31.3) reflecting that origin, age and composition of the manure affect this parameter. The Ea estimate, based on individual slurry materials, was intermediate when compared to published values of 63 and 112.7kJmol(-1) derived from composite data, but was similar to Ea estimated for CH4 production at microbial community level across aquatic ecosystems, wetlands and rice paddies (89.3kJmol(-1)). This supports that the derived temperature sensitivity parameters may be applicable to dynamic modelling of CH4 emissions from livestock manure.

Full GHG balance of a drained fen peatland cropped to spring barley and reed canary grass using comparative assessment of CO2 fluxes

Empirical greenhouse gas (GHG) flux estimates from diverse peatlands are required in order to derive emission factors for managed peatlands. This study on a drained fen peatland quantified the annual GHG balance (Carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and C exported in crop yield) from spring barley (SB) and reed canary grass (RCG) using static opaque chambers for GHG flux measurements and biomass yield for indirectly estimating gross primary production (GPP). Estimates of ecosystem respiration (ER) and GPP were compared with more advanced but costly and labor-intensive dynamic chamber studies. Annual GHG balance for the two cropping systems was 4.0 ± 0.7 and 8.1 ± 0.2 Mg CO2-Ceq ha(-1) from SB and RCG, respectively (mean ± standard error, n = 3). Annual CH4 emissions were negligible (<0.006 Mg CO2-Ceq ha(-1)), and N2O emissions contributed only 4-13 % of the full GHG balance (0.5 and 0.3 Mg CO2-Ceq ha(-1) for SB and RCG, respectively). The statistical significance of low CH4 and N2O fluxes was evaluated by a simulation procedure which showed that most of CH4 fluxes were within the range that could arise from random variation associated with actual zero-flux situations. ER measured by static chamber and dynamic chamber methods was similar, particularly when using nonlinear regression techniques for flux calculations. A comparison of GPP derived from aboveground biomass and from measuring net ecosystem exchange (NEE) showed that GPP estimation from biomass might be useful, or serve as validation, for more advanced flux measurement methods. In conclusion, combining static opaque chambers for measuring ER of CO2 and CH4 and N2O fluxes with biomass yield for GPP estimation worked well in the drained fen peatland cropped to SB and RCG and presented a valid alternative to estimating the full GHG balance by dynamic chambers.

Effects of biochar on air and water permeability and colloid and phosphorus leaching in soils from a natural calcium carbonate gradient

Application of biochar to agricultural fields to improve soil quality has increased in popularity in recent years, but limited attention is generally paid to existing field conditions before biochar application. This study examined the short-term physicochemical effects of biochar amendment in an agricultural field in Denmark with a calcium carbonate (CaCO) gradient. The field comprised four reference plots and four plots to which biochar (birch wood pyrolyzed at 500°C) was applied at a rate of 20 t ha. Five undisturbed soil columns (10 cm diam., 8 cm height) were sampled from each plot 7 mo after biochar application, and a series of leaching experiments was conducted. The leachate was analyzed for tritium (used as a tracer), colloids, and phosphorus concentration. The results revealed that the presence of CaCO has resulted in marked changes in soil structure (bulk density) and soil chemical properties (e.g., pH and ionic strength), which significantly affected air and water transport and colloid and phosphorous leaching. In denser soils (bulk density, 1.57-1.69 g cm) preferential flow dominated the transport and caused an enhanced movement of air and water, whereas in less dense soils (bulk density, 1.38-1.52 g cm) matrix flow predominated the transport. Compared with reference soils, biochar-amended soils showed slightly lower air permeability and a shorter travel time for 5% of the applied tracer (tritium) to leach through the soil columns. Colloid and phosphorus leaching was observed to be time dependent in soils with low CaCO. Biochar-amended soils showed higher colloid and P release than reference soils. Field-scale variations in total colloid and P leaching reflected clear effects of changes in pH and ionic strength due to the presence of CaCO. There was a linear relationship between colloid and P concentrations in the leachate, suggesting that colloid-facilitated P leaching was the dominant P transport mechanism.

