Rhizosphere Phosphatase Activity Research Articles

Soil phosphorus availability mediates facilitation dynamic in maize-grass pea intercropping system

The intercropping-led diversified tillage is crucial for sustainable development in dryland agriculture. However, the plant-plant interaction transition and its effects on soil physiochemical characteristics along soil phosphorus (P) levels are not clear. To address this issue, maize-grass pea intercropping system was used to investigate the variations in plant-plant facilitation along soil P gradients (low- to high-P) and their effects on rhizosphere soil N and P availability and microbial biomass. The data showed that total net effect (NE) was always positive, indicating facilitative effects of intercropping on total community productivity. Specifically, the relative yield (RY) for both species was greater than 0.5 in the P-deficient soils, representing mutually facilitated effects. Yet, under the P-sufficient condition, maize and grass pea acted as facilitated species and facilitator respectively. Rhizosphere phosphatase activity was significantly enhanced in the P-deficient soils, accordingly fostering P mineralization for higher P availability and N utilization. The contents of microbial biomass carbon, nitrogen and phosphorus (i.e., MBC, MBN and MBP) were evidently elevated in intercropping systems than those of monoculture. Also, global meta-analysis verified above phenomenon, showing that high P addition decreased facilitation intensity with type shift, regarding plant productivity, N and P utilization and vice versa. We confirmed a relatively full picture of P-driven facilitation intensity and type shift along stress gradients by field observation and meta-analysis. The findings provided a new insight of biodiversity in soil physiochemical traits and also revealed a critical mechanism affecting resource utilization in the P-limited soils.

New Insight into Phosphorus Release of Rhizosphere Soil in the Water Level Fluctuation Zone

Many phosphorus release experiments have been conducted in the water level fluctuation zone (WLFZ). However, the effects of rhizospheres on the risk of soil phosphorus release remain unclear. In this study, twenty-four soil samples were collected from the WLFZ of the Pengxi River, the largest tributary of the northern Three Gorges Reservoir. Physicochemical properties, phosphorus forms and phosphatase activity in rhizosphere and near-rhizosphere soils were investigated. Both the total phosphorus (TP) and water-extracted organic phosphorus (H2O-Po) contents were significantly higher in rhizosphere soils than in near-rhizosphere soils (p < 0.05). Enrichment rates (ER) of TP and H2O-Po were 42.4% and 180.3%, respectively. Significant increases were also found in activities of acid phosphatase, alkaline phosphatase, and phosphodiesterase in the rhizosphere, and corresponding ERs were 95.3%, 76.1%, and 22.0%, respectively. Environmental factors, such as pH, organic matter, and amorphous manganese, made significant contributions to the variations of phosphorus forms and phosphatase activities. The risk of phosphorus release to water in the rhizosphere soils of the study area increased slightly, according to the water-extracted phosphorus index. This risk may further increase due to flooding, as plant uptake of soil phosphorus would be inhibited while inorganic phosphorus is still generated by phosphatase.

Two-Phase Conceptual Framework of Phosphatase Activity and Phosphorus Bioavailability.

The activity of extracellular phosphatases is a dynamic process controlled by both plant roots and microorganisms, which is responsible for the mineralization of soil phosphorus (P). Plants regulate the availability of soil P through the release of root mucilage and the exudation of low-molecular weight organic acids (LMWOAs). Mucilage increases soil hydraulic conductivity as well as pore connectivity, both of which are associated with increased phosphatase activity. The LMWOAs, in turn, stimulate the mineralization of soil P through their synergistic effects of acidification, chelation, and exchange reactions. This article reviews the catalytic properties of extracellular phosphatases and their interactions with the rhizosphere interfaces. We observed a biphasic effect of root metabolic products on extracellular phosphatases, which notably altered their catalytic mechanism. In accordance with the proposed conceptual framework, soil P is acquired by both plants and microorganisms in a coupled manner that is characterized by the exudation of their metabolic products. Due to inactive or reduced root exudation, plants recycle P through adsorption on the soil matrix, thereby reducing the rhizosphere phosphatase activity. The two-phase conceptual framework might assist in understanding P-acquisition (substrate turnover) and P-restoration (phosphatase adsorption by soil) in various terrestrial ecosystems.

