Effect of bone chars on phosphorus‐cadmium‐interactions as evaluated by three extraction procedures

Foreseen P shortage and contamination problems have stimulated the search for renewable and contaminant‐free P‐fertilizers and amendments that immobilize Cd. We investigated the P‐dissolution and Cd‐immobilizing effect of bone char (pyrolyzed de‐fatted bone chips; BC) and bone char with added reduced S compounds (BC^plus). Five soils varying in pH and low to high Cd‐contamination were incubated with slow‐release P‐fertilizers (BC and BC^plus) and the fast P‐release diammonium phosphate (DAP), and extracted with NH₄NO₃‐, NaHCO₃‐solutions, and H₂O. The P‐concentrations obtained by the three extractants were well correlated and NH₄NO₃ well suited to simultaneously assess the P‐ and Cd‐solubility. The addition of BC increased pH in all soils whereas BC^plus and DAP lowered the pH in soils with pH > 5. Similar trends for NH₄NO₃‐P differences between treatments and control were observed for BC and BC^plus during the incubation period, although BC^plus resulted in much larger P‐concentrations. The highest Cd‐immobilization efficiency was obtained in BC‐treated soils. The addition of BC^plus and DAP decreased the Cd‐concentrations until 34 d of incubation in all soils and remained effective in Cd‐immobilizing in soils that showed a pH raise over 145 d of incubation. Thus, the results indicate that surface modification of BC may promote the P‐dissolution along with a concomitant Cd‐immobilization largely through its pH‐effect but this must be confirmed in studies under non‐equilibrium conditions.     

Fertilizer‐use efficiency of different inorganic polyphosphate sources: effects on soil P availability and plant P acquisition during early growth of corn

Polyphosphate‐based fertilizers are worldwide in use, and their effect on crop yield is often reported to be similar to orthophosphate products, although some studies showed higher yields with polyphosphate applications. However, information on how these fertilizers may influence plant P acquisition is very limited. A pot experiment was carried out under controlled conditions with corn (Zea mays L.) growing on a sandy soil (pH 4.9) and a silty‐loam soil (pH 6.9) differing in P‐sorption properties. The objective was to evaluate phosphorus fertilizer–use efficiency (PFUE) of several polyphosphate (poly‐P) compounds (pyrophosphate [PP], tripolyphosphate [TP], and trimetaphosphate [TMP]) using orthophosphate (OP) as a reference. Focus was put on evaluating plant parameters involved in plant P acquisition, i.e., root length and P uptake per unit of root length. Furthermore, soil P availability was characterized by measuring ortho‐P and poly‐P concentrations in soil solution as well as in CAL (calcium‐acetate‐lactate) extracts. The P availability was differentially influenced by the different P sources and the different soils. In the silty‐loam soil, the application of poly‐P resulted in higher ortho‐P concentrations in soil solution. In the same soil, CAL‐extractable ortho‐P was similar for all P sources, whereas in the sandy soil, this parameter was higher after OP application. In the silty‐loam soil, poly‐P concentrations were very low in soil solution or in CAL extracts, whereas in the sandy soil, poly‐P concentrations were significantly higher. Phosphorus fertilizer–use efficiency was significantly higher for poly‐P treatments in the silty‐loam soil and were related to a higher root length since no differences in the P uptake per unit of root length among poly‐P and OP treatments were found. However, in the sandy soil, no differences in PFUE between OP and poly‐P treatments were observed. Therefore, PFUE of poly‐P compounds could be explained by better root growth, thereby improving plant P acquisition.     

