Showing papers in "Soil Biology & Biochemistry in 1996"

The fumigation-extraction method to estimate soil microbial biomass: Calibration of the kEC value

Abstract: The kEC value (=extractable part of microbial biomass C) of the fumigation-extraction (FE) method was assessed on the basis of 153 soils (94 arable, 46 grassland and 13 forest soils) by indirect calibration using the fumigation-incubation (FI) method. Sixty-six soils were investigated for the first time and the data on a further 87 soils were obtained from the literature. The single kEC values ranged from 0.23 to 0.84. A split according to the form of land use resulted in a significantly (Scheffe, P = 0.05) lower kEC value for the arable soils (0.42; n = 94) in comparison to those for the grassland (0.49; n = 46) and the forest soils (0.51; n = 13). This difference is mainly due to the significant effects of the respiration rate measured in non-fumigated control samples of the FI method which was used for calibration of the kEC value. For that reason, I investigated the effects of incubation temperature (22°, 25° and 28°C) on biomass C data obtained by the FI method, and thus on the kEC value of the FE method, and discuss further problems of direct and indirect calibration. Based on experimental and literature data, I conclude that the kEC values of Vance et al. (Soil Biology & Biochemistry19, 703–707, 1987) and Wu et al. (Soil Biology & Biochemistry22, 1167–1169, 1990) remain valid. A kEC value of 0.38 can be recommended for C analysis by dichromate consumption and a kEC value of 0.45 for that by UV-persulfate or oven oxidation.

Soil aggregate formation and the accrual of particulate and mineral-associated organic matter

Abstract: The degradation of soil aggregates appears to be a primary mechanism in the loss of organic matter caused by long-term cultivation, but little information exists on how the formation and stabilization of macroaggregates control the process of C aggradation when disturbance is reduced or eliminated. A chronosequence of restored tallgrass prairie was used to investigate the relationships between the formation of stable macroaggregates (> 212 μm dia) and the accrual of particulate and mineral-associated organic matter. Changes in the percentage of macroaggregates and in the accumulation of whole-soil organic C across the chronosequence were both described with a simple exponential model. The rate constant (k) for change in aggregation was more than 35 times the k for total organic C accumulation. Thus, the time required to reach 99% of equilibrium was 10.5 y for macroaggregates and 384 y for whole-soil organic C, providing evidence for the existence of a phased relationship between macroaggregate formation and C accrual. The input rate for whole-soil organic C to a 10-cm depth was estimated at 1.16 g kg−1 y−1 or 0.133 kg m−2 y−1 (assuming an average bulk density of 1150 kg m−3 for previously cultivated soils in the chronosequence). An increase in macroaggregate-associated C-to-N ratios with time since cultivation suggested that the accumulating organic matter was not “highly processed”, but less than 20% of the accrued C occurred in the form of particulate organic matter (density ≤ 1.85 g cm−3). Rather, most of the accumulated C occurred in the mineral-associated fraction of macroaggregates, suggesting that inputs of organic debris were rendered relatively rapidly into particles or colloids that are associated with mineral matter and thus are physically protected, slowing decomposition and promoting the development of stable microaggregates within macroaggregates.

Analytical approaches to the characterization of samples of microbial communities using patterns of potential C source utilization.

Abstract: Profiles of potential utilization of 95 separate C sources by microbial communities can be readily generated from direct incubation of environmental samples in BIOLOG microplates. Color formation from a redox indicator dye is used to quantify the degree of C source utilization. I aimed to examine different analytical approaches for classifying microbial communities based on these profiles. Specifically, the relative effects of average rate of color development versus the pattern of relative C source utilization on the classification of rhizosphere samples from different crop types were evaluated. The average rate of color development was correlated to the density of total (acridine-orange) bacterial cells (R2 = 0.52) and active (5-cyano-2,3-ditolyl tetrazolium chloride) bacterial cells (R2 = 0.70) inoculated into the plate. Analysis of plates with different rates of color development after a specific incubation period resulted in samples with variation in the overall extent of color development (expressed as average well color development, or AWCD). Classification of these samples using principal component analysis was significantly influenced by the variation in AWCD, resulting in the classification of samples based on the density of inoculum rather than the pattern of C source utilization. The effect of variation in AWCD was eliminated by normalizing data prior to ordination, or by using an alternative ordination technique, detrended correspondence analysis. Variation in AWCD can be limited through multiple-plate readings and subsequent selection of plates with a common reference point in AWCD. The specific AWCD used for analysis does not appear important for classification purposes; consistent discrimination of rhizosphere samples from different crop types was apparent for analysis across a wide range of AWCD (0.25–1.00 abs. units). The specific differences in C source utilization between rhizosphere sample types did depend on the set point used for analysis due to the differences in the rate of color formation among wells. Results suggest that single-plate readings can be used to classify samples, but only if potential differences in AWCD are accounted for in the data analysis. Repeated plate readings will provide a more complete understanding of differences in C source utilization among samples.

