Phosphorus and Plant Growth

After reading this article you will learn about the role of phosphorus in relation to metabolic functions, nutrition and growth of plants.

The role of phosphorus as a constituent of macro molecular structures is the most prominent in nucleic acids as units of DNA molecule which carry the genetic information and as units of RNA which are responsible for the translation of the genetic information.

In both DNA and RNA, phosphate forms a bridge between ribonucleoside units to form macro-molecules as follows:

As a Structural Component:

Phosphorus is responsible for the acidic nature of nucleic acids and thus for the exceptionally high cation concentration in DNA and RNA structures. The bridging form of phosphorus diester is also present in the phospholipids of bio-membranes forming a bridge between a diglyceride and another molecule (amino acid, amine or alcohol).

The functions of phospholipids are related to their molecular structure. Phosphate esters (C—P) and energy rich phosphates (P~P) represent the metabolic machinery of cells. Out of 50 esters, about 10 of which are present in relatively high concentrations in cells.

The common structure of phosphate esters is appended below:

Role of Phosphorus in Metabolic Function:

Energy Transfer:

Most phosphate enters are intermediates in metabolic pathways of biosynthesis and degradation. Their function and formation are directly related to the energy metabolism of the cells and to energy rich phosphates. As for example, the energy required for the biosynthesis of starch or ion uptake is supplied by an energy rich intermediate or co-enzyme chiefly, ATP:

Energy liberated during glycolysis, respiration and photosynthesis is used for the synthesis of the energy-rich pyrophosphate bond, and on hydrolysis of this bond 30 kJ per mole ATP (Adenosine tri-phosphate) is released.

This energy can be transmitted with the phosphoryl group in a phosphorylation reaction to another compound resulting activation of this compound as follows:

ATP is the principal energy rich phosphate required for starch synthesis. The activity of ATPases carrying out the hydrolysis as well as role in energy transfer has been found to be affected by various factors.

Phosphate is taken up by plant cells against a very steep concentration gradient as the concentration of phosphate in the root cells and xylem sap is about 100 to 1000 times higher than that of the soil solution. The uptake is active. A plasma lemma located ATPase pumps H⁺ ions into the apoplast to protonate the phosphate carrier. The ATPase activity is expected to have an impact on the phosphate uptake.

The capabilities for active uptake of phosphate vary with the nature and type of plants, varieties with the plant species. It is evident that about 80% of the phosphate absorbed is incorporated into organic compounds (hexose phosphates and uridine diphosphate etc.) phosphate is readily mobile in the plant and is trans-located in both directions (upward and downward).

The downward movement occurs mainly in the phloem with the help of main P carrier phosphory/choline. Besides, inorganic P is also present in the phloem sap indicating that inorganic P plays a major role in phloem transport.

Phosphate in the plant occurs in inorganic form as orthophosphate and pyrophosphate.

The organic forms of phosphate are compounds in which orthophosphate is esterified with hydroxyl groups of sugars and alcohols or bound by a pyrophosphate bond to another phosphate group as follows:

Various other organic phosphatic compounds have been discussed earlier along with their metabolic functions. A high import of inorganic phosphate via the phosphate trans-locator into the chloroplast also depresses starch synthesis and promotes the export of phosphoglycerate and triose phosphates from the chloroplast. In phosphorus deficient plants, the amount of inorganic phosphate of stems and leaves and the phytin phosphate content of seeds and fruits are decreased.

P deficiency:

In phosphorus deficient plants, the growth is retarded and shoot/root dry matter ratio usually decreases. In cereal crops, tillering is affected. In case of fruit trees, the fruit and seed formation is drastically affected by P deficiency. All these disorders not only give low yields but also poor quality of products.

Usually the symptoms of P deficiency appear in the older leaves which are darkish green colour. The stems of many plants suffering from P deficiency are characterised by a reddish colouration originating from an increased production of anthocyanins. The leaves of P deficient fruit trees are frequently finged with brownish colour.

The P contents in the P deficient plants are usually low with about 0.1% P or less in the dry matter and in cereals and herbage about 0.3 to 0.4% P during the vegetative growth stage. Under conditions of P deficiency, phosphorus is withdrawn from older tissue and trans-located to meristematic tissue, where metabolism is more rapid.

Role of Phosphorus in Growth of Plant:

The requirement of phosphorus for the optimum growth is in the range of 0.3 to 0.5% (dry weight basis) during the vegetative stage of the plant growth. The possibility of P toxicity increases at a concentration more than 1% in dry matter of the plant. However, this concentration in creating P toxicity varies with the nature and plant varieties.

