Elevated UV-B radiation modifies the extractability of carbohydrates from leaf litter of Quercus robur

We examined the influence of elevated UV-B radiation on the extractability of carbohydrates from leaf litter of Quercus robur. Saplings were exposed to a 30% elevation above the ambient level of erythemally weighted UV-B (280-315 nm) radiation for eight months at an outdoor facility. UV-B radiation was applied under arrays of fluorescent lamps filtered with cellulose diacetate, which transmitted both UV-B and UV-A (315-400 nm) radiation. Saplings were also exposed to elevated UV-A radiation under arrays of polyester-filtered lamps and to ambient radiation under arrays of non-energised lamps. Abscised leaves were collected, ground and sequentially treated with seven solvents in order to fractionate extractable carbohydrates based on the way in which they are held in the cell wall. Elevated UV-B radiation reduced the extractability of carbohydrates from cell walls of Q. robur. Sodium phosphate buffer at pH 7 extracted 10% less total carbohydrate from leaf material exposed during growth to elevated UV-B radiation under cellulose diacetate-filtered lamps than from leaf material grown under polyester-filtered and non-energised lamps. The cumulative amount of carbohydrate released by sequential extraction with phosphate buffer, CDTA, urea and sodium carbonate was between 5.1% and 7.8% lower from leaf material grown under cellulose diacetate-filtered lamps relative to that from leaves grown under non-energised lamps. Abscised leaves were also digested with Driselase, an enzyme mixture extracted from a basidiomycete fungus. No effects of elevated UV radiation were recorded on the amount of carbohydrate released by Driselase digestion. Regression analyses, using data from a previous field decomposition study, suggested that reduced availability of carbohydrates enhanced the colonisation of Q. robur litter by basidiomycete fungi, which then accelerated the decomposition rate of the litter in soil. We recommend that future studies into the effects of UV-B radiation on plant litter decomposition measure not only the concentrations of chemical constituents of litter, but also determine the availability of litter carbon sources to soil microbes, using methods similar to those used here.     

Negligible influence of elevated UV-B radiation on leaf litter quality of Quercus robur

We tested whether elevated UV-B radiation applied to Quercus robur, a principal climax species of northern Europe, would influence concentrations of polyphenolics (Folin–Denis tannins and lignin), phenylpropanoid moieties of lignin, carbohydrates (monosaccharides and holocellulose), or nutrient elements (K, Ca, Mg, P and N) in recently-abscised leaf litter. Saplings of Q. robur were exposed for 2 years at an outdoor facility in the UK to a 30% elevation above the ambient amount of erythemally-weighted UV-B (280–315 nm) radiation under arrays of fluorescent lamps with cellulose diacetate filters, which transmitted both UV-B and UV-A (315–400 nm) radiation. Saplings were also exposed to elevated UV-A alone under arrays of lamps with polyester filters and to ambient radiation under non-energised arrays of lamps. We found little evidence that elevated UV-B radiation influenced leaf litter quality. Data pooled for both years indicated an 8% increase in vanillic acid concentration in litter from polyester-filtered lamp arrays, relative to non-energised arrays, and 8% and 6% increases, respectively, in concentrations of acetovanillone in litter from polyester- and cellulose diacetate-filtered lamp arrays, relative to non-energised lamp arrays. Arabinose concentration in litter from cellulose diacetate-filtered lamp arrays was 3% higher than in litter from polyester-filtered arrays, and glucose concentration in litter from cellulose-diacetate filtered lamp arrays was increased by 6%, relative to non-energised arrays. There were no main effects of elevated UV on the concentrations of holocellulose, polyphenolics or nutrient elements. We conclude that exposure to elevated UV-B does not substantially influence the initial chemical composition of Q. robur leaf litter and that any increases in UV-B radiation arising from ozone depletion over northern mid-latitudes will be unlikely to affect nutrient cycling and decomposition in Quercus woodlands through effects on litter quality alone.     