Microbial response to freeze-thaw cycles in tundra and taiga soils

Abstract: Four tundra and taiga soils were experimentally subjected to three freeze-thaw cycles (5 days each at −5°C and +5°C). After each thaw, there was an initial pulse (

Gas chromatographic determination of muramic acid, glucosamine, mannosamine, and galactosamine in soils

Abstract: Muramic acid, glucosamine, mannosamine and galactosamine in soils may be useful for elucidating the microbial origin of soil organic nitrogen. Therefore, a method was developed to determine the aldononitrile acetate derivatives of the four amino sugars simultaneously in the 6 m HCl hydrolysates of soil samples by means of high resolution gas liquid chromatography. This method was sensitive enough to detect less than 10 μg muramic acid ml−1 and less than 20 μg ml−1 of the other three amino sugars. The maximum release of amino sugars was found after 6 to 8 h hydrolysis at 105°C. Impurities in the acidic hydrolysates were removed simply by neutralisation with KOH solution. The recovery of amino sugars after hydrolysis and purification was more than 90% on average. The method was applied to determine amino sugars in eight soils with different properties. The coefficients of variation averaged 6.1% for glucosamine and galactosamine and 10.9% for muramic acid and mannosamine.

Changes in microbial community structure during long-term incubation in two soils experimentally contaminated with metals

Abstract: The effects of Zn contamination on the microbial community structure of a forest humus and an arable soil, as estimated by phospholipid fatty acid (PLFA) analysis, were followed during 18 months. The soils were contaminated at 10 different metal concentrations and incubated in plastic jars at 22°C. In both soils effects of heavy metal contamination could be detected after 2 weeks. Qualitatively similar changes in the PLFA pattern were found at the later sampling occasions, although the changes became more pronounced with prolonged incubation. In the forest soil the double-unsaturated 18:2ω6, indicating fungi, increased proportionally due to the metal amendment, while there was a strong negative effect of incubation on the fungal biomass in all samples of this soil type. In the arable soil 18:2ω6 showed a strong increase in response to the Zn pollution. As in the forest soil, incubation decreased the mol% of 18:2ω6, although the effect was less pronounced than in the forest soil. The proportions of several individual bacterial PLFAs changed in both soils due to the treatments, indicating shifts within the bacterial community in the soils, but these shifts could not be interpreted in terms of changes in the proportional abundance of specific taxonomic groups of bacteria. The ratio of 16:1ω7t-to-16:1ω7c, which has been proposed as a starvation index, increased in the forest soil due to Zn contamination. In the high-metal samples this ratio decreased during incubation, while it remained unchanged in the uncontaminated control. In the arable soil no clear effect was found on the trans-to-cis ratio either in response to metal contamination or to incubation. The ATP content decreased during incubation. Little or no effect was found on the total amount of PLFAs or on the lipid phosphate content, except after 18 months when these biomass measurements decreased.

Temperature and moisture effects on the production of dissolved organic carbon in a Spodosol

Abstract: Leaching of dissolved organic carbon (DOC) from the forest floor is an important C flux and influences other biogeochemical fluxes in forests. To determine what controls the quantity and quality of the DOC produced, we examined the effects of microclimate on DOC production in Oa-horizon material from a red spruce forest. Samples incubated under different temperature and moisture conditions were leached with a mechanical-vacuum extractor every 7 days for 8 to 10 weeks, or once after 1 to 14 days. The concentration and, in some cases, the composition of the DOC in the extracts were measured. Production of DOC in dry samples (moisture content from 0.5 to 1.7 g g−1) was approximately 1.2 mg g−1 in the first week, but declined by 77% over 8 weeks. In sieved samples, production declined to 40% of initial values, whereas production in unsieved, moist samples declined by less than 30%. In wetter samples (moisture content from 1.8 to 5 mg g−1) DOC production increased by approximately 0.1 mg g−1 week−1 for every g g−1 increase in moisture content. The production of DOC increased exponentially with temperature, with Q10 values of 1.7 for soil with a moisture content of 2.5 g g−1, and 2 for wetter material. The composition of the DOC extracted from the driest samples suggested disruption of microbial biomass. Wetter incubation conditions increased the proportion of hydrophobic acids, whereas warmer incubation conditions increased the proportion of hydrophilic acids. The production of DOC was relatively fast in the first 2 days of incubation, and then slowed to approximately 90 μg g−1 week−1. Production rates in the first 2 days of incubation were higher under warmer conditions. Replicated experiments were useful in constructing precise curves relating the response of DOC production and composition to temperature and moisture.