In plants suffering from P deficiency, leaf expansion, leaf area and number of leaves are reduced. Very oftenly, the chlorophyll content is even increased under P deficiency, and the leaves have a dark green colour as cell and expansion of leaves are more reduced than chloroplast and chlorophyll formation.

However, the photosynthetic efficiency per unit of chlorophyll is much lower in phosphorus deficient plants. The growth of root is relatively less inhibited than that of the shoot growth under phosphorus deficiency decreasing in the shoot/root dry weight ratio.

Despite adaptive responses in increasing phosphorus acquisition by roots, not only is shoot growth rate retarded by phosphorus limitation but also the formation of reproductive organs. Flower initiation is delayed, the number of flowers decreased and the formation of seed restricted due to P deficiency. Premature senescence of leaves is another factor limiting seed yield in phosphorus deficient plants.

Phosphorus is particularly important for leguminous crops because of its influence on the activity of the Rhizobium bacteria. Adequate supply of phosphorus to potatoes enhances the phosphate esterification of starch in potato tubers and thus improves starch quality of potato.

P Dynamics in Soil—Rhizosphere-Plant Continuum:

Soil P Transformation:

Soil P exists in various chemical forms including inorganic P (Pi) and organic P (Po). These P forms differ in their behaviour fate in soils. Pi usually accounts for 35% to 70% of total P in soil.

Primary P minerals including apatites, strengite, and variscite are very stable, and the release of available P from these minerals by weathering is generally too slow to meet the crop demand through direct application of phosphate rocks (i.e. apatites) has proved relatively efficient for crop growth in acidic soils.

In contrast secondary P minerals including calcium (Ca), iron (Fe), and aluminum (Al) phosphates vary in their dissolution rates, depending on size of mineral particles and soil pH. With increasing soil pH, solubility of Fe and Al phosphates increases but solubility of Ca phosphate decreases, except for pH values above 8.

The P adsorbed on various clay and Al/Fe oxides can be released by desorption reactions. All these P forms exist in complex equilibria with each other, representing from very stable, sparingly available, to plant-available P pools such as labile P and solution P (Fig. 21.6).

P Dynamics in the Soil/Rhizosphere-Plant Continuum:

In acidic soils, P can be dominantly adsorbed by Al/Fe oxides and hydroxides and hydroxides, such as gibbsite, hematite and goethite. P can be first adsorbed on the surface of clay minerals and Fe/Al oxides by forming various complexes.

The non-protonated and protonated bi-dentate surface complexes may coexist at pH 4 to 9, while protonated bi-dentate inner-sphere complex is pre-dominant under acidic soil conditions.

Clay minerals and Fe/Al oxides have large specific surface areas, which provide large number of adsorption sites. The adsorption of soil P can be enhanced with increasing ionic strength. With further reactions, P may be occluded in Nano pores that frequently occur in Fe/Al oxides, and thereby become unavailable to plants.

In neutral-to-calcareous soils, P retention is dominated by precipitation reactions, although P can also be adsorbed on the surface of Ca carbonate and clay minerals. Phosphate can precipitate with Ca, generating di-calcium phosphate (DCP) that is available to plants.

Ultimately, DCP can be transformed into more stable forms such as octocalcium phosphate and hydroxyapatite (HAP), which are less available to plants at alkaline pH.

HAP accounts for more than 50% of total Pi in calcareous soils from long-term fertilizer experiments (H, Li, personal communication). HAP dissolution increases with decrease of soil pH, suggesting that rhizosphere acidification may be an efficient strategy to mobilize soil P from calcareous soil.

Po generally accounts for 30% to 65% of the total P in soils. Soils Po mainly exists in stabilized forms as inositol phosphates and pbosphonates, and active forms as orthophosphate diesters, labile orthophosphate monoesters, and organic polyphosphates. The Po can be released through mineralization processes mediated by soil organisms and plant roots in association with phosphatase secretion.

These processes are highly influenced by soil moisture, temperature, surface physical-chemical properties, and soil pH and Eh (for redox potential). Po transformation has a great influence on the overall bioavailability of P in soil.

Therefore, the availability of soil P is extremely complex and needs to be systemically evaluated because it is highly associated with P dynamics and transformation among various P pools (Fig. 21.6).