Effects of soil flooding and organic matter addition on plant accessible phosphorus in a tropical paddy soil: an isotope dilution study

Plant growth experiments were conducted to reveal the mechanism by which organic matter (OM) and soil flooding enhance phosphorus (P) bioavailability for rice. It was postulated that reductive dissolution of iron‐(III) [Fe(III)] oxyhydroxides in soil releases occluded phosphate ions (PO₄), i.e., PO₄ that is not isotopically exchangeable in the original soil prior to flooding. Rice was grown in P‐deficient soil treated with factorial combinations of addition of mineral P (0, 50 mg P kg^?1), OM (0, ≈ 20.5 g OM kg^?1 as cattle manure +/– rice straw) and water treatments (flooded vs. non‐flooded). The OM was either freshly added just before flooding or incubated moist in soil for 6 months prior to flooding; nitrogen and potassium were added in all treatments. The soil exchangeable P was labeled with ³³PO₄ prior to flooding. The plant accessible P in soil, the so‐called L‐value, was determined from the ³³P/³¹P ratio in the plants. The L‐values were inconsistently affected by flooding in contrast with the starting hypothesis. The OM and P addition to soil clearly increased the L‐value and, surprisingly, the increase due to OM application was larger than the total P addition to soil. An additional isotope exchange study in a soil extract (E‐value) at the end of the experiment showed that the E‐value increased less than the total P addition with OM. This suggests that plants preferentially take up unlabeled P from the OM in the rhizosphere compared to labeled labile inorganic P. The effects of soil flooding on P bioavailability is unlikely related to an increase of the quantity of bio‐accessible P in soil (L‐value) but is likely explained by differences in P mobility in soil.     

Effect of long‐term fertilization on the transformations of water‐extractable phosphorus in a fluvo‐aquic soil

Improved information on water‐extractable soil P (P_w) and its distribution in various forms is needed to assess its bioavailability and environmental impact. This study investigated P_w in a fluvo‐aquic soil solution in relation to the continuous application of inorganic fertilizer (NPK) and wheat straw–soybean‐based compost for 15 y. Phosphatase‐hydrolysis techniques were used to fractionate organic P (P_o) in water extracts of soil into phosphomonoester (P_om) and phosphodiester (P_od). In comparison with the noncomposted treatments, compost application significantly increased the levels of inorganic P (P_i) and P_o. P_om was the main form in water‐extractable soil P_o (71%–88%), in which sugar phosphate (P_os) occupied 48%–75%, inositol hexakisphosphate (P_op) comprised 13%–23%, and P_od only accounted for a small percentage (11%–26%). Long‐term compost application significantly increased the content of P_om, P_os, and P_od, but decreased the P_op content; the ratio of P_om to P_o increased significantly in compost‐treated soil, but the ratio of P_op to P_o and P_od to P_o significantly decreased. Thus, the equilibrium of phosphatase involved P transformations shifted to P_i in compost‐treated soil. The phosphomonoesterase and phosphodiesterase activities were significantly higher in compost‐treated soil, which favored the transformations of P_od into P_om and P_om into P_i. The ratio of P_o to P_w in water extracts of compost‐treated soil was similar to that of control soils with no fertilizer input (CK), but was significantly lower than in NPK treatment, which demonstrated that a larger increase occurred for soil P_i in water extracts of compost‐treated soil. Long‐term compost application in the fluvo‐aquic soil changed the composition of P_w, promoted the rate of P transformations in soil solution, and significantly increased soil P bioavailability.     

The equivalence of the Calcium‐Acetate‐Lactate and Double‐Lactate extraction methods to assess soil phosphorus fertility

A large variety of extraction methods are used worldwide for the estimation of “plant‐available P” in soils. In Germany, the standard extractants are Calcium‐Acetate‐Lactate (CAL) and Double‐Lactate (DL). Until now there is no validated transformation procedure available and studies on the comparability of both methods have reported conflicting evidence. The uncertainty about the equivalence of CAL‐P and DL‐P hinders a direct comparison of the P fertility status and P fertilizer recommendations across Germany. Based on 136 datasets for soil samples from an interlaboratory comparison program and three P fertilization field trial sites, for which plant‐available P had been determined by both the CAL and DL method, we assessed the comparability of both extraction methods and derived simple and multiple regression equations to transform DL‐P into CAL‐P values. On average, DL extracted 30% more P than CAL. However, this strongly depended on soil pH and carbonate content. A simple linear regression model explained 70% of the variance. However, if simple linear regression models were fitted to pH‐specific samples (pH range 4.5 to 7.0) the R² increased to 0.96. Based on an independent validation dataset (n = 48) we demonstrated that such pH‐specific models were more accurate than models that did not consider pH when transforming DL‐P to CAL‐P values. Multiple regression results showed that out of soil pH, C_org, N_t, and C : N ratio, only soil pH improved the model. The transformation equations in this study provide a step towards an improved comparability of P fertility status assessments of soils across Germany.     

