Linking field experiments to long-term simulation of impacts of nitrogen deposition on heathlands and moorlands

The results from three long-term field manipulation studies of the impacts of increased nitrogen deposition (0–120 kg N ha^?1 yr^?1) on lowland and upland heathlands in the UK were compared, to test if common responses are observed. Consistent increases in Calluna foliar N content and decreases in litter C:N ratios were found across all sites, while increases in N leaching were not observed at any site over the range 0–80 kg ha^?1 yr^?1. However, the response of Calluna biomass did vary between sites, possibly reflecting site differences in nutrient status and management histories. Five versions of a simulation model of heathland responses to N were developed, each reflecting different assumptions about the fate and turnover of soil N. Model outputs supported the deduction from mass balance calculations at two of the field sites that N additions have resulted in an increase in immobilisation; the latter was needed to prevent the model overestimating measured N leaching. However, this version of the model significantly underestimated Calluna biomass. Model versions, which included uptake of organic N by Callunaand re-mobilisation of N from the soil organic store provided some improvement in the fit between modelled and field biomass data, but re-mobilisation also led to an overestimation of N leaching. Quantification of these processes and their response to increased N deposition are therefore critical to interpreting experimental data and predicting the long-term impacts of atmospheric deposition on heathlands and moorlands.     

Linking field experiments to long-term simulation of impacts of nitrogen deposition on heathlands and moorlands

The results from three long-term field manipulation studies of the impacts of increased nitrogen deposition (0–120 kg N ha^–1 yr^–1) on lowland and upland heathlands in the UK were compared, to test if common responses are observed. Consistent increases in Calluna foliar N content and decreases in litter C:N ratios were found across all sites, while increases in N leaching were not observed at any site over the range 0–80 kg ha^–1 yr^–1. However, the response of Calluna biomass did vary between sites, possibly reflecting site differences in nutrient status and management histories. Five versions of a simulation model of heathland responses to N were developed, each reflecting different assumptions about the fate and turnover of soil N. Model outputs supported the deduction from mass balance calculations at two of the field sites that N additions have resulted in an increase in immobilisation; the latter was needed to prevent the model overestimating measured N leaching. However, this version of the model significantly underestimated Calluna biomass. Model versions, which included uptake of organic N by Callunaand re-mobilisation of N from the soil organic store provided some improvement in the fit between modelled and field biomass data, but re-mobilisation also led to an overestimation of N leaching. Quantification of these processes and their response to increased N deposition are therefore critical to interpreting experimental data and predicting the long-term impacts of atmospheric deposition on heathlands and moorlands.     

Nitrate Leaching From a Mountain Forest Ecosystem with Gleysols Subjected to Experimentally Increased N Deposition

Nitrate leaching was measured over seven years of nitrogen (N) addition in a paired-catchment experiment in Alptal, central Switzerland (altitude: 1200 m, bulk N deposition: 12 kg ha^-1 a^-1). Two forested catchments (1500 m² each) dominated by Picea abies) were delimited by trenches in the Gleysols. NH₄NO₃ was added to one of the catchments using sprinklers. During the first year, the N addition was labelled with ¹⁵N. Additionally, soil N transformationswere studied in replicated plots. Pre-treatment NO₃ ^--N leaching was 4 kg ha^-1 a^-1 from both catchments, and remained between 2.5 and 4.8 kg ha^-1 a^-1 in the control catchment. The first year of treatment induced an additional leaching of 3.1 kg ha^-1, almost 90% of which was labelled with ¹⁵N, indicating that it did not cycle through the large N pools of the ecosystem (soil organic matter and plants). These losses partly correspond to NO₃ ^- from precipitation bypassing the soil due to preferential flow. During rain or snowmelt events, NO₃ ^- concentration peaks as the water table is rising, indicating flushing from the soil. Nitrification occurs temporarily along the water flow paths in the soil and can be the source of NO₃ ^- flushing. Its isotopic signature however, shows that this release mainly affects recently applied N, stored only between runoff events or up to a few weeks. At first, the ecosystem retained 90% of the added N (2/3 in the soil), but NO₃ ^- losses increased from 10 to 30% within 7 yr, indicating that the ecosystem became progressively N saturated.     