Chemical Fertilizer P in Soil:

The modern terrestrial P cycle is dominated by agriculture and human activities. The concentration of available soil Pi seldom exceeds 10 pm, which is much lower than that in plant tissues where the concentration is approximately 5 to 20 mm Pi. Because of the low concentration and poor mobility of plant-available P in soils, applications of chemical P fertilizers are needed to improve crop growth and yield.

The major forms of phosphate fertilizers include mono-calcium phosphate (MCP) and mono-potassium phosphate. Application of MCP can significantly affect soil physicochemical properties. After application to soil, MCP undergoes a wetting process, generates large amounts of protons, phosphate, and DCP, and eventually forms a P-saturated patch.

This Pi-saturated patch forms three different reaction zones including direct reaction, precipitation reaction, and adsorption reaction zones. The direct reaction zone is very acidic (pH = 1.0-1.6), resulting in enhanced mobilization of soil metal ions. These metal ions can also react with high concentrations of Pi in the zone thus causing further precipitation of Pi.

The amorphous Fe-P and Al-P that thereby form can be partly available to plants. In calcareous soil, new complexes of MCP and DCP can be formed and with time DCP is gradually transformed into more stable forms of Ca phosphates (octocalcium phosphate or apatite). Because the Pi concentration is relatively low, P adsorption by soil minerals is dominant is the outer zone.

In contrast, the application of mono-potassium phosphate has little influence on soil physical and chemical properties. Therefore, matching P fertilizer types with soil physical and chemical properties may be an efficient strategy for rational use of chemical fertilizer P.

Manure P in Soil:

Manure can be applied to soil to increase P fertility. The total P content in manure is very variable and nearly 70% of total P in manure is labile. In manure, Pi accounts for 50% to 90%. Manure also contains large amounts of Po, such as phospholipids and nucleic acids, which can be released to increase, soil Pi concentrations by mineralization.

Furthermore, small molecular organic acids from mineralization of humic substances in manure can dissolve Ca phosphate, and especially for citrate, it can efficiently weaken the nanoparticle stability of HAP, by controlling the free Ca availability and thereby the nucleation rate. P adsorption to soil particles can be greatly reduced through applying organic substances.

The humic acids contain large numbers of negative charges, carboxyl and hydroxyl groups, which strongly compete for the adsorption sites with Pi. Manure can also change soil pH and thus alter soil P availability. However, mechanisms of manure-induced P transformation processes between Pi and Po in soil still need further investigation.

P Dynamics in the Rhizosphere:

The rhizosphere is the critical zone of interactions among plants, soils and micro­organisms. Plant roots can greatly modify the rhizosphere environment through their various physiological activities, particularly the exudation of organic compounds such as mucilage, organic acids, phosphatases, and some specific signaling substances, which are key drivers of various rhizosphere processes.

The chemical and biological processes in the rhizosphere not only determine mobilization and acquisition of soil nutrients as well microbial dynamics, but also control nutrient-use efficiency of crops, and thus profoundly influence crop productivity.

Due to its low solubility and mobility in soil, P can be rapidly depleted in the rhizosphere by root uptake, resulting in a gradient of P concentration in a radial direction away from the root surface.

In spite of total soil P content usually exceeding the plant requirements, the low mobility of soil P can restrict its availability to plants. Soluble P in the rhizosphere soil solution should be replaced 20 to 50 times per day by P delivery from bulk soil to the rhizosphere to meet plant demand.

Therefore, P dynamics in the rhizosphere are mainly controlled by plant root growth and function, and also highly related to physical and chemical properties of soil.

Because of the unique properties of P in soil such as low solubility, low mobility and high fixation by the soil matrix, the availability of P to plants is dominantly controlled by two key processes (Fig. 21.6): (1) spatial availability and acquisition of P in terms of plants root architecture as well as mycorrhizal association, and (2) bioavailability and acquisition of P based on the rhizosphere chemical and biological processes.

Spatial Availability and Acquisition of Soil P:

Root Architecture Plants are able to respond to P starvation by changing their root architecture, including root morphology, topology, and distribution patterns. Increases in root/shoot ratio, root branching, root elongation, root topsoil foraging, and root hairs are commonly observed in P-deficient plants, while the formation of specialized roots such as cluster roots occurs in a limited number of species.

P deficiency has been shown to reduce growth of primary roots and enhance length and density of root hairs and lateral roots in many plant species. The P-efficient genotypes of common bean (Phaseolus vulgaris) have more shallow roots in the topsoil where there are relatively high contents of P resources.