The effect of root hairs on rhizosphere phosphatase activity

Abstract Background: Phosphatases in soil are of great importance for plant P acquisition. It is hypothesized that root hairs increase rhizosphere phosphatase activity as they release enzymes into soil and stimulate microbial activity. Methods: To test the effect of root hairs on soil phosphatase activity, we grew barley (Hordeum vulgare ‘Pallas') wild type and its root‐hairless mutant in rhizoboxes and determined phosphatase activity using soil zymography. Measurements were done at three moisture levels (30, 15, and 5% VWC). Rhizosphere phosphatase activity was estimated for the two genotypes and two locations along the root [root tip region (0–4 cm behind tip) and mature roots (> 7 cm behind tip)]. Results: Rhizosphere phosphatase activity was similar in the two locations along the root (root tip region vs. mature root parts). In contrast, rhizosphere phosphatase extension was two times larger for the root tip region of the wild type than for the mutant at 30% and 15% VWC. However, as phosphatase activities at the root surface of tips and mature root parts were slightly higher for the mutant than for the wild type, average enzyme activities were unaffected by the genotype. Conclusions: We conclude that the mutant seems to compensate for the lack of root hairs by increased phosphatase activity close to the root surface. However, the increased rhizosphere phosphatase extension for the wild type may be equally efficient as it allows P mobilization and uptake from large soil volumes.

Phosphorus deficiency tolerance in Oryza sativa: Root and rhizosphere traits

We tested whether root exudation confers high P acquisition efficiency (PAE) in rice by quantifying 13C allocation and rhizosphere phosphatase activity for three contrasting genotypes. Phosphatase activity per root surface was similar for all genotypes, indicating that it is not conferring PAE in rice. DJ123 allocated more 13C from shoots to roots compared to Nerica4 and Chomrong Dhan in accordance to increased root biomass of this genotype. The allocation of 13C from roots to the soil was similar between genotypes indicating no difference of root exudation. However, DJ123 showed lower PAE compared to the other two genotypes. We conclude that in the tested genotypes, root systems size rather than root exudation confers PAE.

Phosphorus mobilization in the Yeyahu Wetland: Phosphatase enzyme activities and organic phosphorus fractions in the rhizosphere soils

Phosphatase enzymes regulate organic phosphorus (OP) turnover in eutrophic wetland soils, but a clear understanding of their relationships remains elusive. In this study, soil samples were collected from rhizosphere and non-rhizosphere soils of three dominant aquatic plants Phragmites communis, Zizania latifolia, and Scirpus planiculmis in Yeyahu Wetland of Beijing, China to investigate the phosphatase enzyme activities, including phosphomonoesterase, phosphodiesterase and phytase, and the impact on OP turnover dynamics in wetlands. Results demonstrated that soil phosphatase activities in rhizosphere were higher than those in non-rhizosphere for all the three plants. Phosphatase activities were correlated strongly with soil labile organic P, although the relationships showed different trends for the three kinds of phosphatases. Moderate liable organic P had a positive correlation to phosphomonoesterase (r = 0.367, P < 0.01) and phosphodiesterase (r = 0.564, P < 0.01). There was a significant negative correlation between highly resistant organic P and phytase (r = −0.438, P < 0.05), indicating that phytase had obvious effects on the process of degrading highly stable P compounds in soils. These results suggest that phosphatase enzymes regulated OP turnover to increase available P and phytase had a strong impact on wetland P cycle system.

Rhizosphere and bulk soil enzyme activities in a Nothotsuga longibracteata forest in the Tianbaoyan National Nature Reserve, Fujian Province, China

The rhizosphere, distinct from bulk soil, is defined as the volume of soil around living roots and influenced by root activities. We investigated protease, invertase, cellulase, urease, and acid phosphatase activities in rhizosphere and bulk soils of six Nothotsuga longibracteata forest communities within Tianbaoyan National Nature Reserve, including N. longibracteata + either Phyllostachys pubescens, Schima superba, Rhododendron simiarum, Cunninghamia lanceolata, or Cyclobalanopsis glauca, and N. longibracteata pure forest. Rhizosphere soils possessed higher protease, invertase, cellulase, urease, and acid phosphatase activities than bulk soils. The highest invertase, urease, and acid phosphatase activities were observed in rhizosphere samples of N. longibracteata + S. superba. Protease was highest in the N. longibracteata + R. simiarum rhizosphere, while cellulase was highest in the pure N. longibracteata forest rhizosphere. All samples exhibited obvious rhizosphere effects on enzyme activities with a significant linear correlation between acid phosphatase and cellulase activities (p < 0.05) in rhizosphere soils and between protease and acid phosphatase activities (p < 0.05) in bulk soils. A principal component analysis, correlating 13 soil chemical properties indices relevant to enzyme activities, showed that protease, invertase, acid phosphatase, total N, and cellulase were the most important variables impacting rhizosphere soil quality.