Responses of soil respiration,soil nutrients,and litter decomposition to inputs from canopy herbivores

We tested whether inputs from canopy herbivores would affect soil processes such as respiration, nutrient cycling, and decomposition along an elevation gradient. The five treatments we used were frass additions, throughfall additions, removal of all litter that fell during the study, removal of greenfall that fell during the study, and controls. Soil respiration was significantly reduced on low and mid elevation sites in litter exclusion, greenfall exclusion and throughfall addition treatments (from 0.846 g CO₂/m²/h for controls to 0.618, 0.667, and 0.708 g CO₂/m²/h, respectively, for the three treatments). Throughfall additions containing PO₄ and NH₄ contributed to significant increases in PO₄ (as much as 0.737 mg/l in 100 ml KCl extract greater then controls), but decreases in NO₃, (0.306  mg/l in 100 ml KCl extract less than con trols), in soil solution samples compared to controls. We observed no significant treatment effects on litter decomposition. Precipitation and temperature influenced soil respiration, but both factors showed a significant interaction with elevation. Phosphate concentrations in soil solutions differed significantly with elevation (low elevation mean 0.097 mg/l, mid elevation mean 0.192 mg/l). Elevation had no significant effect on decomposition.     

Effects of elevated atmospheric CO2 and N fertilization on abundance,diversity and C-isotopic signature of collembolan communities in arable soil

Rising atmospheric CO₂ concentrations are expected to have marked impacts on the carbon (C) turnover in agro-ecosystems through increased plant photosynthetic rates, leading to an enhanced biomass, and wider plant C/N ratios. Through increased carbon allocation below-ground, as well as through changed litter quality, CO₂ enrichment will indirectly affect soil faunal communities. In the present study we investigated how elevated atmospheric CO₂ and two different levels of N fertilization may affect abundance and diversity of collembolans, as important catalysts in decomposition processes, within an agro-ecosystem under winter wheat cultivation. The investigations were carried out in 2002 within a field experiment using the “Free Air CO₂ Enrichment” technique (FACE) at the Federal Agricultural Research Centre (Braunschweig, Germany). Stable C-isotopic analysis of collembolans, soil, and crops gave insight into C translocation. During our investigations δ¹³C values of all components analysed were significantly more negative under FACE compared to ambient air conditions. Stable C-isotopic signatures of collembolans were similar to those of soil under ambient air, but in between those of soil and roots under elevated CO₂ conditions. Our results revealed significant effects of both treatments (CO₂ enrichment and N fertilization) on density and species diversity of collembolans. Overall, collembolans were stimulated under elevated CO₂ conditions, showing an increased abundance of more than 50% (11 240 ind m⁻²) as well as a higher biodiversity (Shannon Weaver index = 2.5; evenness = 0.75) compared to ambient air conditions (7520 ind m⁻²; Shannon Weaver index = 2.2; evenness = 0.72). With regard to N supply, a decrease of about 20–30% under CO₂ enrichment and 45–55% under ambient air conditions in collembolan abundance with no alteration in diversity was recorded under reduced N fertilization. The observed impacts were species-specific.     

Effect of land use types on decomposition of 14C-labelled maize residue (Zea mays L.)

The effect of three land use types on decomposition of ¹⁴C-labelled maize (Zea mays L.) residues and soil organic matter were investigated under laboratory conditions. Samples of three Dystric Cambisols under plow tillage (PT), reduced tillage (RT) and grassland (GL) collected from the upper 5 cm of the soil profile were incubated for 159 days at 20 °C with or without ¹⁴C-labelled maize residue. After 7 days cumulative CO₂ production was highest in GL and lowest in PT, reflecting differences in soil organic C (SOC) concentration among the three land use types and indicating that mineralized C is a sensitive indicator of the effects of land use regime on SOC. ¹⁴CO₂ efflux from maize residue decomposition was higher in GL than in PT, possibly due to higher SOC and microbial biomass C (MBC) in GL than in PT. ¹⁴CO₂ efflux dynamics from RT soil were different from those of PT and GL. RT had the lowest ¹⁴CO₂ efflux from days 2 to 14 and the highest from days 28 to 159. The lowest MBC in RT explained the delayed decomposition of residues at the beginning. A double exponential model gave a good fit to the mineralization of SOC and residue-¹⁴C (R² > 0.99) and allowed estimation of decomposition rates as dependent on land use. Land use affected the decomposition of labile fractions of SOC and of maize residue, but had no effect on the decomposition of recalcitrant fractions. We conclude that land use affected the decomposition dynamics within the first 1.5 months mainly because of differences in soil microbial biomass but had low effect on cumulative decomposition of maize residues within 5 months.     

