Rhizosphere carboxylate concentrations of chickpea are affected by genotype and soil type

We investigated whether concentrations of carboxylates in the rhizosphere of chickpea (Cicer arietinum L.) roots were related to soil phosphorus levels. In a field experiment, cultivar Sona was grown at two P levels on eight soil types at three locations. There were large differences in extractable (0.2 mM CaCl₂) rhizosphere carboxylate concentrations amongst the locations. The effect of P fertiliser was variable and carboxylate concentrations depended on soil type. To examine the effect of soil P in more detail, a glasshouse experiment was carried out, in which three cultivars (Heera, Sona and Tyson) were grown at four P levels on one soil type. The biomass of chickpea plants increased with increasing P level of the soil, and the root mass ratio decreased at the highest soil P level. However, rhizosphere concentrations of the carboxylates malonate, malate and citrate did not differ significantly between P treatments. This implied that there was no simple relation between available P and root exudation rates, in contrast to earlier results in studies using hydroponics. Cultivars differed in carboxylate concentration pattern: Sona and Tyson showed a tendency towards increased rhizosphere carboxylate concentrations at the second harvest, whereas the carboxylate concentration of Heera tended to decrease. It is hypothesised that chickpea roots always exude a basal level of carboxylates into the rhizosphere. They only increase carboxylate exudation considerably when the P availability is extremely low, which may occur in soils that strongly bind P.     

Chickpea and white lupin rhizosphere carboxylates vary with soil properties and enhance phosphorus uptake

Chickpea and white lupin roots are able to exude large amounts of carboxylates, but the resulting concentrations in the rhizosphere vary widely. We grew chickpea in pots in eleven different Western Australian soils, all with low phosphorus concentrations. While final plant mass varied more than two-fold and phosphorus content almost five-fold, there were only minor changes in root morphological traits that potentially enhance phosphorus uptake (e.g., the proportion of plant mass allocated to roots, or the length of roots per unit root mass). In contrast, the concentration of carboxylates (mainly malonate, citrate and malate, extracted using a 0.2 mM CaCl₂ solution) varied ten-fold (averaging 2.3 mol g^–1 dry rhizosphere soil, approximately equivalent to a soil solution concentration of 23 mM). Plant phosphorus uptake was positively correlated with the concentration of carboxylates in the rhizosphere, and it was consistently higher in soils with a smaller capacity to sorb phosphorus. Phosphorus content was not correlated with bicarbonate-extractable phosphorus or any other single soil trait. These results suggest that exuded carboxylates increased the availability of phosphorus to the plant, however, the factors that affected root exudation rates are not known. When grown in the same six soils, three commonly used Western Australian chickpea cultivars had very similar rhizosphere carboxylate concentrations (extracted using a 0.2 mM CaCl₂ solution), suggesting that there is little genetic variation for this trait in chickpea. Variation in the concentration of carboxylates in the rhizosphere of white lupin did not parallel that of chickpea across the six soils. However, in both species the proportion of citrate decreased and that of malate increased at lower soil pH. We conclude that patterns of variation in root exudates need to be understood to optimise the use of this trait in enhancing crop phosphorus uptake.     

Distribution of Carboxylates and Acid Phosphatase and Depletion of Different Phosphorus Fractions in the Rhizosphere of a Cereal and Three Grain Legumes

This study investigates the distribution of carboxylates and acid phosphatases as well as the depletion of different phosphorus (P) fractions in the rhizosphere of three legume crop species and a cereal, grown in a soil with two different levels of residual P. White lupin (Lupinus albus L.), field pea (Pisum sativum L.), faba bean (Vicia faba L.) and spring wheat (Triticum aestivum L.) were grown in small sand-filled PVC tubes to create a dense root mat against a 38-μm mesh nylon cloth at the bottom, where it was in contact with the soil of interest contained in another tube. The soil had either not been fertilised (P0) or fertilised with 15 (P15) kg P ha⁻¹ in previous years. The mesh size did not allow roots to grow into the soil, but penetration of root hairs and diffusion of nutrients and root exudates was possible, and a rhizosphere was established. At harvest, thin (1 mm) slices of this rhizosphere soil were cut, down to a 10-mm distance from the mesh surface. The rhizosphere of white lupin, particularly in the P0 treatment, contained citrate, mostly in the first 3 mm, with concentrations decreasing with distance from the root. Acid phosphatase activity was enhanced in the rhizosphere of all species, as compared with bulk soil, up to a distance of 4 mm. Phosphatase activity was highest in the rhizosphere of white lupin, followed by faba bean, field pea and wheat. Both citrate concentrations and phosphatase activities were higher in P0 compared with P15. The depletion of both inorganic (P_i) and organic (P_o) phosphorus fractions was greatest at the root surface, and decreased gradually with distance from the root. The soil P fractions that were most depleted as a result of root activity were the bicarbonate-extractable (0.5 M) and sodium hydroxide-extractable (0.1 M) pools, irrespective of plant species. This study suggests that differences among the studied species in use of different P pools and in the width of the rhizosphere are relatively small.     