Temporal variations in phosphorus fractions and phosphatase activities in rhizosphere and bulk soil during the development of Larix olgensis plantations

AbstractMany studies have well reported nutrient deficiency and soil degradation focusing on soil N in pure plantations. However, few papers focused on soil P deficiency during pure plantation afforestation. We examined the variations in P fractions and phosphatase (P‐ase) activities in the rhizosphere and bulk soils in Larix olgensis plantations of 16, 23, 34, and 49 y. Correlation analysis was applied to identify the relationships between available P and other P fractions and P‐ase activities in the rhizosphere soil at different developmental stages. The stepwise regression models were established to explore their relationships with tree growth. The results showed that the deficiency of available P occurred in the rhizosphere micro‐region relative to the bulk soil in the 34‐ and 49‐y‐old stands, which was related to the lower acid phosphatase activity, the deficient Fe‐bound P, and occluded phosphate, as well as to soil pH in the rhizosphere soil during these periods. In addition, tree growth in the young and near‐mature stands was significantly correlated with soil P fractions, and the main P fractions closely related to tree growth were total P, organic P, available P and inorganic P, followed by Ca‐bound P and Fe‐bound P. The decreases in average DBH and average height growth in the near‐mature stand were related to the deficiency of available P in this period. Our results indicate that the development of Larix olgensis plantations affects the bioavailability of P through dynamic changes of P fractions and P‐ase activities, which provides a theoretical base in future afforestation with Larix olgensis plantations.

Endophytic establishment of the soil isolate Burkholderia sp. CC-Al74 enhances growth and P-utilization rate in maize (Zea mays L.)

Bacteria are common inhabitants of the rhizosphere or as endophytes in internal plant tissues. Among many bacterial genera, Burkholderia is a genus rich in nitrogen-fixing and phosphate-solubilizing strains that have been isolated from various plant systems. The function of phosphate-solubilizing bacteria in agriculture has been well documented, including enhancements in growth, yield and disease-resistance of crops. However, their response when introduced into a plant system in vitro is poorly studied. Inoculation of Burkholderia sp. CC-Al74, a phosphate-solubilizing soil isolate, on maize seeds resulted in the persistent endophytic establishment as determined by PCR detection method using Burkholderia-specific 16S rRNA gene primers. Burkholderia sp. CC-Al74-inoculated maize showed significant increases in plant height, plant biomass and root length, of about 45%, 48% and 86%, respectively, as compared to non-inoculated controls. After endophytic establishment, inoculated maize seedlings showed 2.4-fold increase in P-utilization rate in planta as well as an increased rhizosphere phosphatase activity as compared to the non-inoculated control. Nutrient analysis of inoculated maize showed 150% and 90% higher contents of P and N as compared to the non-inoculated control, respectively. Multilocus sequence typing (MLST) analysis showed that Burkholderia sp. CC-Al74 had a unique Sequence-Type, ST-730, which was novel when compared with existing STs in the MLST database. The phylogeny produced by recA sequence and MLST data revealed that Burkholderia sp. CC-Al74 is a novel member of Burkholderia cepacia complex. In summary, we provide direct evidence, which shows that the soil isolate Burkholderia sp. CC-Al74 could improve P-utilization rate as well as total P and N contents post-endophytic colonization in maize in vitro.