Soil pH increase due to biological decarboxylation of organic anions

Abstract: Organic matter incorporated into soils may influence soil pH. Until now the detailed processes responsible for a change in soil pH after the incorporation of organic matter were not completely understood. We studied the effects of organic anions, glycine as a representative of amino acids, and glucose as a representative of carbohydrates on soil pH in aerobic incubation experiments. Addition of malate and citrate resulted in an immediate soil pH increase associated with additional CO 2 release. Addition of acetate had no major effect on soil pH but if applied at higher rates decreased soil CO 2 release. Malate added together with ammonium resulted in an immediate soil pH increase, which was followed by a pH decrease associated by an increase in nitrate after 10 days. Also glycine gave an immediate pH increase which was associated with an equivalent decrease of soil organic N extractable with CaCl 2 solution. Addition of glucose gave a slight pH decrease in the first days of incubation followed by an increase to the initial pH value. The results are interpreted in terms of decarboxylation of organic anions, a process which requires one proton per carboxylic group decarboxylated. Under aerobic soil conditions decarboxylation is a major process in organic matter decomposition. The decomposition of carbohydrates in the glycolytic pathway produces carboxylic groups which, after dissociation, may decrease soil pH. As soon as these groups are decarboxylated in the citrate cycle an equivalent amount of protons is required inducing a rise in soil pH. For this reason addition of glucose brought about only a transient soil pH decrease.

Patterns of potential C source utilization by rhizosphere communities

Abstract: Patterns of potential C source utilization by microbial communities were tested as a means for differentiating among and within rhizosphere samples of different crops. Utilization of 95 separate sole C sources was determined by inoculating microbial suspensions into BIOLOG plates and quantifying color production from a redox-sensitive dye. Suspensions were produced by shaking excised root samples from wheat, white potato, soybean and sweetpotato grown in hydroponic culture within controlled environmental growth chambers. Distinctive patterns of C source utilization were apparent for each crop type, and the differences among crops were consistent for experiments repeated over 2 years. A temporal shift in C source utilization related to plant development state was observed in soybean. Results indicate that this rapid method is effective for detecting plant dependent differences in rhizosphere communities, and changes in response to plant developmental state.

Bioavailability of water extractable organic carbon fractions in forest and agricultural soil profiles

Abstract: When forest ecosystems are converted to agriculture, there is generally a marked decline in the amounts of total and labile soil organic carbon (SOC). However, analysis of SOC changes induced by forest conversion to agriculture have generally been restricted to surface horizons and to pools of total and microbial biomass SOC. Changes in water-soluble SOC, which is likely the most labile and mobile form of SOC, have received much less attention. We have compared concentrations of total, water-soluble (both humic and acid-soluble fractions) and bioavailable SOC beneath long-term ( > 50 y) temperate forest and agricultural ecosystems located on the same soil type. Our objectives were (1) to determine if forest conversion to agriculture affects concentrations of total, water-soluble and bioavailable SOC throughout the soil profile and (2) to determine if amounts of water-soluble and bioavailable SOC show similar patterns as total SOC. As expected, total SOC was higher in forest than in crop systems, but the difference was restricted to the surface layers. Surprisingly, concentrations of water-soluble and bioavailable SOC were higher in agricultural soils than in forest soils, again only in the surface layers. The difference in water-soluble and bioavailable SOC between agricultural and wooded soils was largely caused by an increase in the soluble humic acid fraction in the agricultural soils. Our data suggest that while agricultural soils generally have lower amounts of total and microbial C than forest soils, they may support equal or greater rates of microbial activity than forest soils due to increased production of water-soluble carbon.

The Q10 relationship of microbial respiration in a temperate forest soil

Abstract: Our objective was to study the effect of temperature on rates of soil respiration in the A-, E- and B-horizons of a temperate forest (Durham, North Carolina, USA). Soil samples were incubated for several months at 4, 15, 22 and 38°C and respiration was measured frequently during incubation. For each soil horizon, rates of CO2 evolution varied significantly with time of incubation and temperature. The A-horizon had the highest initial rates of soil respiration, followed by the B- and E-horizon soils for each temperature. The initial rates of respiration from the A-soil horizon increased with temperature in accordance with the Arrhenius equation. The Q10 for the A horizon calculated from initial respiration rates varied from 1.9 to 1.7 over the temperature range of 4°C to 28°C. Generally rates of respiration decreased during the incubation, although the total fraction of carbon respired from the soil was small (

Is available carbon limiting microbial respiration in the rhizosphere

Abstract: It is widely known that the carbon availability in the rhizosphere is much higher than in the bulk soil. However, studies have failed to show whether microbial respiration in the rhizosphere is carbon-limited. Precise and timely measurements are lacking. We measured carbon availability index (basal respiration divided by substrate-induced respiration), and water soluble carbon in soils sampled at four spatial points (rhizoplane, rhizosphere, bulk soil and root-free soil) in the rhizosphere continuum in greenhouse and field experiments. Carbon availability index and water soluble carbon were inversely related to the relative distance from the root surface, with several times higher concentrations in the rhizoplane soils. Microbial respiration was not limited by available carbon in the rhizoplane and in the rhizosphere.

