Phosphorus composition of upland soils polluted by long-term atmospheric nitrogen deposition

Atmospheric N deposition can enhance biological P limitation in terrestrial ecosystems and increase the importance of organic P to plants and microorganisms. We used NaOH–EDTA extraction and solution ³¹P NMR spectroscopy to determine the P composition of soils in the Upper Teesdale National Nature Reserve, northern England, an upland region influenced by such deposition for at least 150 years. Three characteristic soil types were sampled on three occasions during an annual cycle: blanket peat (318 mg g^–1 total C, 607 g g^–1 total P, pH 3.9); acid organic soil under grassland (354 mg g^–1 total C, 1190 g g^–1 total P, pH 3.7); calcareous soil under grassland (140 mg g^–1 total C, 649 g g^–1 total P, pH 7.3). Between 58 and 99% of the total P in soil and litter layers was extracted by 0.25 M NaOH + 0.05 M EDTA. Extracts of all soils were dominated by organic P, mainly in the form of orthophosphate monoesters (43–69% extracted P). The two acidic soils also contained large proportions of orthophosphate diesters (6–19% extracted P) and phosphonates (7–16% extracted P), suggesting that these compounds become stabilised at low pH. However, a seasonal trend of increasing orthophosphate monoester-to-diester ratios, most evident in the calcareous grassland soil, indicated the preferential degradation of orthophosphate diesters during the growing season. Orthophosphate was the major inorganic P compound (17–34% extracted P), and all soils contained pyrophosphate (1–5% extracted P). However, orthophosphate determined in the NaOH–EDTA extracts by solution ³¹P NMR spectroscopy was substantially greater than that determined by molybdate colourimetry, suggesting that orthophosphate occurred in complexes with humic compounds that were not detected by conventional procedures. Our results suggest that organisms able to use recalcitrant soil organic P may have a competitive advantage in environments under enhanced atmospheric N deposition.     

Soil phosphorus levels needed for equal P uptake from four soils with different water contents at the same water potential

Soil volumetric water contents, , at –33 kPa potential may vary with soil from 0.06 to 0.70. Because P diffusion depends on , most economic P fertilizer rates required for different soils may require adjusting according to their soil-water relationships. The objective of this study was, after experimentally verifying a mechanistic nutrient uptake model on a series of soils varying in at –33 kPa potential, to use the model to predict labile P levels needed for each of these soils to achieve equal P uptake by maize (Zae mays L.) and verify these predictions. Maize was grown in a pot experiment using four soils having of 0.13, 0.20, 0.26, and 0.40 at –33 kPa each at 0, 200, and 400 mg kg^-1 of added P. When root parameters obtained experimentally were used, predicted P uptake with the uptake model agreed with observed P uptake, y=0.99x+9.08 (r²=0.98). When P uptake was plotted vs. soil solution P, C_li, the relation varied with soil. The higher the the lower the C_li needed for equal P uptake. A similar relation was found between P uptake and diffusible soil P, C_si. Differences between the two plots occurred because of differences among soils in buffer power, C_si/C_li. The C_si vs. P added relation was used to calculate differences among soils in the C_si needed to obtain equal P uptake. The C_si values ranged from 1.3 to 4.0 mmol kg^–1. The calculated values were used in a second pot experiments to verify the predictions. No significant difference (=0.05) in P uptake occurred. The results of this research indicate that the mechanistic nutrient uptake model can be used to predict the degree of adjustments in C_si needed to obtain the most economic P fertilizer rates among soils varying in .Journal Paper No. 13072. Purdue Univ. Agric. Exp. Stn., West Lafayette, IN 47907.     