Rock phosphate enriched post-methanation bio-sludge from kitchen waste based biogas plant as P source for mungbean and its effect on rhizosphere phosphatase activity

Post methanation bio-sludge from a biphasic (aerobic and anaerobic) biogas plant, low grade Indian rock phosphate and two phosphate solubilizing fungi viz., Aspergillus niger TMPS1 and Penicillium oxalicum TMPS3 have been tested for the production of phosphate enriched manure (PEM). A laboratory study revealed that one month incubation was long enough to produce good quality organic P fertilizers having higher water soluble P, lower C:P ratio, higher population of phosphate solubilizing microorganisms and higher phosphatase activity in comparison to biogas sludge. The relative agronomic efficiency of these products was evaluated under greenhouse conditions for mungbean using a 32P radiotracer technique. Relative agronomic efficiencies of phosphate enriched manures inoculated with phosphate solubilizing fungi were better than un-inoculated phosphate enriched manure and equal to single super phosphate (SSP). A significant increase in the population of rhizosphere phosphate solubilizing microorganisms and phosphatase activity was observed in phosphate enriched manure treatments.

Changes of Available Phosphorus and phosphatase activity in the rhizosphere of some field and vegetation crops in the fast growth stage

A large proportion of P is found in organic forms. Phosphatase, plays an essential role in the mineralization of organic phosphorus. Agronomy species can affect phosphatase activity in rhizosphere. The aim ofour study was to determine the effects of some agronomy species (Gramineae, Leguminose, Solanaceae, Labiatae, Cruciferae, Umbellifera, Alliaceae) on phosphatase activity in their rhizosphere. The agronomy species were planted in a semiarid soil in the pots under greenhouse condition. Phosphatase activity in the rhizosphere of any species was studied at middle of their growth. Phosphatase activity was greatly enhanced in the rhizosphere of all species. Alkaline phosphatase activity increased 102-325% and acid phosphatase activity increased 205-455% in the rhizosphere soil compared to the non-rhizosphere soil. This suggests that agronomy species actively promote rhizosphere phosphatase activity either directly by secretion or indirectly by stimulation of microbial activity and/or by depletion of Pi. There were significant differences between phosphatase activities in rhizosphere of plant species. The highest and lowest means of alkaline phosphatase activity were found in rhizosphere of Trifolium repens andOcimum basilicum respectively. The highest and lowest means of acid phosphatase activity were found in rhizosphere of Triticum aestium and Cicer arietinum respectively.

Elevated levels of acid and alkaline phosphatase activity in roots and rhizosphere of cowpea (Vigna unguiculata L. Walp.) genotypes grown in mixed culture and at different densities with sorghum (Sorghum bicolor L.)

The aim of this study was to assess P acquisition efficiency in 5 cowpea genotypes in mixed culture and at different plant densities using assays of acid and alkaline phosphatase activity and measurement of P in organs. Five cowpea genotypes (2 improved cvv. ITH98-46 and TVu1509, and 3 farmer-selected varieties, namely Bensogla, Sanzie, and Omondaw) were grown in the field at 2 planting densities (83 333 and 166 666 plants/ha) under monoculture and mixed culture with sorghum during 2005 and 2006. Fresh plant roots and rhizosphere soils were collected during the 2 years of experimentation, and assayed for acid and alkaline phosphatase activity. P concentrations in root tissue and rhizosphere soil were also determined using inductively coupled plasma-mass spectrometry. The data for 2005 and 2006 were similar, and therefore pooled for statistical analysis. Our results showed that raising cowpea density from 83 333 to 166 666 plants/ha significantly increased both acid and alkaline phosphatase activity in the rhizosphere, just as mixed culture (or intercropping) also increased the acid and alkaline phosphatase activity in cowpea rhizosphere soil. High plant density and mixed culture (or intercropping) also raised the acid phosphatase activity in fresh roots of cowpea plants. The increased enzyme activity in roots and rhizosphere soil resulted in significantly improved P nutrition in cowpea, greater plant growth, and higher grain yield in the farmer-selected varieties, especially cv. Sanzie. This suggests that field-grown legumes can be screened for high P acquisition efficiency by assaying for acid and alkaline phosphatase activities.