Phosphorus acquisition characteristics of cotton (Gossypium hirsutum L.), wheat (Triticum aestivum L.) and white lupin (Lupinus albus L.) under P deficient conditions

A rhizobox experiment was conducted to examine the P acquisition characteristics of cotton (Gossypium hirsutum L.), wheat (Triticum aestivum L.) and white lupin (Lupinus albus L.) under P-deficient conditions. We aimed to identify whether cotton is physiologically efficient at acquiring P through release of protons, phosphatases or carboxylates. Plants were pre-grown in the upper compartment of rhizoboxes filled with a sand and soil mixture to create a dense root mat against a 53 μm polyester mesh. For each species, two P treatments (0 and 20 mg P kg^?1) were applied to the upper compartment in order to create P-deficient and P-sufficient plants. At harvest, the upper compartment with intact plants was used for collection of root exudates while the lower soil compartment was sliced into thin layers (1 mm) parallel to the rhizoplane. Noticeable carboxylates release was only detected for white lupin. All P-deficient plants showed a capacity to acidify their rhizosphere soil to a distance of 3 mm. The activity of acid phosphatase was significantly enhanced in the soil-root interfaces of P-stressed cotton and wheat. Under P-deficient conditions, the P depletion zone of cotton from the lower soil compartment was narrowest (<2 mm) among the species. Phosphorus fractionation of the rhizosphere soil showed that P utilized by cotton mainly come from NaHCO₃–P_i and NaOH–P_o pools while wheat and white lupin markedly depleted NaHCO₃–P_i and HCl–P pools, and the depletion zone extended to 3 mm. Wheat also depleted NaOH–P_o to a significant level irrespective of P supply. The study suggests that acquisition of soil P is enhanced through P mobilization by root exudates for white lupin, and possibly proton release and extensive roots for wheat under P deficiency. In contrast, the P acquisition of cotton was associated with increased activity of phosphatases in rhizosphere soil.     

Sorption of phosphorus by the iron oxide-impregnated filter paper (Pi soil test) embedded in soils

Incubation experiments were carried out to evaluate the feasibility of extracting phosphorus from soil by embedding iron oxide-impregnanted filter paper strips (P_i strips) in soils having a wide range in pH, texture, and extractable-P contents. Under flooded conditions, the amount of P extracted by the P_i strips increased with the period of submergence and embedding time of the P_i strips. Under unsaturated conditions, the P_i strips were found to extract P from soils over a wide range in moisture conditions; however, keeping the soil at moisture level between saturation and field capacity was found to result in maximal sorption of P by the strips. An embedding time of 16 h was found to be adequate.Phosphorus extracted by embedding P_i strips in soil columns for 16 h at field capacity moisture level correlated significantly with P extracted by shaking the soil with 0.01 M CaCl₂ solution and a P_i strip for 16 h in the laboratory (r=0.94^**). The P extracted by embedding P_i strips correlated best with Bray 1 P in acid soils (r=0.97^**) and with Olsen P in alkaline and calcareous soils (r=0.96^**). The results of the studies demonstrate the feasibility of developing a nondestructive method of monitoring changes in plant-available P in situ under field conditions.     

