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Many natural and anthropogenic soils are phosphorus (P) limited, as P is largely immobilized in forms of low bioavailability affecting the plant productivity. One of the strategies to overcome this shortage lies in the symbiosis of plants with mycorrhizal fungi that increases the plant P uptake of these hardly accessible sources in exchange for photoassimilates. Nevertheless, the required investment to acquire the free phosphate from various P sources increases with decreasing P source bioavailability, indicating a different carbon (C) sink potential. Ectomycorrhizae were shown to be able to acquire various P sources, but experimental evidence for their P source dependent C sink potential as well as for their role in resource partitioning for P is missing. The present study aimed to address following objectives: (i) to design a system to observe mycorrhizal mediated nutrient exchange (Study I); and to investigate (ii) whether the C investment from the host plant for the mycorrhizal mediated P derived from different P species is P source dependent (Study II); as well as the (iii) preferences of mycorrhizal plant to specific P sources from a mixed P pool (Study III). Following P sources were tested for the mycorrhizal mediated plant P uptake: PO43- (oP), the primary mineral apatite (AP; Study II) or hydroxyapatite (HAP; Study III), the organic P species (Porg) phytic acid (Phy; Study II) or adenosine monophosphate (AMP; Study III), and goethitebound PO43- (gP) as secondary mineral-P adsorption complex. The practical experience made in Study I revealed that, compared with the in vitro culture system, the construction and maintenance of the axenic rhizotrone and the mesocosm culture systems are less complicated and time consuming and at the same time more robust and very versatile systems that are also suitable for greenhouse conditions. In Study II a P source dependent trend in exchange of host C for ectomycorrhizal mediated P in roots was determined. The exchange of C

Most plants living in tropical acid soils depend on the arbuscular mycorrhizal (AM) symbiosis for mobilizing low-accessible phosphorus (P), due to its strong bonding by iron (Fe) oxides. The roots release low-molecular-weight organic acids (LMWOAs) as a mechanism to increase soil P availability by ligand exchange or dissolution. However, little is known on the LMWOA production by AM fungi (AMF), since most studies conducted on AM plants do not discriminate on the LMWOA origin. This study aimed to determine whether AMF release significant amounts of LMWOAs to liberate P bound to Fe oxides, which is otherwise unavailable for the plant. Solanum lycopersicum L. plants mycorrhized with Rhizophagus irregularis were placed in a bicompartmental mesocosm, with P sources only accessible by AMF. Fingerprinting of LMWOAs in compartments containing free and goethite-bound orthophosphate (OP or GOE-OP) and phytic acid (PA or GOE-PA) was done. To assess P mobilization via AM symbiosis, P content, photosynthesis, and the degree of mycorrhization were determined in the plant; whereas, AM hyphae abundance was determined using lipid biomarkers. The results showing a higher shoot P content, along with a lower N:P ratio and a higher photosynthetic capacity, may be indicative of a higher photosynthetic P-use efficiency, when AM plants mobilized P from less-accessible sources. The presence of mono-, di-, and tricarboxylic LMWOAs in compartments containing OP or GOE-OP and phytic acid (PA or GOE-PA) points toward the occurrence of reductive dissolution and ligand exchange/dissolution reactions. Furthermore, hyphae grown in goethite loaded with OP and PA exhibited an increased content of unsaturated lipids, pointing to an increased membrane fluidity in order to maintain optimal hyphal functionality and facilitate the incorporation of P. Our results underpin the centrality of AM symbiosis in soil biogeochemical processes, by highlighting the ability of the AMF and accompanying microbiota in

