Seasonal changes in abundance and spatial distribution of the soil arthropods in a Japanese cedar (Cryptomeria japonica D.Don) plantation,with special reference to collembola and acarina

Seasonal changes in abundance and distribution pattern of soil micro-arthropods were studied in connection with a few environmental factors in a Japanese cedar (Cryptomeria japonica D. Don) plantation. The soil arthropods were sampled from three different depths at intervals of two months for two years. Of the collected animals (total 51000–155000 m⁻²), Collembola (20000–76000 m⁻²), oribatid mites (19000–55000 m⁻²) and carnivorous mites (6200–21000 m⁻²) were the numerically dominant animal groups. Low seasonal variations in abundance indicated their seasonal stability in population levels. The trends in seasonal fluctuation were similar among these groups and between the two years, showing bimodal pattern with little peaks in early summer and winter. The pattern of seasonal fluctuation in abundance of carnivorous mites (P _d) was significantly synchronized with that in the total abundance of Collembola and oribatid mites (P _τ). Thus, the number-ratios (P _d/P _τ) were fairly constant, ranging from 0.10 to 0.25. Seasonal changes in vertical distribution of the three animal groups showed a similar pattern for both years. The downward migrations were shown to be more affected by low temperatures in winter accompanied by snow coverage rather than by the desiccation of the surface soil in summer. All the three groups demonstrated as a whole slightly aggregated patterns of horizontal distribution throughout the two years. Temporal increases in the patchiness indices were observed from summer to autumn when the moisture content of the surface soil was low.     

A 5 year study of litter decomposition processes in aChamaecyparis obtusa Endl. forest

Decomposition of needle litter in aChamaecyparis obtusa forest was studied over a 5 year period using a litter bag method. Organic matter, nitrogen and carbon mass and faunal abundance were monitored. The pattern of weight loss was represented by three phases: the initial leaching of carbon and nitrogen (0–3 months), nitrogen immobilization (3–15 months), and nitrogen mobilization (15–60 months). The decomposition rate of needle litter was expressed by Olson's decomposition constant (k) which was−0.113 over a 5 year period. The decomposition rate decreased with the advance of decomposition processes. The role of soil fauna in the decomposition process was assessed by comparing decomposition rates between the control and defaunated plots. In the leaching phase, soil animals had no significant role in the decomposition processes. During the immobilization phase, soil animals contributed to the immobilization processes through their grazing activities, and there were significant differences in weight loss between the control and defaunated plots. In the mobilization phase, saprovorous soil animals such as Collembola and Acari contributed to the mobilization processes by feeding on decomposing litter. Decomposition processes observed in this study were compared with other similar studies.     

Effect of burning on selected soil parameters in a grass fallow shifting cultivation system in Bhutan

Grass fallow shifting cultivation is an important land use practice in the highlands of Bhutan. Part of the nutrient pool contained in soil organic matter is made available for the traditional buckwheat (Fagopyrum tataricum) crop through a highly labor intensive system exposing 250–500 MT soil ha⁻¹ to temperatures of 500°C and above. Dry topsoil is collected in mounds and burned using plant biomass or manure and soil organic matter as fuel. Labor input ranged from 150–401 days ha⁻¹ with land preparation accounting for 65–85% of the total requirement. The burning increased soil pH from 6.0 to 6.9 and available K from 34 to 69 mg kg⁻¹. Soil organic C and total N were reduced from 3.3 and 0.17% to 0.8 and 0.08%, respectively. Release of P from soil organic matter and plant material and reduction in C/N ratio resulting in increased N availability are considered the most essential effects required for good crop yields. Through the burning about 16 MT of C and 470 kg N ha⁻¹ are released into the atmosphere. Fallow periods of 15–20 years are required for the system to be sustainable. The research was supported by the Department of Agriculture. Royal Government of Bhuttan and the Swiss Association for Technical Assistance The research was supported by the Department of Agriculture. Royal Government of Bhuttan and the Swiss Association for Technical Assistance     

Carbon, nitrogen and phosphorus in volcanic soils following afforestation with native birch (Betula pubescens) and introduced larch (Larix sibirica) in Iceland