Inhibition of methane oxidation in a slurry surface crust by inorganic nitrogen: an incubation study

Livestock slurry is an important source of methane (CH). However, depending on the dry matter content of the slurry, a floating crust may form where methane-oxidizing bacteria (MOB) and CH oxidation activity have been found, suggesting that surface crusts may reduce CH emissions from slurry. However, it is not known how MOB in this environment interact with inorganic nitrogen (N). We studied inhibitory effects of ammonium (NH), nitrate (NO), and nitrite (NO) on potential CH oxidation in a cattle slurry surface crust. At headspace concentrations of 100 and 10,000 ppmv, CH oxidation was assayed at salt concentrations up to 500 mM. First-order rate constants were used to evaluate the strength of inhibition. Nitrite was the most potent inhibitor, reducing methanotrophic activity by up to 70% at only 1 mM NO. Methane-oxidizing bacteria were least sensitive to NO, tolerating up to 30 mM NO at 100 ppmv CH and 50 mM NO at 10,000 ppmv CH without any decline in activity. The inhibition by NH increased progressively, and no range of tolerance was observed. Methane concentrations of 10,000 ppmv resulted in 50- to 100-fold higher specific CH uptake rates than 100 ppmv CH but did not change the inhibition patterns of N salts. In slurry surface crusts, MOB maintained activity at higher concentrations of NH and NO than reported for MOB in soils and sediments, possibly showing adaptation to high N concentrations in the slurry environment. Yet it appears that the effectiveness of surface crusts as CH sinks will depend on inorganic N concentrations.

Changes in time of sowing, flowering and maturity of cereals in Europe under climate change

The phenological development of cereal crops from emergence through flowering to maturity is largely controlled by temperature, but also affected by day length and potential physiological stresses. Responses may vary between species and varieties. Climate change will affect the timing of cereal crop development, but exact changes will also depend on changes in varieties as affected by plant breeding and variety choices. This study aimed to assess changes in timing of major phenological stages of cereal crops in Northern and Central Europe under climate change. Records on dates of sowing, flowering, and maturity of wheat, oats and maize were collected from field experiments conducted during the period 1985-2009. Data for spring wheat and spring oats covered latitudes from 46 to 64°N, winter wheat from 46 to 61°N, and maize from 47 to 58°N. The number of observations (site-year-variety combinations) varied with phenological phase, but exceeded 2190, 227, 2076 and 1506 for winter wheat, spring wheat, spring oats and maize, respectively. The data were used to fit simple crop development models, assuming that the duration of the period until flowering depends on temperature and day length for wheat and oats, and on temperature for maize, and that the duration of the period from flowering to maturity in all species depends on temperature only. Species-specific base temperatures were used. Sowing date of spring cereals was estimated using a threshold temperature for the mean air temperature during 10 days prior to sowing. The mean estimated temperature thresholds for sowing were 6.1, 7.1 and 10.1°C for oats, wheat and maize, respectively. For spring oats and wheat the temperature threshold increased with latitude. The effective temperature sums required for both flowering and maturity increased with increasing mean annual temperature of the location, indicating that varieties are well adapted to given conditions. The responses of wheat and oats were largest for the period from flowering to maturity. Changes in timing of cereal phenology by 2040 were assessed for two climate model projections according to the observed dependencies on temperature and day length. The results showed advancements of sowing date of spring cereals by 1-3 weeks depending on climate model and region within Europe. The changes were largest in Northern Europe. Timing of flowering and maturity were projected to advance by 1-3 weeks. The changes were largest for grain maize and smallest for winter wheat, and they were generally largest in the western and northern part of the domain. There were considerable differences in predicted timing of sowing, flowering and maturity between the two climate model projections applied.