14C-labelled ryegrass turnover and residence times in soils varying in clay content and mineralogy

Abstract: The influence of soil texture and clay mineral composition on the decomposition of uniformly-labelled ( 14 C) ryegrass ( Lolium hybridum Hausskn) has been investigated. Two clay and two silt loam soils amended with 14 C-labelled ryegrass were allowed to decompose in micro-lysimeters under field conditions for 5 years. Periodically the micro-lysimeters were destructively harvested and the total amount of 14 C remaining in the soil and the fraction of the 14 C incorporated into microbial biomass measured. The influence of the amount and surface area of clay was assessed by calculating the mean and variances of residence times of biomass- 14 C and residual- 14 C. Surface areas were estimated from the mineralogical composition of the soil, and also measured by adsorption of p -nitrophenol. After 5 years' of incubation, 12–25% of the labelled- 14 C was retained by the soils. Decomposition was initially very rapid, between one-third to one-half of the labelled 14 C being lost after 9 weeks. Thereafter, the rate of decomposition was much reduced. During the initial phase of decomposition (9 weeks), a greater portion of the 14 C was retained by the two clay soils than by silt loam soils. In amorphic and smectitic soils the residence times for 14 C were increased because of the protection of microbial metabolites by clay surfaces. The amount of 14 C remaining in the soil did not relate to the amount of clay but correlated with the surface area, as measured by p -nitrophenol adsorption. Apparently, the type of clay, expressed in terms of surface area, controlled carbon turnover rates and residence times. This parameter may provide a reasonable predictor of organic matter decomposition rates and stabilization in different soils.

Nitrogen mobilization from protein-polyphenol complex by ericoid and ectomycorrhizal fungi

Abstract: The horizons of forest and heathland soils in which mycorrhizal roots proliferate typically contain large quantities of polyphenolic materials. These have the potential to bind organic nitrogen in recalcitrant complexes, thereby influencing availability of N to the fungal symbionts. The protein-binding abilities of aqueous extracts of forest soils were examined, and those derived from birch and pine sites were found to bind protein even when present in low concentrations. The effect of polyphenols upon availability of protein to ectomycorrhizal (ECM), ericoid mycorrhizal and wood-decomposing saprophytic fungi was investigated using the soluble polyphenol tannic acid (TA), and protein bovine serum albumin (BSA). While the saprophytic fungi were able to mobilize the precipitate, only three of 18 ECM fungi tested were able to do so. The ericoid mycorrhizal fungi were capable of partial clearance of the precipitate. It was demonstrated that the ericoid mycorrhizal fungi had access to N contained in protein complexed by TA, while ECM fungi did not. The extracellular proteases of ericoid mycorrhizal fungi remained active in the presence of TA, while those of the ECM fungi were inhibited. The ericoid mycorrhizal fungi and several ECM fungi showed abilities to metabolise TA contained in TA-BSA precipitate.

Role of wetland plants in the diurnal control of CH4 and CO2 fluxes in peat

Abstract: We demonstrate that a major portion of the microbial activities in peat monoliths are driven by photosynthetic processes and that effluxes of CH4 and CO2 are regulated by the surface vegetation. CH4 and CO2 monitored continuously at a depth of 15 cm and in the headspace above the peat showed oscillatory concentration changes over a 24-h cycle. Comparative anatomy of the vegetative organs of tracheophytes associated with the peat bog revealed the presence of an extensive lacunar system in the roots, rhizomes and leaves of several monocotyledonous species; this provides the path of minimal resistance for gas transport in waterlogged peat.

13C- and 15N-NMR spectroscopic examination of the transformation of organic nitrogen in plant biomass during thermal treatment

Abstract: Structural changes in lignocellulosic biomass heated under conditions comparable to those encountered in several types of natural or planned burnings have been studied by solid-state 13C- and 15N-CPMAS NMR spectroscopy of 15N-enriched ryegrass (Lolium rigidum) after being subjected to progressive thermal treatment. The solid-state 15N-NMR spectra of biomass subjected to severe heating revealed amide-N in forms which are resistant to the thermal treatment. Progressive burning was found to occur in two well-defined stages: In the early stage the free amino acid and some NH2 groups were removed, but no substantial disruption of the peptide structure was observed. In the final stage of burning the amide-N was converted to heterocyclic structures such as pyrroles, imidazoles and indoles. Some evidence for the presence of pyridines and phenazines was also found at this stage. These findings suggest that a major portion of the N is in forms that may survive most natural fires and that their stability towards further microbial degradation is increased by the heating. The solid-state 13C-NMR spectra revealed that the carbohydrate fraction is converted into condensed dehydrated material producing intense signals in the aromatic region.