Effects of Long-Term Application of Ammonium Sulphate on Nitrogen Fluxes in a Beech Ecosystem at Solling, Germany

To study the effects of elevated inputs of acidity and nitrogen (N), 1000 mmol m^-2 a^-1 of ammonium sulphate (NH₄NO₃) equivalent to an input of potential acidity of 2000 mmol m^-2 a^-1 was applied annually for 11 yr between 1983 and 1993 in a beech forest at Solling, Germany. Most of the applied NH₄ ⁺ was nitrified in the litter layer and in the upper mineral soil. N in soil leachate quickly responded to the elevated input, but most of the applied N was stored in the soil or left the ecosystem via pathways other than soil output. Leaching of N from the soil increased until the last year of N addition. After the last N application, N fluxes decreased rapidly to low values. The buffering of acidity produced by the nitrification of the applied NH₄ ⁺ was caused mainly by three different processes: (i) sulphur (S) retention, (ii) release of aluminium, (iii) release of base cations. Retention of S took place mostly in the subsoil. 72% of the S input was recovered in output after 14 years of the experiment. Due to the increased fluxes of mobile anions with soil solution, outputs of cations increased drastically.     

Fluxes of NO3-, NH4+, NO,NO2, and N2O in an Old Danish Beech Forest

The fluxes of the major nitrogen compounds havebeen investigated in many ecosystem studies over the world.However, only in few studies has attention been drawn to theimportance of the fluxes of minor gaseous nitrogen compoundsto complete the nitrogen cycle. In Denmark a detailed study onthe nitrogen cycle in an old beech forest has been implementedin 1997 at Gyrstinge near Sorø, Zealand. The study includesthe fluxes of the gases NO, N₂O and water mediatedtransport of NO₃ ^- and NH₄ ⁺. Measurementsof the fluxes of the gaseous compounds are performed withmicro-meteorological methods (eddy-correlation and gradient)and with chambers. Water mediated fluxes encompass rain,throughfall, stem-flow and leaching from the root zone. Thehydrological model is verified by TDR measurements. The findings show that the total water mediated N input tothe forest floor with throughfall and stemflow was 25.6 kg Nha^-1 yr ^-1, and open field wet deposition withprecipitation was 19.0 kg N ha^-1 yr ^-1. The internalcycling of N in the ecosystem measured as turnover oflitterfall and plant uptake was 100 kg N ha^-1 yr ^-1and 14 kg N ha^-1 yr ^-1, respectively. The fluxes ofthe gaseous N compounds NO and N₂O were of minorimportance for the total N turnover in the forest, NO_xemission being <1 kg N ha^-1 yr ^-1 and N₂Oemission from the soil being 0.5 kg N ha^-1 yr ^-1 withno significant difference between wet and dry soils.Concentrations of NO₃ ^- and NH₄ ⁺ in thesoil solution beneath the rooting zone are very small andconsequently the N leaching is almost negligible. It isconcluded that the nitrogen mass balance of this old beechforest ecosystem mainly is controlled by the input by dry andwet deposition and a large internal N cycle with a fast litterturnover. The nitrogen input tothe forest ecosystem which currently exceeds the critical loadby 5 kg N ha^-1 yr ^-1is mainly accumulated in the soil and no significant nitrateleaching is occurring.     

Composition of the Saprotrophic Fungi in Calluna Heathland Soil and the Influence of Ammonium Nitrate Application

The composition of saprotrophic soil fungi in the mor layerof a Calluna-dominated, Danish heathland wasinvestigated after two years of fertilization with ammoniumnitrate (0, 35, 50 and 70 kg N ha^1- yr^-1) using asoil washing technique. The most frequently isolated generafrom the soil particles plated were Penicillium, Trichoderma, Mortierella and Mucor. Eightspecies of Penicillium were identified and Penicillium spinulosum was the most frequently isolated.The occurrence of dark, sterile fungi on the soil particleswas low. There were no measurable changes in the specificcomposition of the saprotrophic soil fungal groups due tothe nitrogen treatments, beside from an increasedoccurrence of Absidia californica. I conclude that adirect impact on the composition of the saprotrophic fungiin heathland soil is unlikely under enhanced nitrogen input.     