Shifts in comparative advantages for maize, oat and wheat cropping under climate change in Europe

Climate change is anticipated to affect European agriculture, including the risk of emerging or re-emerging feed and food hazards. Indirectly, climate change may influence such hazards (e.g. the occurrence of mycotoxins) due to geographic shifts in the distribution of major cereal cropping systems and the consequences this may have for crop rotations. This paper analyses the impact of climate on cropping shares of maize, oat and wheat on a 50-km square grid across Europe (45-65°N) and provides model-based estimates of the changes in cropping shares in response to changes in temperature and precipitation as projected for the time period around 2040 by two regional climate models (RCM) with a moderate and a strong climate change signal, respectively. The projected cropping shares are based on the output from the two RCMs and on algorithms derived for the relation between meteorological data and observed cropping shares of maize, oat and wheat. The observed cropping shares show a south-to-north gradient, where maize had its maximum at 45-55°N, oat had its maximum at 55-65°N, and wheat was more evenly distributed along the latitudes in Europe. Under the projected climate changes, there was a general increase in maize cropping shares, whereas for oat no areas showed distinct increases. For wheat, the projected changes indicated a tendency towards higher cropping shares in the northern parts and lower cropping shares in the southern parts of the study area. The present modelling approach represents a simplification of factors determining the distribution of cereal crops, and also some uncertainties in the data basis were apparent. A promising way of future model improvement could be through a systematic analysis and inclusion of other variables, such as key soil properties and socio-economic conditions, influencing the comparative advantages of specific crops.

Toxicity of xenobiotics during sulfate, iron, and nitrate reduction in primary sewage sludge suspensions

The effect and persistence of six organic xenobiotics was tested under sulfate-, iron-, and nitrate-reducing conditions in primary sewage sludge suspensions. The xenobiotics tested were acenaphthene, phenanthrene, di(2-ethylhexyl)phthalate (DEHP), 4-nonylphenol (4-NP), linear alkylbenzene sulfonate (LAS), and 1,2,4-trichlorobenzene (1,2,4-TCB) added to initial analytical concentrations of 54-117 mgL(-1). The suspensions were incubated at 30 degrees C for 15 weeks and rates of sulfate, iron, and nitrate reduction were estimated from the time course of hydrogen sulfide accumulation, Fe(II) accumulation, and nitrate depletion, respectively. Chemical analysis showed that the xenobiotics were persistent under the different electron acceptor regimes for the duration of the experiment. This was partly attributed to low bioavailability and microbial toxicity of the xenobiotics. Rates of anaerobic respiration in control suspensions (without added xenobiotics) showed a weekly reduction potential of 0.84 mM SO(4)(2-), 0.92 mM Fe(III), and 9.25 mM NO(3)(-). All three processes were completely inhibited by 1,2,4-TCB (54 mgL(-1)) whereas there was no significant (P<0.05) toxicity of phenanthrene (109 mgL(-1)) and DEHP (105 mgL(-1)). Sulfate reduction was inhibited completely by LAS (105 mgL(-1)), 76% by acenaphthene (54 mgL(-1)) and 57% by 4-NP (117 mgL(-1)), and likewise iron reduction was inhibited 62% by LAS and 55% by 4-NP (the latter though at P<0.10). Nitrate reduction was not significantly inhibited by acenaphthene and 4-NP and furthermore was resistant to LAS toxicity (105 mgL(-1)). Nitrate reduction also had the highest potential for mineralization of organic matter and thus was the most robust of the tested anaerobic processes in the sewage sludge suspensions.