In an agro-ecosystem, phosphorus (P) is found in organic and an inorganic form which includes soluble P, sorbed P and mineral bounded P. In soil, added P sources undergo various soil processes like mineralization, immobilization, precipitation, weathering, dissolution, sorption and desorption. For understanding the P dynamics in clay loam (Vertic Ustropept) soil, the present study was undertaken in P dynamics under rhizosphere and non-rhizosphere environment of maize in Long Term Fertilizer Experiment at Tamil Nadu Agricultural University, Coimbatore. The results revealed that the fractions of various pools of inorganic NaOH extractable Fe - P, H2SO4 extractable Ca- P, NH4F extractable Al- P, NH4Cl extractable Saloid P and Na citrate - Dithionate extractable Reductant soluble P were dominant in the non-rhizospheric soil than rhizospheric environment. The order of inorganic P fractions in the non-rhizospheric and rhizospheric region of the soil was found as Ca-P > Fe-P > Al-P > Reductant soluble-P > Saloid P and the knee-high stage of the non - rhizosphere soil recorded the highest inorganic as well organic P fractions. Irrespective of P fractions, Ca – P was recorded high (192.5 & 186.7 mg kg-1 ) followed by Fe - P (40.8& 34.9 mg kg-1) at a knee-high stage in non-rhizosphere and rhizosphere, respectively. Practising various nutrient management systems, application of 100% recommended dose of fertilizer along with FYM @ 10 t ha-1 (T8) recorded significant changes in all inorganic (Ca-P, Fe-P, Al-P, Reductant soluble-P, Saloid P), organic fractions and also Total P followed by 150% NPK (T3) in sandy clay loam soil. Nowadays, increasing demand for P fertilizer in India, judicious use of P fertilizer is important. Despite that, intensively cultivated soils have a lot of P reserves like organic and inorganic P pools and effective way of P transformation management could reduce the quantum of P fertilization in soil.

This review covers the nature, characteristics, and reactivity of soil organic matter (SOM) in volcanic soils and the phosphorus (P) accumulation mainly via the formation of stable complexes with organic and inorganic constituents to form P-containing macromolecules derived from both pedogenesis and fertilization. With the time, P accumulates as organic and inorganic compounds with differing lability, but the bulk appears to be recalcitrant. Chilean volcanic soils follow this same trend, subsequently having detrimental characteristics for plant growth, like the highly humified SOM and high P-sorption capacity. In addition, certain Chilean volcanic soils have high acidity, concomitant with a high exchangeable Al. As a result of the continuous application of P fertilizers, together with a low P efficiency of plant root acquisition, a “P reservoir” has built up, giving rise to the so-called residual P. This residual P consists of the inorganic and organic P, as macromolecular structures representing the cumulative average of several decades worth of agronomic P usage. Root modifications are an essential biological intervention to deal with this P accumulation. The general root modifications that are required to mobilize the residual P are discussed in the context of biochemical modifications (root exudations) and the symbiotic alterations by arbuscular mycorrhizal (AM) fungi. For a more efficient utilization of this accumulated P, however, it is essential to investigate the chemical nature and lability of these P forms in order to determine their capacity for plant acquisition and utilization. In this context, attention is focused on P fractionation and on some P-NMR analysis of residual P constituents in Andisols. The major root trait evaluated and discussed here is the AM association, which is able to be extensively modified by management practices. Finally, some potential practices to avoid the excessive application of P fertilizers in volcanic soils by using techno

Objectives The effect on P uptake by plants after inoculation with P-mobilizing 19 microorganisms may be modulated by soil properties, including natural microbiota. 20 However, to put this theory into practical use, research is needed to shed new light on the 21 soil factors which affect the capability of improving P nutrition in plants. The aim of this 22 study was to assess how two P-mobilizing microorganisms, Trichoderma asperellum T34 23 and Bacillus subtilis QST713, influence P uptake by wheat plants in different soils; this 24 will allow us to identify the soil properties which affect the efficiency of P nutrition in 25 plants. 26 Materials and methods. In a completely randomized experiment, wheat was grown in 12 pots in a growing chamber in soil with Olsen P values ranging from 4.8 to 8.7 mg kg-1 27 . 28 The plants were inoculated with 3 treatments: T34, B. subtilis and a non-inoculated 29 control. 30 Results and discussion. Overall, B. subtilis was more effective in increasing plant P 31 uptake and in mobilizing soil P (measured as Olsen P values) than T34. In some soils, B. 32 subtilis was the only treatment which increased Olsen P in the rhizosphere after 33 cultivation. However, the effect of both microorganisms differed depending on the soil. 34 For B. subtilis, phytase hydrolysable P, Olsen P, carbonates, the Feca/Fecbd ratio and citrate 35 soluble P accounted for 92 % of the variation in P uptake in inoculated plants (compared 36 with the non-inoculated control). Most of these soil properties also accounted for 87 % 37 of the variation in the levels of shoot DM in B. subtilis inoculated plants compared with 38 shoot DM in the control plants. In addition, Olsen P, the Feca/Fecbd ratio and phytase 39 hydrolysable P in the NaOH extracts accounted for 82 % and 74 % of the variation in the 40 effect of T34 on P uptake and shoot DM, respectively. Overall, the lower the initial Olsen 41 P in the soil, the higher the P uptake caused by microorganisms. 42 Concl