Afforestation has become an important tool for soil protection and land reclamation in Iceland. Nevertheless, the harsh climate and degraded soils are growth-limiting for trees, and little is know about changes in soil nutrients in maturing forests planted on the volcanic soils. In the present chronosequence study, changes in C, N and total P in soil (0–10 and 10–20 cm depth) and C and N in foliar tissue were investigated in stands of native Downy birch (Betula pubescens Enrh.) and the in Iceland introduced Siberian larch (Larix sibirica Ledeb.). The forest stands were between 14 and 97 years old and were established on heath land that had been treeless for centuries. Soils were Andosols derived from basaltic material and rhyolitic volcanic ash. A significant effect of tree species was only found for the N content in foliar tissue. Foliar N concentrations were significantly higher and foliar C/N ratios significantly lower in larch needles than in birch leaves. There was no effect of stand age. Changes in soil C and the soil nutrient status with time after afforestation were little significant. Soil C concentrations in 0–10 cm depth in forest stands older than 30 years were significantly higher than in heath land and forest stands younger than 30 years. This was attributed to a slow accumulation of organic matter. Soil N concentrations and soil P_tot were not affected by stand age. Nutrient pools in the two soil layers were calculated for an average weight of soil material (400 Mg soil ha⁻¹ in 0–10 cm depth and 600 Mg soil ha⁻¹ in 10–20 cm depth, respectively). Soil nutrient pools did not change significantly with time. Soil C pools were in average 23.6 Mg ha⁻¹ in the upper soil layer and 16.9 Mg ha⁻¹ in the lower soil layer. The highest annual increase in soil C under forest compared to heath land was 0.23 Mg C ha⁻¹ year⁻¹ in 0–10 cm depth calculated for the 53-year-old larch stand. Soil N pools were in average 1.0 Mg N ha⁻¹ in both soil layers and did not decrease with time despite a low N deposition and the uptake and accumulation of N in biomass of the growing trees. Soil P_tot pools were in average 220 and 320 kg P ha⁻¹ in the upper and lower soil layer, respectively. It was assumed that mycorrhizal fungi present in the stands had an influence on the availability of N and P to the trees. Responsible Editor: Hans Lambers.     

The effect of feeding on specific soil algae on the cold-hardiness of two Antarctic micro-arthropods (Alaskozetes antarcticus and Cryptopygus antarcticus)

The effect of consuming terrestrial algae on the cold tolerance of two Antarctic micro-arthropods was examined. From the results of preferential feeding experiments, seven species of Antarctic terrestrial micro-algae were chosen and fed to two common, freeze-avoiding Antarctic micro-arthropods: the springtail Cryptopygus antarcticus (Collembola: Isotomidae), and the mite Alaskozetes antarcticus (Acari: Oribatida). Mites were very selective in their choice of food whereas the springtails were less discriminating. The ice nucleating activity of each species of alga was measured using an ice nucleator spectrometer and a differential scanning calorimeter. Pure cultures of individual species of algae had characteristic supercooling points ranging from ca. −5 to −18 °C. The effect of eating a particular alga on the supercooling point of individual micro-arthropods cultured at two different temperatures (0 and 10 °C) was examined. Neither species showed a preference for algae with low ice-nucleating activity and there was no clear correlation between the supercooling point of food material and that of the whole animal. However, feeding on certain algae such as Prasiola crispa, which contained the most active ice nucleators, decreased the cold tolerance of both species of arthropods. Accepted: 6 May 2000     

Effects of herbicide chlorsulfuron on growth and nutrient uptake parameters of wheat genotypes differing in Zn-efficiency