Establishment of phosphate-solubilizing strains of Azotobacter chroococcum in the rhizosphere and their effect on wheat cultivars under green house conditions

A pot experiment was conducted in the green house to investigate the establishment of phosphate solubilizing strains of Azotobacter chroococcum, including soil isolates and their mutants, in the rhizosphere and their effect on growth parameters and root biomass of three genetically divergent wheat cultivars (Triticum aestivum L.). Five fertilizer treatments were performed: Control, 90 kg N ha^—1, 90 kg N + 60 kg P₂O₅ ha^—1, 120 kg N ha^—1 and 120 kg N + 60 kg P₂O₅ ha^—1. Phosphate solubilizing and phytohormone producing parent soil isolates and mutant strains of A. chroococcum were isolated and selected by an enrichment method. In vitro phosphate solubilization and growth hormone production by mutant strains was increased compared with soil isolates. Seed inoculation of wheat varieties with P solubilizing and phytohormone producing A. chroococcum showed better response compared with controls. Mutant strains of A. chroococcum showed higher increase in grain (12.6%) and straw (11.4%) yield over control and their survival (12—14%) in the rhizosphere as compared to their parent soil isolate (P4). Mutant strain M37 performed better in all three varieties in terms of increase in grain yield (14.0%) and root biomass (11.4%) over control.     

秸秆还田及磷细菌对土壤微生态及豆角产量的影响

为研究秸秆还田及外源添加磷细菌对土壤微生态及农作物产量的影响,以玉米秸秆和具有解磷能力的恶臭假单胞菌(Pseudomonas putida)为研究对象,通过在设施大棚中栽培豆角,研究了不同处理对土壤和豆角根际解磷类细菌数量、土壤有效磷含量、解磷能力以及豆角产量的影响。研究结果显示,玉米秸秆(处理1)、玉米秸秆+磷细菌(处理2)、磷细菌(处理3)三个处理的土壤中解磷类细菌数量有较大差异,其中处理2数量最高,比对照区高31.89%,差异显著(P<0.05);三个处理均能明显增加豆角根际解磷类细菌数量,差异达到显著水平(P<0.05),其中处理2最高,比对照高86.30%,差异达到极显著水平(P<0.01);三个处理土壤有效磷含量均明显高于对照,差异显著(P<0.05),其中处理2最高,比对照高9.8%;三个处理土壤解磷能力差异较大,处理1和处理2可明显提高土壤解磷能力,分别比对照高50.2%和65.2%,差异极显著(P<0.01);三个处理对豆角均有增产效果,但差异较大,处理2比处理1、处理3产量增加明显,分别增产6.8%、10.3%,差异达到显著水平(P&l...     

Phosphate solubilization by organic anion excretion from rice (Oryza sativa L.) growing in aerobic soil

A mathematical model of P solubilization by organic anion excretion from roots is described and used to account for P solubilization by rice (Oryza sativa L.) plants growing in aerobic soil. In previous experiments with rice in an aerobic, highly-weathered, P-deficient soil, we found that the plants were able to solubilize P from an alkali-soluble pool and thereby increase their P uptake. The solubilization could not be explained by pH changes nor by the release of phosphatases. In subsequent experiments we found excretion of citrate from rice roots into nutrient solutions, and the synthesis and excretion of citrate tended to increase under P starvation. The model allows for the diffusion of the organic anion away from a root, its decomposition by soil microbes, its reaction with the soil in solubilizing P, and diffusion of the solubilized P back towards the root as well as away from it. We calculated the rate of citrate excretion from rice roots growing in soil based on measured steady-state citrate concentrations in the rhizosphere and calculated rates of decomposition. Calculations using these and other model parameters obtained independently showed that the observed solubilization and increased P uptake by rice growing in soil could be accounted for. A sensitivity analysis of the model is given. This revised version was published online in June 2006 with corrections to the Cover Date.     

A technique for studying rhizosphere processes in tree crops: soil phosphorus depletion around camellia (Camellia japonica L.) roots

