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Giot Comme G. Laporte, L. Quesnel, CNRS. Hinguant, Boujot, Nouveaux acquis et nouvelles perspectives de recherche 1 2? Gavrinis Table des marchands 2m 3 0 4 Fig. Solers, Joussaume dir. Gomez De Soto, Laporte, CNRS. Constructions circulaires enclos, greniers?

Prototype for Sustainable Agricultures

Laporte, R. Joussaume, CNRS. E groupe de Cerny M.

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Sabourin, Eds. J Agric Sci The World of Organic Agriculture.

Statistics and Emerging Trends Watson Abstract Phosphorus P is a major nutrient for all living organisms and a key production factor in agriculture. In crop production, it is usually supplied to soils through fertilisers or recycled manure and compost. Organic production guidelines ban the use of highly soluble, manufactured P fertilisers and, thus, recommend recycling P from livestock manure and compost.

In this chapter, after an overview of P dynamics in soils, we explore the consequences of such guidelines in terms of fieldand farm-gate P budget, soil P availability and crop productivity. Moreover, we propose some avenues for the more effective use of P resources, ranging from rhizosphere-based processes e.

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Finally, the potential benefits of these options are compared with respect to soil P status, field- and farm-p budgets. Nesme et al. Its scarcity in soils results in P being a limiting factor for crop production in many soils Cordell et al. Crop production results in substantial off-take of P, making it necessary to replace P outputs by P inputs over the long term in order to avoid depleting the soil P reserve except in soils with high P reserves; see Section 4.

IFOAM principles state that organic agricultural production is to be based on ecological processes and recycling. Inputs to organic farms should be reduced by reuse, recycling and efficient management of materials in order to maintain and improve environmental quality and conserve resources. Therefore, organic agriculture should strive to attain ecological balance through the design of farming systems, the establishment of habitats and the maintenance of genetic and agricultural diversity like in natural ecosystems This is particularly true for nutrients such as P since the available P reserve in many soils is not large even if notable exceptions to this exist , and P is not renewable in the same way as nitrogen since there is no notable atmospheric P reservoir.

Moreover, manufactured, chemical P-fertilisers are banned in organic production guidelines. Only some types of P-containing products can thus be used. For example, European organic production regulations allow only two types of products: rock phosphates and P-containing organic materials Council Regulation EC No. Virtually all P-containing organic materials are derived directly or indirectly from rock phosphates.

They are generally extracted from sedimentary deposits that contain apatite-like calcium phosphate minerals and are mainly located in North Africa, China and the USA Cordell et al. However, rock phosphate reserves are facing over-exploitation, dissipation and poor recycling. Their depletion is projected over the next years, depending on food and feed demand Van Vuuren et al.

Therefore, in the coming years, rock phosphate prices are likely to rise. These issues raise questions about the sustainability of P management in organic cropping systems. First, what are the consequences of organic cropping and farming systems for soil P status and crop yields? Second, can we identify some avenues for the better use of soil P reserves by taking advantage of the functional diversity of plants and soil organisms in the rhizosphere i.

In this chapter, we will introduce some basics about the fate of P in soils and P management. We will then focus on the options for better use of soil P reserves through an understanding of the fate of P in low-input soils, particularly considering rhizosphere dynamics.

Aveyron parc régional des Grands Causses, Natuurreservaat Grands Causses

Finally, we will discuss the consequences of current cropping and farming practices for P management in organic systems and will identify options for better P management at both the field and farm levels. The different numbers refer to the main process affecting the P pools: 1, adsorption; 2, desorption; 3, precipitation; 4, dissolution; 5, organisation; 6, mineralisation 2. Functionally, soil solution P is of utmost importance since crop roots can take up phosphate ions from this pool alone. However, these pools are interconnected and their respective dynamics are strongly influenced by cropping practices as is shown below.

The sum of the different pools represents the total soil P. Its content varies considerably with soil type and fertiliser history Richardson et al. It commonly ranges from to mg P kg 1, but can be as little as mg P kg 1 in deeply weathered soils, or reach several thousand mg P kg 1 in heavily fertilised soils that can be found in regions of intensive pig farming and pig slurry application in Denmark, the Netherlands, Catalonia in Spain or Brittany in France.

Inorganic P is bound to a range of P-bearing compounds, namely i positively-charged minerals predominantly, metal oxides and clay minerals onto which phosphate ions are. In neutral to alkaline soils, they are predominantly made up of the least soluble apatite-like calcium phosphates as well as more soluble octocalcium phosphate and dicalcium phosphate Freeman and Rowell ; Lindsay et al. In acidic soils, iron phosphates such as strengite and aluminium phosphates such as variscite can occur as well Hinsinger ; Kizewski et al.

Soil organic matter can also be involved in surface complexation processes that control the fate of phosphate ions in soils. Organic matter contains P that makes up the bulk of soil organic P. Their total amount and proportion can vary according to the content of organic matter, fertiliser history and vegetation.

Organic Farming, Prototype for Sustainable Agricultures

Turner et al. Organic P is not directly available to plants since it requires hydrolysis by phosphatase-like enzymes, which are produced by plants and, more so, by many soil microorganisms. Another pool of soil P is the microbial biomass P, i. However, as explained above, P is strongly bound to the solid fraction of the soil, either as inorganic or organic compounds with low solubility.

Thus, their diffusion hardly extends over distances greater than 1 mm over a few days Hinsinger et al. As a consequence, the P concentration in the soil solution is much lower than the so-called extractable or labile soil P, and considerably lower than the total soil P content Hinsinger ; Pierzynski et al. Typical concentrations of phosphate ions in the soil solution range from 0.

This makes the P concentration in soil solution the first key indicator of soil P availability.

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The phosphate ion concentration in the soil solution is decreased by root uptake but is replenished primarily through desorption of adsorbed ions and diffusion towards roots. The other mechanisms contributing to the replenishment of phosphate ions in the soil solution are the dissolution of phosphate minerals and the mineralisation of organic matter. Thus, the ability of a soil to replenish its P soil solution is referred to. It corresponds to the second key indicator of soil P availability. Indeed, the replenishment of the soil solution and the diffusion of phosphate ions are the limiting steps of P acquisition by crop roots, as has been shown for a long time by plant nutrition models Barber ; Tinker and Nye Phosphorus Management Principles in Agroecosystems Soil P status is strongly influenced by cropping practices 1 through plant uptake and removal from the field via crop products, as well as P inputs of both inorganic and organic fertilisers Fig.