Water-soluble carbon in roots of rape and barley: impacts on labile soil organic carbon, arylsulphatase activity and sulphur mineralization

Investigating the impact of plant species on sulphur (S) availability in the rhizosphere soil is agronomically important to optimize S fertilization. Bulk, rhizosphere soils and the roots of field-grown rape and barley were sampled 7 times (every fortnight), from March to June, at plant maturity. Root carbon (C) and nitrogen (N) in water extract, along with soil SO₄²⁻-S, labile soil organic-C (HWC) and -N (HWN) in hot water extract, as well as soil arylsulphatase activity were then monitored. The average concentrations of both HWC and HWN were observed in the following decreasing order: rape rhizosphere soil >barley rhizosphere soil >bulk soil. In parallel, the average contents of water extractable-C and -N in rape roots were higher than those in barley roots. These results suggest that soil C and N contents in hot water extract (including rhizodeposition) were correlated with C and N released by roots. Great ARS activities found in rape rhizosphere soil were accompanied by great SO₄²⁻-S mineralization over time. Finally, bulk and rhizosphere soils of rape and barley were pooled from the seven samplings and incubated with the corresponding pooled root water-soluble C of both plant species and glucose-C. After 1 and 9 weeks, a greater net S mineralization (gross mineralization - immobilization) was observed with rape root water-soluble C than with barley root water-soluble C and glucose-C. Conjointly, we found a higher average value of ARS activity in rape rhizosphere than in barley rhizosphere soil. Our findings suggest that plant species, via their rhizodeposition, determine the dynamic of S in soil.     

Root-induced modifications of the exchange of phosphate ion between soil solution and soil solid phase

The uptake of phosphorus (P) by roots results in a depletion of phosphate ions (PO₄) in the rhizosphere. The corresponding decrease in PO₄ concentration in the soil solution (C_P) gives rise to a replenishment of P from the solid phase which is time- and C_P-dependent. This PO₄ exchange which reflects the buffer power of the soil for PO₄ also varies with the composition and the physico-chemical conditions of the soil. As root activity can modify these physico-chemical conditions in the rhizosphere, the question arises whether these modifications affect the ability of PO₄ bound to the soil solid phase to exchange with PO₄ in soil solution. The aim of the present work was to measure and compare the parameters which describe the amount of PO₄ bound to soil solid phase that is capable to replenish solution P for both rhizosphere and bulk soils. The soil sample was a P-enriched, calcareous topsoil collected from a long-term fertiliser trial. Rhizosphere soil samples were obtained by growing dense mats of roots at the surface of 3 mm thick soil layer for one week. Three plant species were compared: oilseed rape (Brassica napus L., cv Goeland) pea (Pisum sativum L., cv. Solara) and maize ( Zea mays L., cv. Volga). The time- and C_P-dependence of the PO₄ exchange from soil to solution were described using an isotopic dilution method. The measured C_P values were 0.165 mg P L⁻¹ for bulk soil and 0.111, 0.101 and 0.081 mg P L⁻¹ for rhizosphere soils of maize, pea and rape, respectively. The kinetics of the PO₄ exchange between liquid and solid phases of soil were significantly different between rhizosphere and bulk soils. However, when changes in C_P were accounted for, the parameters describing the PO₄ exchange with time and C_P between soil solution and soil solid phase were found to be very close for bulk and rhizosphere soils. For this calcareous and P-enriched soil, plant species differed in their ability to deplete PO₄ in solution. The resulting changes in the ability of the soil solid phase to replenish solution PO₄ were almost fully explained by the depletion of soil solution P. This revised version was published online in June 2006 with corrections to the Cover Date.     