Prunings of Calliandra calothyrsus, Grevillea robusta, Leucaena diversifolia and farm yard manure were applied each cropping season at 3 and 6 t dry matter ha⁻¹ to an Oxisol in Burundi. The field plots also received basal applications of nitrogen (N), phosphorus (P) and potassium (K). Application of the tree prunings or farm yard manure decreased the concentration of monomeric inorganic aluminium (Al) in soil solution from 2.92 mg Al dm⁻³ in the control plots to 0.75 mg Al dm⁻³ in the plots receiving 6 t ha⁻¹ Calliandra prunings. The other organic materials also decreased the concentration of monomeric inorganic aluminium in the soil solution. The lowered Al concentration led to a corresponding decrease in the percentage Al saturation of the 0–10 cm soil layer from 80% to 68%. Grain yields of maize and beans were strongly inversely related to the percentage Al saturation of the soil. This confirms that soil acidity was the main constraint to maize and beans production. The yield improvement was mainly attributed to the ameliorating effects of the organic matter application on Al toxicity. The nutrient content had less effect presumably because of fertilizer use. In the best treatments, the yield of maize increased from 0.9 to 2.2 t ha⁻¹ and the corresponding beans yield increased from 0.2 to 1.2 t ha⁻¹. A C Borstlap Section editor     

Seasonal changes in soil arthropod abundance in the dry evergreen forest of north-east Thailand,with special reference to collembolan communities

A soil arthropod community was studied in a dry evergreen forest over a 3-year period from May 1998 to April 2001. Population abundance, species composition, and community structure were investigated over the 3-year study period. The soil arthropods consisted of Acari (75.38%), Collembola (16.11%), and others (8.51%), and their abundances showed a clear difference between the rainy and dry seasons. Population abundance of Collembola and Acari were low during drought conditions. The humidity was the most important factor determining distribution, abundance, and survival of soil Collembola in this tropical forest. High predation and low accumulation of organic matter caused low population abundance of Collembola in the tropical habitat. The collembolan community was dominated by a few dominant species over the study period. The pattern of seasonal changes in numbers of Collembola was similar over the 3-year study period. The species composition of the collembolan community was constant and persistent throughout a 3-year study period. Thus, the collembolan community showed constancy in its species composition with seasonal variability over the 3-year study period.     

Zn uptake by maize under the influence of AM-fungi and Collembola Folsomia candida

Zn uptake by maize plants may be affected by the presence of arbuscular mycorrhizal fungi (AMF). Collembola often play an important controlling role in the inter-relationship between AMF and host plants. The objective of this experiment was to examine whether the presence of Collembola at different densities (0.4 and 1 individuals g⁻¹ dry soil) and their activity have any effect on Zn uptake by maize through the plant–AMF system. The presence of the AMF (Glomus intraradices) and of the Collembola species Folsomia candida was studied in a laboratory microcosm experiment, applying a Zn exposure level of 250 mg kg⁻¹ dry soil. Biomass and water content of the plants were no different when only AMF or when both AMF and Collembola were present. In the presence of AMF the Zn content of the plant shoots and roots was significantly higher than without AMF. This effect was reduced by Collembola at both low and high density. High densities of Collembola reduced the extent of AMF colonization of the plant roots and hyphal length in the soil, but low densities had no effect on either. The results of this experiment reveal that the F. candida–G. intraradices interaction affects Zn uptake by maize, but the mechanisms are still unknown.     

Soil enzyme activities and organic matter composition in a turfgrass chronosequence

Highly managed turfgrass systems accumulate considerable soil organic C, which supports a diverse and robust soil microbial community. Degradation of this soil organic C is mediated by a suite of soil enzymes. The relationship between these enzyme activities and the quality of soil organic C is central to understanding the dynamics of soil organic matter. We examined the activities of several soil enzymes involved in microbial C acquisition, including β-glucosidase, N-acetyl-β-glucosaminidase, cellulase, chitinase, and phenol oxidase, and characterized the chemical composition of soil organic matter using Fourier transform infrared spectroscopy (FTIR) in a turfgrass chronosequence (1–95 years old) and adjacent native pines. Non-metric multidimensional scaling analysis showed that the chemical composition of soil organic matter varied with turf age and land use (turf versus pines). Using the polysaccharide peak (1,060 cm⁻¹) as a reference, both aliphatic (2,930 cm⁻¹) and carboxylic (1,650 and 1,380 cm⁻¹) compounds increased with turf age, indicating that soil organic matter became more recalcitrant. Soil enzyme activities per unit soil mass increased with turf age and were correlated to soil C content. Most soil enzyme activities in native pines were similar to those in young turf, but the cellulase activity was similar to or greater than the activity in old turfgrass systems. On a soil C basis, however, the activities of N-acetyl-β-glucosaminidase and cellulase decreased with turf age; this reduction was correlated to the relative changes in the chemical composition of soil organic matter. We observed that the chemical composition of soil organic matter was significantly correlated with the enzyme activity profile when expressed per unit microbial biomass C, but not per unit soil organic C. Our results suggest that chemical composition of soil organic matter changes with turf age and this change partially determines the relative abundance of C-degrading soil enzymes, likely through the influence on microbial community composition.     