Effects of organic matter addition on phosphorus availability to flooded and nonflooded rice in a P‐deficient tropical soil: a greenhouse study

Addition of organic matter (OM) to flooded soils stimulates reductive dissolution of Fe(III) minerals, thereby mobilizing associated phosphate (P). Hence, OM management has the potential to overcome P deficiency. This study assessed if OM applications increases soil or mineral fertilizer P availability to rice under anaerobic (flooded) condition and if that effect is different relative to that in aerobic (nonflooded) soils. Rice was grown in P‐deficient soil treated with combinations of addition of mineral P (0, 26 mg P/kg), OM (0, ~9 g OM/kg as rice straw + cattle manure) and water treatments (flooded vs nonflooded) in a factorial pot experiment. The OM was either freshly added just before flooding or incubated moist in soil for 6 months prior to flooding; blanket N and K was added in all treatments. Fresh addition of OM promoted reductive dissolution of Fe(III) minerals in flooded soils, whereas no such effect was found when OM had been incubated for 6 months before flooding. Yield and shoot P uptake largely increased with mineral P addition in all soils, whereas OM addition increased yield and P uptake only in flooded soils following fresh OM addition. The combination of mineral P and OM gave the largest yield and P uptake. Addition of OM just prior to soil flooding increased P uptake but was insufficient to overcome P deficiency in the absence of mineral P. Larger applications of OM are unlikely to be more successful in flooded soils due to side effects, such as Fe toxicity.     

Phosphorus bioavailability of sewage sludge‐based recycled fertilizers

Six phosphorus (P) fertilizers recycled from sewage sludge [Struvite SSL, Struvite AirPrex®, P‐RoC®, Mephrec®, Pyrolysis coal and Ash (Mg‐SSA)] were tested for their plant availability in potted soil of pH 7.2 under greenhouse conditions. The crop sequence simulated a rotation of red clover (Trifolium pratense L.), maize (Zea maize L.), and ryegrass (Lolium perenne L.). Other P fertilizer treatments included: Phosphate Rock (PR), Calcium dihydrogen phosphate [Ca(H₂PO₄)₂], and an unfertilized control. Additionally, soil was regularly inoculated with two strains of plant growth‐promoting rhizobacteria (PGPR; Pseudomonas sp. Proradix, and Bacillus amyloliquefaciens) to test their ability to increase P availability to plants. Sequential P fractionation was conducted to link the amount of readily available P in fertilizers to plant P acquisition. Shoot P content and dry matter of maize decreased in the following order: Struvite SSL ≥ Ca(H₂PO₄)₂ > P‐RoC® ≥ Struvite AirPrex® ≥ Mephrec® > Pyrolysis coal ≥ Mg‐SSA ≥ PR ≥ unfertilized. Rhizobacteria did not affect shoot biomass or P content. The results show that red clover might have mobilized substantial amounts of P. Sequential P fractionation was not suitable to predict the efficacy of the fertilizers. Generally, the sewage sludge‐based fertilizers tested proved to be suitable alternative P sources relevant to organic farming systems. However, the efficacy of recycled fertilizers is strongly dependent on their specific production conditions.     