Field-scale variation in microbial activity and soil properties in relation to mineralization and sorption of pesticides in a sandy soil

Pesticides applied to agricultural soils are subject to environmental concerns because leaching to groundwater reservoirs and aquatic habitats may occur. Knowledge of field variation of pesticide-related parameters is required to evaluate the vulnerability of pesticide leaching. The mineralization and sorption of the pesticides glyphosate and metribuzin and the pesticide degradation product triazinamin in a field were measured and compared with the field-scale variation of geochemical and microbiological parameters. We focused on the soil parameters clay and organic carbon (C) content and on soil respiratory and enzymatic processes and microbial biomass. These parameters were measured in soil samples taken at two depths (Ap and Bs horizon) in 51 sampling points from a 4-ha agricultural fine sandy soil field. The results indicated that the spatial variation of the soil parameters, and in particular the content of organic C, had a major influence on the variability of the microbial parameters and on sorption and pesticide mineralization in the soil. For glyphosate, with a co-metabolic pathway for degradation, the mineralization was increased in soils with high microbial activity. The spatial variability, expressed as the CV, was about five times higher in the Bs horizon than in the Ap horizon, and the local-scale variation within 100 m(2) areas were two to three times lower than the field-scale variation within the entire field of about 4 ha.

Spatial variation in 2-methyl-4-chlorophenoxyacetic acid mineralization and sorption in a sandy soil at field level

The phenoxyacetic acid herbicide MCPA (2-methyl-4-chlorophenoxyacetic acid) is frequently detected in groundwater beneath Danish agricultural fields. We investigated spatial variation in microbial MCPA mineralization potential in a flat agricultural field of fine sandy soil (USDA classification: Humic Dystrudept) located on the Yoldia plains of Northern Jutland, Denmark. Samples for determination of MCPA mineralization and sorption were collected from the Ap and Bs horizons at 51 sampling sites located in a 200 x 220 m grid. Spatial variation in sorption was low in both horizons (distribution coefficient, 0.36-4.16 L kg(-1)). Sorption correlated strongly with soil organic carbon content in both horizons (CV, 93 and 83%, respectively) and negatively with soil pH. [Ring-(14)C]-MCPA mineralized readily in the Ap horizon, with 49 to 62% of the (14)C-MCPA being converted to (14)CO(2) during the 67-d incubation period. With the subsoil, mineralization of (14)C-MCPA varied considerably between samples (0.5-72.8%). At neither depth was there correlation between (14)C-MCPA mineralization and sorption, soil pH, organic carbon content, clay content, number of colony-forming units (CFU), pseudomonad CFU, or any of the four microbial activity parameters measured. The presence of microbial genes encoding for the TfdA enzyme was quantified using real-time polymerase chain reaction. No correlation was found between MCPA mineralization potential and the natural background number of tfdA genes present in the soil samples. The degradation kinetics suggests that the high (14)C-MCPA mineralization rate detected in soil samples was linked to growth of the MCPA-degrading soil microbial community.

The fate of sulfate in acidified pig slurry during storage and following application to cropped soil

Acidification of slurry with sulfuric acid is a recent agricultural practice that may serve a double purpose: reducing ammonia emission and ensuring crop sulfur sufficiency. We investigated S transformations in untreated and acidified pig slurry stored for up to 11 mo at 2, 10, or 20 degrees C. Furthermore, the fertilizer efficiency of sulfuric acid in acidified slurry was investigated in a pot experiment with spring barley. The sulfate content from acidification with sulfuric acid was relatively stable and even after 11 mo of storage the majority was in the plant-available sulfate form. Microbial sulfate reduction during storage of acidified pig slurry was limited, presumably due to initial pH effects and a limitation in the availability of easily degradable organic matter. Sulfide accumulation was observed during storage but the sulfide levels in acidified slurry did not exceed those of the untreated slurry for several months after addition. The S fertilizer value of the acidified slurry was considerable as a result of the stable sulfate pool during storage. The high content of inorganic S in the acidified slurry may potentially lead to development of odorous volatile sulfur-containing compounds and investigations are needed into the relationship between odor development and the C and S composition of the slurry.