Alkaline Phosphatase−Polyresorcinol Complex: Characterization and Application to Seed Coating

An alkaline phosphatase (EC 3.1.3.1) from Escherichia coli ATCC27257 was immobilized by copolymerization with resorcinol. The phosphatase-polyresorcinol complex synthesized retained about 74% of the original enzymatic activity. The pH and temperature profile of the immobilized and free enzyme revealed a similar behavior. Kinetic parameters were determined: K(m) and K(i) values were 2.44 and 0.423 mM, respectively, for the phosphatase-polyresorcinol complex and 1.07 and 0.069 mM, respectively, for free phosphatase. The thermal and storage stabilities of the immobilized phosphatase were higher than those of the native one. On addition to soil, free enzyme was completely inactivated in 4 days, whereas the phosphatase-polyresorcinol complex was comparatively stable. Barley seed coated with the immobilized enzyme exhibited higher rhizosphere phosphatase activity. Under pot culture conditions, an increase in the soil inorganic phosphorus was detected when the seed was encapsulated with the phosphatase-polyresorcinol complex, and a positive influence on biomass and inorganic phosphorus concentration of shoot was observed.

Changes in Phosphorus Fractions, pH, and Phosphatase Activity in Rhizosphere of Two Rice Genotypes

A rhizobox experiment with two phosphorus (P) treatments, zero-P (0 mg P kg −1) and plus-P (100 mg P kg −1) as Ca(H2PO4)2·H 2O, was conducted to study the chemical and biochemical properties in the rhizosphere of two rice genotypes (cv. Zhongbu 51 and Pembe) different in P uptake ability and their relationship with the depletion of soil P fractions. Plant P uptake, pH, phosphatase activity, and soil P fractions in the rhizosphere were measured. Both total dry weight and total P uptake of Pembe were significantly ( P< 0.05) higher than those of Zhongbu 51 in the zero-P and plus-P treatments. Significant depletions of resin-Pi, NaHCO3-Pi, NaHCO3-Po, and NaOH-Pi, where Pi stands for inorganic PandPo for organic P, were observed in the rhizosphere of both Zhongbu 51 and Pembe under both P treatments. Pembe showed a greater ability than Zhongbu 51 in depleting resin-Pi, NaHCO3-Pi, NaHCO3-Po, NaOH-Pi, and NaOH- Po in the rhizosphere. HCl-Pi and residual-P were not depleted in the rhizosphere of both genotypes, regardless of P treatments despite significant acidification in the rhizosphere of Pembe under zero-P treatment. Higher acid phosphatase (AcPME) activity and alkaline phosphatase (AlPME) activity were observed in the rhizosphere of both Zhongbu 51 and Pembe compared to the corresponding controls without plant. AcPME activity was negatively ( P< 0.01) correlated to NaHCO3-Po concentration in the rhizosphere of both Zhongbu 51 and Pembe, suggesting that AcPME was associated with the mineralization of soil organic P.

Role of Phosphatases, Iron Reducing, and Solubilizing Activity on the Nutrient Acquisition in Mixed Cropped Peanut and Barley

ABSTRACT The effect of interspecific complementary and competitive root interactions and rhizosphere effects on primarily phosphorus (P) and iron (Fe) but also nitrogen (N), potassium (K), calcium (Ca), zinc (Zn), and manganese (Mn) nutrition between mixed cropped peanut (Arachis hypogaea L.) and barley (Hordeum vulgare L.). In order to provide more physiological evidence on the mechanisms of interspecific facilitation, phosphatase activities in plant and rhizosphere, root ferric reducing capacity (FR), Fe-solubilizing activity (Fe-SA), and rhizosphere pH were determined. The results of the experiment revealed that biomass yield of peanut and barley was decreased by associated plant species as compared to their monoculture. Rhizosphere chemistry was strongly and differentially modified by the roots of peanut and barley and their mixed culture. In the mixed cropping of peanut/barley, intracellular alkaline and acid phosphatases (AlPase and APase), root secreted acid phosphatases (S-APase), acid phosphatases activity in rhizosphere (RS-APase), and bulk soil (BS-APase) were higher than that of monocultured barley. Regardless of plant species and cropping system, the rhizosphere pH was acidified and concomitantly to this available P and Fe concentrations in the rhizosphere were also increased. The secretion Fe-solubilizing activity (Fe-SA) and ferric reducing (FR) capacity of the roots were generally higher in mixed culture relative to that in monoculture treatments which may improve Fe and Zn nutrition of peanut. Furthermore, mixed cropping improved N and K nutrition of peanut plants, while Ca nutrition was negatively affected by mixed cropping.