Nitrogen and phosphor compounds in bottom sediments: mechanisms of accumulation,transformation and release

This paper is an overview of Russian literature dealing with the accumulation, the transformations and the release of phosphate and nitrogen compounds in a great number of Russian lakes and reservoirs. A considerable data bank has been analysed. Special attention is given to the relations of N- and P-accumulation with the input and transformation of organic carbon, as well as to the release mechanisms, often in relation to eutrophication of the lakes and reservoirs. It is shown that the major input of organic matter into the sediments comes from autochthonous material, and is usually > 70 %. The relative importance of phytoplankton and macrophytes as sources of organic matter is discussed; it appears that trophic state, depth and other factors may have a large influence on this ratio. In shallow eutrophic lakes macrophytes may be the source of organic matter, which source can amount to 1.5–2.5 times that of phytoplankton. It is also shown that the C/N ratio is not a good indicator of the source of the organic matter, because their C/N ratios often are not very different. The decomposition rate of organic matter was analysed; it depends on trophic state and other factors. Sediment N accumulation is mostly (> 90%) in organic form, and depends on nitrogen and organic matter inputs coming from phytoplankton or macrophytes. A correlation coefficient of 0.9–0.95 was found in 176 lakes. In 113 lakes the N accumulation was 0.11 x C accumulation, with C/N ratios between 7.4 and 12.9. Ammonification was rather constant in different groups of lakes; values were often about 20–25 mg m⁻² d⁻¹. The presence of the different forms of nitrogen in interstitial water and in adsorbed forms is discussed. The N in interstitial water is usually in the form of NH₃. Sediment P-accumulation is usually in inorganic form and is related to primary production. Three different groups of sediments could be distinguished with C/P ratios of 31–100, of 101–350 and > 350. In hard water lakes P sedimentation was found to be 0.3–0.5 times that in soft water lakes with comparable primary production. The relative occurrence of apatite, non-apatite and residual P in sediments was calculated. In the interstitial water the P concentration appeared to be controlled by the input and decomposition of organic matter. The concentration of phosphate dissolved in the interstitial water of the top 2 cm layer is often 10–100 times lower than that of the dissolved N. The concentrations of interstitial phosphate are from a few μgl⁻¹ up to 15 mgl⁻¹, but the higher concentrations occur only rarely. Different types of vertical profiles of P compounds in the sediments were shown to be related with the presence of an oxidised zone, the presence of clay etc. Autochthonous apatite and non-apatite phosphates are more mobile than the allochthonous ones and are in equilibrium with interstitial phosphate. Accumulation of autochthonous apatite in sediments is controlled by decomposition of organic matter and accumulation of carbonates.     

Sulfur speciation and distribution in soils and aboveground biomass of a boreal coniferous forest

Major sulfur pools are quantified in soils and aboveground biomass of a coniferous boreal forest. Total ecosystem S averages 1395 kg·ha⁻¹ of which 98% is found in the soil, with 89% being in the mineral horizons. Organic S dominates in soil, tree parts (trunks, branches + foliage, roots) and litterfall, ranging from 77 to 99% of total S concentration. Carbon-bonded S, ester sulfate and SO₄-S in soil profiles range respectively from 51–68%, 29–37% and 1–14% of total S concentrations and account respectively for 57, 33 and 10% of total S on an areal basis. Adsorbed SO₄ accounts for 82% of total SO₄, and can be predicted from Al bound to organic matter, amorphous Al and pH (r^{2 = 0.81}). There is a strong relationship between % carbon and carbon-bonded S in 4 of the 5 soil horizons studied which represent over 95% of the total soil organic matter, whereas ester sulfate is related to % carbon in 3 soil horizons representing only 37% of the soil organic matter. An analysis of atmospheric S loading and S data for 10 forested sites in Europe and North America suggests that the size of the organic S pool in forested systems is independent of atmospheric loading.     

