Estimating crop N uptake from organic residues using a new approach to the 15N isotope dilution technique

Experiments were conducted to test a new approach to the ¹⁵N isotope dilution technique for estimating crop N uptake from organic inputs. Soils were pre-labelled with ¹⁵N fertiliser and a carbon source. These were then incubated until there was stabilisation of the ¹⁵N abundance of the inorganic N pool and resumption of inorganic N concentrations. Residues were then applied to the soils and planted with ryegrass (Lolium perenneL.) to determine the nitrogen derived from the residue (Ndfr) using the isotope dilution equations. This method was compared with the direct method, i.e. where ¹⁵N-labelled residues were added to the soil and Ndfr in the ryegrass calculated directly. Estimates of percentage nitrogen derived from the residue (%Ndfr) alfalfa (Medicago sativaL.) in the ryegrass, were similar, 22 and 23% for the direct and soil pre-labelling methods, respectively, in the Wechsel sandy loam. Also, estimates of the %Ndfr from soybean (Glycine max (L.) Merr) residues in the Krumbach sandy loam were similar 34% (direct) and 36% (soil pre-labelling approach). However, in the Seibersdorf clay loam, the %Ndfr from soybean was 49% using the direct method and 61% using the soil pre-labelling method; yet Ndfr from common bean residue was 46% using the direct approach and 40% using the pre-labelling, not significantly different (P > 0.05). The soil pre-labelling approach appears to give realistic values for Ndfr. It was not possible to obtain an estimate of Ndfr using the soil pre-labelling method from the maize residues (Zea mays L.) in two of the soils, as there was no increase in the total N of the ryegrass over the growing period. This was probably due to microbial immobilisation of inorganic N, as a result of the wide C:N ratio of the residue added. The results suggest that the new soil pre-labelling method is feasible and that it is a potentially useful technique for measuring N release from a wide range or organic residues, but it requires further field-testing. This revised version was published online in June 2006 with corrections to the Cover Date.     

Phosphorus availability to maize as influenced by organic manures and fertilizer P associated phosphatase activity in soils

Laboratory incubation and green house studies were conducted to compare the P availability of organic manures and P uptake from organic manures by maize. Various organic manures viz. Poultry manure (PM), Farmyard manure (FYM), Green manure (GM) and Crop residue (CR) and graded levels of fertilizer P were applied in Samana sandy loam and Ladhowal silt loam soils and incubated for 7, 15, 30, 60 and 90 days. Samples were analyzed for P availability, P uptake and alkaline phosphatase activity. The overall, phosphatase activity, Paranitrophenyl phosphate (PNP h⁻¹ g⁻¹), in the Ladhowal silt loam soil was higher than in the Samana sandy loam soil. As the level of inorganic P increased, the release of PNP h⁻¹ g⁻¹ soil also increased. Among different organic manures, PM registered the highest enzyme activity followed by FYM, GM and CR. Compared to 7 days incubation a slightly higher increase in PNP was noticed in samples from 90 days incubation in both soils. The differential phosphatase activity in the organic manures was further reflected in dynamic P availability. The highest amount of Olsen extractable P was in PM-treated soil followed by FYM, GM and field pea crop residue. Organic manure addition along with inorganic P, irrespective of the source, increased the Olsen extractable P throughout the incubation period. Total P uptake by maize increased with the increasing level of inorganic P in both soils. The highest uptake was obtained in PM-treated soil and lowest in the CR-amended soil. We conclude that PM more readily supplies P to plants than other organic manure sources.     

Direct evidence on participation of root hairs in phosphorus (32P) uptake from soil

Root hairs substantially extend root surface for ion uptake. Although many reports suggest a relationship between root hairs and phosphorus (P) uptake of plants, the role of root hairs in phosphorus uptake from soils is still debated. We measured uptake of phosphorus from soil directly via root hairs. Root hairs only were allowed to penetrate through a tightly stretched nylon screen (53 µm) glued to the bottom of a PVC tube. The penetrating root hairs grew for 2 and 4 days in soil labelled with radioisotope phosphorus (P) tracer ³²P (185 kBq g^-1 dry soil) filled in another PVC tube. Transparent plastic rings of thickness ranging from 0.25 mm to 2.0 mm were inserted between the two PVC tubes. This provided slit width for microscopic observations in situ, which confirmed that only root hairs were growing into the ³²P labelled soil. In some cases no rings were inserted (slit width = 0) where both root hairs and root surface were in contact with the labelled soil (total ³²P uptake). The uptake of³² P from soil via the root hairs only was quantified by measuring activity of ³²P in the plant shoot (³²P uptake only via root hairs).The results showed that when 70 percent of the root hairs grew into the labelled soil, they contributed to 63 percent of the total P uptake. With decreasing number of root hairs growing into the ³²P labelled soil, the quantity of ³²P in the plant shoot decreased. In this study, P uptake via root hairs was measured in a soil-based system, where root hairs were the only pathway of ³²P from soil to the plant shoot. Therefore, this study provides a strong evidence on the substantial participation of root hairs in uptake of phosphorus from soil.     