Soil Acidity Parameters and Defoliation Degree in Six Norway Spruce Stands in Finland

Soil acidity parameters (pH, basesaturation, exchangeable Al) in the organic and mineralsoil layers and in soil water (pH, dissolved organiccarbon, total Al, Al³⁺ and molar Ca/Al ratios) insix Norway spruce stands in different parts of Finlandwere compared. An attempt was also made to relate thedegree of defoliation in the tree stand to N and Sdeposition and soil parameters. No relationship was foundbetween soil acidity parameters and defoliation in thesix stands. Defoliation was positively correlated withstand age and the C/N ratio of the organic layer, andnegatively with the cation exchange capacity. The plotlocated on a so-called sulphate soil on the west coast ofFinland had very low soil pH values, and extremely highAl and SO₄ ^2- concentrations and molar Ca/Alratios of well below 1.0 in soil water. Despite the highnatural acidity in the soil on this plot, defoliation inthe spruce stand was the lowest (mean 8.6%) of all sixplots. The results of this study indicate that soilacidity is not a major factor affecting stand conditionin these spruce stands, and that the variation in soilacidity parameters is closely related to climatic factorsand natural soil formation processes.     

An Assessment of Management Practices to Control Nutrient and Pesticide Levels in Vegetable Production Areas in the Rattaphum Catchment, Thailand

The over-use of agrochemicals may have adverse effects on soils along with surface and groundwater. An assessment of alternative management practices to reduce the application of agrochemicals and minimise the pollution of water resources was carried out in the vegetable agro-ecosystem in the Rattaphum Catchment. This paper details the factors that govern the volume of agrochemicals used on high intensity crops in this area, covering the choice of crop and cropping patterns, level of pest and weed infestation and the socio-economic status of farmers such as capital, size of holding and labour availability. Alternative management practices tested included (i) a reduction in the use of pesticides and chemical fertilisers; (ii) the use of bio-insecticides and bio-fertilisers; and (iii) the development of buffer zones near streams to reduce nutrient leaching into surface water systems. The results showed that when compared with bio-insecticides, synthetic insecticides were more effective in controlling major insect pests during chaisim vegetable production and were associated with higher net incomes. The high rate of application of chemical fertilisers in home gardens and commercial farms led to the accumulation of phosphorous and potassium in the top soil, with the traditional method of combining organic and chemical fertilisers producing the highest total nitrogen soil content. A fifty percent reduction in chemical fertilisers in the commercial farms produced the lowest total nitrogen soil content, without any apparent change in crop yield. The three types of tested buffers seem to be effective in reducing runoff and sediment load and were particularly efficient in lowering nutrient leaching to streams; the best results were obtained with an effective length of 2.5 m.     

What factors control soil profile nitrogen storage?

Factors influencing soil profile nitrogen storage (SPNS) in GB have been investigated. The SPNS values of moorland peats and podzols in the South Pennines, of permanent grassland in Yorkshire, and of adjacent arable-, semi-permanent grassland- and ancient woodland-soils in Worcestershire have been compared to assess land use effects. The soils exhibited similar SPNS values, because changes in bulk density and organic matter composition offset increases in N concentrations in highly organic soils. Data from the Soil Survey of Scotland were used to show effects of soil parent material on SPNS were also small. Data on Scottish podzol soils under heather moorland and derived from sandstone or quartzite were used to show that SPNS declined with increasing precipitation, but increased with the amount of pollutant N deposited from the atmosphere.     

What factors control soil profile nitrogen storage?

Factors influencing soil profile nitrogen storage (SPNS) in GB have been investigated. The SPNS values of moorland peats and podzols in the South Pennines, of permanent grassland in Yorkshire, and of adjacent arable-, semi-permanent grassland- and ancient woodland-soils in Worcestershire have been compared to assess land use effects. The soils exhibited similar SPNS values, because changes in bulk density and organic matter composition offset increases in N concentrations in highly organic soils. Data from the Soil Survey of Scotland were used to show effects of soil parent material on SPNS were also small. Data on Scottish podzol soils under heather moorland and derived from sandstone or quartzite were used to show that SPNS declined with increasing precipitation, but increased with the amount of pollutant N deposited from the atmosphere.     