Sorption of linear alkylbenzene sulfonate to soil components and effects on microbial iron reduction

When sewage sludge is applied to arable land, linear alkylbenzene sulfonate (LAS) is released into the environment. In soils, LAS has been shown to impede microbial processes, such as bacterial iron reduction. The aim of the present study was to quantify LAS adsorption and desorption to agricultural soils and iron oxides and relate this to the inhibition of microbial iron reduction. Two agricultural soils were used, namely, Askov (coarse sandy loam soil) and Lundgaard (coarse sandy soil). In both soils, LAS inhibited microbial iron reduction even at low LAS concentrations with 10% effect concentrations of 6 to 7 and 26 to 32 mg LAS/kg dry-weight soil for Lundgaard and Askov soil, respectively. The sorption isotherms showed that sorption of LAS to iron oxides was 10 to 100 times stronger than sorption to the agricultural soils. Also, it appeared that at low LAS concentrations (< 10 mg/kg dry-wt soil), Lundgaard soil adsorbed approximately 10 times more LAS than Askov soil. Thus, the inhibitory effect of LAS on microbial iron reduction was highest in the Lundgaard soil, which exhibited both the strongest sorption and the lowest desorption of the two soils. A possible hypothesis to explain this correlation was that LAS toxicity toward bacterial iron reduction was, at least partly, caused by LAS adsorbed to iron oxides, which could interfere with transfer of electrons between the bacteria and their respiratory electron acceptor.

Biodegradation of linear alkylbenzene sulfonates in sulfate-leached soil mesocosms

Aromatic sulfonates (R-SO(3)(-)) can be used as sulfur sources by sulfate-starved bacteria in laboratory cultures and the corresponding phenols are excreted from the cells. The present study was conducted to demonstrate whether such desulfonation reactions also occur in sulfate-leached agricultural soil, where desulfonation of organic sulfur compounds may have agronomic importance as a S source for plants. Xenobiotic linear alkylbenzene sulfonates (LAS) were added to nominal concentrations of 0, 10 and 100 mgkg(-1) dry weight in a sandy soil that was depleted in sulfate by leaching the soil with water (sulfate depletion, approximately 75%). The soil was incubated at 20 degrees C in duplicate 3-dm(3) mesocosms for 8 weeks. Primary degradation of LAS was rapid with half-lives of 1-4 days. Sulfophenylcarboxylates were identified and quantified as intermediates, whereas linear alkylphenols (the expected primary desulfonation products) were not detected by high-pressure liquid chromatography coupled with both fluorescence and electrospray ionization-mass spectrometry. Thus, LAS was used by the bacteria as a source of energy and carbon, rather than as a source of sulfur. Measurements of soil pH, fluorescein diacetate (FDA) hydrolysis and arylsulfatase activity showed that stable microbial conditions prevailed in the soil mesocosms. FDA hydrolysis (a measure of total microbial activity) was transiently inhibited at the highest LAS concentrations. Arylsulfatase activity (i.e., hydrolysis of aromatic sulfate esters) was not significantly affected by the soil incubation, although arylsulfatases may be upregulated in sulfate-starved bacteria. However, an increased production of arylsulfatase may be difficult to detect due to the background of extracellular arylsulfatases stabilised in the soil. Therefore, the present data does not exclude a regulatory response to sulfate depletion by the soil microorganisms. However, the importance of desulfonation reactions in natural environments still needs to be demonstrated.

Effects of linear alkylbenzene sulfonates on functional diversity of microbial communities in soil