Rapid estimation of microbial biomass in grassland soils by ultra-violet absorbance

A reliable and simple technique for estimating soil microbial biomass (SMB) is essential if the role of microbes in many soil processes is to be quantified. Conventional techniques are notoriously time-consuming and unreproducible. A technique was investigated that uses the UV absorbance at 280 nm of 0.5 M K₂SO₄ extracts of fumigated and unfumigated soils to estimate the concentrations of carbon, nitrogen and phosphorus in the SMB. The procedure is based on the fact that compounds released after chloroform fumigation from lysed microbial cells absorb in the near UV region. Using 29 UK permanent grassland soils, with a wide range of organic matter (2.9–8.0%) and clay contents (22–68%), it was demonstrated that the increase in UV absorbance at 280 nm after soil fumigation was strongly correlated with the SMB C (r=0.92), SMB N (r=0.90) and SMB P (r=0.89), as determined by conventional methods. The soils contained a wide range of SMB C (412–3412 μg g⁻¹ dry soil), N (57–346 μg g⁻¹ dry soil) and P (31–239 μg g⁻¹ dry soil) concentrations. It was thus confirmed that the UV absorbance technique described was a rapid, simple, precise and relatively inexpensive method of estimating soil microbial biomass.     

Applying an oxygen-based respiratory assay to assess soil microbial responses to substrate and N availability

Documented approaches for measuring soil microbial activities and their controlling factors under field conditions are needed to advance understanding of soil microbial processes for numerous applications. We manipulated field plots with carbon (C) and nitrogen (N) additions to test the capability of a respiratory assay to: (1) measure respiration of endogenous soil C in comparison to field-measured CO₂ fluxes; (2) determine substrate-induced respiratory (SIR) activities that are consistent with substrate availability in the field; and, (3) report N availability in the field based on assay responses with and without added N. The respiratory assay utilizes a microplate containing an oxygen-sensitive fluorescent ruthenium dye. Respiratory activities measured with this approach have previously been shown to occur within short (6–8 h) incubation periods using low substrate concentrations that minimize enrichment during the assay. Field treatments were conducted in a randomized full-factorial design with C substrate (casamino acids, glucose, or none) and inorganic N (±) as the treatment factors. With one exception, we found that respiration of endogenous soil C in the assay responded to the field treatments in a similar manner to CO₂ fluxes measured in the field. Patterns of SIR with low concentrations of added amino acid or carbohydrate substrate (200 μg C g⁻¹ soil) were consistent with field treatments. The ratio (N_ratio) of carbohydrate respiration with added N (25 μg N g⁻¹ soil) to the same without N in the assay was significantly (P < 0.05) decreased by field N amendment. The carbohydrate N_ratio exhibited a logarithmic relationship (r = 0.64, P < 0.05) with extractable inorganic soil nitrate and ammonium concentrations. These data significantly extend and support the capability of this oxygen-based respiratory assay to evaluate in situ soil activities and examine factors that limit these activities.     

Comparison of the eddy covariance and automated closed chamber methods for evaluating nocturnal CO2 exchange in a Japanese cypress forest