Forest Ecosystem Responses to Atmospheric Pollution: Linking Comparative With Experimental Studies

The impact on an ecosystem of an environmental stress, such as climate change or air pollution, can be studied through experimentation, through comparisons of sites across a gradient of the stress, through long-term studies at a single site, or through theoretical or modelling approaches. Although the former three techniques often are used to develop and test models, it is much rarer to explicitly link experimental, comparative or long-term studies together. Here we present a concept for combining experimental and comparative research to assess the direction and rate of change, the expected long-term state, and the rate at which the long-term state is achieved after an ecosystem is exposed to an environmental stress. We do this by comparing the response of a forest in Denmark to experimentally increased N deposition with the expected long-term response based on a European database of forests exposed to different levels of N deposition over long time periods. The analysis suggests that if N deposition were to increase by 3-fold to about 50 kg N ha^-1 a^-1 at the Danish site, and remain at this level, the N concentration in needles would respond within 2–4 yr after the onset of the enhanced N deposition, and would rapidly plateau to an expected mean value of 18.0 mg N g^-1 dry mass (95% confidence interval ± 2.5 mg g^-1). The N concentration of new litter also would respond rapidly (1–2 yr) to reach an expected value of 16.6 mg N kg^-1 dry mass (± 3). The N concentration of the organic layer in the soil would increase much more slowly, but a significant increase would be expected within 5–10 yr. Mineral soil pH would take more than 7 yr to change. Finally, the flux of dissolved inorganic N in leachate wouldbegin to increase immediately, but would take many years to reach the expected level of 22.4 kg N ha^-1 a^-1(± 4).     

A conceptual model of spatially heterogeneous nitrogen leaching from a welsh moorland catchment

Soil- and stream-water data from the Plynlimon research area, mid-Wales, have been used to develop a conceptual model of spatial variations in nitrogen (N) leaching within moorland catchments. Extensive peats, in both hilltop and valley locations, are considered near-complete sinks for inorganic N, but leach the most dissolved organic nitrogen (DON). Peaty mineral soils on hillslopes also retain inorganic N within upper organic horizons, but a proportion percolates into mineral horizons as nitrate (NO^? ₃), either through incomplete immobilisation in the organic layer, or in water bypassing the organic soil matrix via macropores. This NO^? ₃ reaches the stream where mineral soilwaters discharge (via matrix throughflow or pipeflow) directly to the drainage network, or via small N-enriched flush wetlands. NO^? ₃ in hillslope waters discharging into larger valley wetlands will be removed before reaching the stream. A concept of catchment ‘nitrate leaching zones’ is proposed, whereby most stream NO^? ₃ derives from localised areas of mineral soil hillslope draining directly to the stream; the extent of these zones within a catchment may thus determine its overall susceptibility to elevated surface water NO^? ₃ concentrations.     

Characterisation of Organic Matter and Carbon Cycling in Rehabilitated Lignite-rich Mine Soils

Open-cast lignite mining in the Lusatian mining district resulted in rehabilitated mine soils containing up to four organic matter types: (1) recent plant litter, (2) lignite deposited by mining activity, (3) carbonaceous ash particles deposited during amelioration of the lignite-containing parent substrate and (4) airborne carbonaceous particles deposited during contamination. The influence of lignite-derived carbon types on the organic matter development and their role in the soil carbon cycle was unknown. This paper presents the findings obtained during a six year project concerning the impact of lignite on soil organic matter composition and the biogeochemical functioning of the ecosystem. The organic matter development after rehabilitation was followed in a chronosequence of rehabilitated mine soils afforested in 1966, 1981 and 1987. A differentiation of the organic matter types and an evaluation of their role within the ecosystem was achieved by the use of ¹⁴C activity measurements, ¹³C CPMAS NMR spectroscopy and wet chemical analysis of plant litter compounds. The results showed that the amount and degree of decomposition of the recent organic matter derived from plant material of the 30 year old mine soil was similar to natural uncontaminated forest soil which suggests complete rehabilitation of the ecosystem. The decomposition and humification processes were not influenced by the presence of lignite. On the other hand it was shown that lignite, which was thought to be recalcitrant because of its chemical structure, was part of the carbon cycle in these soils. This demonstrates the need to elucidate further the stabilisation mechanisms of organic matter in soils.     