Phosphorus intensity determines short-term P uptake by pigeon pea (<Emphasis Type="Italic">Cajanus cajan</Emphasis> L.) grown in soils with differing P buffering capacity

Phosphorus (P) uptake by plant roots depends on P intensity (I) and P quantity (Q) in the soil. The relative importance of Q and I on P uptake is unknown for soils with large P sorption capacities because of difficulties in determining trace levels of P in the soil solution. We applied a new isotope based method to detect low P concentrations (<20 μg P l⁻¹). The Q factor was determined by assessment of the isotopically exchangeable P in the soil (E-value) and the I factor was determined by measurement of the P concentration in the pore water. A pot trial was set up using four soils with similar labile P quantities but contrasting P buffering capacities. Soils were amended with KH₂PO₄ at various rates and pigeon pea (Cajanus cajan L.) was grown for 25 days. The P intensity ranged between 0.0008 and 50 mg P l⁻¹ and the P quantity ranged between 10 and 500 mg P kg⁻¹. Shoot dry matter (DM) yield and P uptake significantly increased with increasing P application rates in all soils. Shoot DM yield and P uptake, relative to the maximal yield or P uptake, were better correlated with the P concentration in the pore water (R ² = 0.83–0.90) than with the E-value (R ²=0.40–0.53). The observed P uptakes were strongly correlated to values simulated using a mechanistic rhizosphere model (NST 3.0). A sensitivity analysis reveals that the effect of P intensity on the short-term P uptake by pigeon pea exceeded the effect of P quantity both at low and high P levels. However, DM yield and P uptake at a given P intensity consistently increased with increasing P buffering capacity (PBC). The experimental data showed that the intensity yielding 80% of the maximal P uptake was 4 times larger in the soil with the smallest PBC compared to the soil with the largest PBC. This study confirms that short-term P uptake by legumes is principally controlled by the P intensity in the soil, but is to a large extent also affected by the PBC of the soil. Section Editor: N. J. Barrow     

The Diagnosis and Recommendation Integrated System (DRIS) for diagnosing the nutrient status of grassland swards: III Practical applications

Perennial ryegrass is the most important species of forage grass in both continental Europe and the British Isles. An investigation was carried out to see if the DRIS model developed for this species was able to diagnose crop nutrient sufficiency status, at harvest time, using data for herbage samples collected 2 weeks earlier. A re-evaluation of P fertiliser recommendations for silage, based on the ‘Olsen’ soil P-test, was then carried out using DRIS diagnoses of P sufficiency status as the criteria with which to judge if swards had been adequately, under, or over-supplied with fertiliser P. The results confirmed that reliable (DRIS) diagnoses of N, P, K and S sufficiency statuses of silage swards may be made from herbage clippings taken 2 weeks prior to harvest. Current P recommendations for silage swards proved to be excessive for non-basaltic sandy textured soils at first cut, correct for this group of soils at second cut, and more or less correct for non-basaltic clay textured soils at both cuts. For basaltic soils, however, P recommendations at both cuts appeared to be unrelated to plant P status, and it was concluded that the ‘Olsen’ soil P-test had provided an erroneous assessment of plant available P in these exceptionally iron-rich soils. This revised version was published online in June 2006 with corrections to the Cover Date.     

Impact of synthetic sheep urine on N and P in two pastures in the Scottish uplands

Synthetic sheep urine additions (59 g N m⁻²) were made to pastures on two soils, at Fasset, a semi-natural grassland, and Strathfinella, an improved pasture. Urine was applied to microplots in May and the soil, grass and soil solution analyzed 1, 2, 4, 5, 12 and 23 weeks after the addition. At Fasset, the grass was scorched by urine and the standing biomass decreased compared to the control, increasing only after 5 weeks. The absence of scorching at the Strathfinella site was attributed to a greater biomass of root mat to buffer the roots from exposure to NH₃and a greater rainfall immediately following treatment. Scorching reduced the uptake of N and it was not clear if the greater contents of NH₄ ⁺ and the increases in soil pH at Fasset compared with Strathfinella were the causes or symptoms of the scorch effect. Amounts of extractable organic N (DON) were similar in both soils and increased during the first 4 weeks and then decreased. Urine addition both increased and decreased DON at different times, but the overall mean values were unchanged. Urine application changed the distribution of P in the two soils, increasing the soil solution P at Fasset by 80 mg P m⁻² and raising the P content of herbage at Strathfinella by 600 mg P m⁻². In the soil solution, dissolved forms of molybdate reactive P, organic P and condensed P fractions were all increased by the urine addition. After 23 weeks, condensed P made the greatest contribution to soil solution P in both soils indicating that this fraction was the least available for plant uptake. This revised version was published online in June 2006 with corrections to the Cover Date.     