Soil surface CO2 flux as an index of soil respiration in situ: a comparison of two chamber methods.

Abstract: Predictions of global climate change have recently focused attention on soils as major sources and sinks for atmospheric CO2, and various methodologies exist for measuring soil surface CO2 flux. A static (passive CO2 absorption in an alkali trap over 24 h) and a dynamic (portable infra-red CO2 gas analyzer over 1–2 min) chamber method were compared. Both methods were used for 100 different site × treatment × time combinations in temperate arable, forest and pasture ecosystems. Soil surface CO2 flux estimates covered a wide range from 0 to ca. 300 mg CO2C m−2 h−1 by the static method and from 0 to ca. 2500 mg CO2C m−2 h−1 by the dynamic method. The relationship between results from the two methods was highly non-linear, and was best explained by an exponential equation. When compared to the dynamic method, the static method gave on average 12% higher flux rates below 100 mg CO2C m−2 h−1, but much lower flux rates above 100 mg CO2C m−2 h−1. Spatial variability was large for both methods, necessitating a large number of replicates for reliable field data, with typical coefficients of variation being in the range 10–60%, usually higher with the dynamic than the static method. Diurnal variability in soil surface CO2 flux was partly correlated with soil temperature, whereas day-to-day variability was more unpredictable. However, use of a mechanistic simulation model of CO2 transport in soil, SOILCO2, showed that very large day-to-day changes in soil surface CO2 flux can result from rainfall events causing relatively small changes in soil water content above field capacity (ca. −10 kPa), even if CO2 production rates remained relatively unaffected.

Moisture and temperature sensitivity of CH4 oxidation in boreal soils

Abstract: We used laboratory experiments to evaluate CH4 uptake kinetics and the influence of soil moisture and temperature on rates of CH4-oxidation by boreal soils at in situ CH4 concentrations. Two upland forest sites (AS2 and BS2) were atmospheric CH4 sinks; a bog site (LB) was an atmospheric CH4 source characterized by distinct depth zonation of CH4 production and consumption. Apparent half-saturation constants (Ks) for CH4-oxidation showed relatively well-adapted communities. The Ks for the high CH4-source soil (LB) was 1.1 μ m , about 10-fold higher than values for CH4-sink soils (AS2 and BS2), 37 and 124 n m . Experiments assessing the individual effects of moisture and temperature on CH4-oxidation indicated that moisture was the primary control in CH4-sink soils at AS2 and BS2, while temperature was more important in CH4-source soil at LB. A combination of the highest moisture content and lowest temperature for each soil gave the lowest CH4-oxidation rates in experiments evaluating the interactive effects of these two variables. Conversely, a soil moisture content close to the optimum identified in moisture dependence experiments combined with the highest soil temperature consistently gave the highest CH4-oxidation rate.

Methane emission from a landfill and the methane oxidising capacity of its covering soil

Abstract: Methane emission from a small covered landfill site showed, seasonally varying fluxes, ranging from −5.9 to 914.3 mg CH4 m−2 d−1. The moisture content of the CH4-oxidising cover soil was thought to cause this variation. Comparing gross and net CH4 emission rates, it was found that the cover soil, due to its CH4 oxidising capacity, had a large mitigating effect on the CH4 emission. In laboratory experiments the effects of soil moisture, temperature and different ammonium amendments on CH4 oxidation were investigated. When the moisture content and temperature were combined, CH4 oxidation rates between 0.88 and 10.86 ng CH4 g−1 h−1 were observed. The optimum moisture content ranged between 15.6 and 18.8% w/w (±12 WHC). The optimum incubation temperature (30-20°C) decreased with increasing moisture contents. For the oxidation rates at 10 and 20°C, we found an average Q10 value of 1.88 ± 0.14. The activation energy for moisture contents between 5 and 25% was 83.0 ± 4.4 kJ mol−1. Increased ammonium additions reduced the CH4-oxidising capacity. This reduction decreased with increasing moisture contents. A high correlation (R2 > 0.98) was found between the moisture content and the reduction of the CH4 uptake rate mg−1 NH4+ −N kg−1 added. Because the nitrification rate was also lower at higher moisture contents, it was thought that the CH4 oxidation rate was more closely connected with the NH4+ turnover rate than with its actual concentration. Multiple linear regression analysis of the CH4 oxidation rates under the different incubation conditions showed the following decreasing effect on the CH4-oxidising capacity of the soil: amount of NH4+ added > moisture content > incubation temperature.