Gradational Forest Change along the Climatically Dry Valley Slopes of Bhutan in the Midst of Humid Eastern Himalaya

Altitudinal forest and climate changes from warm, dry valley bottom (1250 m a.s.l.) to cool, humid ridge top (3550 m a.s.l.) along the typical dry valley slopes of the Bhutan Himalaya were studied. Annual mean temperature decreased upslope with a lapse rate of 0.62 °C·100 m⁻¹ from 18.2 °C at the valley bottom to 4.3 °C at the ridge top. On the contrary volumetric soil moisture content increased from 14.7 to 75.0%. This inverse relationship is the major determinant factor for the distribution of different forest types along the altitudinal gradient. Based on the quantitative vegetation data from 15 plots arranged ca. 200 m in altitude interval (1520–3370 m a.s.l.), a total of 83 tree species belonging to 35 families were recorded. Three major formation types of lower and upper coniferous forests, and a mid-altitude evergreen and deciduous broad-leaved forest were contrasted. Including two transitional types, five forest zones were categorized based on cluster analysis, and each zone can be characterized by the dominants and their phytogeographical traits, viz. (1) west Himalayan warm, dry pine (1520–1760 m a.s.l.), (2) wide ranging east-west Himalayan mixed broad-leaved (1860–2540 m a.s.l.), (3) humid east Himalayan evergreen broad-leaved (2640–2820 m a.s.l.), (4) cool, humid east Himalayan conifer (2950–3210 m a.s.l.), and (5) wide ranging cold, humid conifer (3305–3370 m a.s.l.). Structurally, total basal area (biomass) increased from 15.2 m² ha⁻¹ in the pine forest (1520 m) to 101.7 m² ha⁻¹, in the conifer forest (3370 m a.s.l.). Similarly, soil organic carbon increased from 2.7 to 11.3% and nitrogen from 0.2 to 1.9% indicating dry, poor nutrient fragile ecosystem at the dry valley bottom. We concluded that low soil moisture content (<20%) limits downslope extension of broad-leaved species below 1650 m a.s.l. while coldest month’s mean temperature of −1 °C restricted the upslope extension of evergreen broad-leaved species above 3000 m a.s.l. Along the dry valley slopes, the transition from dry pine forest in the valley bottom, to a mixture of dry west Himalayan evergreen and deciduous east Himalayan broad-leaved, and to humid evergreen oak–laurel forests feature a unique pattern of forest type distribution.     

Landscape Controls on Organic and Inorganic Nitrogen Leaching across an Alpine/Subalpine Ecotone,Green Lakes Valley,Colorado Front Range

Here we report measurements of organic and inorganic nitrogen (N) fluxes from the high-elevation Green Lakes Valley catchment in the Colorado Front Range for two snowmelt seasons (1998 and 1999). Surface water and soil samples were collected along an elevational gradient extending from the lightly vegetated alpine to the forested subalpine to assess how changes in land cover and basin area affect yields and concentrations of ammonium-N (NH₄-N), nitrate-N (NO₃-N), dissolved organic N (DON), and particulate organic N (PON). Streamwater yields of NO₃-N decreased downstream from 4.3 kg ha⁻¹ in the alpine to 0.75 kg ha⁻¹ at treeline, while yields of DON were much less variable (0.40–0.34 kg ha⁻¹). Yields of NH₄-N and PON were low and showed little variation with basin area. NO₃-N accounted for 40%–90% of total N along the sample transect and was the dominant form of N at all but the lowest elevation site. Concentrations of DON ranged from approximately 10% of total N in the alpine to 45% in the subalpine. For all sites, volume-weighted mean concentrations of total dissolved nitrogen (TDN) were significantly related to the DIN:DON ratio (R ² = 0.81, P < 0.001) Concentrations of NO₃-N were significantly higher at forested sites that received streamflow from the lightly vegetated alpine reaches of the catchment than in a control catchment that was entirely subalpine forest, suggesting that the alpine may subsidize downstream forested systems with inorganic N. KCl-extractable inorganic N and microbial biomass N showed no relationship to changes in soil properties and vegetative cover moving downstream in catchment. In contrast, soil carbon–nitrogen (C:N) ratios increased with increasing vegetative cover in catchment and were significantly higher in the subalpine compared to the alpine (P < 0.0001) Soil C:N ratios along the sample transect explained 78% of the variation in dissolved organic carbon (DOC) concentrations and 70% of the variation in DON concentrations. These findings suggest that DON is an important vector for N loss in high-elevation ecosystems and that streamwater losses of DON are at least partially dependent on catchment soil organic matter stoichiometry. Received 26 July 2001; accepted 6 May 2002.     