Changes and availability of P fractions following 65 years of P application to a calcareous soil in a Mediterranean climate

The fate and availability of P derived from granular fertilisers in an alkaline Calcarosol soil were examined in a 65-year field trial in a semi-arid environment (annual rainfall 325 mm). Sequential P fractionation was conducted in the soils collected from the trial plots receiving 0–12 kg P ha⁻¹crop⁻¹, and the rhizosphere soil after growing wheat (Triticum aestivum L. cv. Yitpi) and chickpea (Cicer arietinum L. cv. Genesis 836) for one or two 60-day cycles in the glasshouse. Increasing long-term P application rate over 65 years significantly increased all inorganic P (Pi) fractions except HCl–Pi. By contrast, P application did not affect or tended to decrease organic P (Po) fractions. Increasing P application also increased Olsen-P and resin-P but decreased the P buffer capacity and sorption maxima. Residual P, Pi and Po fractions accounted for an average of 32, 16 and 52% of total P, respectively. All soil P fractions including residual P in the rhizosphere soil declined following 60-day growth of either wheat or chickpea. The decreases were greater in soils with a history of high P application than low P. An exception was water-extractable Po, which increased following plant growth. Changes in various P fractions in the rhizosphere followed the same pattern for both plant species. Biomass production and P uptake of the plants grown in the glasshouse correlated positively with the residual P and inorganic fractions (except HCl–Pi) but negatively with Po in the H₂O-, NaOH- and H₂SO₄-fractions of the original soils. The results suggest that the long-term application of fertiliser P to the calcareous sandy soil built up residual P and non-labile Pi fractions, but these P fractions are potentially available to crops.     

The determination of labile soil phosphate as influenced by the time of application of labelled phosphate

Summary An experiment was conducted to determine the effect on the L-values of pre-equilibrating P³² with three soil types for 2, 1, and 0 months before sowing ryegrass. Resin and phosphoric acid were used as carriers. Equilibrium was established some twelve weeks after sowing and this time was virtually unaffected by the pre-equilibration treatments. The phosphate source was found to affect both P uptake and L-value; higher uptake and lower L-values were recorded from the resin.     

Phosphate transport by hyphae of field communities of arbuscular mycorrhizal fungi at two levels of P fertilization

This study was conducted to elucidate the effect of P fertilisation on the function of field communities of arbuscular mycorrhizal fungi (AMF) measured as P transport to flax. Two methods were applied to soil from a long-term field experiment with NaHCO₃-extractable soil P levels of 24 and 50 mg kg^-1in an experiment under controlled conditions: i) Measurement of plant growth and P uptake in the presence or absence of the fungicide benomyl and ii) measurement of hyphal P transport from a root-free compartment labelled with ³²P. Benomyl successfully prevented mycorrhizal function. The absolute contribution of AMF to plant P uptake was of the same magnitude with or without P fertilisation at 27 days after sowing. Therefore, even though plants grown at the higher soil P level had greater P uptake, the relative contribution of AMF to P uptake was greater at the lower P level than at the higher P level (77 and 49% of total P uptake, respectively). The AMF in P-fertilized soil transported less P³² from the root-free compartment to the plant after 23 days than the AMF in unfertilized soil, but this difference disappeared in plants harvested after 27 and 32 days. The production of hyphae was largely similar in both fertilization treatments, indicating that the capacity for P uptake and transport by hyphae of the two AMF communities was similar. This revised version was published online in June 2006 with corrections to the Cover Date.     