Linear alkylbenzene sulfonates (LAS) often occur in sewage sludge that is applied to agricultural soil. Here LAS may affect the microbial activity, which is an important basis for nutrient cycling. Previous studies have shown that single bacterial species and specific soil processes can be very sensitive to LAS. Here we report that two levels of LAS, 22 and 174 mg/kg dry weight soil, had little or no significant influence of the functional diversity of bacteria in a sandy soil, as tested by community-level physiological profiles. Briefly, these profiles are a characterization of the microbial communities based on the pattern of substrate utilization in 96-well microtiter plates (Biolog EcoPlates). Sandy agricultural soil was incubated in duplicate 1-L mesocosms with or without LAS, and bacteria were extracted after one, two, and four weeks. During incubation, more than 98 and 93% of LAS added to 22 and 174 mg/kg dry weight soil was degraded, respectively. The presence of LAS at 174 mg/kg dry weight caused a higher number of bacteria in the soil extracts, maximally corresponding to 2.5 times the numbers in LAS-free soil (1.8 x 10(7) cells/g dry wt soil) after four weeks of incubation. No inhibitory effect of LAS was observed when the substrate utilization data were analyzed for substrate richness and diversity (Shannon-Weaver indices). Principal component analysis, however, showed that the pattern of substrate utilization in soil with the highest LAS content (174 mg/kg dry wt) could be distinguished from control soil and soil with 22 mg LAS/kg dry weight. Yet the overall conclusion was that the functional diversity of the aerobic, heterotrophic bacterial community was rather insensitive to LAS.

A sandwich-designed temperature-gradient incubator for studies of microbial temperature responses

A temperature-gradient incubator (TGI) is described, which produces a thermal gradient over 34 aluminium modules (15x30x5 cm) intersected by 2-mm layers of partly insulating graphite foil (SigraFlex Universal). The new, sandwich-designed TGI has 30 rows of six replicate sample wells for incubation of 28-ml test tubes. An electric plate heats one end of the TGI, and the other end is cooled by thermoelectric Peltier elements in combination with a liquid cooling system. The TGI is equipped with 24 calibrated Pt-100 temperature sensors and insulated by polyurethane plates. A PC-operated SCADA (Supervisory Control And Data Acquisition) software (Genesis 4.20) is applied for temperature control using three advanced control loops. The precision of the TGI temperature measurements was better than +/-0.12 degrees C, and for a 0-40 degrees C gradient, the temperature at the six replicate sample wells varied less than +/-0.04 degrees C. Temperatures measured in incubated water samples closely matched the TGI temperatures, which showed a linear relationship to the sample row number. During operation for 8 days with a gradient of 0-40 degrees C, the temperature at the cold end was stable within +/-0.02 degrees C, while the temperatures at the middle and the warm end were stable within +/-0.08 degrees C (n=2370). Using the new TGI, it was shown that the fine-scale (1 degrees C) temperature dependence of S(o) oxidation rates in agricultural soil (0-29 degrees C) could be described by the Arrhenius relationship. The apparent activation energy (E(a)) for S(o) oxidation was 79 kJ mol(-1), which corresponded to a temperature coefficient (Q(10)) of 3.1. These data demonstrated that oxidation of S(o) in soil is strongly temperature-dependent. In conclusion, the new TGI allowed a detailed study of microbial temperature responses as it produced a precise, stable, and certifiable temperature gradient by the new and combined use of sandwich-design, thermoelectric cooling, and advanced control loops. The sandwich-design alone reduced the disadvantageous thermal gradient over individual sample wells by 56%.

Effects and risk assessment of linear alkylbenzene sulfonates in agricultural soil. 1. Short-term effects on soil microbiology