不同耐盐植物根际土壤盐分的动态变化

以甘肃秦王川引大灌区盐渍化土壤为研究背景,用盆栽根袋法对4种耐盐植物根际和非根际土壤pH和盐分离子的动态变化进行了分析比较。结果表明:4种待测植物随着培养时间的延长土壤pH和EC值呈降低趋势。新疆大叶(Medicago Sativa L.cv.Xinjiangdaye)、向日葵(Helianthus annuus)和霸王(Zygophyllum xanthoxylum)生长90 d后根际土壤pH明显低于非根际,而裸麦(Hordeum vulgare var. vulgare)根际较非根际pH差异不大。霸王和新疆大叶根际土壤EC值较非根际高,而裸麦和向日葵的根际与非根际差异不大。4种供试植物根际K⁺均出现亏缺,Ca²⁺、Na⁺、Mg²⁺、SO^2-₄和Cl^-在新疆大叶、霸王和向日葵3种植物根际均出现富集,对于裸麦:Ca²⁺、Mg²⁺和SO^2-₄ 3种离子在植物根际富集,而Cl^-和Na⁺在根际亏缺。随着待测植物培养时间的增加Na⁺/K⁺、Na⁺/Ca²⁺和Na⁺/Mg²⁺ 这3个比值呈降低趋势,说明Na⁺相对于K⁺、Ca²⁺和Mg²⁺的含量降低,生物措施对Na⁺的移除效果较显著。     

Studies on soil rhizosphere: speciation and availability of Cd

ABSTRACT

The rhizosphere soils of two durum wheat (Triticum turgidum var. durum L.) cultivars Kyle and Areola grown in two selected soils of southern Saskatchewan were collected both at 2-week and 7-week plant growth stages. The cadmium availability index (CAI), determined as M NH₄CI-extractable Cd, pH and the distribution of the particulate- bound Cd species of the soils were carried out and the data were discussed in comparison with those of the corresponding bulk soil. At the 2-week growth stage, the pH of the rhizosphere soil was less than that of the corresponding bulk soil and the CAI values were higher in the rhizosphere soil, indicating that more Cd was complexed with the low-molecular-weight organic acids (LMWOAs) at the soil-root interface and was extractable by M NH₄CI. Compared with the bulk soils, the CAI values were 2–9 times higher in the soil rhizosphere of the plots fertilized with Idaho monoammonium phosphate fertilizer at 2-week growth stage, which is attributed to the combined effects of the Cd introduced into the soil rhizosphere from the fertilizer (Cd content of the fertilizer was 144 mg kg^?1) and complexation reactions of phosphate and LMWOAs with soil Cd. At 7-week plant growth stage, such differences were not observed. The increased amounts of carbonate-bound and metal-organic complex-bound Cd species of the rhizosphere soils are due to the increased amounts of carbonate, a product of plant respiration, and the LMWOAs at the soil-root interface, respectively. Simple correlation analysis of the data showed that the CAI of the rhizosphere soils of the control plots correlated at least two orders of magnitude better with the metal-organic complex-bound Cd whereas the CAI of the rhizosphere soils treated with Idaho phosphate correlated better with carbonate-bound Cd species in comparison to other species.     

Barley genotypes differ in activity of soluble extracellular phosphatase and depletion of organic phosphorus in the rhizosphere soil

In the present investigation we studied the extent of variation among barley genotypes (Hordeum vulgare L. cv. Alexis, Canut, Digger, Etna, Peel) in their ability: i) to induce activity of soluble extracellular phosphatase in rhizosphere soil. ii) to withdraw bicarbonate extractable organic phosphorus (NaHCO₃-P₀). All the genotypes induced 3–4 times higher phosphatase activities in rhizosphere soil as compared to bulk soil. Among the genotypes, there were significant (p>0.01) differences in soluble extracellular and non-soluble phosphatase activities and depletion of NaHCO₃-P₀ in soil near their root mats. Etna induced highest phosphatase activities and depleted most NaHCO₃-P₀ from the rhizosphere soil. A high correlation (r=0.79) was found between the activity of soluble extracellular phosphatase and the quantity of NaHCO₃-P₀ withdrawn from the rhizosphere soil by the barley genotypes.     