Stimulation of sulfate reduction rates in Mediterranean fish farm sediments inhabited by the seagrass <Emphasis Type="Italic">Posidonia oceanica</Emphasis>

Sulfate reduction rates and biogeochemical parameters of fish farm sediments across the Mediterranean were investigated in the order to evaluate the potential effects of organic matter inputs on habitat quality for the common seagrass Posidonia oceanica. Four study sites were selected in Spain, Italy, Greece and Cyprus to represent the Mediterranean basin. P. oceanica was found in immediate vicinity of all the farms, which were located at physically exposed sites about 1 km from the shore lines. Organic matter accumulation, sulfate reduction rates and sulfur pools were measured in depth profiles along transects from the farms in both bare and vegetated sediments. Results show that although the organic matter accumulation was minor at the sites (POC < 2.8% DW), the sulfate reduction rates were high, in particular at the largest farm in Italy (up to 212 mmol m⁻² d⁻¹), similar to rates found at shallower, temperate fish farm sites, where higher sedimentation rates can be expected. Sulfate reducing bacteria in these low-organic, carbonate-rich Mediterranean sediments respond strongly to organic matter loadings and cause habitat degradation. Sulfate reduction rates measured in the P. oceanica sediments were among the highest recorded (7.8–42.0 mmol m⁻² d⁻¹) similar to rates found in degrading meadows impacted by organic matter loadings. As sulfate reduction rates were correlated with the sedimentation rates along the transects rather than organic matter pools this suggests mineralization processes were controlled by organic matter loading in fish farm sediments. The vegetated sediments near the net cages were more reduced due to accumulation of sulfides compared to control sites, which is a possible contributing factor to the observed seagrass decline in the farm surroundings. It is recommended that Mediterranean fish farms are placed in areas with rapid dispersal of particulate waste products to minimize organic matter loading of the sediments and thereby preserve habitat quality for benthic fauna and flora.     

Long-term effects of continuous direct seeding mulch-based cropping systems on soil nitrogen supply in the Cerrado region of Brazil

In the Cerrado region of Brazil conventional soybean monoculture is since the 1980s being replaced by direct seeding mulch-based cropping (DMC) with two crops per year and absence of tillage practices. The objective of this study was to assess the long-term impact of DMC on soil organic matter accumulation and nitrogen (N) mineralization. Measurements of soil organic carbon (C) content, soil total N content and soil N mineralization, both under laboratory conditions using disturbed soil samples and under field conditions using intact soil cores were conducted on a chronosequence of 2-, 6-, 9- and 14-year-old DMC fields (DMC-2, DMC-6, DMC-9 and DMC-14, respectively). The average increase of organic C in the 0–30 cm topsoil layer under DMC was 1.91 Mg C ha⁻¹ year⁻¹. Soil total N increased with 103 kg N ha⁻¹ year⁻¹ (0–30 cm). The potential N mineralization rate under laboratory conditions (28°C, 75% of soil moisture at field capacity) was 0.27, 0.28, 0.39 and 0.36 mg N kg soil⁻¹ day⁻¹ for, respectively, the DMC-2, DMC-6, DMC-9 and DMC-14 soils. The corresponding specific N mineralization rates were 0.16, 0.15, 0.22 and 0.17 mg N g N⁻¹ day⁻¹. There was no obvious explanation for the higher specific N mineralization rate of soils under DMC-9, given the similar soil conditions and land-use history before DMC was introduced. Results from the in situ N incubation experiments were in good agreement with those from the laboratory incubations. We estimated that soil N mineralization increases with about 2.0 kg N ha⁻¹ year⁻¹ under DMC. The increase was mainly attributed to the larger soil total N content. These results indicate that even in the medium term (10 years), continuous DMC cropping has limited implications for N fertilization recommendations, since the extra soil N supply represents less than 20% of the common N fertilization dose for maize in the region.     

