Roles of Soil Biota In P-transformation

A.  INTRODUCTION

Healthy soil is the foundation of the food system. It produces healthy crops that in turn nourish people. Maintaining a healthy soil demands care and effort from farmers because farming is not benign. By definition, farming disturbs the natural soil processes including that of nutrient cycling – the release and uptake of nutrients.

Soil is a living, dynamic ecosystem. Healthy soil is teeming with microscopic and larger organisms that perform many vital functions including converting dead and decaying matter as well as minerals to plant nutrients. Different soil organisms feed on different organic substrates. Their biological activity depends on the organic matter supply.

The soil biota consist of a large number and range of micro- and macro-organisms and are the living part of soils. They interact with each other and with plants, directly providing nutrition and other benefits. They regulate their own populations as well as those of incoming microorganisms by biological control mechanisms. Microorganisms are responsible for organic matter decomposition and for the transformations of organically bound nitrogen and minerals to forms that are available to plants. Their physical structure and products contribute significantly to soil structure. Management practices have a significant impact on micro- and macro-organism populations and activities.

Phosphorus (P) is one of the major plant growth-limiting nutrients although it is abundant in soils in both inorganic and organic forms. Phosphate solubilizing micro-organisms (PSMs) are ubiquitous in soils and could play an important role in supplying P to plants in a more environmentally friendly and sustainable manner. Although solubilization of P compounds by microbes is very common under laboratory conditions, results in the field have been highly variable.

B.  DISCUSSION

P in soils is present in pools varying in availability, and pools with the lowest availability are the largest in Oxisols. The quantity of P present in the soil solution represents only a small fraction of plant needs, and the remainder must be obtained from the solid phase by a combination of abiotic and biotic processes. The processes involved in soil P transformation are precipitation- dissolution and adsorption-desorption which control the abiotic transfer of P between the solid phase and soil solution, and biological immobilization-mineralization processes that control the transformations of P between inorganic and organic forms.

With increasing demand of agricultural production and as the peak in global production will occur in the next decades, phosphorus (P) is receiving more attention as a nonrenewable resource. One unique characteristic of P is its low availability due to slow diffusion and high fixation in soils. All of this means that P can be a major limiting factor for plant growth. Applications of chemical P fertilizers and animal manure to agricultural land have improved soil P fertility and crop production, but caused environmental damage in the past decades. Maintaining a proper P-supplying level at the root zone can maximize the efficiency of plant roots to mobilize and acquire P from the rhizosphere by an integration of root morphological and physiological adaptive strategies. Furthermore, P uptake and utilization by plants plays a vital role in the determination of final crop yield. Taken together, overall P dynamics in the soil-plant system is a function of the integrative effects of P transformation, availability, and utilization caused by soil, rhizosphere, and plant processes.

Soil P exists in various chemical forms including inorganic P (Pi) and organic P (Po). These P forms differ in their behavior and 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 though 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. 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 clays 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.

In acidic soils, P can be dominantly adsorbed by Al/Fe oxides 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 nonprotonated and protonated bidentate surface complexes may coexist at pH 4 to 9, while protonated bidentate innersphere complex is predominant 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 nanopores 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 dicalcium 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. Soil Po mainly exists in stabilized forms as inositol phosphates and phosphonates, 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.

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 cereal crops in the North China. This may be of significant importance for saving P resources without sacrificing crop yields though it may cause P accumulation in soil due to high threshold levels and low P-use efficiency by crops.

C.  CONCLUSIONS

Phosphorus losses from soils occur by leaching at very low rates in undisturbed. The implementation of  intensive agricultural production has markedly increased P losses from soils through increased runoff, erosion and leaching, which in turn can have adverse effects on water quality. These losses are further increased by the excessive accumulation of bioavailable P in the upper soil horizons, due either to application of inorganic and/or organic P fertilizers in excess of plant needs and/or to inappropriate fertilizer applications.

The P nutrition of plants is predominantly controlled by P dynamics in the soil/rhizosphere-plant continuum. The distribution and dynamics of P in soil has a significant spatio-temporal variation. Root architecture that distributes more roots to the place where P resources are located plays an important role in efficiently exploiting these P resources. Furthermore, root architecture can exhibit functional coordination with root exudation of carboxylates, protons, and phosphatases in P mobilization and acquisition. The coordination of plant adaptations in root morphology and root physiology to P-limiting environments may effectively match heterogeneous P supply and distribution in soil, resulting in increased spatial availability and bioavailability of soil P. The integration of P dynamics from soil to plant via the rhizosphere provides a comprehensive picture of available P behavior and efficient acquisition in association with plant adaptive strategies. In the past two decades, significant progress has been made in understanding soil, rhizosphere, and plant processes associated with soil P transformation, P mobilization and acquisition, and P-deficiency responses.

Given the importance of P to plants and its importance as a strategic resource, a better understanding of P dynamics in the soil/rhizosphere-plant continuum is necessary to guide establishment of integrated P-management strategies involving manipulation of soil and rhizosphere processes, development of P-efficient crops, and improving P-recycling efficiency.

BIBLIOGRAPHY

Bot, Alexandra. and Jose Benites. 2005. The Importance of Soil Organic Matter. Food and Agriculture Organization of The United Nations (FAO). Rome, Italy. ftp://ftp.fao.org/agl/agll/docs/sb80e.pdf

Cardoso, Irene M. and Thomas W. 2006. Kupyer. Mychorrhizas and tropical soil fertility. Agriculture, Ecosystems and Environment 116 (2006) 72–84. http://dels-old.nas.edu/banr/gates1/docs/mtg5docs/bgdocs/mycorrhizas_tropical_soil.pdf

Frossard, et al. 2000. Processes Governing Phosphorus Availability in Temperate Soils. J. Environ. Qual. 29:15-23. http://researcharchive.lincoln.ac.nz/dspace/bitstream/10182/1134/1/Temperate_Soils.pdf

Gyaneshwar, et al. 2002. Role of soil microorganisms in improving P nutrition of plants. Journal of Plant and Soil. Volume 245, Number 1 (2002), 83-93, DOI: 10.1023/A:1020663916259. http://www.springerlink.com/content/g0710v2x185727kp/

Roper, MM. and V Gupta. 2012. Management-practices and soil biota. Australian Journal of Soil Research 33(2) 321 – 339. http://www.publish.csiro.au/paper/SR9950321.htm

Shen, et al. 2011. Phosphorus Dynamics: From Soil to Plant. Plant Physiology July 2011 vol. 156 no. 3 997-1005. http://www.plantphysiol.org/content/156/3/997.full