The influence of plants grown under elevated CO2 and N fertilization on soil nitrogen dynamics

We investigated the effects of spring barley growth on nitrogen (N) transformations and rhizosphere microbial processes in a controlled system under elevated carbon dioxide (CO₂) at two levels of N fertilization (applied with ¹⁵N labelling). After 25 d, elevated CO₂ (twice ambient) increased plant growth (dry weight, DW) by 141% at low‐N fertilization and by 60% at high‐N fertilization, but its positive effect on the root‐to‐shoot ratio was only significant at low‐N input. As a result of this plant response, elevated CO₂ caused a greater soil CO₂ efflux, rhizosphere soil DW, and soil microbial biomass under N‐limiting conditions than under high N availability. Elevated CO₂ also caused a significant (P < 0.001) increase in the N recovered by the plant from both the labelled (N_f) and unlabelled (N_s + N_uf) N pools. The dynamics of N in the system as affected by elevated CO₂ were driven principally by mineralization–immobilization turnover, with little loss by denitrification. Under N‐limiting conditions, there is evidence to suggest enhanced nutrient release from soil organic matter (SOM) pools—a process which could be defined as priming. The results of our experiment did not indicate a direct plant‐mediated effect of elevated CO₂ on nitrous oxide (N₂O) fluxes or denitrification activity.     

The fate of spring applied fertilizer N during the autumn-winter period: comparison between winter-fallow and green manure cropped soil

A field experiment was carried out at a pilot plot that was cropped with oilseed rape, and then left partly fallow and partly cropped with a green manure (mustard) during the autumn after harvest of the oilseed rape. The rape residues were incorporated in the soil. Methods used to quantify the N fluxes from harvest until sowing of the next crop were (1) ¹⁵N balance method, (2) total mineral N analysis and (3) NO emission measurements. Losses of spring applied fertilizer N were negligible in cropped plots and minimal in fallow plots during the following autumn-winter period. Most of the plant-N residues was retained by the organic N pool of the upper 30-cm soil layer. The green manure contributed slightly to soil available N at sowing of the next crop. However, the incorporation of plant material resulted in a nitrate flux that was at risk of leaching on the fallow plots, and on the green manure plots after incorporation of the green manure. This nitrate was largely derived from soil organic N, not from unused fertilizer applied in spring or from immobilized fertilizer. The NO emissions from the green manure plots were significantly higher than emissions from the fallow plots. The plants had a stimulating effect on the NO emission. A relationship between the NO emission and the soil nitrate concentration could not be established. No emissions were measured after green manure incorporation due to the low temperatures at the pilot plot. However, a greenhouse experiment showed an increased emission after incorporation. The NO emissions seemed to be related with the soil ammonium concentration.     

硫肥对春油菜幼苗生理指标和土壤酶活性的影响

该研究以春油菜幼苗为材料,采用土壤盆栽试验,设7个不同施硫(0、35、70、105、140、175、210mg·kg^-1)处理,通过测定春油菜幼苗的株高、植株鲜重、叶绿素含量、MDA含量、SOD、POD、CAT活性、土壤全氮含量、pH、蔗糖酶、过氧化氢酶和脲酶活性指标,分析不同施硫量对春油菜幼苗生理生化指标和土壤相关酶活性的影响。结果表明:在春油菜苗期施用硫肥对幼苗的农艺性状、生理生化指标和土壤酶活性均产生了一定影响。施硫量在35~105mg·kg^-1范围时,对植株鲜重有明显的促进作用;施硫量在70~105mg·kg^-1范围时,类胡萝卜素含量达到最高;施硫量在70~105mg·kg^-1范围时,叶片中POD和CAT的活性明显升高,而MDA含量明显下降;经相关分析,MDA含量与POD活性呈极显著负相关(r=-0.92,P<0.01),与CAT活性呈显著负相关(r=-0.72,P<0.05),说明叶片MDA含量受POD和CAT活性变化的影响;施硫量高于105mg·kg^-1时,土壤脲酶和蔗糖酶活性受到抑制;施硫量高于140mg·kg^-1时,土壤过氧化氢酶活性受到抑制;随着施硫量的增加,土壤pH值和叶片SOD活性逐渐下降;经相关性分析,土壤脲酶活性和全氮含量间呈极显著正相关(r=1,P<0.01),表明土壤全氮含量受土壤脲酶活性变化的影响。由此可知,在低硫(35~105mg·kg^-1)条件下对春油菜幼苗生理生化指标及土壤酶活性具有一定的促进作用,而在高硫(>105mg·kg^-1)条件下则产生抑制。     