Phosphorus uptake by root systems: mathematical modelling in ecosys

The uptake of P by plant root systems is believed to be controlled by the concentration of soluble orthophosphate at the root surface. If a P transformation model in which this concentration is calculated were coupled to a root and mycorrhizal growth model in which this concentration is used to calculate P uptake, then it should be possible to simulate P uptake under different soil and climate conditions if soil properties relevant to the control of P concentration are known. To test this idea, models for the transformation and transport of inorganic and organic P were coupled to ones for root growth and nutrient uptake as part of the ecosys modelling program. Seasonal estimates of soluble P concentration, root growth and P uptake from the combined models were tested with data measured from barley under fertilized and unfertilized treatments in a long term P fertilizer experiment conducted on two different soils. In both soils the fertilizer treatment increased simulated and measured soluble P concentrations from 0.1-0.2 to 0.2-0.4 g m^-3, annual P uptake from 0.6-0.7 to 1.2-1.4 g m^-2, and annual DM accumulation from 400-500 to 700-800 g m^-2. Increases in soluble P concentrations caused by fertilizer P were reproduced in the model from changes in the balance between the desorption and dissolution of solid P on one hand, and the uptake of P by root and mycorrhizal systems on the other. Increases in P uptake caused by fertilizer P were reproduced in the model from higher solution P concentrations, root uptake kinetics, and from functional equilibria for C and P exchange simulated among mycorrhizal, root and shoot components of the plant. There was a tendency in the model to overestimate P uptake later in the growing season in the unfertilized treatment which could be corrected if parameters for root uptake kinetics were reduced after anthesis. Because the model is constructed independently of data for P uptake, and avoids the use of site-specific parameters, it may provide a means of estimating uptake under different managements and climates from soils of known properties.     

A new method for estimating gross phosphorus mineralization and immobilization rates in soils

Phosphorus availability in soils is controlled by both the sizes of P pools and the transformation rates among these pools. Rates of gross P mineralization and immobilization are poorly known due to the limitations of available analytical techniques. We developed a new method to estimate P transformation rates in three forest soils and one grassland soil representing an Alfisol, an Ultisol, and Andisol, and a Mollisol. Three treatments were applied to each soil in order to separate the processes of mineral P solubilization, organic P mineralization, and solution P immobilization. One set of soils was retained as control, a second set was irradiated with -rays to stop microbial immobilization, and a third was irradiated and then autoclaved, also stop phosphatase activity. All three sets of samples were then incubated with anion exchange resin bags under aerobic conditions. Differences in resin P among the three treatments were used to estimate gross P mineralization and immobilization rates. Autoclaving did not affect resin-extractable P in any of the soils. Radiation did not alter resin-extractable P in the forest soils but increased resin-extractable P in the grassland soil. This increase was corrected in the calculation of potential P transformation rates. Effects of radiation on phosphatase activity varied with soils but was within 30% of the original values. Rates of P gross mineralization and immobilization ranged from 0.6–3.8 and 0–4.3 mg kg-soil^-1 d^-1, respectively, for the four soils. The net rates of solubilization of mineral P in the grassland soil were 7–10 times higher than the rates in forest soils. Mineralization of organic P contributed from 20–60% of total available P in the acid forest soils compared with 6% in the grassland soil, suggesting that the P mineralization processes are more important in controlling P availability in these forest ecosystems. This new method does not require an assumption of equilibrium among P pools, and is safer and simpler in operation than isotopic techniques.     

Phosphate sorption isotherms for evaluating phosphorus requirement of wetland rice soils