Testing mechanisms underlying the Hedley sequential phosphorus extraction of soils

The Hedley fractionation has become the preeminent measure for estimating the bioavailability of phosphorus (P) in soils. However, mechanisms underlying P extractability have never been tested. We hypothesize that P sequentially extracted by individual steps can either be referred to a specific mineral source (Hypothesis 1) or to its binding strength to minerals (Hypothesis 2). We prepared mineral‐P associations in the laboratory using various secondary mineral phases and P forms (orthophosphate, phytic acid, ribonucleic acid), which were then subject to the Hedley sequential extraction scheme (anion exchange resin in form, 0.5 M NaHCO₃, 0.1 M NaOH, 1 M HCl, and concentrated HCl at 80°C). Extracts were analyzed for P as well as for the main mineral‐borne elements by inductively coupled plasma–optical emission spectroscopy (ICP–OES). In order to test if the observed mineral dissolution patterns match those of natural soils, we applied the Hedley fractionation to forest soils comprising various P stocks and measured in addition to extracted P also iron, aluminum, and calcium by ICP – OES. Phosphorus extractability from mineral‐P associations differed between P forms and mineral phases. Adsorbed P always contributed to several or all extracts, Hypothesis 1 was thus not tenable. Aluminum hydroxide, allophane, ferrihydrite, and goethite completely dissolved during Hedley fractionation from the third extraction step onwards. Successive mineral dissolution also occurred for the soil samples. Thus, extracted P represents partly desorbed P from various soil constituents and partly P co‐released upon dissolution of various minerals. Consequently, also Hypothesis 2 could not be confirmed, i.e., the sequential extraction is not suitable to assess different binding strengths between P forms and minerals. We conclude that the method hardly provides information for studies aiming at the mechanistic understanding of P bioavailability in soil.     

Phosphorus bioavailability of sewage sludge‐based recycled fertilizers in an organically managed field experiment

Recycled phosphorus (P) fertilizers from sewage sludge can contribute to the ongoing effort of closing the P cycle. Five recycled P fertilizers (Struvite SSL, Struvite AirPrex®, P‐RoC®, Pyrolysis coal, and Na‐SSA) were tested for their P availability in a two‐year field experiment with maize. The experiment was conducted on an organic certified research station at soil pH 6.5. Other P fertilizer treatments included: phosphate rock (PR), compost, and an unfertilized control. In addition, the rhizobacteria strain Bacillus sp. Proradix (Proradix®) was applied to test its ability to increase P bioavailability. Each year, shoot DM and P offtake of maize was measured and P use efficiency of the tested fertilizers was calculated. No significant differences in shoot DM were found among fertilized treatments and the unfertilized control in both years of experiment. Fertilization with recycled fertilizers increased P offtake by between 0% (Na‐SSA) and 27.5% (Struvite SSL) compared to the unfertilized control. Rhizobacteria application led to an increase in P offtake of maize from 25.9 to 38.7 kg P ha^?1 when combined with PR fertilization in the year of fertilizer application, while no significant effect was found for the recycled fertilizers. Some of the tested recycled fertilizers from urban waste water can be considered as effective fertilizers for their use in organic agriculture.     

Molecular Characterization of Corn Starch Using an Aqueous HPSEC‐MALLS‐RI System Under Various Dissolution and Analytical Conditions

Molecular characteristics based on absolute weight‐average molecular weight (M_w) and z‐average radius of gyration (R_g) of normal corn starch were analyzed by high‐performance size‐exclusion chromatography (HPSEC) attached to multiangle laser‐light scattering (MALLS) and refractive index (RI) detectors under different starch dissolution and analytical conditions. Autoclaving (121°C, 20 min) or microwave heating (35 sec) provided better HPSEC recovery and higher M_w for starch molecules than simple dissolution in hot water. The M_w for the autoclaved corn amylopectin and amylose fractions separated with a TSK G5,000 column at 60°C were 201 × 10⁶ and 3.3 × 10⁶, respectively. The specific volume for gyration (SV_g) calculated from M_w and R_g could be used for the comparison of molecular compactness which was inversely related to the degree of branching. The SV_g values of amylopectin and amylose fractions in the chromatogram (TSK G5,000, autoclaved for 20 min) were 0.092 and 0.529, respectively. But a portion (20–30%) of large amylopectin molecules did not pass the injection membrane filter (3.0 μm) and the SEC column, resulting in incomplete recovery. The unfiltered portion varied according to the dissolution treatment. Homogenization (7,000 rpm, 5 or 10 min) of the starch solution improved the recovery of the amylopectin fraction, but significantly increased the M_w of the amylose fraction (17 × 10⁶). Sonication for 5 min degraded starch molecules. For accurate analysis of a native starch using an aqueous SEC, the starch should be fully dissolved with proper treatment such as autoclaving or microwaving, and the column should be improved for full recovery of large amylopectin molecules.     