Underestimation of nocturnal CO₂ respiration using the eddy covariance method under calm conditions remains an unsolved problem at many flux observation sites in forests. To evaluate nocturnal CO₂ exchange in a Japanese cypress forest, we observed CO₂ flux above the canopy (F_c), changes in CO₂ storage in the canopy (S_t) and soil, and trunk and foliar respiration for 2 years (2003–2004). We scaled these chamber data to the soil, trunk, and foliar respiration per unit of ground area (F_s, F_t, F_f, respectively) and used the relationships of F_s, F_t, and F_f with air or soil temperature for comparison with canopy-scale CO₂ exchange measurements (=F_c + S_t). The annual average F_s, F_t, and F_f were 714 g C m⁻² year⁻¹, 170 g C m⁻² year⁻¹, and 575 g C m⁻² year⁻¹, respectively. At small friction velocity (u_*), nocturnal F_c + S_t was smaller than F_s + F_t + F_f estimated using the chamber method, whereas the two values were almost the same at large u_*. We replaced F_c + S_t measured during calm nocturnal periods with a value simulated using a temperature response function derived during well-mixed nocturnal periods. With this correction, the estimated net ecosystem exchange (NEE) from F_c + S_t data ranged from −713 g C m⁻² year⁻¹ to −412 g C m⁻² year⁻¹ in 2003 and from −883 g C m⁻² year⁻¹ to −603 g C m⁻² year⁻¹ in 2004, depending on the u_* threshold. When we replaced all nocturnal F_c + S_t data with F_s + F_t + F_f estimated using the chamber method, NEE was −506 g C m⁻² year⁻¹ and −682 g C m⁻² year⁻¹ for 2003 and 2004, respectively.     

Measurements of ozone deposition to a potato canopy

Potatoes are an important staple crop, grown in many parts of the world. Although ozone deposition to many vegetation types has been measured in the field, no data have been reported for potatoes. Such measurements, including the latent-heat flux, were made over a fully grown potato field in central Scotland during the summer of 2006, covering a 4-week period just after rainfall and then dry, sunny weather. The magnitude of the flux was typical of many canopies showing the expected diurnal cycles. Although the bulk-canopy stomatal conductance declined as the field dried out (～300 mmol-O₃ m^?2 s^?1 to ～70 mmol-O₃ m^?2 s^?1), the total ozone flux did not follow the same trend, indicating that non-stomatal deposition was significant. Over a dry surface non-stomatal resistance (R_ns) was 270–450 s m^?1, while over a wet surface R_ns was ～50% smaller and both decreased with increasing surface temperature and friction velocity. From the variation with relative humidity (RH) it is suggested that three processes occur on leaf surfaces: on a very dry surface ozone is removed by thermal decomposition, possibly enhanced by photolytic reactions in the daytime and so R_ns decreases as temperature increases; at 50–70% RH a thin film of liquid blocks the “dry” process and resistance increases; above 60–70% RH sufficient surface water is present for aqueous reactions to remove ozone and resistance decreases.     

Solar global UVB (280–315 nm) and UVA (315–380 nm) radiant fluxes and their relationships with broadband global radiant flux at an eastern Mediterranean site

The depletion in the concentration of ozone layer in the stratosphere may significantly increase the UV radiant flux levels (mainly the UVB, 280–315 nm) reaching the earth's surface. These modifications have raised concern among scientists and policy makers during the last two decades so that numerous field experiments have been conducted illustrating deleterious UVB effects on both ecosystems and human beings. This study aiming to explore the UV radiant flux levels in the eastern Mediterranean basin, hourly measurements of global UVB (G_uvb), UVA (G_uva) and broadband (G_h) radiant fluxes collected at Athalassa, Cyprus (35°N, 33°E, 165 m above MSL) from 1 January 2004 to 31 December 2006, during a joint research project between the Athens University and the Meteorological Service of Cyprus, are employed. These data were used to determine temporal variability of the UV radiant components further addressing their dependence on several atmospheric parameters.The analyses of hourly UVB and UVA values indicated significant diurnal variation of these radiant fluxes during daylight hours. The seasonal variation of the ratios obtained from hourly correlations ranged from 0.0299 ± 0.003 (winter) to 0.0311 ± 0.002 (autumn) with an annual mean of 0.0309 ± 0.003 for (G_uva/G_h), and from 0.00160 ± 0.0003 (winter) to 0.00168 ± 0.0003 (autumn) with an annual mean of 0.00164 ± 0.0003 for (G_uvb/G_h). The hourly ratio (G_uva/G_h) increased from 0.0306 ± 0.003 to 0.0318 ± 0.003 as sky conditions changed from clear to overcast; the hourly (G_uvb/G_h) ratio also increased from 0.00162 ± 0.0003 to 0.00170 ± 0.0004, as sky conditions changed from clear to overcast skies. Several atmospheric parameters such as sky clearness (?), brightness (Δ) and path length (sin h) were found to cause changes to both G_uva/G_h and G_uva/G_h ratios. The present data set indicated also a seasonal contribution of the aerosol extinction on these radiometric ratios. Finally, the inverse correlation between ozone and the UVB radiant flux by means of daily G_uvb/G_uva ratio has been verified for the Athalassa's environment.     