Relationship of physicochemical properties of rhizosphere soils with native population of mineral phosphate solubilizing fungi

Phosphate solubilizing fungi (PSF) were enumerated in 78 rhizosphere soil samples collected from various sites of Bhavnagar district with agro climatic zone of hot, semi-arid region of Gujarat. 81% of samples were inhabited with indigenous PSF; the count varied ranging from 0.01 to 90 × 10³ PSF g⁻¹. PSF population in the rhizosphere soils were maximal at pH 7.4–8.4, electrical conductivity (EC) 1.2 dS m⁻¹ wherein PSF population decreased with a rise in EC. Optimal moisture content ranged ≤10–30% where the abundance of PSF always decreased with increase in moisture levels while suitable organic matter content ranged from ≤0.5–4.5%. Available P range 100–300 mg kg⁻¹ was the most suitable for occurrence of PSF as being noticed from number of soil samples harboring them in rhizosphere soils. Amongst the various rhizosphere soil properties studied, a highly signifi cant positive correlation was established between PSF and soil available P as well as pH. A signifi cant positive association observed between total fungal population and organic matter as well as soil available P. Both abundance and number of PSF were more pronounced in descending order in plant covers: oilseeds, flowers, orchards, vegetables, pulses and cereals.     

Elevated [CO2], temperature increase and N supply effects on the accumulation of below-ground carbon in a temperate grassland ecosystem

The effects of elevated [CO₂] (700 μl l⁻¹ [CO₂]) and temperature increase (+3 °C) on carbon accumulation in a grassland soil were studied at two N-fertiliser supplies (160 and 530 kgN ha⁻¹ year⁻¹) in a long-term experiment (2.5 years) on well established ryegrass swards (Lolium perenne L.,) supplied with the same amounts of irrigation water. For all experimental treatments, the C:N ratio of the top soil organic matter fractions increased with their particle size. Elevated CO₂ concentration increased the C:N ratios of the below-ground phytomass and of the macro-organic matter. A supplemental fertiliser N or a 3 °C increase in elevated [CO₂] reduced it. At the last sampling date, elevated [CO₂] did not affect the C:N ratio of the soil organic matter fractions, but increased significantly the accumulation of roots and of macro-organic matter above 200 μm (MOM). An increased N-fertiliser supply stimulated the accumulation of the non harvested plant phytomass and of the OM between 2 and 50 μm, without positive effect on the macro-organic matter >200 μm. Elevated [CO₂2] increased C accumulation in the OM fractions above 50 μm by +2.1 tC ha⁻¹, on average, whereas increasing the fertiliser N supply led to an average supplemental accumulation of +0.8 tC ha⁻¹. There was no significant effect of a 3 °C temperature increase under elevated [CO₂] on C accumulation in the OM fractions above 50 μm. This revised version was published online in June 2006 with corrections to the Cover Date.     

Transport of Carbon and Nitrogen Between Litter and Soil Organic Matter in a Northern Hardwood Forest