Enhancing phosphorus availability,soil organic carbon,maize productivity and farm profitability through biochar and organic–inorganic fertilizers in an irrigated maize agroecosystem under semi‐arid climate

Biochar amendments offer promising potential to improve soil fertility, soil organic carbon (SOC) and crop yields; however, a limited research has explored these benefits of biochar in the arid and semi‐arid regions. This two‐year field study investigated the effects of Acacia tree biomass‐derived biochar, applied at 0 and 10 t ha^?1 rates with farmyard manure (FYM) or poultry manure (PM) and mineral phosphorus (P) fertilizer combinations (100 kg P ha^‐1), on maize (Zea mays L.) productivity, P use efficiency (PUE) and farm profitability. The application of biochar with organic–inorganic P fertilizers significantly increased soil P and SOC contents than the sole organic or inorganic P fertilizers. Addition of biochar and PM as 100% P source resulted in the highest soil P (104% increase over control) and SOC contents (203% higher than control). However, maize productivity and PUE were significantly higher under balanced P fertilizer (50% organic + 50% mineral fertilizer) with biochar and the increase was 110%, 94% and 170% than 100%‐FYM, 100%‐PM and 100% mineral fertilizer, respectively. Maize productivity and yield correlated significantly positively with soil P and SOC contents These positive effects were possibly due to the ability of biochar to improve soil properties, P availability from organic–inorganic fertilizers and SOC which resulted in higher PUE and maize productivity. Despite the significant positive relationship of PUE with net economic returns, biochar incorporation with PM and mineral fertilizer combination was economically profitable, whereas FYM along biochar was not profitable due to short duration of the field experiments.     

Application of low‐phosphorus‐containing legume residues reduces extractable phosphorus in a tropical Ultisol

Application of legume green manure (GM) is suggested to be effective in increasing the availability of native soil phosphorus (P) and the dissolution and utilization of phosphate rock (PR)‐P by food crops. Experiments were conducted to study the dynamics of extractable P (P extracted by Bray‐1‐extracting solution) of an Ultisol amended with or without GM residues of contrasting P concentrations in the absence of growing plants. In two separate experiments, GM residues of Aschynomene afraspera (a flood‐tolerant legume) and of Crotalaria micans (upland) with varying P concentrations were added to an acidic soil amended with PR‐P or triple superphosphate (TSP) in plastic bottles. Soil moisture was brought to field capacity of the soil in the upland experiment and saturated with distilled water in the lowland setup. This was done to simulate aerobic upland and anaerobic lowland soil conditions in the relevant plastic bottles. Only P concentration of the residues added varied, while lignin and C : N ratios were similar. A temperature of 25°C was maintained throughout the experiment. Changes in soil extractable Bray‐1‐P were measured at the end of the incubation period (60 or 80 d). In the aerobic soils, extractable P in the combined PR+GM or TSP+GM treatments was significantly lower than in the PR‐ or TSP‐ treated soils. The amendment with GM residues alone significantly increased Bray‐1‐P over the unamended control in the case of the inorganic P‐fertilized GM residues. The trend in extractable P was similar in the soils incubated under anaerobic conditions. However, in the case of PR, concentrations of P extracted by Bray‐1 solution did not significantly change in the presence or absence of GM. The results suggest that the incorporation of GM residues with low P concentration does not lead to a net P release in upland or lowland soils. These results have implications for nutrient cycling in farming systems in W Africa as most of the soils are poor and very low in available P.     