Variation in acquisition of soil phosphorus among wheat and barley genotypes

To assess the extent of variation in phosphorus acquisition efficiency of some winter wheat (Triticum aestivum L.), winter and spring barley (Hordeum vulgare L.) genotypes, depletion of inorganic phosphorus (P) extractable with 0.5 M NaHCO₃ (NaHCO₃-P_i) from the rhizosphere soil was studied. Nutrients supply, rhizosphere soil pH and soil water content was kept equal for all the genotypes with the aim to reduce the confounding variation due to these factors. The experimental set up implied that no difference in the relative growth rates, nitrogen, potassium and calcium content of shoot dry matter occurred among the genotypes.The winter wheat, winter barley and spring barley genotypes differed significantly (p>0.05) in their efficiency to acquire NaHCO₃-P_i from the rhizosphere soil. The efficiency of the winter wheat genotypes to acquire NaHCO₃-P_i from rhizosphere soil ranked Kraka > Gawain > Foreman > Sleipner = Obelisk > Kosack > Pepital > Arum. Winter wheat genotypes differed in extent of P depletion profiles in the rhizosphere, indicating variation in root hair length. The winter barley and spring barley genotypes also showed significant differences in their P depletion profiles near roots. The efficiency of the winter barley genotypes to acquire soil P in the rhizosphere ranked Hamu > Frost > Marinka > Astrid > Clarine = Angora. The efficiency of spring barley genotypes to acquire NaHCO₃-P_i in the rhizosphere ranked Canut > Etna Riga > Digger > Peel > Semal > Alexis. The rhizosphere pH remained unchanged, suggesting that additional mechanisms such as root hair formation and root exudates play a significant role in causing variation in P acquisition among the genotypes.     

The effect of cropping and fertiliser nitrogen rates on nitrogen balance in soil

Summary Fertilizer/soil N balance of cropped and fallow soil has been studied in a pot experiment carried out with grey forest soil (southern part of Moscow region) at increasing rates of¹⁵N labelled ammonium sulfate (0; 8; 16; 32 mg N/100 g of soil). The fertilizer¹⁵N balance has been shown to depend upon its application rate and the presence of growing plants. Fertilizer N uptake efficiency was maximum (72.5%) and gaseous losses-minimum (12.5%) at the application rate of 16 mg N/100 g of soil. Fertilizer N losses from the fallow soil were 130–220% versus those from the cropped soil. At the application of fertilizer N the plant uptake of soil N was 170–240% and the amount of soil N as N–NH₄ exchangeable + N–NO₃ in fallow was 350–440% as compared to the control treatment without nitrogen (PK).After cropping without or with N fertilizer application at the rates of 8 and 32 mg N/100 g of soil, a positive nitrogen balance has been found which is likely due to nonsymbiotic (associative) N-fixation. It has been shown that biologically fixed nitrogen contributes to plant nutrition.     

Effect of inoculation ofAzospirillum lipoferum on nitrogen fixation in rhizosphere soil,their association with root,yield and nitrogen uptake by mustard (Brassica juncea)

Summary A microplot field experiment was conducted in the presence or absence of P and N application to evaluate the influence of the seed inoculation of mustard (cv. Baruna T₅₉) withAzospirillum lipoferum on N₂-fixation in rhizosphere, association of the bacteria with the roots and grain yield and N uptake. Inoculation significantly increased the N content in rhizosphere soil particularly at early stage (40 days) of plant growth, which was accompanied by the increased association of the bacteria (A. lipoferum) in rhizosphere soil, root surface washing and surface-sterilized macerated root. A significant increase in grain yield and N uptake was also observed due to inoculation. Application of P particularly at the 20 kg. ha^–1 level further enhanced the beneficial effect ofAzospirillum lipoferum inoculation, while N addition markedly reduced such an effect.     