Linear alkylbenzene sulfonates (LAS) may occur in sewage sludge that is applied to agricultural soil, in which LAS can be inhibitory to biological activity. As a part of a broader risk assessment of LAS in the terrestrial environment, we tested the short-term effects of aqueous LAS on microbial parameters in a sandy agricultural soil that was incubated for up to 11 d. The assays included 10 microbial soil parameters; ethylene degradation; potential ammonium oxidation; potential dehydrogenase activity; beta-glucosidase activity; iron reduction; the populations of cellulolytic bacteria, fungi and actinomycetes; the basal soil respiration; and the phospholipid fatty acid (PLFA) content. Except for beta-glucosidase activity, basal respiration, and total PLFA content, all soil parameters were sensitive to LAS, with EC10 values in the range of less than 8 to 22 mg/kg dry weight. This probably reflected a similar mode of LAS toxicity, ascribed to cell membrane interactions, and showed that sensitivity to LAS was common for various soil microorganisms. The extracellular beta-glucosidase activity was rather insensitive to LAS (ECI10, 47 mg/kg dry wt), whereas the basal soil respiration was not inhibited even at 793 mg/kg dry weight. This was interpreted as a combined response of inhibited and stimulated compartments of the microbial community. The PLFA content, surprisingly, showed no decrease even at 488 mg/kg. In conclusion, LAS inhibited specific microbial activities, although this could not be deduced from the basal respiration or the total PLFA content. The lowest EC10 values for microbial soil parameters were slightly higher than the predicted no-effect concentrations recently derived for plants and soil fauna (approximately 5 mg/kg dry wt).

Effects and risk assessment of linear alkylbenzene sulfonates in agricultural soil. 2. Effects on soil microbiology as influenced by sewage sludge and incubation time

The anionic surfactant linear alkylbenzene sulfonate (LAS) may inhibit soil microorganisms and may occur in agricultural soil through the application of sewage sludge. For five microbial parameters (microbial biomass C and the potentials of iron reduction, ammonium oxidation, dehydrogenase activity, and arylsulfatase activity), we compared the effects of aqueous LAS and LAS-spiked sewage sludge added to existing levels of 0, 3, 8, 22, 22, 62, 174, and 488 mg/kg soil (dry wt) in a Danish sandy agricultural soil that was incubated for 5 d to eight weeks. Arylsulfatase activity (measured after four weeks of incubation) was rather insensitive to LAS, with an EC 10 of 222 and more than 488 mg/kg in soil samples treated with aqueous LAS and LAS-spiked sewage sludge, respectively. For the other microbial parameters, the short-term effects (approximately one to two weeks) of aqueous LAS were characterized by an EC10 in the range of 3 to 39 mg/kg. Application of LAS via sewage sludge generally reduced the short-term effects for the microbial parameters, and the EC10 for LAS in sludge-amended soil after approximately one to two weeks of incubation ranged from less than 8 to 102 mg/kg. Recovery potential was seen for most microbial parameters as a result of prolonged incubation, both under conditions of LAS persistence (anaerobic conditions, the iron-reduction test) and LAS depletion (aerobic incubations, all other assays). In conclusion, the short-term inhibitory effects of LAS on soil microbiology were decreased in the presence of sewage sludge and by a prolonged (two to eight weeks) laboratory incubation period.

Effects and risk assessment of linear alkylbenzene sulfonates in agricultural soil. 5. Probabilistic risk assessment of linear alkylbenzene sulfonates in sludge-amended soils

Linear alkylbenzene sulfonates (LAS) can be found in high concentrations in sewage sludge and, hence, may enter the soil compartment as a result of sludge application. Here, LAS may pose a risk for soil-dwelling organisms. In the present probabilistic risk assessment, statistical extrapolation has been used to assess the risk of LAS to soil ecosystems. By use of a log-normal distribution model, the predicted no-effect concentration (PNEC) was estimated for soil fauna, plants, and a combination of these. Due to the heterogeneous endpoints for microorganisms, including functional as well as structural parameters, the use of sensitivity distributions is not considered to be applicable to this group of organisms, and a direct, expert evaluation of toxicity data was used instead. The soil concentration after sludge application was predicted for a number of scenarios and used as the predicted environmental concentration (PEC) in the risk characterization and calculation of risk quotients (RQ = PEC/PNEC). A LAS concentration of 4.6 mg/kg was used as the current best estimate of PNEC in all RQ calculations. Three levels of LAS contamination (530, 2,600, and 16,100 mg/kg), three half-lives (10, 25, and 40 d), and five different sludge loads (2, 4, 6, 8, and 10 t/ha) were included in the risk scenarios. In Denmark, the initial risk ratio would reach 1.5 in a realistic worst-case consideration. For countries not having similar sludge regulations, the estimated risk ratio may initially be considerably higher. However, even in the most extreme scenarios, the level of LAS is expected to be well beyond the estimated PNEC one year after application. The present risk assessment, therefore, concludes that LAS does not pose a significant risk to fauna, plants, and essential functions of agricultural soils as a result of normal sewage sludge amendment. However, risks have been identified in worst-case scenarios.