Kinetic Study Of The Composting Of Evergreen Oak Forestry Waste

The successive stages in the composting process of forestry waste from evergreen oak (Quercus ilx sbsp. ballota) were studied under controlled conditions (initial) carbon to nitrogen ratio = 30, T = 27°C). The original material was composted for 6 months and sampled every 15 days. The variables measured on the oak biomass in the course of the experiment showed different kinetics: the weight loss and germination index underwent a monotonic increase whereas the reducing sugars, phenols and E₄₆₅/E₆₆₅ extinction ratio of the water-soluble fraction stabilized at their lowest values after the first 2 weeks. Other variables, such as alkali solubility, water repellency, pH and particle size, showed maximum or minimum values at intermediate stages of the experiment. In contrast to the adverse agrobiological effects of the direct application to soil of the original waste, germination biotests and greenhouse experiments showed that plant response improved from the 2 first weeks of composting. The kinetics observed for the parameters studied suggested that the less favourable effect on plant yield may come from phytotoxic substances in compost but also from the microbial use of soil N required for the transformation of the most biodegradable compost fractions in special hemicelluloses.     

Impact of land use on soluble organic nitrogen in soil

Although it has been hypothesized that soluble organic nitrogen (SON) plays a central role in regulating productivity in some terrestrial ecosystems, the factors controlling the size of the SON pool in soil remain poorly understood. Therefore our principal aim in this work was to assess the impact of seven different land use systems (rough and managed grassland, deciduous and coniferous woodland, heathland, wetland and tilled land) on the size of the SON and inorganic N (NO^? ₃, NH⁺ ₄) pools in the surface soil layer (0–15 cm). After extraction with deionised water, we found that in most cases the size of the water extractable organic N (WEON) pool was similar in size to the inorganic N pool. In contrast, the KCl extractable organic N (KClEON) pool constituted the dominant form of soluble N in soils under all land uses, perhaps indicating that significant amounts were held on the soil exchange phase. In contrast to inorganic N, which varied significantly with land use, the size of the KClEON and WEON pool was similar for all land uses with the exception of KClEON in tilled land, where significantly lower amounts were observed. We conclude that SON constitutes an important soil N pool in a broad range of land uses, and that its role in microbial N assimilation, plant nutrition and ecosystem responses to atmospheric N deposition warrants further attention.     

Characteristics of environmental factors and their effects on CH4 and CO2 emissions from a closed landfill: An ecological case study of Shanghai

To elucidate the influence of landfill gas (LFG) emission on environmental factors, an ecological investigation that was primarily concerned with the characteristics of vegetation, cover soil, and solid waste in the landfill was carried out. Temporal and spatial variations in vegetation diversity and coverage and their effects on reducing the emission of methane in the landfill were investigated. The results showed that both vegetation coverage and diversity increased with elapsed landfill closure time. The transition trend of the vegetation species was from perennial plant (Phragmites australis) to annual plants. Perennial vegetation was the dominant type of vegetation during the early closure period, and annual vegetation coverage increased with closure time. Vegetation preferentially appeared in areas of comparatively high depth of cover soil, which was characterized by high moisture retentiveness that enabled vegetation growth. The concentrations of methane and carbon dioxide in the cover soil significantly decreased with increasing closure time. The concentrations of methane and carbon dioxide from bare cover soil were higher than those from vegetated cover soil whereas the CO₂ flux of bare cover soil was less than that of vegetated cover soil.     

Bio fuel ash in a road construction: impact on soil solution chemistry

Limited natural resources and landfill space, as well as increasing amounts of ash produced from incineration of bio fuel and municipal solid waste, have created a demand for useful applications of ash, of which road construction is one application. Along national road 90, situated about 20 km west of Sollefteå in the middle of Sweden, an experiment road was constructed with a 40 cm bio fuel ash layer. The environmental impact of the ash layer was evaluated from soil solutions obtained by centrifugation of soil samples taken on four occasions during 2001–2003. Soil samples were taken in the ash layer, below the ash layer at two depths in the road and in the ditch. In the soil solutions, pH, conductivity, dissolved organic carbon (DOC) and the total concentration of cations (metals) and anions were determined. Two years after the application of the ash layers in the test road, the concentrations in the ash layer of K, SO₄, Zn, and Hg had increased significantly while the concentration of Se, Mo and Cd had decreased significantly. Below the ash layer in the road an initial increase of pH was observed and the concentrations of K, SO₄, Se, Mo and Cd increased significantly, while the concentrations of Cu and Hg decreased significantly in the road and also in the ditch. Cd was the element showing a potential risk of contamination of the groundwater. The concentrations of Ca in the ash layer indicated an ongoing hardening, which is important for the leaching rate and the strength of the road construction.     