We used sugar maple litter double-labeled with ¹³C and ¹⁵N to quantify fluxes of carbon (C) and nitrogen (N) between litter and soil in a northern hardwood forest and the retention of litter C and N in soil. Two cohorts of litter were compared, one in which the label was preferentially incorporated into non-structural tissue and the other structural tissue. Loss of ¹³C from this litter generally followed dry mass and total C loss whereas loss of ¹⁵N (20–30% in 1 year) was accompanied by large increases of total N content of this decaying litter (26–32%). Enrichment of ¹³C and ¹⁵N was detected in soil down to 10–15 cm depth. After 6 months of decay (November–May) 36–43% of the ¹³C released from the litter was recovered in the soil, with no differences between the structural and non-structural labeled litter. By October the percentage recovery of litter ¹³C in soil was much lower (16%). The C released from litter and remaining in soil organic matter (SOM) after 1 year represented over 30 g C m⁻² y⁻¹ of SOM accumulation. Recovery of litter ¹⁵N in soil was much higher than for C (over 90%) and in May ¹⁵N was mostly in organic horizons whereas by October it was mostly in 0–10 cm mineral soil. A small proportion of this N was recovered as inorganic N (2–6%). Recovery of ¹⁵N in microbial biomass was higher in May (13–15%) than in October (about 5%). The C:N ratio of the SOM and microbial biomass derived from the labeled litter was much higher for the structural than the non-structural litter and for the forest floor than mineral SOM, illustrating the interactive role of substrates and microbial activity in regulating the C:N stoichiometry of forest SOM formation. These results for a forest ecosystem long exposed to chronically high atmospheric N deposition (ca. 10 kg N ha⁻¹ y⁻¹) suggest possible mechanisms of N retention in soil: increased organic N leaching from fresh litter and reduced fungal transport of N from soil to decaying litter may promote N stabilization in mineral SOM even at a relatively low C:N ratio.     

Nitrogen dynamics of a boreal black spruce wildfire chronosequence

This study examined the nitrogen (N) dynamics of a black spruce (Picea mariana (Mill.) BSP)-dominated chronosequence in Manitoba, Canada. The seven sites studied each contained separate well- and poorly drained stands, originated from stand-killing wildfires, and were between 3 and 151 years old. Our goals were to (i) measure total N concentration ([N]) of all biomass components and major soil horizons; (ii) compare N content and select vegetation N cycle processes among the stands; and (iii) examine relationships between ecosystem C and N cycling for these stands. Vegetation [N] varied significantly by tissue type, species, soil drainage, and stand age; woody debris [N] increased with decay state and decreased with debris size. Soil [N] declined with horizon depth but did not vary with stand age. Total (live + dead) biomass N content ranged from 18.4 to 99.7 g N m⁻² in the well-drained stands and 37.8–154.6 g N m⁻² in the poorly drained stands. Mean soil N content (380.6 g N m⁻²) was unaffected by stand age. Annual vegetation N requirement (5.9 and 8.4 g N m⁻² yr⁻¹ in the middle-aged well- and poorly drained stands, respectively) was dominated by trees and fine roots in the well-drained stands, and bryophytes in the poorly drained stands. Fraction N retranslocated was significantly higher in deciduous than evergreen tree species, and in older than younger stands. Nitrogen use efficiency (NUE) was significantly lower in bryophytes than in trees, and in deciduous than in evergreen trees. Tree NUE increased with stand age, but overall stand NUE was roughly constant (∼ ∼150 g g⁻¹ N) across the entire chronosequence.     

Cadmium accumulation in relation to organic acids in leaves of Solanum nigrum L. as a newly found cadmium hyperaccumulator

The influence of various cadmium concentrations on organic acid levels in leaves of the Cd hyperaccumulator, Solanum nigrum L. and a closely related species, Solanum melongena L., were investigated. In particular, the relationship of organic acids with Cd accumulation in the two plants was investigated. The results showed that Cd accumulation in the shoots of S. nigrum was significantly higher than that of S. melongena. The accumulation of Cd in the leaves of S. nigrum ranged from 2.0 to 167.8 μg g⁻¹ dry weight (DW), but only from 1.2 to 64.0 μg g⁻¹ DW in S. melongena. Solanum melongena was considerably less tolerant to Cd than S. nigrum. Approximately 20% of the total Cd in S. nigrum leaves was water-soluble, suggesting that some accumulated Cd was associated with water-soluble compounds such as organic acids. Malic acid in the leaves of S. nigrum was the most abundant organic acid [up to 115.6–145.7 μmol g⁻¹ fresh weight (FW)], but this acid was not significantly affected by the Cd concentration in soil. However, the level of malic acid in S. melongena plants was much lower, only 16.3–75.4 μmol g⁻¹ FW. The significant positive correlations between total Cd and water-soluble Cd concentrations and both acetic and citric acid concentrations in the leaves of S. nigrum were observed. In contrast, there was no correlation between concentrations of the two acids and Cd concentrations in the leaves of S. melongena. These results indicated that acetic and citric acids in the leaves of S. nigrum might be related to its Cd hyperaccumulation.