Depletion of organic phosphorus from Oxisols in relation to phosphatase activities in the rhizosphere

SummaryPhosphorus (P) deficiency is a major limitation to agricultural production in many parts of the world. It is therefore desirable to identify plants with enhanced abilities to utilize P more efficiently. Exudation of phosphatase from roots may improve P availability, yet there is little direct evidence for this. Here we report the dynamics of organic P in the rhizosphere of plants that have enhanced rhizosphere phosphatase activity. Agroforestry species and transgenic subterranean clover (engineered to produce phytase) were compared with crop and wild‐type plant controls, respectively. Depletion of organic P was measured in pools defined by chemical extraction, solution 31P NMR spectroscopy, and microbial immobilization of radio‐isotopic P. Plants that had greater extracellular phosphatase activity depleted more organic P from P‐deficient Oxisols than control plants. Depleted organic P forms were primarily phosphate monoesters. Plants with enhanced extracellular phosphatase activity also had access to a pool of soil P that was less isotopically exchangeable. Transgenic subterranean clover that expresses a microbial phytase gene appeared to have greater access to recently immobilized P, whereas plants expressing endogenous phosphatases utilized the unlabelled portion of soil organic P to a greater extent. Collectively, these results indicate that the enhancement of phosphatase activity in the rhizosphere of plants is implicated in the depletion of organic P forms from soils, most notably orthophosphate monoesters, whilst also suggesting that there is some exclusivity to the pools of organic P utilized by plants and microorganisms.

Root traits, nutrient uptake, multi-location grain yield and benefit–cost ratio of two lentil (Lens culinaris, Medikus.) varieties

Lentil is a protein-rich pulse, grown mainly in developing countries as a rain-fed crop in nutrient-poor soils. Hence, the importance of root traits for efficient capture of soil nutrients and water can be crucial to its economical yield. Little is known about the lentil root system and even less about its relationship to grain yield. We compared the root system of two Bangladeshi lentil varieties, Barimasur-3 (BM-3) and Barimasur-4 (BM-4), in a pot experiment and related it to their multi-location grain yield in the fields. BM-4 maintained faster root development both at an early growth stage (20 days after sowing) and at flowering (60 days) compared to BM-3. The roots of BM-4 penetrated the 25 cm depth of the soil profile after 19 ± 1 days and while those of BM-3 took 24 ± 2 days to reach the same depth. The roots of BM-4 were covered with denser (26 ± 3 mm−1) and longer (0.48 ± 0.11) root hairs than BM-3 (density 17 ± 2 mm−1, length 0.32 ± 0.09 mm). The differential presence of root hairs increased the effective length of root system of BM-4 by 12 times and that of BM-3 by five times. The lentil varieties did not differ in their ability to induce pH change and acid phosphatase activity in rhizosphere. In the pot experiment, the uptake of macro-nutrients (K, P, Ca, and Mg) as well as micro-nutrients (Fe, Mn, Zn, Cu, B and Mo) by BM-4 was significantly higher, compared to BM-3. The varieties produced the same amount of shoot biomass. At five of six agro-ecological distinct field locations in Bangladesh, BM-4 gave significantly higher (10–20%) grain yield than BM-3. Linked with the higher grain yield, the benefit-cost ratio (BCR) of BM-4 was 3.14 and that of BM-3 were 2.62, indicating that BM-4 provided better return per unit investment, compared to BM-3, supported by the better root morphology and higher nutrient uptake. This may be one of the reasons supporting the popularity and preferred adoption of BM-4 among the Bangladeshi farmers, who grow lentil mainly on nutrient-poor soils. The results indicate the benefits of selection and breeding for superior root traits for better agro-economics.

Phosphatase activity and organic acids in the rhizosphere of potential agroforestry species and maize