Acclimatization of K+ uptake to changes in root temperature: experiments with oilseed rape and barley in flowing solution culture

Abstract Changes in the net uptake rate of K⁺ and in the average tissue concentration of K⁺ were measured over 14 d in response to changes in root temperature with oilseed rape (Brassica napus L. cv. Bien venu) and barley (Hordeum vulgare L. cv. Atem). Plants were grown in flowing nutrient solutions containing 2.5 mmol m^?3 K⁺ and were acclimatized over 49 d (rape) or 28 d (barley) to low root temperature (5°C) prior to steady–state treatments at root temperatures between 3 °C and 25 °C, with common air temperature. Uptake of K⁺ was monitored continuously over 14 d and nitrogen was supplied as NH₄⁺+ NO^?₃ or NH⁺₄ or NO^?₃. Unit absorption rates of K⁺ increased with time and with root temperature up to Day 4 or 5 following the change in root temperature. Thereafter they usually approached steady-state, with Q₁₀? 2.0 between 7 °C and 17°C, although rates became similar between 7 °C and 13°C. Uptake of K⁺ by rape plants was invariably greater under NO^?₃ nutrition compared with NH⁺₄. The percentage K⁺ in the plant dry matter increased with temperature from 2% at 3 °C to 4% at 25 °C in rape, but there was less effect of temperature on the average concentrations of K⁺ in the plant fresh weight or plant water content. Concentrations of K⁺ in the leaf water fraction of rape plants decreased with increasing root temperature, but in barley they increased with increasing root temperature. Concentrations of K⁺ in the root water fraction were relatively stable with respect to root temperature. The results are discussed in terms of compensatory changes in K⁺ uptake following a change in root temperature and the relationships between growth, shoot: root ratio and K⁺ composition of the plant.     

Plant uptake of sulphur as related to changes in the HI-reducible and total sulphur fractions in soil

Although considerable progress has been made in relating extractable soil S to plant S availability, most of these studies determined the extractable soil S at the beginning of the experiment to use as an index of soil S status. This bears little or no relationship to the S taken up by plants during the entire growing season. The present study investigates the changes in extractable soil S with time and relates these to changes in plant S uptake. Six soils with different long-term fertiliser histories (0, 21, 42 kg of S as superphosphate ha^–1 applied since 1952) and animal camping treatments (camp and non-camp) were used in two pot systems (with and without plants). Carrier-free ³⁵SO₄–S was added to the soils, to provide the information on the transformations of recently added S between the different extractable S forms in soils and whether these transformations could predict plant-available S. The soils were pre-conditioned and then transferred to the glasshouse, where one set of pots were planted with perennial ryegrass (Lolium perenne L.) while the other set was left uncropped. Periodic plant harvests and soil samplings at four weekly intervals were conducted over a period of 20 weeks to determine plant S uptake and amounts of extractable soil S and ³⁵S forms using five extractants. Same extractions of soil S and ³⁵S were conducted for the initial soils. Results showed that HI-reducible and total soil S extracted by CaCl₂, KH₂PO₄ and by KCl at 40°C were utilised significantly by plants but not those extracted by NaHCO₃ and NaOH extractants. However, after the 8th week, plants continued to take up S even though levels of S extracted from the soil by CaCl₂, KH₂PO₄ and by KCl at 40°C remained low and unchanged. These results suggest that soil S taken up by plants after the 8th week period originated directly from the mineralisation of soil organic S from S pools other than those present in the extractable soil S forms. Similar results were shown by ³⁵S data, thereby confirming the complexity of determining plant S availability based on soil S extraction methods.     