Rhizosphere studies on tree crops have been hampered by the lack of a satisfactory method of sampling soils at various distances in the rhizosphere. A modified root study container (RSC) technique developed for annual crops, grasses and legumes was used to study the mechanisms by which camellia plants (Camellia japonica L.) utilise soil P in the glasshouse and field. Plants belonging to the Camellia family (e.g. tea) have the ability to utilise P from relatively unavailable native P sources and for this reason camellia plants were selected for this study.In the glasshouse trial, the RSCs were filled with a Recent soil, treated with P fertilisers; North Carolina phosphate rock (NCPR), diammonium phosphate (DAP), mono calcium phosphate (MCP) and single superphosphate (SSP) at 200 g P g^-1 soil. A planar mat of roots was physically separated by a 24 m polyester mesh and the soil on the other side of this mesh was cut into thin slices parallel to the rhizoplane and analysed for pH, and different forms of P (organic, P_o and inorganic, P_i) to understand P depletion at different distances from camellia roots. In the field trial this technique was modified and used to study the rhizosphere processes in mature camellia trees fertilised with only SSP and NCPR.In both field and glasshouse trials, all P fertilisers increased all the bulk soil P fractions except NaOH-P_o over unfertilised soil with the greatest increases being in the H₂SO₄-P_i fraction in the NCPR treatment and NaOH-P_i in the SSP treatment. Resin-P, NaOH-P_i and H₂SO₄-P_i were significantly lower in the rhizosphere soil compared to the bulk soil whereas NaOH-P_o was higher in the rhizosphere soil than in the bulk soil. Plant and microbial P uptake were thought to be the major causes for the low resin-P rather than P fixation by Fe and Al because the NaOH-P_i fraction which is a measure of Fe-P and Al-P, also decreased in the rhizosphere soil. The rhizo-deposition of NaOH-P_o suggests that labile inorganic P was immobilized by rhizosphere microbes which were believed to have multiplied as a result of carbon exudates from the roots. A marked reduction in pH (about 0.2–0.4 in the glasshouse and 0.2 in the field trial) was observed near the rhizoplane compared to that in the bulk soil in all treatments. The pH near the rhizoplane as well as in the bulk soil was highest for NCPR treated soil. The increase in pH in the NCPR treatment over the control was consistent with the number of protons consumed during the dissolution of NCPR. In both trials, the dissolution of NCPR in the rhizosphere was higher than in the bulk soil due to lower pH and plant uptake of solution P in the rhizosphere. The RSC technique proved to be a viable aid to study the rhizosphere processes in tree crops.     

Fate and effects of phosphorus additions in soils under N2-fixing red alder

Soil phosphorus (P) dynamics are controlled by the interaction of geochemical, biochemical and biological processes. Changes in species composition or management could alter the relative importance of these processes. We examined soil P dynamics in two plantations of N₂-fixing red alder (Alnus rubra) by determining the fate and effects of added fertilizer P. History of the plantations varied such that sites were previously occupied by 60-yr-old stands of alder or non-fixing Douglas-fir (Pseudotsuga menziesii). Without fertilization, the soil with a longer period of alder influence had more organic P (P_o) and less sorbed inorganic P (Hydroxide- and Bicarb-extractable P_i). Fertilization increased soil total P, and 88% of the fertilizer was accounted for in the surface mineral soil (0–15 cm). Sorbed P_i was the major sink for fertilizer P (55–60%), independent of site history. Although P_o was 35–70% of soil P in unfertilized plots, added P did not accumulate as P_o. Neither site history nor P addition influenced phosphatase activity. Fertilization increased decomposition during incubation of the organic horizon, suggesting that late-stage decomposition is P-limited in these N-rich soils. On the time-scale of a few years, geochemical sorption and desorption of inorganic P were the most important processes controlling the distribution of added P. Organic P accumulation is expected to occur over a longer time frame, linked to the production and turnover of organic matter.     

Recovery of soil phosphorus on former bauxite mines through tropical forest restoration

Soil phosphorus (P) is a major driver of forest development and a critically limited nutrient in tropical soils, especially when topsoil is removed by mining. This nutrient can be present in soils in the form of different fractions, which have direct consequences for P availability to plants and, consequently, for restoration success. Therefore, understanding how the stocks of different soil P fractions change over the restoration process can be essential for guiding restoration interventions, monitoring, and adaptive management. Here, we investigated the recovery of soil P fractions by forest restoration interventions on bauxite mine sites in the Brazilian Atlantic Forest. We assessed the concentration of different fractions of soil organic and inorganic P at (1) a bauxite mine prepared for restoration; (2) two former bauxite mines undergoing forest restoration for 6 and 24 years; and (3) an old‐growth forest remnant. Overall, restored areas recovered levels of labile organic P (P_o‐NaHCO₃) at 5–40 cm and of moderately labile organic P (P_o‐NaOH) at different depths, exhibiting concentrations similar to those found in a conserved forest. The use of P‐rich fertilizers and forest topsoil may have greatly contributed to this outcome. Some other fractions, however, recovered only after 24 years of restoration. Other inorganic P fractions did not differ among mined, restored, and conserved sites: nonlabile P_i (residual P and P‐HCl), labile P_i (P_i‐NaHCO₃), and moderately labile P_i (P_i‐NaOH). Forest restoration was able to promote efficient recovery of important soil P fractions, highlighting the value of restoration efforts to mitigate soil degradation by mining.     