Objectives The effect on P uptake by plants after inoculation with P-mobilizing 19 microorganisms may be modulated by soil properties, including natural microbiota. 20 However, to put this theory into practical use, research is needed to shed new light on the 21 soil factors which affect the capability of improving P nutrition in plants. The aim of this 22 study was to assess how two P-mobilizing microorganisms, Trichoderma asperellum T34 23 and Bacillus subtilis QST713, influence P uptake by wheat plants in different soils; this 24 will allow us to identify the soil properties which affect the efficiency of P nutrition in 25 plants. 26 Materials and methods. In a completely randomized experiment, wheat was grown in 12 pots in a growing chamber in soil with Olsen P values ranging from 4.8 to 8.7 mg kg-1 27 . 28 The plants were inoculated with 3 treatments: T34, B. subtilis and a non-inoculated 29 control. 30 Results and discussion. Overall, B. subtilis was more effective in increasing plant P 31 uptake and in mobilizing soil P (measured as Olsen P values) than T34. In some soils, B. 32 subtilis was the only treatment which increased Olsen P in the rhizosphere after 33 cultivation. However, the effect of both microorganisms differed depending on the soil. 34 For B. subtilis, phytase hydrolysable P, Olsen P, carbonates, the Feca/Fecbd ratio and citrate 35 soluble P accounted for 92 % of the variation in P uptake in inoculated plants (compared 36 with the non-inoculated control). Most of these soil properties also accounted for 87 % 37 of the variation in the levels of shoot DM in B. subtilis inoculated plants compared with 38 shoot DM in the control plants. In addition, Olsen P, the Feca/Fecbd ratio and phytase 39 hydrolysable P in the NaOH extracts accounted for 82 % and 74 % of the variation in the 40 effect of T34 on P uptake and shoot DM, respectively. Overall, the lower the initial Olsen 41 P in the soil, the higher the P uptake caused by microorganisms. 42 Concl

Sedimentary burial of the essential nutrient phosphorus (P) under anoxic and sulfidic conditions is incompletely understood. Here, we use chemical and micro-scale spectroscopic methods to characterize sedimentary P burial along a water column redox transect (six stations, 78–2107 m water depth) in the Black Sea from the shelf with its oxygenated waters to the anoxic and sulfidic deep basin. Organic P is an important P pool under all redox regimes, accounting for up to 60% of P burial. We find a general down-core increase in the relative importance of organic P, especially on the shelf where P bound to iron (Fe) and manganese (Mn) (oxyhydr)oxides is abundant in the uppermost sediment but rapidly declines in concentration with sediment depth. Our chemical and spectroscopic data indicate that the carbonate-rich sediments (Unit I, ∼3000 years, ∼0–30 cm depth) of the sulfidic deep Black Sea contain three major P pools: calcium phosphate (apatite), organic P and P that is strongly associated with CaCO3 and possibly clay surfaces. Apatite concentrations increase from 5% to 25% of total P in the uppermost centimeters of the deep basin sediments, highlighting the importance of apatite formation for long-term P burial. Iron(II)-associated P (ludlamite) was detected with X-ray absorption spectroscopy but was shown to be a minor P pool (∼5%), indicating that lateral Fe–P transport from the shelf (“shuttling”) likely occurs but does not impact the P burial budget of the deep Black Sea. The CaCO3–P pool was relatively constant throughout the Unit I sediment interval and accounted for up to 55% of total P. Our results highlight that carbonate-bound P can be an important sink for P in CaCO3-rich sediments of anoxic, sulfidic basins and should also be considered as a potential P sink (and P source in case of CaCO3 dissolution) when reconstructing past ocean P dynamics from geological records.