Active fractions of organic matter in soils with different texture

Abstract: Summary-Relationships between soil organic C (SOC), soil microbial biomass C (SMBC), mineralizable C and N during a 21 d incubation, and basal soil respiration (BSR) were evaluated on eight soil types from Texas that varied in soil texture (745% clay) and organic matter. The portion of SOC as SMBC increased with increasing clay content, whereas the relationships of mineralizable C and N and BSR to SGC were not affected by soil texture. The ratio of BSR-to-SOC averaged 1.4 + 0.4 mg mineralizable C g-r SOC d-r. The amount of mineralizable C and N and BSR per unit of SMBC, however, decreased with increasing clay content, indicating that the soil microbial biomass (SMB) was more active in coarse-textured soils than in fine-textured soils. The average specific respiratory activit was 29 mg mineralizable C g-’ SMBC d-’ with 10% clay and 11 mg mineralizable C g-’ SMBC d- Y wrth 40% clay. The C-to-N ratio of the mineralizable fraction was 10 f 3 and not affected by soil texture. The established relationships between active soil organic matter (SOM) fractions and soil texture could be used in models predicting SOM turnover. Published by Elsevier Science Ltd

Sequential reduction processes and initiation of CH4 production upon flooding of oxic upland soils

Abstract: Sequential reduction processes were studied in four oxic upland soils (cultivated, forest, savanna and desert soil) which were slurried and incubated under anoxic conditions. NO3−1 reduction began almost immediately and was followed by reduction of manganese(IV), sulfate and iron(III). The phases of reduction of Mn4+, SO42− and Fe3+ overlapped, with SO42− being depleted long before accumulation of Mn2+ and Fe2+ was finished. CH4 production and growth of methanogenic bacteria began when all the other reduction processes were finished. Radiotracer experiments showed that CH4 was produced from H2 (29–42%) and acetate. The respiratory index indicated that the acetate was predominantly degraded by methanogenic bacteria. The late onset of methanogenesis was not a consequence of limitation by the methanogenogenic precursors, since H2 and acetate were present long before the initiation of methanogenesis. Thermodynamic calculations showed that the concentrations of these substrates were always sufficient to allow exergonic production of CH4 at Gibbs free energies of ΔG + 400 mV to final values of < − 150 mV, except in the forest soil where the redox potential stayed at + 50 mV. The onset of methanogenesis and of growth of methanogenic bacteria coincided with redox potentials between +70 and 0 mV, which is much higher than claimed in literature. We speculate that the redox-active substances in soil were the signal for methanogenic bacteria to initiate activity.

Comparison of CH4 oxidation rates in woodland, arable and set aside soils

Abstract: CH4 fluxes and various soil properties were measured over three successive years at a field site on a loamy sand soil in eastern Scotland, to determine which factors influence CH4 oxidation rate. This site included three adjacent areas with contrasting land use: woodland, arable land and set aside land. The CH4 oxidation rates in the arable soil were less than half the corresponding rates in the woodland soil. The CH4 oxidation rates in the set aside soil were even lower, indicating that there is no immediate recovery when cultivation and fertilisation are abandoned. In the woodland and set aside soils, a seasonal variation in CH4 oxidation rate was found, but in the arable soil there was no such trend. The CH4 oxidation rate was negatively correlated with soil moisture content (P < 0.001) in the woodland soil and positively correlated with soil temperature (P < 0.001) in the set aside soil. In the arable soil, CH4 oxidation rate was related to moisture content only in dry summer conditions, when the relationship was positive (P < 0.001). These relationships suggest that CH4 oxidation was controlled partly by diffusion and partly by biological activity. A negative correlation was found between soil ammonium concentration and CH4 oxidation rate in the woodland soil (P < 0.001), indicating that ammonium inhibited CH4 oxidation in that environment.

Kinetic characteristics of ammonium-oxidizer communities in a California oak woodland-annual grassland