Ethylene removal at low temperatures under biofilter and batch conditions

Removal of the plant hormone ethylene (C(2)H(4)) is often required by horticultural storage facilities, which are operated at temperatures below 10 degrees C. The aim of this study was to demonstrate an efficient, biological C(2)H(4) removal under such low-temperature conditions. Peat-soil, acclimated to degradation of C(2)H(4), was packed in a biofilter (687 cm(3)) and subjected to an airflow ( approximately 73 ml min(-1)) with 2 ppm (microl liter(-1)) C(2)H(4). The C(2)H(4) removal efficiencies achieved at 20, 10, and 5 degrees C, respectively, were 99.0, 98.8, and 98.4%. This corresponded to C(2)H(4) levels of 0.022 to 0.032 ppm in the biofilter outlet air. At 2 degrees C, the average C(2)H(4) removal efficiency dropped to 83%. The detailed temperature response of C(2)H(4) removal was tested under batch conditions by incubation of 1-g soil samples in a temperature gradient ranging from 0 to 29 degrees C with increments of 1 degrees C. The C(2)H(4) removal rate was highest at 26 degrees C (0.85 microg of C(2)H(4) g [dry weight](-1) h(-1)), but remained at levels of 0.14 to 0.28 microg of C(2)H(4) g (dry weight)(-1) h(-1) at 0 to 10 degrees C. At 35 to 40 degrees C, the C(2)H(4) removal rate was negligible (0.02 to 0.06 microg of C(2)H(4) g [dry weight](-1) h(-1)). The Q(10) (i.e., the ratio of rates 10 degrees C apart) for C(2)H(4) removal was 1.9 for the interval 0 to 10 degrees C. In conclusion, the present results demonstrated microbial C(2)H(4) removal, which proceeded at 0 to 2 degrees C and produced a moderately psychrophilic temperature response.

Thermophilic Sulfate Reduction in Hydrothermal Sediment of Lake Tanganyika, East Africa

In environments with temperatures above 60°C, thermophilic prokaryotes are the only metabolically active life-forms. By using the 35 SO 4 2- tracer technique, we studied the activity of sulfate-reducing microorganisms (SRM) in hot sediment from a hydrothermal vent site in the northern part of freshwater Lake Tanganyika (East Africa). Incubation of slurry samples at 8 to 90°C demonstrated meso- and thermophilic sulfate reduction with optimum temperatures of 34 to 45°C and 56 to 65°C, respectively, and with an upper temperature limit of 80°C. Sulfate reduction was stimulated at all temperatures by the addition of short-chain fatty acids and benzoate or complex substrates (yeast extract and peptone). A time course experiment showed that linear thermophilic sulfate consumption occurred after a lag phase (12 h) and indicated the presence of a large population of SRM in the hydrothermal sediment. Thermophilic sulfate reduction had a pH optimum of about 7 and was completely inhibited at pH 8.8 to 9.2. SRM could be enriched from hydrothermal chimney and sediment samples at 60 and 75°C. In lactate-grown enrichments, sulfide production occurred at up to 70 and 75°C, with optima at 63 and 71°C, respectively. Several sporulating thermophilic enrichments were morphologically similar to Desulfotomaculum spp. Dissimilatory sulfate reduction in the studied hydrothermal area of Lake Tanganyika apparently has an upper temperature limit of 80°C.