Summary Phosphate sorption isotherms were developed for five Philippine wetland rice soils using the conventional technique and a modified one. In the conventional method, P requirements of soils varied between 280 and 810 g P/g soil. In the modified method, they varied from 160 to 540 g P/g soil at 0.2 ppm P in solution. Soils with high P-sorption capacities had vermiculite and halloysite as the dominant clay minerals. Soil reduction by flooding decreased P-sorption by 28–70 percent at 0.2 ppm P in solution. The decrease in P-sorption due to soil reduction was greatest in a crystalline soil with vermiculite and halloysite as the dominant clay minerals and least in a soil with dominant X-ray amorphous silicates in the clay fraction.Desorption of freshly adsorbed P under reduction was greater in HCO ₃ ^– solution than in CaCl₂ and it increased with level of applied P. Desorption patterns of freshly adsorbed P were similar to adsorption patterns but values of P in solution were lower at desorption. Soils varied with respect to desorption of freshly sorbed P. Desorption studies indicate that soils vary in intensity factor with respect to P and thus influence P availability to plants. Use of P-sorption and P-desorption data obtained under reduced soil condition was proposed for detecting P needs of submerged rice soils.Results of a pot study with IR36 at different levels of solution P (reduced) in one soil indicated a high degree of correlation between adjusted P levels and the measured growth parameters. About 0.12 ppm P in the soil solution or 0.46 ppm P desorbed in HCO ₃ ^– solution (equivalent to 100 mg P/kg soil) was adequate for near-maximum plant height, tiller production, total dry matter yield, plant P content, and total P uptake.     

City refuse compost as a phosphorus source to overcome the P-fixation capacity of sesquioxide-rich soils

In sesquioxide-rich soils of tropical and subtropical areas and volcanic-ash soils with high levels of active Al(Fe), large amounts of phosphate fertilizers are needed to overcome their high P-fixation capacity (quenching strategy). A greenhouse pot experiment has been used to evaluate the effectiveness of city refuse compost (CRC) as a P-source for these variable-charge soils, compared to inorganic P. Mature CRC and K₂HPO₄ were applied at rates equivalent to 125, 250, 375, 500 and 625 kg P ha^–1 to a ferrallitic soils from Tenerife Island (Andeptic Paleudult) with a high content in active Al+Fe (4.82%) and a high P-fixation capacity (87%). Perennial ryegrass (Lolium perenne L.) was grown in pots and plants were harvested at regular intervals after seedling emergence. CRC increases plant P concentration and soil labile-P proportional to the applied rate. The best results were obtained from a compost application of 30 t ha^–1 equivalent-rate, after a residence time of at least three months. An important residual effect in the supply capacity of P in relation to the phosphate fertilizer was also observed. The relative agronomic effectiveness (RAE) in comparison to K₂HPO₄ was 66% after 6 months, considering P uptake + soil labile-P. The soil P-fixation capacity was significantly reduced from a compost application of 40 t ha^–1 equivalent-rate. Competition in adsorption between organic ligands and phosphate, in combination with net mineralization of organic P in compost, might account for the high RAE value obtained. The main conclusion is that the city refuse compost could be a suitable P-amendment for resquioxic soils due to its high RAE, and the residual effect on P-supply. ei]H. Lambers     

Increased P diffusion as an explanation of increased p availability in flooded rice soils

Summary Phosphorus supply factors (capacity, kinetic, intensity, and diffusivity) and plant growth were the approaches used to assess P supply of flooded rice soils. Increases in the capacity, intensity, and kinetic factors, as measured by E-value, solution P concentration, and soil P release rate to a distilled water sink respectively, were unpronouced and infrequent upon water-saturation of ten soils. However, increases in the diffusitivity factor, as measured by ³²P diffusion coefficients, were at least ten-fold as soil moisture increased. The greatest increases in P diffusion occurred as soil moisture increased beyond one-third bar.Using a P fertilized soil or P treated powdered cellulose as the P source and a minus P nutrient solution to nourish a split root system with water and nutrients, data were obtained which suggested that P uptake and rice-shoot growth (indicators of P availability) increased with increasing moisture level. Phosphorus uptake and rice-shoot growth were greatst when the soil or P treated cellulose were water-saturated. These data indicate that increased soil P availability upon flooding can be attributed to an increase in the diffusivity factor.Paper Number 4532 of the Journal Series of the North Carolina Agricultural Experiment Station.Paper Number 4532 of the Journal Series of the North Carolina Agricultural Experiment Station.     