Enhanced mineralization and denitrification as a result of heterogeneous distribution of clover residues in soil

Batch studies were conducted with Mn oxides (birnessite-hausmannite mixture, BHM) and samples of four soil series from the Mid-Atlantic region of the USA to determine effects of reducing organic acids, similar to those found in the rhizosphere, on the SeO₃/SeO₄ distribution. Jackland (Typic Hapludalf), Myersville (Ultic Hapludalf), Christiana (Aeric Paleaquult), and Evesboro (Typic Quartizipsamment) A and B horizon soil samples with and without prior Mn oxide reduction were incubated aerobically for 10 d with 0.1 mmol kg^-1 SeO₃ and 0 or 25 mmol kg^-1 of ascorbic acid, gallic acid, oxalic acid, or citric acid. Selenite was also added to BHM (10 mmol kg^-1) with 0 or 0.1 mmol kg^-1 ascorbic acid. The availability of Se for plant uptake as a result of root-soil interactions was examined using growth chamber studies with barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) seedlings grown in 150-mL cone-shaped containers to maximize root-soil surface interactions and to create rhizosphere soil throughout the root zone. In the BHM system ascorbic acid increased oxidation of SeO₃ to SeO₄ to 33% of added SeO₃. In the presence of ascorbic and gallic acids and Mn oxides, oxidation of SeO₃ to SeO₄ occurred in the B horizons of all the soils and in the A horizons of Jackland and Myersville soils. Removal of Mn oxides decreased the oxidation in some samples. Wheat and barley plants were able to accumulate up to 20 mol Se kg^-1 from the Jackland soil when soluble Se was not measurable. The root-soil interactions in the Jackland soil with barley and wheat provided the plant with Se from insoluble sources. The results also indicate that Mn oxides coming in contact with reducing root exudates have a greater ability to oxidize SeO₃ to SeO₄. Thus, rhizosphere processes play an important role in the availability of Se for plant uptake.Maryland Agricultural Experiment Station Scientific Article A 6381.Maryland Agricultural Experiment Station Scientific Article A 6381.     

Plant induced alteration in the rhizosphere and the utilisation of soil heterogeneity

Plant-soil interactions result in a special rhizosphere soil chemistry, differing from that of the bulk soil found only a few mm from the root. The aim of this study was to investigate adaptation mechanisms of herbs growing in acid soils through studying their rhizosphere chemistry in a greenhouse experiment and in a field study. Ten herbs were grown in acid soil (pH 4.2 in the soil solution) in the greenhouse. The concentrations of NO₃ ^-, SO₄ ^2-, phosphates, Ca²⁺, Mg²⁺, Mn²⁺, K⁺, Na⁺, NH₄ ⁺ and pH were analysed in soil solutions obtained by centrifugation. The general pattern found was a depletion of nutrients in the rhizosphere compared with their concentrations in the bulk soil. The pH increase (up to 0.7 units) in the rhizosphere soil appeared to be caused by plant uptake of NO₃ ^- (r²=0.88). The ion concentrations in the soil solution of the rhizosphere were dependent on plant species and biomass increase. Although species with a larger biomass and higher growth rates showed a higher degree of ion depletion (except for Na⁺, SO₄ ^2-) in the rhizosphere, there were also species specific responses. A field study of five herbs at five oak forest sites in Southern Sweden (Scania) was also carried out. In addition to the soil solution concentrations, the loss on ignition (LOI) and the concentrations of 0.1 M BaCl₂ extractable K⁺, Mg²⁺, Mn²⁺, Ca²⁺, and Al ions were measured. The amount of soil solution Al was determined as free ionic (quickly reacting) Al. For all species and sites, the LOI and the concentrations of exchangeable cations were higher in the rhizosphere than in the bulk soil, apparently due to the roots preferably growing at organic-rich microsites. The concentrations of the ions as measured in the centrifuged soil solution, were either higher in the rhizosphere than in the bulk soil or were the same in both, except for NO₃ ^- and quickly reacting Al. The lower concentrations of quickly reacting Al in the rhizosphere, compared with the bulk soil could indicate the uptake of Al by the plant or the exudation of complexing substances. The pH differences were only small and mostly non-significant. Plant-soil interactions and the ability of plants to utilise heterogeneity of the soil appear to be more important for plant growth in acid soils than recognised heretofore. Rhizosphere studies provide an important means of understanding plant strategies in acid soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Field calibration of liming responses of four crops using soil pH,Al and Mn