Stable and labile soil organic compounds play different roles in the soil. It is a question of how far soil organic matter (SOM) fractions with different labilities vary as a function of climatic and management conditions. In order to answer this question stable (organic C -C-, total N -N-, organic P -Po-), and labile SOM fractions (total carbohydrates -CHt- and hot water soluble carbohydrates -CHw-) were measured monthly for two years in the 10-cm soil top-layer of an Entic Haplustoll, under conventional tillage (CT), vertical tillage (VT) and no-till (NT). Results showed that contents of all analyzed organic fractions were higher in NT than in VT and CT in almost all sampling dates. All organic compounds were less variable with time in NT and VT than in CT, in agreement with the smaller soil disturbance of NT and VT compared to CT. The more labile fractions varied as a function of short term changes in the climatic conditions, mainly temperature. Under soil disturbing tillage systems, the most stable fractions tended to decrease and the more labile to increase with time. This was attributed to the transformation of the more stable into the more labile fractions, possibly due to the disruption of aggregates produced by tillage that favored SOM mineralization. Po was the less variable compound, even under the most disturbing tillage conditions. The quotients C/N, CHt/C and CHw/C evolved similarly in all tillage systems, indicating that that tillage systems change the amount but not the quality of SOM., Os compostos estáveis e lábeis da matéria orgânica têm papeis diferentes no solo. A forma como as frações da matéria orgânica do solo (SOM) com diferente labilidade variam em função das condições climáticas e das condições de maneio do solo é pouco conhecida. Para responder a esta questão mediram-se mensalmente, ao longo de dois anos, as frações estáveis (C orgânico –C-, N total-N-, P orgânico-Po-), e as frações lábeis da SOM (hidratos de carbono totais CHt- e hidratos de carbono solúveis -CHw-) na camada dos primeiros 10 cm de um Entic Haplustoll, submetido a lavoura convencional (CT), lavoura vertical (VT) e plantio direto (NT). Os resultados mostraram que os teores de todas as frações da matéria orgânica analisadas eram maiores no NT do que no VT e no CT na maioria das datas de amostragem. Em função do tempo, as concentrações de todos os compostos orgânicos eram menos variáveis em NT e VT do que em CT, o que está de acordo com a menor perturbação do solo em NT e VT comparada com a do CT. As frações mais lábeis variaram em função das alterações de curto prazo nas condições climáticas, nomeadamente a temperatura. Em condições de lavoura de maior perturbação do solo, as frações mais estáveis tenderam a diminuir e as mãos lábeis a aumentar com o tempo. Este facto foi atribuído à transformação das frações mais estáveis em frações mais lábeis, possivelmente devido à quebra dos agregados produzida pela lavoura, o que teria favorecido a mineralização da SOM. O Po foi o componente menos variável, mesmo sob condições de lavoura com alta perturbação do solo. As relações C/N, CHt/C e CHw/C evoluíram de maneira semelhante em todos os sistemas, indicando que os sistemas de lavoura mudam a quantidade mas não a qualidade da SOM., Los compuestos estables y lábiles de la materia orgánica juegan papeles diferentes en el suelo. Se desconoce cómo las fracciones de materia orgánica del suelo (SOM) de diferente labilidad varían en función de las condiciones climáticas y de manejo. Para dar respuesta a esta cuestión se midieron mensualmente a lo largo de dos años las fracciones estables (C orgánico -C-, N total-N-, P orgánico -Po-), y las fracciones lábiles (carbohidratos totales -CHt- y carbohidratos solubles -CHw-) en los primeros 10 cm de un Entic Haplustoll, bajo labranza convencional (CT), labranza vertical (VT) y siembra directa (NT). Los resultados mostraron que los contenidos de todas las fracciones de la materia orgánica analizadas fueron mayores en NT que en VT y CT en la mayoría de las fechas de muestreo. Todos los compuestos orgánicos fueron menos variables con el tiempo en NT y VT que en CT, de acuerdo con la menor alteración del suelo en NT y VT que en CT. Las fracciones más lábiles variaron en función de los cambios a corto plazo en las condiciones climáticas, principalmente la temperatura. Bajo sistemas de labranza que incluyeron perturbación del suelo, las fracciones más estables tendieron a decrecer y las más lábiles a aumentar con el tiempo. Esto fue atribuido a la transformación de las fracciones más estables en fracciones más lábiles, posiblemente debido a la ruptura de los agregados producida por la labranza que habría favorecido la mineralización. El Po fue el componente menos variable, aún bajo condiciones de labranza con alta perturbación del suelo. Las relaciones C/N, CHt/C y CHw/C evolucionaron de forma similar en todos los sistemas de labranza, indicando que los sistemas de labranza cambian la cantidad pero no la calidad de la SOM.