Temporal variations in some plant and soil P pools in two pasture soils of widely different P fertility status

Temporal variations in plant production, plant P and some soil P (and N) pools were followed over 21 months in two New Zealand pasture soils of widely different P fertility status. Plant growth rates, and herbage composition at the high-fertility site, were closely linked to soil water use, with growth rates falling when soil water deficits exceeded 60 mm. Herbage P concentrations reflected P fertility, and varied with season, being generally higher in winter and lower in summer. A similar temporal pattern was also observed for labile organic P (NaHCO₃-extractable P₀) in both soils. In the low-fertility soil in spring, net mineralization was especially strong, but from early winter net immobilization occurred. Surprisingly, Olsen P also changed temporally in the high-fertility soil. The microbial biomass remained fairly constant throughout the year, whereas the P content of the biomass varied seasonally. Although microbial biomass was not a useful index of soil fertility, highest microbial P₀ contents coincided with periods of maximum labile P₀ mineralization, when herbage production was also at a peak. Net N-mineralization in the low-fertility soil, in contrast to the high-fertility soil, was low but varied seasonally, under standardised incubation conditions. Soil P and N dynamics were apparently synchronised in the low-fertility soil through soil microbial processes, with mineral N being negatively correlated with microbial P₀ in samples collected two months later. The results of this investigation suggest that the demands of rapid and sustained pasture growth in spring and early summer can best be met by maximising the build-up of organic matter during the preceding autumn and winter. This practice could help to alleviate the common problem of feed shortage in North Island hill country pastures in late winter-early spring.     

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.     

The effect of soil water status on fertiliser, topsoil and subsoil phosphorus utilisation by wheat

Background and aims

Crop phosphorus (P) content is controlled by P uptake from both banded P fertiliser and from P throughout the soil profile. These P supply factors are in turn controlled by soil, climatic and plant factors. The aim of this experiment was to measure the contribution of fertiliser, topsoil and subsoil P to wheat plants under wet and dry growing season conditions.

Methods

An isotopic tracer technique was used to measure P uptake from fertiliser at seven agricultural field sites under wet and dry growing season conditions. At three of these sites a dual isotopic technique was used to distinguish between wheat uptake of P from fertiliser, topsoil (0–15 cm) and subsoil (below 15 cm).

Results

The amount of P fertiliser used by wheat was in the order of 3–30% of the P applied and increased with increasing rainfall. Topsoil P was the most important P source, but when sufficient P was present in the subsoil, P fertiliser addition stimulated the use of subsoil P.

Conclusions

Most crop P uptake was from the topsoil, however P fertiliser banded below the seed increased plant P uptake and stimulated the use of subsoil P in one soil type in a decile 7 (above average rainfall) growing season.     

Growth and nutrient dynamics of beech (Fagus sylvatica L.) seedlings in acid soils

The relative uptake rates of N, P, K, S, Ca, Mg, Fe, Mn, Zn, Cu, and Al were estimated in beech seedlings pot cultured in the field in six acid soils (treatments). The relative uptake rates were compared with the relative growth rates. The relative uptake rates of N, K and Ca agreed well with the growth rates of the seedlings irrespective of widely differing soil conditions (acid sand-clayey till, pH 4–6). The relative uptake rates of P, Fe, and Al differed from that predicted by the growth rate. The uptake rates of Fe and Al were highest at the lowest growth rates, and the P uptake rate was lower than the growth rate in these treatments. Thus the P availability probably limited growth in an eluvial (E) horizon of a podzol, and possibly in the illuvial (B) horizon of a podzol and in an acid clayey till (Dystric Cambisol). Low P uptake was associated with a tendency towards higher relative root growth rates. In terms of the concept of steady state nutrition the high relative root growth rate in some treatments may be interpreted as an acclimation to low P supply. The P limitation seemed to be related to interactions among Fe, Al and organic compounds of the soil solution.FAX no: +4646104423     

Effect of salinity on mycorrhizal onion and tomato in soil with and without additional phosphate