不同磷浓度植株残体降解对紫色土磷素有效性的影响

植株残体降解可直接或间接地影响土壤磷素的有效性,为探讨不同磷浓度植株残体降解对紫色土磷分级体系的影响,结合31P核磁共振分析技术,选取了3种磷浓度不同的植物残体与两种紫色土进行室内模拟培养试验,得出了以下研究结论:(1)添加植株残体显著增强了紫色土呼吸强度,且紫色土分级体系中的活性磷含量均高于对照处理(2)31P-NMR分析结果得知,植株残体的正磷酸盐、磷酸单酯占浓缩液全磷比例的90%以上,高磷植株的正磷酸盐和磷酸单酯含量显著高于中磷和低磷植株,土壤磷素有效性的变化与植株残体的正磷酸盐和磷酸单酯含量有关;(3)紫色土分级体系中的活性磷在0 d含量最高,随着培养周期的延长,土壤磷素有效性会出现降低的趋势;酸性紫色土的累积呼吸强度、分级体系中活性磷(Resin-P、Na HCO3-Pt)所占比例均高于中性紫色土,与土壤钙含量有关。综上所述,植株残体的磷浓度越高,更有利于提高土壤磷素的有效性,本研究结果为农业生产中秸秆还田技术提供了理论参考。     

Root competition for phosphorus between the tree and herbaceous components of silvopastoral systems in Kerala, India

Root competition in polyculture systems involving combinations of four tree species and four grass species was evaluated based on ³²P recovery by each species in mixed and sole crop situations. The tree species were: Leucaena leucocephala, Casuarina equisetifolia, Acacia auriculiformis and Ailanthus triphysa, and the grass species were: Pennisetum purpureum (hybrid napier), Brachiaria ruziziensis (congo signal), Panicum maximum (guinea grass) and Zea mexicana (teosinte). Four lateral distance (25 and 50 cm) and depth (15 and 50 cm) treatments were included in the study to characterize the relative fine root distribution of trees. Absorption of ³²P was monitored through radioassay of leaves. Regardless of the species, ³²P uptake from 50 cm soil depth was lower than that of 15 cm depth. Absorption of ³²P from 50 cm lateral distance was also less than that of 25 cm distance in Acacia and Casuarina. Grass species in sole crop situations absorbed more ³²P than in mixed systems. None of the grass species when grown in association with tree components affected the absorption of ³²P by trees. All grass species exerted a complementary effect on ³²P absorption by Casuarina. Leucaena also benefited in the same way when grown in association with congo signal and/or teosinte. Of the tree species, Acacia and Leucaena adversely affected the ³²P uptake by grass species.     

Mobilization of non-exchangeable K by ryegrass in five Moroccan soils with and without mica

Intensive cropping of Italian ryegrass (Lolium multiforum L.) in pots was used to assess the contribution of non-exchangeable K to plant uptake. The soils used were: two soils high in mica (illite) developed on recent alluvium plus two smectitic (beidellitic) soils and a soil of mixed mineralogy rich in mica. Four K treatments were used (0, 28.6, 143, and 286 mg kg^-1 soil) with 8 successive monthly cuttings. A response of plant K uptake to added K was observed in all soils. Both 1.0 M NH₄0Ac and 0.2 M CaCl₂ extractable K were depleted to a minimum level specific for each soil. The minima were lower in the old upland soils compared to the young alluvial soils. Uptake of K by Italian ryegrass induced K release from the non-exchangeable K to replenish the plant available pool of K ions. The release of mica interlayer K in the alluvial and in the high K smectitic soil supplied sufficient K to plants even under intensive cropping. The rate of mobilization of interlayer K was low in the smectitic soil with lower K. The lowest release rate was in the old high mica soil. Iron coatings may have inhibited mobilization of interlayer K. The rates of mobilization cannot be predicted from mineralogical and K-extraction data only. The rates of K uptake and the rates of K release by ryegrass under intensive cropping are potential values which can be used for modelling K availability to plants in the soils studied.     