Abstract: We must have information on the kinetic characteristics of soil biological processes if we are accurately to predict how environmental change will affect soil nutrient cycles. Little information is available on the characteristics of NH 4 + -oxidizing bacteria in uncultivated soils. In addition, little is known about how much physiological diversity occurs in NH 4 + -oxidizer communities within a single ecosystem. Therefore, we evaluated how NH 4 + -oxidizer communities from an oak woodland-annual grass ecosystem in north-eastern California respond to changes in temperature, osmotic potential and substrate concentrations. We used nitrification potential assays to determine the effects of temperature and osmotic potential on rates of NH 4 + oxidation in two soil depths, under two types of vegetative cover and during two seasons. We determined the kinetics of substrate utilization using soil slurries and enrichment cultures. In slurries, NH 4 + oxidation rates were measured using 15 N-isotope dilution to avoid confounding the effects of NO 3 − consumption with NO 3 − production. Ammonium-oxidizer communities beneath the canopies of oaks had lower temperature optima, greater activities and greater seasonal fluctuations in activity than communities in open grassy interspaces. Temperature optima of communities beneath oak canopies (31.8°C) and in open grassy interspaces (35.9°C) were as different as those reported for communities from tropical and temperate climatic zones. Ammonium-oxidizer communities from beneath oak canopies and open interspaces showed no difference in tolerance of low osmotic potential. The effect of substrate concentration on NH 4 + oxidation rates in slurries were best described by the Michaelis-Menten equation. Rates in liquid cultures were best described by the Haldane equation because of substrate inhibition. The half-saturation constant ( K m ) for NH 4 + oxidation in these soils averaged 15 μ m NH 4 + (=0.012 μ m NH 3 ), which is substantially lower than values reported in the literature for agricultural soils, sediments and sewage sludge. Enrichment cultures were inhibited by lower substrate concentrations (1600 μ m NH 4 + or 1.3 μ m NH 3 ) than reported for isolates from sewage systems. These results suggest that NH 4 + -oxidizer communities in uncultivated soils are more oligotrophic in nature, and thus kinetic parameters reported in the literature for agricultural soils, sediments and sewage sludge are not appropriate for describing NH 4 + oxidation rates in these soils.

Nitrogen mineralization and microbial biomass as affected by soil compaction

Abstract: Soil compaction may retard decomposition of organic matter and N mineralization and increase gaseous losses of N We studied the effect of soil compaction on the turnover of N from added organic materials in pots with Italian-ryegrass (Lolium multiflorum Lam) plants Solid cattle manure or 15N-labelled white-clover (Trifolium repens L) material was incubated at controlled temperature (15°C) and moisture (pF 24 or pF 18) in a sandy loam with a bulk density of 11 or 14 g cm−3 The distribution of labelled clover N was determined after 22, 42, 64 and 98 days Also, net N mineralization from manure and clover was determined by subtraction of the values for unamended soil Hydrogen sulphide, volatile fatty acids, soil acidity, phytotoxicity (bioassay), soil atmosphere composition (N2O, O2, CO2), and colony-forming bacteria after anaerobic and aerobic incubation of dilution plates were determined as selected indicators of anaerobicity in the soil After 98 days at pF 24, soil compaction (14 g cm−3) had reduced the net mineralization of clover 15N by 18% compared to uncompacted soil, a reduction corresponding to 4% of added 15N Total 15N recovery was not reduced by compaction, and there was no evidence of anaerobic metabolism Consequently, increased gaseous N losses or retarded decomposition due to O2 deficiency could not account for the difference Compaction increased 15N retention in soil organic matter by 8% and in microbial biomass (chloroform fumigation-extraction) by 1% of added 15N The compaction effects increased successively during the incubation The negative effect of compaction on N mineralization was stronger at the higher soil moisture content (pF 18, sampled on day 64 only), but no evidence of anaerobicity was detected Compaction effects on N mineralization, bacterial biomass (microscopy) and microbial biomass determined by difference (amended minus unamended soil) agreed with the 15N results Soil compaction reduced the volume of pores with neck dia > 30 μm, ie pores available to nematodes, from 304 to 146% of total bulk volume The volume of pores

Rapid meneralization of the s-triazine ring of atrazine in soils in relation to soil management

Abstract: Mineralization of the triazine ring of atrazine was studied in soils with similar physicochemical properties, from experimental plots under different crop rotations located in Grignon, France. Rapid mineralization rates were found inplots under continuous maize receiving atrazine every year. On the contrary, low mineralization rates were measured in plots under continuous wheat or permanent grass that had never received atrazine. The rapid mineralization of the atrazine-ring was observed without any previous laboratory microbial enrichment. It was also related to the presence of a chloro-substituent on the ring: rapid mineralization was also observed with simazine, another chloro-s-triazine, but not with terbutry, a thiomethyltriazine. The characterization of the extractable metabolites during the incubation experiments did not allow determination of the degradation pathway. In these soils, two competitive dissipation processes are proposed: (1) rapid dissipation through ring cleavage and mineralization in soil from the plot receiving atrazine every year, (2) more progressive dissipation through formation of bound residues in the other soils. The rate of mineralization of the atrazine-ring varied during the year and was rather sensitive to soil storage conditions.