Summary Vesicular-arbuscular mycorrhizal fungi (VAM) are known to increase plant growth in saline soils. Previous studies, however, have not distinguished whether this growth response is due to enhanced P uptake or a direct mechanism of increased plant salt tolerance by VAM. In a glasshouse experiment onions (Allium cepa L.) were grown in sterilized, low-P sandy loam soil amended with 0, 0.8, 1.6 mmol P kg^–1 soil with and without mycorrhizal inoculum. Pots were irrigated with saline waters having conductivities of 1.0, 2.8, 4.3, and 5.9 dS m^–1. Onion colonized withGlomus deserticola (Trappe, Bloss, and Menge) increased growth from 394% to 100% over non-inoculated control plants when soil P was low ( 0.2 mmol kg^–1 NaHCO₃-extractable P) at soil saturation extract salinities from 1.1 dS m^–1 to 8.8 dS m^–1. When 0.8 and 1.6 mM P was added no dry weight differences due to VAM were observed, however, K and P concentrations were higher in VAM plants in saline treatments.Glomus fasciculatum (Gerdeman and Trappe) andGlomus mosseae (Nicol. and Gerd.) isolates increased growth of VAM tomato 44% to 193% in non-sterilized, saline soil (10 dS m^–1 saturation extract) despite having little effect on growth in less saline conditions when soil P was low. Higher tomato water potentials, along with improved K nutrition by VAM in onion, indicate mechanisms other than increased P nutrition may be important for VAM plants growing under saline stress. These effects appear to be secondary to the effects of VAM on P uptake.     

Phosphorus availability in chicken manure is lower with increased stockpiling period, despite a larger orthophosphate content

Background and aims

The relative proportions of phosphorus (P) forms present in manure will determine the overall availability of manure P to plants; however, the link between the forms of P in manures and manure P availability is unclear. This study compares the bioavailability and P speciation of three manures of different stockpiling duration: less than 1 month, 6 months and 12 months; manures were collected concurrently from a single poultry farm.

Methods

Bioavailability to wheat in a glasshouse trial was measured using an isotopic dilution method with manure added at an application rate equivalent to 20 kg P ha^?1. Phosphorus speciation was measured by ³¹P nuclear magnetic resonance (NMR) spectroscopic analysis of NaOH-EDTA extracts of the manures.

Results

The addition of all manures significantly increased shoot biomass and P concentration, with the fresh manure having the greatest effect. Addition of the fresh manure resulted in the largest labile P pool, highest manure P uptake and manure P recovery, while the manure stockpiled for 12 months resulted in the lowest manure P uptake and manure P recovery. NMR analysis indicated that there was more monoester organic P, especially phytate, in manure stockpiled for shorter periods, while the proportion of manure P that was orthophosphate increased with stockpiling time.

Conclusions

Together, these results imply that although the proportion of total P in the manures detected as orthophosphate was higher with longer stockpiling, only a fraction of this orthophosphate was plant-available. This suggests the availability of P from orthophosphate in manures decreases with longer stockpiling time in much the same way that P from orthophosphate in mineral fertilizer becomes less available in soil over time.     

Exploring the pattern of phenotypic and genetic polymorphism in the arsenic hyperaccumulator Pteris vittata L. (Chinese brake fern)

Background and aims

The trace element uptake process of plants is a key factor in assessing the risks of trace element build-up in agricultural soils. Scarce information exists on the trace element dynamic uptake of plants grown in the field, especially on those potentially hazardous. In this study, the uptake process of As, Cd, Cu, and Zn in maize plants was quantified and characterized throughout the entire season.

Methods

Along two seasons, the uptake dynamics of field-grown maize plants in absorbing the soil borne trace elements was examined. Biomass production and the concentration of the elements in plant and soil solution samples were determined. A kinetic model was employed to characterize the uptake by plants.

Results

The kinetic parameters of the uptake process, maximum cumulative uptake rate, U _max , time to reach 50 % of U _max , t _U50 , and reciprocal of the uptake rate, b _U when followed throughout the season in terms of the plant’s growing degree days remained constant between seasons and were element specific. In spite of the large amount biomass produced, maize plants extracted minute quantities of Cd and As. Increasing cumulative uptake rates of As, Cd, Cu, and Zn from the soil took place primarily in the early half of the growing season when the biomass accumulation was still less than 50 % of the maximum harvested biomass. The element-specific plant uptake factor (PUF), which denote the partition of trace elements between the soil solution and plant phases, decreased following a first-order kinetics along the growing period, did not show any significant difference between seasons, and, at maturity stage, followed the sequence Cd≥Zn>Cu≥As.