High nighttime temperatures affect rice productivity through altered pollen germination and spikelet fertility

Seasonally high nighttime temperatures (HNTs) along the United States Gulf Coast and in regions of similar climate, occurring during the critical stages of development, reduce rice (Oryza sativa L.) yield and quality. The objective of this study was to determine the effects of HNT and preventive exogenous effectors (α-tocopherol, glycine betaine and salicylic acid) on growth, development, physiology and yield of rice plants. Plants were subjected to ambient nighttime temperature (ANT) (27 °C) or HNT (32 °C) through use of continuously controlled infrared heaters, starting from 2000 h until 0600 h. The HNT did not affect leaf photosynthetic rates; however, profound effects on chlorophyll content, leaf nitrogen content, percent pollen germination and spikelet fertility were observed. In addition, HNT hastened plant development rates, as indicated by the panicle emergence date. Plants grown under HNT showed a 90% decrease in yield compared to plants grown under ANT. Dry matter partitioning to the grains of cv. Cocodrie decreased under HNT mainly due to effects on pollen germination and spikelet fertility, but not photosynthesis. Our findings indicate that exogenous application of salicylic acid reduced the negative effects of HNT by 16%.     

Life-history traits and population growth rate in the laboratory of the earthworm Dendrobaena octaedra cultured in copper-contaminated soil

A study on the widespread earthworm Dendrobaena octaedra was conducted to determine which individual life history traits were the most sensitive to copper and to determine the contribution of changes in individual traits to changes in the population growth rate (λ). The study showed that the effect of copper on population growth rate mirrored the effects seen on growth, maturation and reproductive output, with stimulation at the lowest concentrations and inhibition at the highest concentration. A decomposition analysis showed that the mean change in λ was mainly driven by time between consecutive cocoon productions, except at the highest copper concentration (200 mg/kg dry soil) where decreased production of fertile cocoons also contributed to the reductions in λ. The highest population growth rate (λ = 1.18 week⁻¹) occurred at 80 mg Cu/kg dry soil. At higher concentrations λ became gradually smaller, and was almost 1 week⁻¹ (where no population increase or decrease occurs) at the highest exposure concentration of 200 mg Cu/kg dry soil suggesting that extinction would occur if a population of D. octaedra were to be exposed to copper concentrations only slightly higher than this level.     

Influence of foliar phosphorus and nitrogen contents on chemical properties of water extractable organic matter derived from fresh and decomposed sugar maple leaves

The effects of watershed-scale experimental acidification on the macronutrient content and decomposition of sugar maple (Acer saccharum Marsh) leaves were investigated. Bear Brook Watershed in Maine (BBWM) is a paired forest watershed study where the West Bear (WB) watershed has been treated bi-monthly with 1800 eq ha^?1 yr^?1 of (NH₄)₂SO₄ since 1989, and the adjacent East Bear (EB) watershed has acted as a reference. Leaf samples collected from the treated WB watershed had significantly higher concentrations of N and P than leaves from the reference EB watershed. Leaves from both watersheds were decomposed for a 10-day laboratory incubation. Extractable total soluble carbon (C_TS) content of the leaves decreased following decomposition to a greater extent in WB leaves than in EB leaves. Spectroscopic and chromatographic chemical analyses indicated similar chemical properties for the fresh WB and EB WEOM. However, after decomposition, the WB WEOM was more humified as compared to EB WEOM indicating that the watershed treatment resulted in leaves which were more biodegradable than those in the reference watershed. Multi-dimensional fluorescence spectroscopy with parallel factor analysis (PARAFAC) modeled five components: tyrosine-like, three humic substance-like, and terrestrial/anthropogenic associated-like fluorophores. Following decomposition, the relative concentrations of two of the humic-associated components increased to a significantly greater extent for WB than for EB WEOM. These observations were consistent with greater decomposition-related changes to the WEOM from WB samples relative to EB samples. Pearson correlation analysis showed that foliar N and P concentrations were positively correlated with indices of humification. Adsorption of WEOM to goethite and gibbsite was significantly greater for decomposed WB WEOM than EB WEOM. These results demonstrate that greater leaf N and P contents can increase short-term decomposition, accelerate production of more humic-like WEOM, and thereby potentially influence the distribution of organic matter within the soil carbon pool.     