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     

Compared cycling in a soil-plant system of pea and barley residue nitrogen

Field experiments were carried out on a temperate soil to determine the decline rate, the stabilization in soil organic matter and the plant uptake of N from ¹⁵N-labelled crop residues. The fate of N from field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) residues was followed in unplanted and planted plots and related to their chemical composition. In the top 10 cm of unplanted plots, inorganic N was immobilized after barley residue incorporation, whereas the inorganic N pool was increased during the initial 30 days after incorporation (DAI) of pea residues. Initial net mineralization of N was highly correlated to the concentrations of soluble C and N and the lignin: N ratio of residues. The contribution of residue-derived N to the inorganic N pool was at its maximum 30 DAI (10–55%) and declined to on average 5% after 3 years of decomposition.Residual organic labelled N in the top 10 cm soil declined rapidly during the initial 86 DAI for all residue types. Leaching of soluble organic materials may have contributed to this decline. At 216 DAI 72, 59 and 45% of the barley, mature pea and green pea residue N, respectively, were present in organic N-forms in the topsoil. During the 1–3 year period, residual organic labelled N from different residues declined at similar rates, mean decay constant: 0.18 yr^-1. After 3 years, 45% of the barley and on average 32% of the pea residue N were present as soil organic N. The proportion of residue N remaining in the soil after 3 years of decomposition was most strongly correlated with the total and soluble N concentrations in the residue. The ratio (% inorganic N derived from residues): (% organic N derived from residues) was used as a measure of the rate residue N stabilization. From initial values of 3–7 the ratios declined to on average 1.9 and 1.6 after 2 and 3 yrs, respectively, indicating that a major part of the residue N was stabilized after 2 years of decomposition. Even though the largest proportion of residue N stabilized after 3 years was found for barley, the largest amount of residue N stabilized was found with incorporation of pea residues, since much more N was incorporated with these residues.In planted plots and after one year of decomposition, 7% of the pea and 5% of the barley residue N were recovered in perennial ryegrass (Lolium perenne L.) shoots. After 2 years the cumulative recovery of residue N in ryegrass shoots and roots was 14% for pea and 15% for barley residue N. The total uptake of non-labelled soil N after 2 years of growth was similar in the two residue treatments, but the amount of soil N taken up in each growth period varied between the treatments, apparently because the soil N immobilized during initial decomposition of residues was remineralized later in the barley than in the pea residue treatment. Balances were established for the amounts of barley and mature pea residue N remaining in the 0–10 cm soil layer and taken up in ryegrass after 2 years of decomposition. About 24% of the barley and 35% of the pea residue N were unaccounted for. Since these apparent losses are comparable to almost twice the amounts of pea and barley residue N taken up by the perennial ryegrass crop, there seems to be a potential for improved crop residue management in order to conserve nutrients in the soil-plant system.     

Functional diversity in arbuscular mycorrhizas: exploitation of soil patches with different phosphate enrichment differs among fungal species

Most terrestrial plant species form associations with arbuscular mycorrhizal fungi (AMF) that transfer soil P to the plant via their external hyphae. The distribution of nutrients in soils is typically patchy (heterogeneous) but little is known about the ability of AMF to exploit P patches in soil. This was studied by growing symbioses of Linum usitatissimum and three AMF (Glomus intraradices, G. mosseae and Gigaspora margarita) in pots with two side-arms, which were accessible to hyphae, but not to roots. Soil in one side-arm was either unamended (P0) or enriched with P; simultaneous labelling of this soil with ³²P revealed that G. intraradices responded to P enrichment both in terms of hyphal proliferation and P uptake, whereas the other AMF did not. Labelling with ³³P of P0 soil in the other side arm revealed that the increased P uptake by G. intraradices from the P-enriched patch was paralleled by decreased P uptake by other parts of the mycelium. This is the first demonstration of variation in growth and nutrient uptake by an AMF as influenced by a localized P enrichment of the soil. The results are discussed in the context of functional diversity of AMF.     

Effects of residue quality and climate on plant residue decomposition and nutrient release along the transect from humid forest to Sahel of West Africa