Conclusions

The uptake process of the elements was adequately described by the kinetic model, showing similar patterns but different magnitude and distribution in the plant. The extraction of Cd and As by plants is low in comparison to common inputs through fertilizer applications into maize production systems, indicating potential risk of trace element accumulation in soils. The PUF may be estimated according to the kinetics parameters of the uptake process. On a per-unit-soil solution element basis, Cd and Zn would be more susceptible to the soil-to-plant transfer than As and Cu.     

Phosphorus efficiency and the forms of soil phosphorus utilized by upland rice cultivars

Experimental measurements of phosphorus (P) uptake and the forms of soil P depleted from an Ultisol by 6 upland rice cultivars are reported. In both P-fertilized and-unfertilized soil, the majority of P taken up was solubilized from a 0.1 M NaOH-soluble pool by root-induced changes. The soil pH within 4 mm of the roots was lowered by up to 0.5 units (from 4.6), but this by itself could not account for the P solubilized, and nor could increased phosphatase activity near the roots. The possible role of root-released low molecular weight organic acid anions in P solubilization is discussed. No significant differences in the extent of solubilization by a given root mass could be detected between cultivars. In P-unfertilized soil, but not in P-fertilized soil, there were significant differences between cultivars in internal P efficiency as measured by shoot dry weight per unit total plant P. In unfertilized soil, root growth and P uptake were strongly correlated with the P content of the seeds from which the plants were grown.     

Nutrient mobilization and processing in Sonoran desert riparian soils following artificial re-wetting

Research in river-floodplain systems has emphasized the importance of nutrient delivery by floodwaters, but the mechanisms by which floods make nutrients available are rarely evaluated. Using a laboratory re-wetting experiment, we evaluated the alternative hypotheses that increased nutrient concentrations in riparian groundwater during flash floods are due to (H1) elevated nutrient concentrations in surface floodwaters entering the riparian zone or (H2) re-mobilization of nutrients from riparian soils. We sampled soils from the riparian zone of a 400m reach of Sycamore Creek, AZ. Two sub-samples from each soil were re-wetted with a solution that mimicked the chemistry of floodwaters, with one sub-sample simultaneously treated with a biocide. We also measured structural characteristics of soils (texture, organic matter, moisture, and extractable nutrients) to investigate relationships between these characteristics and response to re-wetting. Riparian soils exhibited considerable variation in physical and chemical structure. Soil organic matter, moisture, and texture co-varied among samples. Re-wetting increased concentrations of nitrate and ammonium, and decreased SRP, relative to initial concentrations. Live soils were significantly lower in NO3-and SRP than biocide-treated samples. Extractable DIN pools were the best predictors of mobilization, and soil organic matter was strongly correlated with nitrate losses, probably via its relationship with microbial uptake. Nutrient mobilization and processing also varied considerably with depth, lateral position, and among plots. We estimate that 70–80% of N in riparian groundwater during flash floods is re-mobilized from riparian soils, but are unable to reject the hypothesis that flood inputs may be important sources of nutrients to riparian soils over longer time scales.     

The influence of some cattle and pig slurries on the uptake of nitrogen by ryegrass in relation to fractionation of the slurry N

Ryegrass was grown, in pots under controlled-environment conditions, on soil mixed with each of ten slurries, eight from dairy farms and two from pig farms. In addition, ryegrass was grown under the same conditions but with the water-insoluble material separated from each slurry. Incorporation of the whole slurries increased the yield of herbage, the concentration of N in the herbage and N uptake, compared with plants grown on soil alone, the effects being greatest at the first of six successive harvests. In contrast, incorporation of the water-insoluble material of the cattle slurries decreased herbage yield and N uptake, particularly at the first harvest, but the water-insoluble material of the pig slurries produced some increase in herbage yield and N uptake.The results indicate that the water-insoluble material of the cattle slurries immobilized N that would otherwise have been available from the water-soluble fraction of the whole slurries and/or from the soil. The recovery by the ryegrass of the water-soluble N from the whole slurries was closely correlated with the concentration of N in the water-insoluble material (r=0.863^***) and negatively correlated with the CN ratio (r=0.892^***). Correlations between the recovery of the water-soluble N and the concentrations of N in five particle size fractions of the water-insoluble material indicated that the fraction of smallest particle size (<0.2 mm) had the greatest effect.