Transfer of N and P from intact or decomposing roots of pea to barley interconnected by an arbuscular mycorrhizal fungus

Abstract: The role of arbuscular mycorrhizas in the transfer of N and P between pea ( Pisum sativum L.) and barley ( Hordeum vulgare L.) plants was studied in a controlled environment. The plants were grown together in PVC containers, either in symbiosis with Glomus intraradices Schenck and Smith or as non-mycorrhizal controls, and with their root systems separated by an intermediate buffer zone (2 cm), confined by fine nylon mesh. The pea donor plants were supplied simultaneously with 15 N and 32 P, using a split-root labelling technique, in order to follow the flow of N and P to the barley receiver plants during 60 d of growth. In half of the containers, the donor-plant shoot was removed 42 d after the start of labelling and the roots were left in the soil to decompose. The reverse transfer of N and P, from barley donor to pea receiver plants was also measured to allow calculation of the net transfer through hyphae between mycorrhizal donor and receiver plants. No significant transfer of N was detected from intact pea donor plants to the barley receiver plants in the non-mycorrhizal controls. Mycorrhizal colonization slightly increased the transfer of N. However, the net transfer of N was almost insignificant since N was also transferred in the reverse direction, from barley to pea. Removal of the pea donor-plant shoots increased the N transfer to 4% of the donor-root N in the non-mycorrhizal controls. Contrastingly, 15% of the donor-root N was transferred to the receiver plants, when plants were colonized by G. intraradices . The results for P transfer followed the same patterns as was observed for N, although in smaller proportions. The results indicate that arbuscular mycorrhizas may play a significant role in the flow of N and P between two plants interconnected by hyphae, when the root system of one of the plants is decomposing.

Decomposition of eucalyptus leaves in litter mixtures

Abstract: Leaf litter from Eucalyptus globulus was decomposed alone and in mixture with either oak (Quercus petraea), ash (Fraxinus excelsior) or birch (Betula pendula) leaf litter under laboratory conditions. Decomposition was monitored as CO2 release and leaching of inorganic N over 13 weeks. At the end of the experiment, litters were separated into their species components and analyzed for mineral composition (K, Ca, Mg, P and N) and mass loss. Differences between expected and measured rates of decomposition were evaluated, based on a comparison between the results from the pure litters and the mixtures. Mixing eucalyptus litter with oak litter resulted in enhanced total CO2 release from the litter mixture when compared with the pure components. Similar, but less marked, positive interactions were observed in mixtures with eucalyptus + birch and eucalyptus + ash. Decomposition of eucalyptus litter in the presence of the other litters also influenced N mineralisation, resulting in greater net N retention in the mixtures with eucalyptus + oak and eucalyptus + birch, but a decrease in mixture with ash. The results support the conclusion that the decomposition of litters in mixtures cannot be readily predicted from the behaviour of the component litters decomposing in isolation. We suggest that mixtures of eucalyptus with other litters could be one mechanism by which the high productivity rates of eucalyptus plantations may be maintained, and that manipulation of litter mixtures could assist in synchronising nutrient release and plant uptake.

Carbon mobilization from the forest floor under red spruce in the northeastern U.S.A.

Abstract: Global climate change may alter soil temperature and moisture conditions, increasing the need to understand how these basic factors affect C dynamics. This is particularly important in boreal forests, which often have large C pools in the forest floor and mineral horizons. We examined the effects of temperature and precipitation frequency on C dynamics in forest floor horizons from eight red spruce sites in the northeastern U.S. using column leaching experiments. Intact and sieved forest floor samples were incubated at 3, 10 or 20°C and leached either daily, once per week, or twice per week during 14 to 39 days using simulated throughfall solutions (pH 2.7 or 4.0). Leachate DOC and CO2 production were measured along with soil C and N concentrations. For intact samples, losses of C as DOC and as CO2 increased with increasing temperature, and the increase (Q10) was usually greater between 3 and 10°C than between 10 and 20°C. There was a greater response of CO2 to temperature than of DOC (e.g. Howland sieved soil Q10s of 1.9 and 7.2 for CO2 and 1.5 and 2.0 for DOC at 3–10 and 10–20°C ranges, respectively). More frequent leaching increased steady state DOC mobilization (e.g. 145 and 58 μg g−1 forest floor d−1 for daily and weekly leachings at 10°C, respectively), but not CO2 evolution (e.g. 79 and 74 μg CO2C g−1 forest floor d−1 for daily and weekly leachings at 10°C, respectively). Across the eight sites DOC loss and CO2 evolution varied by factors of 3.6 and 4.0, respectively. Both CO2 evolution and DOC in leachates calculated as fluxes were correlated (r = 0.73 and 0.87 respectively, n = 8) with the C-to-N ratios of the samples (C-to-N ratios ranged from 27 to 58), which could be explained by N limitations that triggered selective lignin degradation, differences in degree of humification of the material, or position on a west-to-east pollution gradient. Although higher temperatures and more frequent leaching increased DOC mobilization, and higher temperatures increased CO2 evolution, both treatments and site to site variation illustrate the complexity of the response of forest-floor C pools to manipulations.