Relationship between vertical distributions of Bray II P content and organic P content in the Ap horizon of paddy rice fields

Abstract

We examined the vertical distributions of total, Bray II and organic phosphorus (P) in the Ap horizons of five paddy rice fields including no tillage treatment. Soil samples were collected from the Ap horizons as nine thin layers of 0–1, 1–2, 2–3, 3–4, 4–6, 6–8, 8–10, 10–15 and 15–20 cm from the soil surface after harvest. In three lowland paddy soils of no-tillage Hachirogata (HA-NT), conventional-tillage Hachirogata (HA-T) and conventional-tillage Furukawa (FU-T), the total P (TP) content was relatively low. In these soils, the modified Bray II P (soil : solution = 1:20; shaking time 60s) content decreased and the organic P (OP) content increased at a depth of 0–1 cm compared with the underlying layers, possibly because of high microbial activities as suggested from the increased total C and N contents in this layer. In contrast, the decrease in Bray II P content and the increase in the OP content of the surface layer were not evident in two Andosols of the conventional-tillage Utsunomiya (UM-T) and conventional-tillage Kawatabi (KA-T). Even TP content fluctuated in the thin surface layers of 0–3 cm depending on the sampling site. Accordingly, percentages of Bray II P and OP to TP were calculated to compare the vertical distribution curves between Bray II P and OP. The percentage of Bray II P to TP of the 0–1 cm layer was lower and the percentage of OP to TP in the layer was higher than the underlying layers for HA-T, HA-NT and FU-T. The decrease in Bray II P was nearly compatible with the increase in OP for the surface of the 0–1 cm layers of HA-NT, HA-T and FU-T. This result suggests that a part of Bray II P was converted to OP in the thin surface layer of the Ap horizons in these paddy soils. This change in chemical form of P was more pronounced in the HA-NT by accumulating the effect of the microbial activities over a longer period of time because of no tillage compared with HA-T and FU-T. These findings add a new dimension to P behavior in submerged rice soils under field conditions, particularly near the soil surface. Moreover, we must be careful in soil sampling for P availability evaluation, and soil samples should represent the vertical distribution of P in the whole Ap horizon of a paddy field. Changes in OP with depth in UM-T and KA-T may have been masked by the high OP content as well as by the high TP content in these Andosols.     

Fate of phosphorus in soil during a long‐term fertilization experiment in Finland

Accumulation and depletion of soil phosphorus (P) was studied in a long‐term (37 y) field experiment in Southern Finland. The loam soil had a high pH (7.5–7.7) due to an earlier liming. Spring barley, spring wheat, oat, and ryegrass, grown in rotation, were annually fertilized with 0, 32, or 67 kg P ha^?1 y^?1 (P₀, P₁, and P₂K) and sufficient N. The average dry matter grain yield 2,600 kg ha^?1 of the P₀ plots increased by about 500 kg ha^?1 at P₁ treatment and another 600 kg ha^?1 by P₂K. Soil samples were collected in 1978 (beginning), 1995, 2005, and 2015. According to the Chang and Jackson sequential extraction, the P₂K and P₁ treatments increased the inorganic soil P by 732 and 32 kg P ha ^?1 in 37 years, respectively, while the P₀ plots were depleted by –459 kg P ha ^?1. The P₂K treatment increased all four P fractions, extracted with NH₄Cl (easily soluble), NH₄F (Al‐P), NaOH (Fe‐P), and H₂SO₄ (Ca‐P). Continuous depletion (P₀) decreased the NH₄Cl‐P and NH₄F‐P pools, NaOH‐P and H₂SO₄‐P pools remaining stable. None of the P pools changed significantly at P₁. The remarkable gap between the measured change and the balance for the P₂K and P₁ treatments cannot be explained solely by lateral soil movement, meaning that a significant proportion of the applied P was lost either in surface runoff or transported below the investigated depth of 40 cm. Despite large P applications, the degree of P saturation reached only 20% in the P₂K topsoil, assuming a 50% reactivity of Fe and Al oxides. As derived from sorption isotherms, a high EPC₀ (i.e., equilibrium P concentration at zero net P sorption or desorption) of 1.30 mg L^?1 had been built up in the P₂K treatment, while in the P₁ treatment EPC₀ (0.33 mg L^?1) had remained unchanged and P depletion (P₀) had caused a decrease to 0.12 mg L^?1. These results demonstrate that P sorption and desorption properties respond strongly to both P fertilization and null fertilization treatments and that in a long‐term field experiment only a low proportion of the residual fertilizer P can be recovered from soil.     