Changes in soil phosphorus fractions following positive and negative phosphorus balances for long periods

The effect of phosphorus (P) balance (addition, in both fertilizers and farmyard manure (FYM), minus removal in crops) on eight soil P fractions determined by sequential extraction, was measured on archived soils from various long-term experiments run by Rothamsted Experimental Station in the United Kingdom. It has been established unequivocally that, for all the soils investigated, no one of the eight P fractions was increased or decreased during long periods of P addition or depletion, respectively. However, changes were mainly in the resin (24–30%) and the inorganic (P_i) component of the four fractions extracted sequentially by 0.5 M NaHCO₃, 0.1 M NaOH, 1.0 M NaOH, 0.5 M H₂SO₄ (41–60%). For the sandy loam there were also consistent changes in the organic (P_o) fraction (25%), especially that extracted by bicarbonate, presumably because the soil contained only a little clay and presumably had low sorption capacity. When the soils were cropped without P addition the largest proportional change was in the P extracted by resin, 0.5 M NaHCO₃ and 0.1 M NaOH, suggesting that the P in these fractions is readily available, or has the potential to become available, for crop growth. This was supported by changes in the overall P balance. On the heavier textured soils, 50–80% of the change in total soil P (P_T) was in these fractions; on the sandy soil this increased to more than 90%. The change in the sum of the first five fractions accounted, on average, for 90% of the P balance. However these changes in the P in the plough layer frequently left large amounts of P unaccounted for in some of the excessively P enriched soils. The amount of P_i extracted by resin and bicarbonate (P_i(r+b)) ranged between 14 and 50% of the sum of the P_i fractions. Soils with the lower percentages were those known to be most responsive to P fertilizers. P_i(r+b) accounted for an average of 70% of the P balance (negative) in P depleting soils where crop offtake was not offset or exceeded by annual P additions (positive balance). The ratio between P_i(r+b) and P_i(sum) could be a guide in defining soils deficient in P and those which are excessively enriched.     

Effects of plant species on phosphorus availability in a range of grassland soils

Vegetative conversion from grass to forest may influence soil nutrient dynamics and availability. A short-term (40 weeks) glasshouse experiment was carried out to investigate the impacts of ryegrass (Lolium perenne) and radiata pine (Pinus radiata) on soil phosphorus (P) availability in 15 grassland soils collected across New Zealand using ³³P isotopic exchange kinetics (IEK) and chemical extraction methods. Results from this study showed that radiata pine took up more P (4.5–33.5 mg P pot^–1) than ryegrass (1.1–15.6 mg pot^–1) from the soil except in the Temuka soil in which the level of available P (e.g., E _1min P_i, bicarbonate extractable P_i) was very high. Radiata pine tended to be better able to access different forms of soil P, compared with ryegrass. There were no significant differences in the level of water soluble P (Cp, intensity factor) between soils under ryegrass and radiata pine, but the levels of Cp were generally lower compared with original soils due to plant uptake. The growth of both ryegrass and radiata pine resulted in the redistribution of soil P from the slowly exchangeable P_i pool (E _{> 10m} P_i, reduced by 31.8% on the average) to the rapidly exchangeable P_i (E _1min-1d P_i, E _1d-10m P_i) pools in most soils. The values of R/r ₁ (the capacity factor) were also generally greater in most soils under radiata pine compared with ryegrass. Specific P mineralisation rates were significantly greater for soils under radiata pine (8.4–21.9%) compared with ryegrass (0.5–10.8%), indicating that the growth of radiata pine enhanced mineralisation of soil organic P. This may partly be ascribed to greater root phosphatase activity for radiata pine than for ryegrass. Plant species × soil type interactions for most soil variables measured indicate that the impacts of plant species on soil P dynamics was strongly influenced by soil properties.     