Limitless decomposition in leaf litter of Common beech: Patterns,nutrients’ and heavy metal's dynamics

Long-term studies of Common beech litter decomposition are scarce and the relationship of its limit values to nutrients/heavy metals dynamics has not been sufficiently studied. The present study is a rare case in which beech litter decomposes almost entirely and enables analyses of the impacts of nutrients and heavy metals on litter decomposition. The aim of the present paper is to (i) determine a decomposition pattern of leaf litter and estimate the limit values and to (ii) determine the dynamics of the main nutrients and heavy metals (concentration and net amounts, based on ash-free litter) in an unpolluted stand of Common beech.Common beech (Fagus sylvatica L.) leaf litter was incubated in polyester litterbags (1.5 mm mesh size) and 41 samplings were made over a period of 6.5 years until a mass loss of 88.9% was achieved. Carbon (C) plus 12 more nutrients and heavy metals were analyzed.Mass losses of both whole litter and of C were used in order to estimate the limit values as well as to determine significant differences between the two approaches. An asymptotic function gave significant limit values that were close to 100% (p < 0.0001). These results were also supported by a single exponential function (p < 0.0001). The initial increase in concentrations of nutrients was followed by a decrease of N, P, K, Ca, Na and Mn. A similar pattern was observed for some of the heavy metals (Cu, Cd and Fe) while Zn concentrations decreased continuously. A net release (e.g. a decrease in the net amounts) was observed for all nutrients and heavy metals except for Cd. The litter fraction did not leave any stable residues (i.e. limit values were close to 100%), which was at least partly due to the low initial N and very high Mn concentration (20 times higher than in other studies).     

Effects of ivermectin in dung pats on earthworm (Megascolecidae) populations and pat degradation in Japanese grassland

The effects of residual ivermectin in dung pats on earthworm activity and dung decomposition in Japanese grassland, where Megascolecidae are the dominant group of dung decomposers, were studied. Artificial cowpats containing 0, 0.1, and 1 mg ivermectin kg⁻¹ dung were prepared and deposited on grassland in October 2003. Pats were collected again for analysis 1, 3, 5, and 7 weeks after deposition. Earthworms were collected from the soil around pats at a depth of 0–10 cm. The Megascolecidae, Pheretima (Amynthas) heteropoda and Pheretima (Amynthas) divergens, together accounted for more than 90 and 99% of earthworm individuals and biomass, respectively. Earthworms aggregated around the pats regardless of the ivermectin treatment. Dung-degradation rate was also unaffected by the ivermectin treatments. Dung decomposition appeared to be due mainly to earthworm activity, as dung beetles were rare at this site. These results suggest that ivermectin may have no adverse effects on Megascolecidae activity and on the degradation of cowpats in pastureland sites where earthworms dominate the dung decomposer community.     