Field litterbag studies were conducted in the 2000 rainy season and the 2000/2001 dry season along the transect of West African major agroecological zones (agroeco-zones) to measure the decomposition of, and N and P release from 5 plant residues (leaves of woody species) with increasing quality: Dactyladenia barteri, Pterocarpus santalinoides, Alchornea cordifolia, Senna siamea and Gliricidia sepium. The decomposition rate constant (wk⁻¹) ranged from 0.034 (Dactyladenia, subhumid zone) to 0.49 (Gliricidia, humid zone) in the rainy season, and from 0.01 (Dactyladenia, subhumid zone) to 0.235 (Pterocarpus, arid zone) in the dry season. The direct correlation between the decomposition rate of plant residues and their quality was only valid in agroeco-zones where there is not moisture stress. Similarly, the direct correlation between the decomposition rate of plant residues and moisture availability was only valid for plant residues with high quality. The decomposition rate of the low quality plant residue could increase from humid to arid zone in West Africa. In the arid zone, the low quality plant residue could also decompose faster than high quality plant residue. The climate-residue quality interactive effects on plant residue decomposition in West Africa were attributed to the feedback of low quality plant residue’s mulching effect, soil fauna and appreciable photodegradation in dry regions. A decomposition equation that could be used to predict the decomposition rate of plant residues with various qualities across agroeco-zones in West Africa was obtained from this study. The equation was expressed as follow: k = 0.122 − 0.000747*PRQI²− 0.0233*PRQI*CI + 0.00337*CI* PRQI², in which k is the decomposition rate constant (wk⁻¹), PRQI the plant residue quality index, and CI the climate index (ratio of rainfall to sunshine hours cumulative during the entire decomposition). The response of N and P release from plant residues to residue quality and climate was similar to that of residue decomposition. At the late stage of the dry season decomposition, the high C/N and C/P ratio plant residue (Dactyladenia leaves) that immobilized N and P in wet zones showed a release of N and P in the dry zone. The research was conducted when G. Tian, G. O. Kolawole and F. K. Salako were employees of the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria.     

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.     

Nitrogen release from eucalypt leaves and legume residues as influenced by their biochemical quality and degree of contact with soil

Large areas of short-rotation eucalypt plantations are being established in south-western Australia on land previously used for agriculture. Options for maintaining soil N supply include retention of harvest residues and legume inter-cropping. We evaluated the effects of adding the residues of five legume species and Eucalyptus globulusleaves on inorganic N dynamics in two soils (a Rhodic Ferralsol or red earth and a Haplic Podzol or grey sand) using two modes of residue application in a laboratory incubation experiment (519 days). The time course of net N immobilisation and mineralisation in both soils was strongly influenced by the type and mode of application. Eucalypt leaves caused strong N immobilisation (–7 mg N g^–1 residue-C) over the entire 519-day incubation, whereas for the legume species, N that was eventually immobilised at the start of the incubation, remineralised later to different degrees. Amongst the legumes, largest amounts of N were released from lupin residues (18 mg N g^–1 residue-C) and lowest amounts from field pea (2 mg N g^–1 residue-C). However, initial residue quality parameters were not significantly (P > 0.05) correlated with N release from the residues. Grinding and incorporating of the residues caused a much greater immobilisation of N than when residues were cut and surface applied. When ground residues were incorporated, immobilisation of N was more severe and endured for longer in the finer textured red earth than in the coarse textured grey sand. Where residues were surface applied, N dynamics were similar for both soil types. The results of this study suggest that legumes used as a mulch in eucalypt plantations are a readily available source of N for trees, and that the benefits from retention of harvest residues are more likely in maintaining soil N fertility on the long-term.     

Transformations between organic and inorganic sediment phosphorus in Lake Balaton

In order to estimate microbial P content and biological P uptake in sediments, the tungstate precipitation method of Orrett & Karl (1987) was used in sediment extracts. This method allows a simple and rapid separation of organic and inorganic ³²P radioactivity. Either inorganic ³²P (as carrierfree H₃ ³²PO₄) or organic ³²P (as ³²P-labelled algal material) was added to surface sediment suspensions of shallow Lake Balaton. Inorganic ³²P was rapidly transformed into organic ³²P, and this process was completely inhibited by formaline. P content of living benthic microorganisms was estimated from steady state distribution of the radioactivity. Transformation of algal organic P into inorganic P could also be detected.In extremely P limited Lake Balaton benthic microorganisms were shown to supplement their high P requirements by inorganic P uptake. The velocity of the inorganic into organic P transformation, i.e. the rate of microbial P uptake, was comparable to P uptake in the water column. Microbial P uptake contributed significantly to total P fixation by sediments, particularly at low ( 100 µg P l^–1) phosphate additions.     

Effects of vesicular-arbuscular mycorrhiza on the size of the labile pool of soil phosphate

Summary Inoculation of lettuce, onion and clover with VA mycorrhizal fungus (Glomus mosseae) increased plant yields and phosphate uptake in three soils that had been depleted in phosphate. From two soils in which the labile pool of phosphate had been labelled with³²P, the specific activity of plant phosphate was the same whether the plants were mycorrhizal or non-mycorrhizal. In a third soil (Sonning) the specific activity was lower in lettuce and clover when the plants were mycorrhizal. When the experiment was repeated with the same soil under conditions that gave lower growth rates, the specific activity was the same in mycorrhizal and non-mycorrhizal plants. The lower specific activity in lettuce and clover in the first experiment is atributed to greater release of slowly exchanging phosphate (which is not in equilibrium with the added³²P), caused by the high uptake of phosphate by the mycorrhizal plants. When they occur, lower specific activities in mycorrhizal plants may therefore not necessarily indicate a solubilizing effect of the mycorrhiza on soil phosphate.     