Iron oxide solubilization by organic matter and its effect on iron availability

The solubility of Fe in soils is largely controlled by Fe oxides; ferrihydrite, amorphous ferric hydroxide, and soil-Fe are generally believed to exert the major control. Fe(III) hydrolysis species constitute the major Fe species in solution. Other inorganic Fe complexes are present, but their concentrations are much less than the hydrolysis species. Organic complexes of Fe including those of organic acids like citrate, oxalate, and malate contribute slightly to increased Fe solubility in acid soils, but not in alkaline soils.The most important influence that organic matter has on the solubilization of Fe is through reduction. Respiration of organic matter creates reduction microsites in soil where Fe²⁺ concentrations increase above those of the Fe(III) hydrolysis species. Fluctuating redox conditions in these microsites are conducive to the formation of a mixed valency ferrosic hydroxide. This metastable precipitate maintains an elevated level of soluble inorganic Fe for prolonged periods and increases Fe availability to plants. The release of reducing agents and acids next to roots, as well as the production of siderophores by microorganisms within the rhizosphere, contribute to the solubilization and increased availability of Fe to plants.     

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.     

Effects of liming and mycorrhizal colonization on soil phosphate depletion and phosphate uptake by maize (Zea mays L.) and soybean (Glycine max L.) grown in two tropical acid soils

The effects of liming and inoculation with the arbuscular mycorrhizal fungus, Glomus intraradices Schenck and Smith on the uptake of phosphate (P) by maize (Zea mays L.) and soybean (Glycine max [L.] Merr.) and on depletion of inorganic phosphate fractions in rhizosphere soil (Al-P, Fe-P, and Ca-P) were studied in flat plastic containers using two acid soils, an Oxisol and an Ultisol, from Indonesia. The bulk soil pH was adjusted in both soils to 4.7, 5.6, and 6.4 by liming with different amounts of CaCO₃.In both soils, liming increased shoot dry weight, total root length, and mycorrhizal colonization of roots in the two plant species. Mycorrhizal inoculation significantly increased root dry weight in some cases, but much more markedly increased shoot dry weight and P concentration in shoot and roots, and also the calculated P uptake per unit root length. In the rhizosphere soil of mycorrhizal and non-mycorrhizal plants, the depletion of Al-P, Fe-P, and Ca-P depended in some cases on the soil pH. At all pH levels, the extent of P depletion in the rhizosphere soil was greater in mycorrhizal than in non-mycorrhizal plants. Despite these quantitative differences in exploitation of soil P, mycorrhizal roots used the same inorganic P sources as non-mycorrhizal roots. These results do not suggest that mycorrhizal roots have specific properties for P solubilization. Rather, the efficient P uptake from soil solution by the roots determines the effectiveness of the use of the different soil P sources. The results indicate also that both liming and mycorrhizal colonization are important for enhancing P uptake and plant growth in tropical acid soils.     

Medium pH, carbon and nitrogen concentrations modulate the phosphate solubilization efficiency of Penicillium purpurogenum through organic acid production

Aims: To study phosphate solubilization in Penicillium purpurogenum as function of medium pH, and carbon and nitrogen concentrations. Methods and Results: Tricalcium phosphate (CP) solubilization efficiency of P. purpurogenum was evaluated at acid or alkaline pH using different C and N sources. Glucose‐ and (NH₄)₂SO₄‐based media showed the highest P solubilization values followed by fructose. P. purpurogenum solubilizing ability was higher in cultures grown at pH 6·5 than cultures at pH 8·5. Organic acids were detected in both alkaline and neutral media, but the relative percentages of each organic acid differed. Highest P release coincided with the highest organic acids production peak, especially gluconic acid. When P. purpurogenum grew in alkaline media, the nature and concentration of organic acids changed at different N and C concentrations. A factorial categorical experimental design showed that the highest P‐solubilizing activity, coinciding with the highest organic acid production, corresponded to the highest C concentration and lowest N concentration. Conclusions: The results described in the present study show that medium pH and carbon and nitrogen concentrations modulate the P solubilization efficiency of P. purpurogenum through the production of organic acids and particularly that of gluconic acid. In the P solubilization optimization studies, glucose and (NH₄)₂SO₄ as C and N sources allowed a higher solubilization efficiency at high pH. Significance and Impact of the Study: This organism is a potentially proficient soil inoculant, especially in P‐poor alkaline soils where other P solubilizers fail to release soluble P. Further work is necessary to elucidate whether these results can be extrapolated to natural soil ecosystems, where different pH values are present. Penicillium purpurogenum could be used to develop a bioprocess for the manufacture of phosphatic fertilizer with phosphate calcium minerals.     