Deep-burrowing earthworm additions changed the distribution of soil organic carbon in a chisel-tilled soil

We investigated the influence of earthworms on the three-dimensional distribution of soil organic carbon (SOC) in a chisel-tilled soil. By burrowing, foraging, and casting at the surface and throughout the soil, anecic earthworms such as Lumbricus terrestris L. may play a major role in regulating the spatial distribution of organic matter resources both at the surface and within the soil. In the fall of 1994, we manipulated ambient earthworm communities, which were without deep burrowing species, by adding 100 earthworm individuals m⁻² in spring and fall for 3 years. Overall, the biomass of L. terrestris was increased with earthworm additions and total earthworm biomass declined compared with ambient control treatments. To investigate the spatial variability in soil organic carbon due to this shift in earthworm community structure, we sampled soil on a 28×24 cm grid from the surface to 40 cm in four layers, 10 cm deep. Samples were analyzed for total carbon. We found that additions of anecic earthworms significantly increased average soil organic carbon content from 16.1 to 17.9 g C kg⁻¹ for the 0–10 cm soil, and from 12.4 to 14.7 g kg⁻¹ at 10–20-cm depth, and also changed the spatial distribution of soil organic carbon from uniform to patchy, compared with the ambient treatment.     

Climatic influences on active fractions of soil organic matter

Biologically active fractions of soil organic matter are important in understanding decomposition potential of organic materials, nutrient cycling dynamics, and biophysical manipulation of soil structure. We evaluated the quantitative relationships among potential C and net N mineralization, soil microbial biomass C (SMBC), and soil organic C (SOC) under four contrasting climatic conditions. Mean SOC values were 28±11 mg g⁻¹ (n=24) in a frigid–dry region (Alberta/British Columbia), 25±5 mg g⁻¹ (n=12) in a frigid–wet region (Maine), 11±4 mg g⁻¹ (n=117) in a thermic–dry region (Texas), and 12±5 mg g⁻¹ (n=131) in a thermic–wet region (Georgia). Higher mean annual temperature resulted in consistently greater basal soil respiration (1.7 vs 0.8 mg CO₂–C g⁻¹ SOC d⁻¹ in the thermic compared with the frigid regions, P<0.001), greater net N mineralization (2.8 vs 1.3 mg inorganic N g⁻¹ SOC 24 d⁻¹, P<0.001), and greater SMBC (53 vs 21 mg SMBC g⁻¹ SOC, P<0.001). Specific respiratory activity of SMBC was, however, consistently lower in the thermic than in the frigid regions (29 vs 34 mg CO₂–C g⁻¹ SMBC d⁻¹, P<0.01). Higher mean annual precipitation resulted in consistently lower basal soil respiration (1.1 vs 1.3 mg CO₂–C g⁻¹ SOC d⁻¹ in the wet compared with the dry regions, P<0.01) and lower SMBC (31 vs 43 mg SMBC g⁻¹ SOC, P<0.001), but had inconsistent effects on net N mineralization that depended upon temperature regime. Specific respiratory activity of SMBC was consistently greater in the wet than the dry regions (≈33 vs 29 mg CO₂–C g⁻¹ SMBC d⁻¹, P<0.01). Although the thermic regions were not able to retain as high a level of SOC as the frigid regions, due likely to high annual decomposition rates, biologically active soil fractions were as high per mass of soil and even 2–3-times greater per unit of SOC in the thermic compared with the frigid regions. These results suggest that macroclimate has a large impact on the portion of soil organic matter that is potentially active, but a relatively small impact on the specific respiratory activity of SMBC.     

Mercury affects the distribution of culturable species of Pseudomonas in soil

Pseudomonas bacteria isolated during 52 days on Gould's S1 agar from soil spiked with 0, 3.5 and 15 mg Hg(II) kg soil⁻¹ were characterised to reveal whether mercury affected them differently. Isolates from the treatments with 0 and 15 mg Hg kg⁻¹ were characterised using FT-IR characterisation and subsequent 16S rDNA partial sequencing of representative isolates. To verify the selectivity of Gould's S1 agar and the FT-IR characterisation, all 450 isolates were subjected to the following tests: Gram-determination, catalase and oxidase activity, pigment production on PDA and growth at different temperatures. Furthermore, the isolates were tested for their ability to grow on agar amended with 10 mg Hg kg⁻¹ as an indication of mercury resistance. We found that up to 80% of the isolates in soil amended with 15 mg Hg kg⁻¹ were mercury-resistant, whereas only up to 20% were resistant in the treatments with 0 and 3.5 mg Hg kg⁻¹. We found two groups of Pseudomonas, which probably represent non-described species since they did not group closely with any known species of Pseudomonas in the dendrogram. Hg-enhanced isolates were closely related to P. frederiksbergensis. Furthermore, Hg resistance was almost exclusively restricted to P. frederiksbergensis and P. migulae groups. We conclude that Hg caused a shift in the dominating species of culturable Pseudomonas.     