Impact of Land Use on Soluble Organic Nitrogen in Soil

Although it has been hypothesized that soluble organic nitrogen (SON) plays a central role in regulating productivity in some terrestrial ecosystems, the factors controlling the size of the SON pool in soil remain poorly understood. Therefore our principal aim in this work was to assess the impact of seven different land use systems (rough and managed grassland, deciduous and coniferous woodland, heathland, wetland and tilled land) on the size of the SON and inorganic N (NO ₃ ^– , NH ₄ ⁺ ) pools in the surface soil layer (0–15 cm). After extraction with deionised water, we found that in most cases the size of the water extractable organic N (WEON) pool was similar in size to the inorganic N pool. In contrast, the KCl extractable organic N (KClEON) pool constituted the dominant form of soluble N in soils under all land uses, perhaps indicating that significant amounts were held on the soil exchange phase. In contrast to inorganic N, which varied significantly with land use, the size of the KClEON and WEON pool was similar for all land uses with the exception of KClEON in tilled land, where significantly lower amounts were observed. We conclude that SON constitutes an important soil N pool in a broad range of land uses, and that its role in microbial N assimilation, plant nutrition and ecosystem responses to atmospheric N deposition warrants further attention. SAFRD, University of Newcastle, Newcastle-upon-Tyne, NE1 7RU, U.K.     

A Conceptual Model of Spatially Heterogeneous Nitrogen Leaching from a Welsh Moorland Catchment

Soil- and stream-water data from the Plynlimon research area, mid-Wales, have been used to develop a conceptual model of spatial variations in nitrogen (N) leaching within moorland catchments. Extensive peats, in both hilltop and valley locations, are considered near-complete sinks for inorganic N, but leach the most dissolved organic nitrogen (DON). Peaty mineral soils on hillslopes also retain inorganic N within upper organic horizons, but a proportion percolates into mineral horizons as nitrate (NO ₃ ^– ), either through incomplete immobilisation in the organic layer, or in water bypassing the organic soil matrix via macropores. This NO ₃ ^– reaches the stream where mineral soilwaters discharge (via matrix throughflow or pipeflow) directly to the drainage network, or via small N-enriched flush wetlands. NO ₃ ^– in hillslope waters discharging into larger valley wetlands will be removed before reaching the stream. A concept of catchment nitrate leaching zones is proposed, whereby most stream NO ₃ ^– derives from localised areas of mineral soil hillslope draining directly to the stream; the extent of these zones within a catchment may thus determine its overall susceptibility to elevated surface water NO ₃ ^– concentrations.     

Dynamic of functional microbial groups during mesophilic composting of agro-industrial wastes and free-living (N2)-fixing bacteria application

Although several reports are available concerning the composition and dynamics of the microflora during the composting of municipal solid wastes, little is known about the microbial diversity during the composting of agro-industrial refuse. For this reason, the first parts of this study included the quantification of microbial generic groups and of the main functional groups of C and N cycle during composting of agro-industrial refuse. After a generalized decrease observed during the initial phases, a new bacterial growth was observed in the final phase of the process. Ammonifiers and (N₂)-fixing aerobic groups predominated outside of the piles whereas, nitrate-reducing group increased inside the piles during the first 23 days of composting. Ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), showed an opposite trend of growth since ammonia oxidation decreased with the increase of the nitrite oxidation activity. Pectinolytics, amylolytics and aerobic cellulolytic were present in greater quantities and showed an upward trend in both the internal and external part of the heaps.Several free-living (N₂)-fixing bacteria were molecularly identify as belonging especially to uncommon genera of nitrogen-fixing bacteria as Stenotrophomonas, Xanthomonas, Pseudomonas, Klebsiella, Alcaligenes, Achromobacter and Caulobacter. They were investigated for their ability to fix atmospheric nitrogen to employ as improvers of quality of compost. Some strains of Azotobacter chrococcum and Azotobacter salinestris were also tested. When different diazotrophic bacterial species were added in compost, the increase of total N ranged from 16% to 27% depending on the selected microbial strain being used. Such microorganisms may be used alone or in mixtures to provide an allocation of plant growth promoting rhizobacteria in soil.