Some plant species, for example while lupin (Lupinus albus), can develop cluster roots with dense and determinative lateral roots, which are covered by large numbers of root hairs. Therefore, root architecture plays an important role in maximizing P acquisition because root systems with higher surface area are able to explore a given volume of soil more effectively.

Some adaptive modifications in root architecture in response to P deficiency are well documented in Arbidopsis (Arabidopsis thaliana) and in those species forming cluster roots.

Adaptive changes or root growth and architecture under P starvation are related to altered carbohydrate distribution between roots and shoots, and these changes may be caused by plant hormones, sugar signaling and nitric oxide in the case of cluster-root formation in white lupin.

Root proliferation is stimulated when plant roots encounter nutrient-rich patches, particularly when the patches are rich in P and/or nitrogen. The root proliferation in P-rich topsoil layers is related to a decreased root gravitropic response under P limitation, and ethylene may be involved in the regulation of these responses.

Root proliferation can be greatly stimulated in the P-enriches soil patches. However, the mechanisms of P-dependent changes in root proliferation in response to local P supply are not fully understood. Localized application of phosphates plus ammonium significantly enhances P uptake and crop growth – through stimulating root proliferation and rhizosphere acidification in calcareous soil.

Mycorrhizal Association:

Mycorrhizal symbioses can increase the spatial availability of P, extending the nutrient absorptive surface by formation of mycorrhizal hyphae. Arbuscular mycorrhizal fungi (AMF) form symbiotic associations with the roots of about 74% of angiosperms. In the symbioses, nutrients are transferred by AMF via their extensive mycorrhizal mycelium to plants while in return the fungi receive carbon from the plant.

AMF not only influence plant growth through increased uptake of nutrients (e.g. P, zinc and copper), but may also have non-nutritional effects in terms of stabilization of soil aggregates and alleviation of plant stresses caused by biotic and abiotic factors.

The beneficial effects of AMF and other micro-organisms on plant performance and soil health can be very important for the sustainable management of agricultural ecosystems.

A primary benefit of AMF is the improved P uptake conferred on symbiotic plants. In low-P soils mycorrhizal plants usually grow better than non-mycorrhizal plants as a consequence of enhanced direct P uptake of plant roots via the AM pathway.

However, plant growth can be suppressed even though the AM pathway contributes greatly to plant P uptake. The growth inhibitions might be caused by the down-regulation of the direct root P-uptake pathway.

Bioavailability and Acquisition of Soil P:

Root-induced chemical and biological changes in the rhizosphere play a vital role in enhancing the bioavailability of soil P. These root-induced changes mainly involve proton release to acidify the rhizosphere, carboxylate exudation to mobilize sparingly available P by chelation and ligand exchange, and secretion of phosphatases or phytases to mobilize Po by enzyme-catalyzed hydrolysis.

Root-induced acidification can decrease rhizosphere pH by 2 to 3 units relative to the bulk soil, resulting in substantial dissolution of sparingly available soil P. The pH change in the rhizosphere is mainly affected by cation/anion uptake ratios and nitrogen assimilation. Ammonium supply to plant roots causes rhizosphere acidification, whereas nitrate supply causes alkalization.

Legumes take up excess cations over anions, resulting in proton release. P deficiency in white lupin stimulates proton release and citrate exudation by cluster roots in association with an inhibition of nitrate uptake. The changes of rhizosphere pH are also related to soil-buffering capacity, microbial activities, and plant genotypes.

Besides proton release, carboxylate exudation such as that of citrate, malate, and oxalate greatly enhances Pi acquisition through chelation as well as by ligand exchange. Organic acid excretion and function in increasing P mobilization is well documented.

However, the mechanisms of soil P mobilization by carboxylates especially the relative contributions of ligand exchange, ligand- promoted dissolution of P-bearing minerals such as Fe/Al oxides, complexation of Al, Ca, or Fe and changes in solution P speciation, and carboxylate adsorption promoting changes in surface changes on clays and Al/Fe oxides are not fully understood despite some progress on the physiological control of carboxylate synthesis and excretion.

Involvement of anion channels in organic acid excretion is confirmed by the action of anion channel blockers. It is found that the Al-activated malate transporter mediates malate exudation and the multi­drug and toxic compound extrusion transporter mediates citrate exudation and both may confer Al resistance.