Emission of nitrous oxide from hydrocarbon contaminated soil amended with waste water sludge and earthworms

Soils in Mexico are often contaminated with hydrocarbons and addition of waste water sludge and earthworms accelerates their removal. However, little is known how contamination and subsequent bioremediation affects emissions of N₂O and CO₂. A laboratory study was done to investigate the effect of waste water sludge and the earthworm Eisenia fetida on emission of N₂O and CO₂ in a sandy loam soil contaminated with the polycyclic aromatic hydrocarbons (PAHs): phenanthrene, anthracene and benzo(a)pyrene. Emissions of N₂O and CO₂, and concentrations of inorganic N (ammonium (NH₄⁺), nitrite (NO₂^?) nitrate (NO₃^?)) were monitored after 0, 5, 24, 72 and 168 h. Adding E. fetida to the PAHs contaminated soil increased CO₂ production rate significantly 2.0 times independent of the addition of sludge. The N₂O emission rate from unamended soil expressed on a daily base was 5 μg N kg^?1 d^?1 for the first 2 h and increased to a maximum of 325 μg N kg^?1 d^?1 after 48 h and then decreased to 10 μg N kg^?1 d^?1 after 168 h. Addition of PAHs, E. fetida or PAHs + E. fetida had no significant effect on the N₂O emission rate. Adding sludge to the soil sharply increased the N₂O emission rate to >400 μg N kg^?1 d^?1 for the entire incubation with a maximum of 1134 μg N kg^?1 d^?1 after 48 h. Addition of E. fetida, PAHs or PAHs + E. fetida to the sludge-amended soil reduced the N₂O emission rate significantly compared to soil amended with sludge after 24 h. It was found that contaminating soil with PAHs and adding earthworms had no effect on emissions of N₂O. Emission of N₂O, however, increased in sludge-amended soil, but addition of earthworms to this soil and contamination reduced it.     

Detection of water stress in an olive orchard with thermal remote sensing imagery

An investigation of the detection of water stress in non-homogeneous crop canopies such as orchards using high-spatial resolution remote sensing thermal imagery is presented. An airborne campaign was conducted with the Airborne Hyperspectral Scanner (AHS) acquiring imagery in 38 spectral bands in the 0.43–12.5 μm spectral range at 2.5 m spatial resolution. The AHS sensor was flown at 7:30, 9:30 and 12:30 GMT in 25 July 2004 over an olive orchard with three different water-deficit irrigation treatments to study the spatial and diurnal variability of temperature as a function of water stress. A total of 10 AHS bands located within the thermal-infrared region were assessed for the retrieval of the land surface temperature using the split-window algorithm, separating pure crowns from shadows and sunlit soil pixels using the reflectance bands. Ground truth validation was conducted with infrared thermal sensors placed on top of the trees for continuous thermal data acquisition. Crown temperature (T_c), crown minus air temperature (T_c − T_a), and relative temperature difference to well-irrigated trees (T_c − T_R, where T_R is the mean temperature of the well-irrigated trees) were calculated from the ground sensors and from the AHS imagery at the crown spatial resolution. Correlation coefficients for T_c − T_R between ground IRT sensors and airborne image-based AHS estimations were R² = 0.50 (7:30 GMT), R² = 0.45 (9:30 GMT) and R² = 0.57 (12:30 GMT). Relationships between leaf water potential and crown T_c − T_a measured with the airborne sensor obtained determination coefficients of R² = 0.62 (7:30 GMT), R² = 0.35 (9:30 GMT) and R² = 0.25 (12:30 GMT). Images of T_c − T_a and T_c − T_R for the entire field were obtained at the three times during the day of the overflight, showing the spatial and temporal distribution of the thermal variability as a function of the water deficit irrigation schemes.