Liming trials were conducted at 28 sites in the western Great Plains of Canada for barley, rape, red clover and alfalfa. Yield increases from liming correlated with soil pH and Al but not with Mn. When all sites were included, yield increases from liming correlated closely (r=0.86 to 0.94) with exchangeable Al, percent Al saturation and 0.02M CaCl₂-Al for barley, rape and red clover, these responses having correlated less well (R=0.56 to 0.72) with soil pH. Alfalfa yield responses gave low correlations with both pH and the Al measurements. When only the sites with soil pH≥5 were used, the yield responses to lime of barley and rape still correlated better with the Al measurements than with pH even though the correlations, in general, were much lower than when all sites were included. For the sites with soil pH>-5, the correlations were highest for yield responses of barley and rape with 0.02M CaCl₂-Al. It is suggested that the use of toxic Al and Mn for routinely diagnosing the limiting factor by soil acidity could improve on the economy of liming. Contribution Number 653.     

Urea hydrolysis and transformations in coastal dune sands and soil

Summary Urea hydrolysis was studied in samples taken from a coastal sand dune succession, from uncolonized sand; the rhizosphere ofAmmophila arenaria and soil from the mature dune. Comparisons were made with urea hydrolysis in a fertile loam soil. Urea was hydrolyzed in all sand and soil samples, with complete hydrolysis occurring after 6 and 3 weeks in the rhizosphere sand and dune soil compared with only 4 days in the fertile loam. A third of the added urea, however, was still present in the uncolonized sand samples 6 weeks after the beginning of the incubation period. Urea hydrolysis broadly correlated with urease activity.The liberated NH ₄ ⁺ was oxidized to NO ₃ ^– –N in all samples. Urea stimulated the release of N from native organic matter in the two soils, but not sands, due presumably to the low organic matter content of the latter. Nitrite accumulated in the dune sands and soil, but not in the fertile loam.Although N-Serve (Nitrapyrin) had no effect on urea hydrolysis in any of the treated samples, it inhibited the nitrification of released NH ₄ ⁺ –N. The relevance of these findings to the use of urea as a fertilizer to improve plant growth and dune stabilization is commented upon.     

Soil nitrogen heterogeneity in a Dehesa ecosystem

The C mineralization and N transformations during the decomposition of sunflower stalks (Helianthus annuus L.) and wheat straw (Triticum aestivum L.) with and without addition of (NH₄)₂SO₄ (27.53 atom% ¹⁵N) were studied in a Vertisol. Soil samples were incubated under aerobic conditions for 224 days at 22 °C. The plant residues were added at a rate of 5.2 g kg^-1 soil. Nitrogen was applied at a rate of 50.7 mg N kg^-1 soil. Carbon dioxide emission and inorganic N content in soil were periodically determined. Gross N immobilization and remineralization were calculated on the basis of the isotopic dilution technique. At the end of the incubation period a ¹⁵N balance was established. Respectively, 68 and 45% of the applied residue-C mineralized from the sunflower stalks and wheat straw after 224 days. Both crop residues caused losses of up to 25% of added ¹⁵N after 224 days of incubation. These ¹⁵N losses were about three times larger than in the control soil, and were probably due to denitrification. The net immobilization of soil derived N following residue incorporation was largest in the case of wheat straw, depleting all soil inorganic N. In the wheat straw treatment with added (NH₄)₂SO₄ soil inorganic N remained available, resulting in an enhanced initial C mineralization and N immobilization compared to the treatment without added N. In the case of the sunflower stalks, the high inorganic N content of the stalks suppressed the effects of N addition on C mineralization and N immobilization/mineralization. Gross N immobilization amounted to 31.9 and 28.2 mg N g^-1 added C after 14 days for wheat straw and sunflower stalks, respectively. At the end of the incubation, about 35% of the newly immobilized N was remineralized in both plant residue treatments. Gross N immobilization plotted against decomposed C suggests that fairly uniform C-N relationships exist during the decomposition of divers C substrates. The results demonstrate that low fertilizer N use efficiencies may be expected in a wheat-sunflower cropping system with incorporation of crop residues, as the fertilizer N applied becomes largely immobilized in the soil organic fraction. This revised version was published online in August 2006 with corrections to the Cover Date.     