Stable and labile soil organic compounds play different roles in the soil. It is a question of how far soil organic matter (SOM) fractions with different labilities vary as a function of climatic and management conditions. In order to answer this question stable (organic C -C-, total N -N-, organic P -Po-), and labile SOM fractions (total carbohydrates -CHt- and hot water soluble carbohydrates -CHw-) were measured monthly for two years in the 10-cm soil top-layer of an Entic Haplustoll, under conventional tillage (CT), vertical tillage (VT) and no-till (NT). Results showed that contents of all analyzed organic fractions were higher in NT than in VT and CT in almost all sampling dates. All organic compounds were less variable with time in NT and VT than in CT, in agreement with the smaller soil disturbance of NT and VT compared to CT. The more labile fractions varied as a function of short term changes in the climatic conditions, mainly temperature. Under soil disturbing tillage systems, the most stable fractions tended to decrease and the more labile to increase with time. This was attributed to the transformation of the more stable into the more labile fractions, possibly due to the disruption of aggregates produced by tillage that favored SOM mineralization. Po was the less variable compound, even under the most disturbing tillage conditions. The quotients C/N, CHt/C and CHw/C evolved similarly in all tillage systems, indicating that that tillage systems change the amount but not the quality of SOM., Os compostos estáveis e lábeis da matéria orgânica têm papeis diferentes no solo. A forma como as frações da matéria orgânica do solo (SOM) com diferente labilidade variam em função das condições climáticas e das condições de maneio do solo é pouco conhecida. Para responder a esta questão mediram-se mensalmente, ao longo de dois anos, as frações estáveis (C orgânico –C-, N total-N-, P orgânico-Po-), e as frações lábeis da SOM (hidratos de carbono totais CHt- e hidratos de carbono solúveis -CHw-) na camada dos primeiros 10 cm de um Entic Haplustoll, submetido a lavoura convencional (CT), lavoura vertical (VT) e plantio direto (NT). Os resultados mostraram que os teores de todas as frações da matéria orgânica analisadas eram maiores no NT do que no VT e no CT na maioria das datas de amostragem. Em função do tempo, as concentrações de todos os compostos orgânicos eram menos variáveis em NT e VT do que em CT, o que está de acordo com a menor perturbação do solo em NT e VT comparada com a do CT. As frações mais lábeis variaram em função das alterações de curto prazo nas condições climáticas, nomeadamente a temperatura. Em condições de lavoura de maior perturbação do solo, as frações mais estáveis tenderam a diminuir e as mãos lábeis a aumentar com o tempo. Este facto foi atribuído à transformação das frações mais estáveis em frações mais lábeis, possivelmente devido à quebra dos agregados produzida pela lavoura, o que teria favorecido a mineralização da SOM. O Po foi o componente menos variável, mesmo sob condições de lavoura com alta perturbação do solo. As relações C/N, CHt/C e CHw/C evoluíram de maneira semelhante em todos os sistemas, indicando que os sistemas de lavoura mudam a quantidade mas não a qualidade da SOM., Los compuestos estables y lábiles de la materia orgánica juegan papeles diferentes en el suelo. Se desconoce cómo las fracciones de materia orgánica del suelo (SOM) de diferente labilidad varían en función de las condiciones climáticas y de manejo. Para dar respuesta a esta cuestión se midieron mensualmente a lo largo de dos años las fracciones estables (C orgánico -C-, N total-N-, P orgánico -Po-), y las fracciones lábiles (carbohidratos totales -CHt- y carbohidratos solubles -CHw-) en los primeros 10 cm de un Entic Haplustoll, bajo labranza convencional (CT), labranza vertical (VT) y siembra directa (NT). Los resultados mostraron que los contenidos de todas las fracciones de la materia orgánica analizadas fueron mayores en NT que en VT y CT en la mayoría de las fechas de muestreo. Todos los compuestos orgánicos fueron menos variables con el tiempo en NT y VT que en CT, de acuerdo con la menor alteración del suelo en NT y VT que en CT. Las fracciones más lábiles variaron en función de los cambios a corto plazo en las condiciones climáticas, principalmente la temperatura. Bajo sistemas de labranza que incluyeron perturbación del suelo, las fracciones más estables tendieron a decrecer y las más lábiles a aumentar con el tiempo. Esto fue atribuido a la transformación de las fracciones más estables en fracciones más lábiles, posiblemente debido a la ruptura de los agregados producida por la labranza que habría favorecido la mineralización. El Po fue el componente menos variable, aún bajo condiciones de labranza con alta perturbación del suelo. Las relaciones C/N, CHt/C y CHw/C evolucionaron de forma similar en todos los sistemas de labranza, indicando que los sistemas de labranza cambian la cantidad pero no la calidad de la SOM.