Plants can secrete phosphatase to mobilize Po through enzyme-catalyzed hydrolysis. The activities of phosphatases are up-regulated under P deficiency. However, the efficacy of these phosphohydrolases can be greatly altered by the availability of substrate, interactions with soil micro-organisms, and soil pH, depending on soil physical and chemical environments.

Therefore, there is often on significant correlation between the phosphatase activity and plant growth performance in acidic or calcareous soils. Moreover, carboxylate exudation may have strong interactions with soil, resulting in a low efficiency in P mobilization.

Therefore, root- induced bioavailability and acquisition of P in association with root exudation should be systemically evaluated in the soil/rhizosphere-plant continuum (Fig. 21.6).

Some soil and rhizosphere micro-organisms except mycorrhizal fungi (for example, plant growth promoting rhizobacteria, particularly P-solubilizing bacteria [PSB] and fungi [PSF] can also enhance plant P acquisition by directly increasing solubilization of P to plants, or by indirect homone-induced stimulation of plant growth.

P-solubilizing micro-organisms (PSM; PSB plus PSF) account for approximately 1% to 50% in P solubilization potential. The PSB or PSF may mobilize soil P by the acidification of soil, the release of enzymes (such as phosphatases and phytases), or the production of carboxylates such as gluconate, citrate, and oxalate.

P Uptake and Utilization by Plants:

Plant roots absorb P as either of H₂PO₄^– or HPO₄^2-. Because the concentrations of these ions in soils are in the micro-molar range, high-affinity active transport systems are required for Pi uptake against a steep chemical potential gradient across the plasma membrane of root epidermal and cortical cells.

This process is mediated by high-affinity Pi/H⁺ symporters that belong to the PHT1 gene family. Disruption of PHT1 gene expression results in a significant decrease of P acquisition by roots. Most of the Pi taken up by roots is loaded into the xylem and subsequently trans-located into shoots.

Two rice (Oryza sativa) phosphate transporters OsPht1; 2 and OsPht1; 6 with different kinetic properties are involved in Pi translocation from roots to shoots. The putative regulators PHO1 and PHO1; H1 containing the SPX (for SYG/PHO81/XPR1) tripartite domain also contribute to Pi translocation through loading Pi to the xylem.

Within plant cells, P is a major component of nucleic acids, membrane lipids, and phosphorylated intermediates of energy metabolism. Thus, the cellular Pi homeostasis is essential for physiological and biochemical processes.

Under P deficiency, plants can develop adaptive responses not only to facilitate efficient Pi acquisition and translocation, but also to utilize efficiently stored P by adjusting Pi recycling internally, limiting P consumption, and reallocating P from old tissues to young and/or actively growing tissues.

Although 85% to 95% of the cellular P is present in the vacuole, ³¹P-NMR studies reveal that the Pi efflux from the vacuole is insufficient to compensate for a rapid decrease of the cytosolic Pi concentration during P starvation. By contrast, a phosphate transporter PHT4;6 is located in the Golgi membrane, probably transporting Pi out of the Golgi luminal space for the recycling of the Pi released from glycosylation.

Another phosphate transporter PHT2; 1 is present in the chloroplast, and can affect allocation of Pi within the plant. Releasing Pi from organic sources, such as phospho-monoesters and nucleic acids, is also an important step for internal P-recycling processes.

Phosphatases are needed to release Pi from phsophomonoesters, and a dual-targetted purple acid phsophatase isozyme AtPAP26 (intracellular and secreted APase) is essential for efficient acclimation of Arabidopsis to P deprivation.

Ribonucleases are responsible for P mobilization from RNA, and two genes (AtRNSl and AtRNS2) are up- regulated by P starvation. These phosphatase and ribonuclease genes are also induced by leaf senescence, further supporting their important role in the P remobilization process.

To limit P consumption, membrane lipid composition can be altered to some extent through a decrease of phospholipids and an increase of non-P lipids under P limitation. Degradation of phospholipids into Pi and diacylglycerol is mediated by phospholipases C and D, which are essential for lipid turnover in plants acclimating to P deficiency.

Diacylglycerol is subsequently converted into galactolipids or sulfolipids by two enzymes SQD1 and SQD2 to functionally substitute for phospholipids. In addition, plants can also use the alternative cellular respiratory pathways bypassing the adenylate and Pi reaction for reduction of P demand under P starvation.

Taken together, plants have developed a series of adaptive responses to take up and utilize P efficiently, including morphological, physiological, and biochemical responses (Fig.21.6). This complex network is required to control Pi nutrition in plants either locally or systemically.