Phytolith‐associated potassium in fern: characterization,dissolution properties and implications for slash‐and‐burn agriculture

In recent time, phytoliths (silicon deposition between plant cells) have been recognized as an important nutrient source for crops. The work presented here aims at highlighting the potential of phytolith‐occluded K pool in ferns. Dicranopteris linearis (D. linearis ) is a common fern in the humid subtropical and tropical regions. Burning of the fern D. linearis is, in slash‐and‐burn regions, a common practice to prepare the soil before planting. We characterised the phytolith‐rich ash derived from the fern D. linearis and phytolith‐associated potassium (K) (phytK), using X‐ray tomographic microscopy in combination with kinetic batch experiments. D. linearis contains up to 3.9 g K/kg d.wt, including K subcompartmented in phytoliths. X‐ray tomographic microscopy visualized an interembedding structure between organic matter and silica, particularly in leaves. Corelease of K and Si observed in the batch experiments confirmed that the dissolution of ash phytoliths is one of major factors controlling K release. Under heat treatment, a part of the K is made available, while the remainder entrapped into phytoliths (ca. 2.0–3.3%) is unavailable until the phytoliths are dissolved. By enhanced removal of organic phases, or forming more stable silica phases, heat treatment changes dissolution properties of the phytoliths, affecting K release for crops and soils. The maximum releases of soluble K and Si were observed for the phytoliths treated at 500–800 °C. For quantitative approaches for the K provision of plants from the soil phytK pool in soils, factors regulating phytolith dissolution rate have to be considered.     

Phosphorus in agricultural constructed wetland sediment is sparingly plant‐available

Agricultural constructed wetlands (CWs) are intended to retain sediment and phosphorus (P) carried off with runoff and drainage water. The accumulated sediment, with adsorbed P, is often advised to be recycled to agricultural land, but little is known about the fertilizer value of sediment‐associated P. This study examined the effects on P adsorption characteristics and P plant availability of mixing CW sediment into soil. Although the total P content in the sediment was approximately equal to that in catchment soil and the NaOH‐extractable P content was higher to that in catchment soil, in adsorption‐desorption tests sediment P solubility decreased and affinity for P increased with increasing addition rate of CW sediment to soil. Already the lowest sediment addition rate (12.5% of dry weight) decreased the equilibrium P concentration (EPC₀') by 60% on average compared to unamended catchment soil. In a greenhouse pot experiment, Italian ryegrass (Lolium multiflorum L.) yield was largely unaffected by CW sediment application, but P uptake systematically decreased when the rate of sediment application to soil increased. When 12.5% dry weight of sediment was added, plant P uptake decreased by 6–50% in P‐unfertilized pots and by 6–17% in P‐fertilized pots (150 mg P kg⁻¹) compared with P uptake of ryegrass grown in unamended field soil. Our other results suggest that the plant availability of P in CW sediments is very low due to high clay content and high concentrations of aluminium (Al) and iron (Fe) (hydr)oxides in the sediment. Thus, if applied to agricultural fields in large quantities, dredged CW sediment may impair crop P supply.     

A combination method of hydrofluoric acid,oxalate and NaOH‐EDTA extractions to determine phosphorus species in soil by 31P‐NMR spectroscopy

The objective of this study was to investigate the applicability of selective extraction methods in combination with solution phosphorus‐31 (³¹P) nuclear magnetic resonance (NMR) for the determination of organic phosphorus (P_o) species in soil. Inositol hexakisphosphates (IHP) and glucose 6‐phosphate (GP) were identified in the soil. IHP adsorbed to both soil organic matter (SOM) and short‐range order (SRO) minerals, whereas GP adsorbed only to SOM. These results suggest that P_o hosting phases are different depending on P_o species.     