Phosphorus (P), a non-renewable resource, needs to be used more efficiently in agriculture. This requires using soil P tests. However, the P test threshold values for fertilizer response depend on many soil properties, some of which may be useful to estimate these threshold values, others not. Therefore, we searched here which soil properties are useful to estimate P threshold values. We calculated the threshold values for Olsen P and 0.01 M CaCl2 extractable P of 18 representative agricultural soils of the Mediterranean region of Spain. For that, we performed a P starvation experiment in which wheat and sunflower were alternatively pot-cropped. Results show that Olsen P threshold values are negatively correlated to P buffer capacity (r of −0.74, P lower than 0.001), clay content (−0.82, 0.001), pH (−0.76, 0.001), and Fe oxide content (−0.55, 0.05). Multiple regression models involving clay, pH or soil organic C, and phosphatase activity or organic hydrolysable P accounted for as much as 87 % of the variance in calculated Olsen P threshold values. In particular, there is a major effect of organic P on Olsen P threshold values. Single models based on routinely measured soil properties such as clay content and pH made accurate predictions of Olsen P threshold values with r 2 of 0.81 and P lower than 0.001.

Phosphorus (P), a non-renewable resource, needs to be used more efficiently in agriculture. This requires using soil P tests. However, the P test threshold values for fertilizer response depend on many soil properties, some of which may be useful to estimate these threshold values, others not. Therefore, we searched here which soil properties are useful to estimate P threshold values. We calculated the threshold values for Olsen P and 0.01 M CaCl2 extractable P of 18 representative agricultural soils of the Mediterranean region of Spain. For that, we performed a P starvation experiment in which wheat and sunflower were alternatively pot-cropped. Results show that Olsen P threshold values are negatively correlated to P buffer capacity (r of −0.74, P lower than 0.001), clay content (−0.82, 0.001), pH (−0.76, 0.001), and Fe oxide content (−0.55, 0.05). Multiple regression models involving clay, pH or soil organic C, and phosphatase activity or organic hydrolysable P accounted for as much as 87 % of the variance in calculated Olsen P threshold values. In particular, there is a major effect of organic P on Olsen P threshold values. Single models based on routinely measured soil properties such as clay content and pH made accurate predictions of Olsen P threshold values with r 2 of 0.81 and P lower than 0.001.

Phosphorus (P), a non-renewable resource, needs to be used more efficiently in agriculture. This requires using soil P tests. However, the P test threshold values for fertilizer response depend on many soil properties, some of which may be useful to estimate these threshold values, others not. Therefore, we searched here which soil properties are useful to estimate P threshold values. We calculated the threshold values for Olsen P and 0.01 M CaCl2 extractable P of 18 representative agricultural soils of the Mediterranean region of Spain. For that, we performed a P starvation experiment in which wheat and sunflower were alternatively pot-cropped. Results show that Olsen P threshold values are negatively correlated to P buffer capacity (r of −0.74, P lower than 0.001), clay content (−0.82, 0.001), pH (−0.76, 0.001), and Fe oxide content (−0.55, 0.05). Multiple regression models involving clay, pH or soil organic C, and phosphatase activity or organic hydrolysable P accounted for as much as 87 % of the variance in calculated Olsen P threshold values. In particular, there is a major effect of organic P on Olsen P threshold values. Single models based on routinely measured soil properties such as clay content and pH made accurate predictions of Olsen P threshold values with r 2 of 0.81 and P lower than 0.001.

Phosphorus (P), a non-renewable resource, needs to be used more efficiently in agriculture. This requires using soil P tests. However, the P test threshold values for fertilizer response depend on many soil properties, some of which may be useful to estimate these threshold values, others not. Therefore, we searched here which soil properties are useful to estimate P threshold values. We calculated the threshold values for Olsen P and 0.01 M CaCl2 extractable P of 18 representative agricultural soils of the Mediterranean region of Spain. For that, we performed a P starvation experiment in which wheat and sunflower were alternatively pot-cropped. Results show that Olsen P threshold values are negatively correlated to P buffer capacity (r of −0.74, P lower than 0.001), clay content (−0.82, 0.001), pH (−0.76, 0.001), and Fe oxide content (−0.55, 0.05). Multiple regression models involving clay, pH or soil organic C, and phosphatase activity or organic hydrolysable P accounted for as much as 87 % of the variance in calculated Olsen P threshold values. In particular, there is a major effect of organic P on Olsen P threshold values. Single models based on routinely measured soil properties such as clay content and pH made accurate predictions of Olsen P threshold values with r 2 of 0.81 and P lower than 0.001.