Microbial communities in sugarcane field soils with and without a sugarcane cropping history

Microbial communities in rhizosphere soil from sugarcane (Saccharum inter-specific hybrids) and bulk soil were compared at paired field sites with and without a sugarcane cropping history to determine whether monoculture affects soil microbial community composition. Differences were evaluated for culturable microorganisms and functional diversity indicated by community level physiological profiles (CLPP). Qualitative differences in rhizosphere bacterial communities were detected between sites with no sugarcane cropping history (N_site) and sites with a long-term sugarcane cropping history (L_site). More fluorescent pseudomonads were detected in N_site than L_site rhizosphere soil at two of three sites, and Actinobacteria were more numerous in N_site than L_site rhizosphere soil at one site. Fusarial fungi were more numerous in N_site than L_site rhizosphere soils. Bacteria were more numerous in rhizosphere soil compared to bulk soil. Total bacterial, pseudomonad, and Actinobacteria population densities were greater in bulk soil from an N_site compared to an L_site. CLPP distinguished bulk from rhizosphere soil at one of two sites and N_site and L_site rhizosphere soils at two of four sites. Site affected CLPP similarity more than cropping history. The results demonstrated that sugarcane monoculture can affect the composition of the microbial community in field soil. The findings have possible implications for reduced yields associated with sugarcane monoculture.     

Modelling root-induced solubilization of nutrients

Theory for coupled diffusion processes in soil is briefly described and three examples of its application to understand root-induced solubilization of nutrients given. The examples are: (1) solubilization of P through root-induced pH changes in the rhizosphere of rice plants growing in flooded soil; (2) solubilization of P through excretion of organic chelating agents from rice roots growing in aerobic soil; and (3) the effects of root geometry on P solubilization, particularly cylindrical versus planar geometry and the effect of excretion of a solubilizing agent being localized along the root axis. The theory is tested by comparing measured concentration profiles of P near roots with the predictions of the theory made using independently measured parameter values. In the examples given, the agreement between the observed and predicted concentration profiles is very good, indicating that the theory is sound and the processes involved well understood.     

Cellulose amendment promotes P solubilization by Penicillium aculeatum in non-sterilized soil

Successful application of microbial biofertilizers, such as phosphorus (P) solubilizing fungi to agroecosystems, is constrained from the lack of knowledge about their ecology; for example in terms of how they respond to an external input of carbon (C) to get established in the soil. In two soil incubation experiments we examined the performance of the P solubilizing fungus Penicillium aculeatum in non-sterile and semi-sterile (γ-irradiated) soil with different C and P sources. Results from the first experiment with C sources showed that starch and cellulose generally improved P solubilization by P. aculeatum measured as water extractable P (P_wep), though only significantly in non-sterile soil. This coincided with an increased population density of P. aculeatum measured with a hygromycin B resistant strain of this fungus. Soil respiration used to measure soil microbial activity was overall much higher in treatments with C compounds than without C in both non-sterile and semi-sterile soil. However, soil respiration was highest with cellulose in semi-sterile soil, especially in combination with P. aculeatum. Hence, for the second experiment with P sources (tricalcium phosphate (TCP) and sewage sludge ash) cellulose was used as a C source for P. aculeatum growth in all treatments. Main results showed that P. aculeatum in combination with cellulose soil amendment increased soil P_wep independent of soil sterilization and P source treatments. Soil resin P (P_res) and microbial P (P_mic), which represents stocks of potentially plant available P, were also affected from P. aculeatum inoculation. Increased soil P_res from TCP and sewage sludge ash was observed with P. aculeatum independent of soil type. On the other hand soil P_mic was higher after P. aculeatum inoculation only in semi-sterile soil. Population density of P. aculeatum measured with qPCR was maintained or increased in non-sterile and semi-sterile soil, respectively, compared to the original inoculum load of P. aculeatum. In conclusion, our results underline the importance of C source addition for P. aculeatum if used as a biofertilizer. For this, cellulose seems to be a promising option promoting P. aculeatum growth and P solubilization also in non-sterilized soil.