Experimental and mathematical simulation of plant growth promoting rhizobacteria and plant interaction under cadmium stress

Bacterial inoculants of the commercially available plant growth promoting rhizobacteria (PGPR) Arthrobacter mysorens 7, Flavobacterium sp. L30, and Klebsiella mobilis CIAM 880 were selected to obtain ecologically safe barley crop production on cadmium (Cd) polluted soils. All the PGPR immobilized 24–68% soluble cadmium from soil suspension. A. mysorens 7 and K. mobilis CIAM 880 were highly resistant to Cd and grew in up to 1 and 3 mmol CdCl₂ on DAS medium respectively. All PGPR were able to fix nitrogen (276–1014 nmol mg^–1 bacterial DW) and to produce indole acetic acid (IAA) (126–330 nmol mg^–1 bacterial DW) or ethylene (4.6–13.5 nmol bacterial DW). All the PGPR actively colonized barley root system and rhizosphere and significantly stimulated root elongation of barley seedlings (up to 25%), growing on soil containing 5 or 15 mg Cd kg^–1 of soil. Created in the simulation mathematical model confirms our hypothesis that PGPR beneficial effect on barley growing under Cd-stress is a complex process. One of mechanisms underlying this effect might be increase of bacterial migration from rhizoplane to rhizosphere, where PGPR bind soluble free Cd ions in biologically unavailable complex forms. Among the studied PGPR K. mobilis CIAM 880 was the most effective inoculant. Inoculation with K. mobilis CIAM 880 of barley plants growing on Cd contaminated soil (5 mg Cd kg^–1 of soil) under field conditions increased by 120% grain yield and 2-fold decreased Cd content in barley grain. The results suggest that the using K. mobilis CIAM 880 is an effective way to increase the plant yield on poor and polluted areas.     

Pyoverdin production by the plant growth beneficial Pseudomonas strain 7NSK2: Ecological significance in soil

The P. aeruginosa-related plant growth beneficial strain 7NSK₂ was able to increase the yield of barley, wheat, 4 varieties of maize, cucumber, spinach and corn salad from 10 to 24%. Siderophore deficient mutants of 7NSK₂, obtained by Tn5-mutagenesis were never able to stimulate plant growth, although their root colonizing capacities were not impaired. The effect of 7NSK₂ inoculation on cucumber dry weight was more pronounced in EDDHA-treated soil (+18%) than in the control soil (+10%). Soil inoculation with 7NSK₂ in a FeEDDHA-treated soil had no effect on plant growth.The 7NSK₂ strain and a sid^- mutant MPFM1 could make use of a variety of Fe(III) siderophore complexes, while the pyoverdin produced by 7NSK₂ was only used by a limited number of other fluorescent Pseudomonas strains. Many more root than soil bacteria were able to use Fe-desferal or Fe-pyoverdin to overcome iron-shortage. It is postulated that pyoverdin production plays an important role in plant growth stimulation, but is not the only factor involved. Pyoverdin production might be very strategic, especially in the rhizosphere, not only to supply the cell with iron, but also to keep other microorganisms, which might be harmful to 7NSK₂ and to the plant, at bay.