Reversibility of phosphorus sorption by ferruginous nodules

Ferruginous nodules sorb significant amounts of available soil and fertiliser phosphate. The effect of this sorption on phosphorus availability of an agricultural soil was tested by sequential extraction and by exhaustive cropping with millet (Pennisetum typhoides) in a greenhouse trial following fertilisation of the original soil containing 70% nodules and of prepared samples containing various mixes of separated soil fines and nodules. Phosphorus sorption maxima by the soil fines and nodules were 190 mg kg⁻¹ and 380 mg kg⁻¹ respectively. Samples of fines and nodules which had sorbed 110 and 194 mg kg⁻¹ were submitted to 8 successive extractions with 0.01 M KCl, after which P desorption amounted to 117 mg kg⁻¹ and 103 mg kg⁻¹ respectively. Hysteresis between sorption and desorption was negligible for the soil fines and increased with increasing nodule content of the samples. In the greenhouse experiment, P uptake at the first cropping was highest in the soil fines at all levels of phosphorus applied. Subsequent croppings, however, showed higher P uptake in the concretionary soils. These results indicate a higher initial P release from the soil fines with cropping followed by an earlier exhaustion of phosphorus. At the end of the greenhouse experiment, yields were low in spite of the large quantities of P still remaining in the soils. Phosphorus fractionation showed that, of the P left in the soil after cropping 20% was in labile, 29% in Fe or Al-associated, and 51% in low-availability forms.     

Uptake of phosphate from soils and fertilizers as affected by soil P availability and solubility of phosphorus fertilizers

Fertilizers labelled with ³²P were used to measure amounts of phosphorus, P_s and P_F, taken up by Lolium perenne from available soil P and from P fertilizer respectively, when applied at a rate of 66 mg P·(kg soil^–1) in greenhouse experiments. The quantity P_s of phosphorus taken up from soil in the presence of P fertilizer was compared to the quantity P_o taken up from soil without P fertilizer. The quantity (P_s–P_o) is positive for low P_o values, i.e. in soils poor in available phosphorus, but is negative for high P_o values indicating that an input of P fertilizer can induce a decrease in the utilization of available soil phosphorus. Moreover, for a given soil, the quantity (P_s–P_o) depends on the chemical form of the fertilizer. The standard method of evaluation of P fertilizer efficiency is based on the assumption that P_s=P_o, but P_s can differ from P_o. This result can explain the contradictory data published from field experiments about the efficiency of the various P fertilizers.     

Contribution by two arbuscular mycorrhizal fungi to P uptake by cucumber (Cucumis sativus L.) from32P-labelled organic matter during mineralization in soil

An experiment was set up to investigate the role of arbuscular mycorrhiza (AM) in utilization of P from organic matter during mineralization in soil. Cucumber (Cucumis sativus L.) inoculated with one of two AM fungi or left uninoculated were grown for 30 days in cross-shaped PVC pots. One of two horizontal compartments contained 100 g soil (quartz sand: clay loam, 1:1) with 0.5 g ground clover leaves labelled with³²P. The labelled soil received microbial inoculum without AM fungi to ensure mineralization of the added organic matter. The labelling compartment was separated from a central root compartment by either 37 m or 700 m nylon mesh giving only hyphae or both roots and hyphae, respectively, access to the labelled soil. The recovery of³²P from the hyphal compartment was 5.5 and 8.6% for plants colonized withGlomus sp. andG. caledonium, respectively, but only 0.6 % for the non-mycorrhizal controls. Interfungal differences were not related to root colonization or hyphal length densities, which were lowest forG. caledonium. Both fungi depleted the labelled soil of NaHCO₃-extractable P and³²P compared to controls. A 15–25% recovery of³²P by roots was not enhanced in the presence of mycorrhizas, probably due to high root densities in the labelled soil. The experiment confirms that AM fungi differ in P uptake characteristics, and that mycorrhizal hyphae can intercept some P immobilization by other microorganisms and P-sorbing clay minerals.