Phosphorus (P), a non-renewable resource, needs to be used more efficiently in agriculture. This requires using soil P tests. However, the P test threshold values for fertilizer response depend on many soil properties, some of which may be useful to estimate these threshold values, others not. Therefore, we searched here which soil properties are useful to estimate P threshold values. We calculated the threshold values for Olsen P and 0.01 M CaCl2 extractable P of 18 representative agricultural soils of the Mediterranean region of Spain. For that, we performed a P starvation experiment in which wheat and sunflower were alternatively pot-cropped. Results show that Olsen P threshold values are negatively correlated to P buffer capacity (r of −0.74, P lower than 0.001), clay content (−0.82, 0.001), pH (−0.76, 0.001), and Fe oxide content (−0.55, 0.05). Multiple regression models involving clay, pH or soil organic C, and phosphatase activity or organic hydrolysable P accounted for as much as 87 % of the variance in calculated Olsen P threshold values. In particular, there is a major effect of organic P on Olsen P threshold values. Single models based on routinely measured soil properties such as clay content and pH made accurate predictions of Olsen P threshold values with r 2 of 0.81 and P lower than 0.001.

Phosphorus (P), a non-renewable resource, needs to be used more efficiently in agriculture. This requires using soil P tests. However, the P test threshold values for fertilizer response depend on many soil properties, some of which may be useful to estimate these threshold values, others not. Therefore, we searched here which soil properties are useful to estimate P threshold values. We calculated the threshold values for Olsen P and 0.01 M CaCl2 extractable P of 18 representative agricultural soils of the Mediterranean region of Spain. For that, we performed a P starvation experiment in which wheat and sunflower were alternatively pot-cropped. Results show that Olsen P threshold values are negatively correlated to P buffer capacity (r of −0.74, P lower than 0.001), clay content (−0.82, 0.001), pH (−0.76, 0.001), and Fe oxide content (−0.55, 0.05). Multiple regression models involving clay, pH or soil organic C, and phosphatase activity or organic hydrolysable P accounted for as much as 87 % of the variance in calculated Olsen P threshold values. In particular, there is a major effect of organic P on Olsen P threshold values. Single models based on routinely measured soil properties such as clay content and pH made accurate predictions of Olsen P threshold values with r 2 of 0.81 and P lower than 0.001.

Phosphorus (P) binding groups were identified in phytoplankton, settling particles, and sediment profiles by 31P NMR spectroscopy from the Swedish mesotrophic Lake Erken. The 31P NMR analysis revealed that polyphosphates and pyrophosphates were abundant in the water column, but rapidly mineralized in the sediment. Orthophosphate monoesters and teichoic acids degraded more slowly than DNA-P, polyphosphates, and P lipids. Humic acids and organic acids from phytoplankton were precipitated from the NaOH extract by acidification and identified by 31P NMR spectroscopy. The precipitated P was significantly more recalcitrant than the P compound groups remaining in solution, but does not constitute a major sink of P as it did not reach a stable concentration with depth, which indicates that it may eventually be degraded. Since P also precipitated from phytoplankton, the origin of humic-P can not be related solely to allochthonous P.

Phosphorus (P) binding groups were identified in phytoplankton, settling particles, and sediment profiles by P-31 NMR spectroscopy from the Swedish mesotrophic Lake Erken. The P-31 NMR analysis revealed that polyphosphates and pyrophosphates were abundant in the water column, but rapidly mineralized in the sediment. Orthophosphate monoesters and teichoic acids degraded more slowly than DNA-P, polyphosphates, and P lipids. Humic acids and organic acids from phytoplankton were precipitated from the NaOH extract by acidification and identified by P-31 NMR spectroscopy. The precipitated P was significantly more recalcitrant than the P compound groups remaining in solution, but does not constitute a major sink of P as it did not reach a stable concentration with depth, which indicates that it may eventually be degraded. Since P also precipitated from phytoplankton, the origin of humic-P can not be related solely to allochthonous P.