Indeed, many key players within the network, such as transcriptional factors (PHR1), microRNA (MiR399), and ubiquitin E2 conjugase (PHO2), are able to regulate PHT, PHO1, and RNA genes at different regulatory levels. In addition, the sugar-signaling and hormonal networks are also involved in the Pi response.

Strategies for Improving P Efficiency in the Soil/Rhizosphere-Plant Continuum:

Better understanding of P dynamics in the soil/rhizosphere-plant continuum provides an important basis for optimizing P management to improve P-use efficiency in crop production. The effective strategies for P management may involve a series of multiple-level approaches in association with soil, rhizosphere, and plant processes.

P input into farmland can be optimized based on the balance of inputs/outputs of P. Soil-based P management requires a long-term management strategy to maintain the soil-available P supply at an appropriate level through monitoring soil P fertility because of the relative stability of P within soils.

By using this approach, the P fertilizer application can be generally reduced by 20% compared to farmer practice for the high-yielding cereals crops. This may be of significant important for saving P resources without sacrificing crop yields (Fig. 21.7) though it may cause P accumulation in soil due to high threshold levels and low P-use efficiency by crops.

Gap 1 for saving P input can be achieved by soil-based nutrient management for optimizing P supply to meet crop demand. Gap 2 can be realized by root/rhizosphere management for improving P-use efficiency and crop production through exploitation of root/ rhizosphere efficiency and further saving P resource input.

The solid curve represents crop productivity response to high-P input under intensive agriculture. The dotted curve represents crop productivity response to P input under soil-based P management. The dashed curve represents crop productivity response to P input under root/rhizosphere management.

Rhizosphere-based P management provides an effective approach to improving P-use efficiency and crop yield through exploitation of biological potential for efficient mobilization and acquisition of P by crops, and reducing the over-reliance on application of chemical fertilizer P (Fig. 21.7).

Localized application of P plus ammonium improved maize (Zea mays) growth by stimulating root proliferation and rhizosphere acidification in a calcareous soil, indicating the potential for field-scale modification of rhizosphere processes to improve nutrient use and crop growth. Faba bean (Vicia faba) can acidify its rhizosphere, whereas maize does not.

The enhanced P uptake and maize yields in the faba bean/maize intercropping system are mainly attributed to the rhizosphere interactions between the two plant species. Some soil and rhizosphere micro-organisms such as AMF and plant growth promoting rhizobacteria also contribute to plant P acquisition.

Alternatively, successful P management can be achieved by breeding crop cultivars or genotypes more efficient for P acquisition and use. Great progress has been made in traditional plant breeding programs towards selecting crop varieties for high P-use efficiency.

An example of efficient genotype was the wheat (Triticum aestivum) variety Xiaoyan 54 that secreted more carboxylates (e.g. malate and citrate) into the rhizospher than P-inefficient genotypes.

Another promising example was soybean (Glycine max) “BX10” with superior root traits that enable better adaptation to low-P soils. Some important root genetic traits have been identified with potential utility in breeding P-efficient crops, including root exudates, root hair traits, and topsoil foraging through basal or adventitious rooting.

In addition, the ability to use insoluble P compounds in soils can be enhanced by engineering crops to exude more phytase, which results from over expression of a fungal phytase gene.

The integration of genetically improved P efficient crops with advanced P management in the soil-plant system is important for improving nutrient use efficiency and sustainable crop production. This approach requires cooperative work between scientists from different disciplines in the crop, plant and soil sciences.

Issues involving P use in agriculture are becoming important in various fields beyond agronomy. In the past two decades, the amount of P cycling in intensive agriculture has been significantly changed.

The amount of P fertilizer (P) applied to farmland has increased dramatically from 1.18 Mt P in 1985 to 4.80 Mt P in 2005, and meanwhile the amount of P entering animal-production systems from crop-production systems in the form of phytate feed has increased by 3.7 fold from 0.31 to 1.44 Mt. P.

However, the proportion of animal manure P returned to the field decreased from 78% to 41%, resulting in 1.57 Mt P released into the environment. This is clearly a waste of P resources as well as an environmental risk.

The holistic P management involves a series of strategies such as increasing P uptake efficiency by plant and animal, reducing overuse of chemical fertilizer P, and improving recycling efficiency of manure P. It is suggested that employing the integrated approach of P management may reduce the use of chemical P fertilizer.