A simple,low‐cost technique for in situ measurement of leaf P concentration in field‐grown rice

Simple tools for diagnosing crop nutritional status are in great demand by agricultural extension staff, particularly in low‐fertility environments. We developed the first practical method for in situ diagnosis of phosphorus (P) nutrition in field‐grown cereal crops by using a handheld colorimeter. The concentration of P in extracts from fresh leaves of lowland rice grown under various P availabilities was closely correlated with that of oven‐dried leaves measured with the standard molybdenum‐blue method in the laboratory, over a range of values from 0.319 to 1.420 mg P g ^?1 DW (r = 0.885^**, n = 14). It takes only 3–5 min to estimate the P concentration of plant samples using the new technique. Although not as accurate as the laboratory method, the new method can easily detect P deficiency of rice in the field (and possibly other cereal crops) without requiring costly, off‐site equipment. Thus, extension staff and agronomists can easily adopt this technique.     

Assessing extractable soil phosphorus methods in estimating phosphorus concentrations in surface run‐off from Calcic Hapludolls

Understanding soil test phosphorus (STP) and surface run‐off phosphorus (P) relationships for soils is necessary for P management. The objective of the study was to evaluate the efficacy of various soil test indices to predict P losses in surface run‐off. Selected sites were subjected to in situ rainfall simulations according to the protocol of the National Phosphorus Research Project ( NPRP, 2001 ). P from a composite of twenty‐four 2.0‐cm‐diameter core soil samples (0–5 cm) was extracted using the Olsen, Bray–Kurtz, Mehlich III, distilled water and 0.01 m calcium chloride procedures. All of these P extraction methods explained a significant amount of variability in surface run‐off total dissolved P [TP (<0.45)] (r² ≥ 0.67; P ≤ 0.01), where 0.45 is the filter pore diameter in microns. Multiple regression models showed extractable P to be the best soil predictor of surface run‐off TP (<0.45) among the studied soils. Despite extraction method or soil type, extractable P was the best soil predictor of surface run‐off TP (<0.45). Either agronomic (0.92 ≤ r² ≤ 0.96) or environmental (0.94 ≤ r² ≤ 0.96) soil tests were effective in estimating surface run‐off TP (<0.45) in select Mollisols.     

The phosphorus status of German cropland—An inventory of top‐ and subsoils

Background : In search for more sustainable crop production, the subsoil has recently come into focus as considerable reservoir of nutrients and water. Aims : Dimensions of subsoil phosphorus (P) reserves are yet largely unknown but crucial for identifying regions suitable to include subsoil into sustainable management strategies. Methods : We analyzed stocks of total and plant‐available (calcium acetate lactate‐extractable) P in 96 representative soil profiles of German arable land down to 1 m depth. Results : We found that the German arable soils stored, on average, 8 t ha^?1 of total P, of which nearly 500 kg ha^?1 were readily plant‐available. Notably, one third of plant‐available P was located below the plow layer and one fifth even at depths below 0.5 m. The depth gradients of plant‐available P stocks were affected more by major reference soil group than by texture. Generally, Chernozem but also Anthrosol, Gleysol and Fluvisol exhibited the largest P stocks in German cropland. The contribution of plant‐available P to total P stocks was larger in sandy and extremely acidic (pH < 4.5) soils compared with more fine‐grained and slightly acidic to alkaline soils, possibly because fertilization compensated for overall lower total P stocks at these sites. Generally, the more P was stored in topsoils, the more P was stored also in subsoils. Conclusions : A hypothetical crop utilization of 10% from plant‐available P stocks and 0.1% from total P stocks from shallow subsoil could compensate for P fertilization by ca. 8 kg ha^?1, but the rate of plant‐available P replenishment in subsoil likely remains the crucial factor for the role of subsoil P stocks in crop nutrition. Generally, the large P reserves found in subsoil could act as an ‘insurance' system for crops.