Previous work has hypothesised that Tithonia diversifolia produces both organic acids and phosphatase. Simultaneous exudation of these compounds could increase both P solubility, by releasing bound Po, and Po mineralisation by increasing the rate of hydrolytic cleavage. The objectives of this work were to identify and quantify the P active root exudates released into the rhizosphere of agroforestry species compared to maize. Agroforestry species and maize were grown in a rhizopot system and thin soil slices were analysed for phosphatase activity using a fluorescent enzyme assay based on methylumbelliferone-phosphate (MUB-phosphate) and organic acid concentration by methylation and GCMS. Tithonia (137 μM) and Tephrosia (108 μM) had higher concentrations of citric acid in the rhizosphere than maize (96 μM). However, subsequent analysis of the citric acid chromatograph peak with a mass spectrum detector showed that peaks were actually attributable to a fatty acid. Therefore, there was no evidence for enhanced organic acid concentration in the rhizosphere. In contrast, phosphatase activity was greatly enhanced in the rhizosphere of all agroforestry species. Tithonia (29.64 nmol MUB min −1 g −1 soil) and Crotalaria (36.08 nmol MUB min −1 g −1 soil) had greater activity at 0–1 mm from their root than did maize (15.75 nmol MUB min −1 g −1 soil). Agroforestry species enhanced the activity of acid phosphatase, while maize enhanced the activity of alkaline phosphatase. Acid phosphatase activity increased exponentially with increasing proximity to the root mat of the agroforestry species. This suggests that agroforestry species are actively increasing rhizosphere phosphatase activity either directly by secretion or indirectly by stimulation of microbial activity and/or depletion of Pi. Overall, agroforestry species enhanced phosphatase activity in their rhizosphere. However, this does not necessarily lead to increased Po mineralisation unless there is a concurrent increase in the solubility of Po forms.

Extracellular Acid Phosphatase Activities in Eriophorum vaginatum Tussocks: A Modeling Synthesis

Analyses of Eriophorum vaginatum tussocks provided mass and kinetic parameters for a Michaelis-Menten model of phosphatase activities in Alaskan tussock tundra. This model was used to simulate the temporal patterns of phosphatase activities, given a 90-d thawing season and organic phosphorus concentrations of 30 ,LM in the first and last 10-d intervals; 15 uM at other times. Results indicated that about 28% of the total annual tussock activity (155 mg P released) occurred during the brief period of high substrate availability in autumn; little occurred in spring because most of the tussock was frozen and live root mass was low. Phosphatases associated with living roots of E. vaginatum were responsible for about 4% of the total activity in tussocks (ca. 6 mg P), which is almost twice the annual plant demand (ca. 3.5 mg). These results suggest that (1) E. vaginatum may obtain much of its phosphorus requirement from the activities of root surface phosphatases, and (2) the timing of maximum plant phosphorus uptake (late in year) and growth (early in year) are asynchronous, i.e., E. vaginatum integrates nutrient availabilities across years. and is regrown each summer. Therefore, living root mass increases throughout the growing season. The total depth of soil thaw also increases during this period, expanding the potential rooting volume and stimulating enzyme activities. In addition, soil solution organic phosphorus concentrations change with time. For these reasons, any assessment of tussock phosphatase activities must account for these dynamically changing factors. The goals of this study were to evaluate the potential contributions of E. vaginatum root surface phosphatases to (1) total rhizosphere phosphatase activity and (2) satisfying plant phosphorus requirement. A combination of experimental and modeling approaches was used to address these objectives. The various components of E. vaginatum tussocks (e.g., live roots, dead roots, soil, etc.) were separated, quantified, and their associated phosphatase activities assayed. This information was then used to develop a model of rhizosphere enzyme activity, including the effects of soil thawing, root growth, and fluctuating organic phosphorus concentrations. Simulations were performed to elucidate the dynamics of tussock phosphatase activities throughout the growing season.

Rhizosphere phosphatase activity and phosphatase hydrolyzable organic phosphorus in two forested spodosols

The cycling of organic phosphorus has a large effect on P availability in forest ecosystems. Organic P must be hydrolyzed to inorganic P before it can be utilized by plants. We examined the acid phosphatase activity in the rhizosphere of slash pine (Pinus elliottii) growing in A and Bh horizon soil from two forested Spodosols; and determined the amount of soluble organic P in these soils that could be hydrolyzed following the addition of an acid phosphomonoesterase. Significant acid phosphatase activity was observed in the A and Bh horizons of the two soils. Acid phosphatase activity was greater in the rhizosphere of the Leon A and Bh horizons and the Pomona Bh horizon soil. Phosphorus fertilization decreased the phosphatase activity in the rhizosphere soil. Soluble organic P comprised 15–25% of the total soluble P in the two A horizon soils and over 75% in the Bh horizon soils. Between 20 and 30% of the water-soluble organic P in the A and Bh horizons of the two soils was hydrolyzed when an acid phosphomonoesterase was added. Our results suggest that organic P may contribute to the P nutrition of slash pine growing on these soils.