Soil phosphorus cycle in a grazing system (Transcript)

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Scene one

Shows an animation of a landscape featuring gum trees and a kangaroo; the upper part of the soil horizon shown as highly weathered with leached surface soil.

This scene is accompanied with the text "Phosphorus (P) is necessary for all life. P is an essential element in genetic molecules, the structure of cell membranes, and for the transfer of energy. The transformation, utilisation and recycling of P forms a complex cycle through soil, water and living organisms. Most Australian soils are generally old, highly weathered, and have low levels of P. Native vegetation has evolved to cope with these low levels."

Scene two

The scene shifts to show a grazing system on the same weathered soil; a tractor dispensing P fertiliser, and a cow defecating, a pasture growing.

The scene is accompanied with the text "Productive agriculture is dependent on the availability of P in the soil. Introduced crops and pastures often require additional inputs of P. The inputs can be through fertilizer and organic inputs such as manure."

Scene three

Using the same features above ground as Scene 2, the focus changes to below the soil surface by darkening the soil and highlighting two pools of Phosphorus. Inorganic P is shown as a fuzzy pool of blue on the soil profile, and organic P is shown as a fuzzy pool of orange on the soil profile.

This scene is accompanied with the text "There are two major 'pools' of P in soil - Organic P and Inorganic P. These two pools are not exclusive and P can cross between them."

Scene four

Using the same features as Scene three, text appears over the pools of P identifying one as Inorganic P and one as Organic P. An orange arrow demonstrates the addition of cow manure as organic P to the organic P pool, while a blue arrow demonstrates P being added to the inorganic P pool also.

This scene is accompanies with the text "The Organic P pool consists of living and dead organic material, in varying degrees of decomposition. In animal production systems, manure is an important contributor to inputs of P. Organic P is not readily available to plants and needs to be mineralised into available forms."

Scene five

Small icons appear one by one in the soil to demonstrate dung beetles, earth worms, fungi, and bacteria, accompanied by an arrow from the Organic P pool to the Inorganic P pool and shaded from orange through to blue.

The scene is accompanied with the text "A host of living organisms (including dung beetles, earthworms, fungi and bacteria) play key roles in breaking down organic matter and mineralising Organic P into Inorganic P."

Scene six

An additional arrow from the Inorganic P pool back to the Organic P pool also appears, shaded from blue through to orange. A cycle of arrows now shows on the page accompanied by the four icons of the living organisms in the soil responsible for these processes.

The scene is accompanied with the text "These living organisms are also responsible for utilising the P present in the Inorganic P pool and are therefore competing with plants for available P."

Scene seven

The screen shifts slightly to the left to show the tractor above the soil distributing P fertiliser, and the blue pool of Inorganic P within the soil profile.

The scene is accompanied with the text "There are three broad forms of Inorganic P. Soil solution P is the P occurring in the soil water and that is readily available to plants and microorganisms. Labile P is a soluble form of P that is readily exchangeable with the soil solution. Non-labile P is poorly soluble and only slowly exchanges with the soil solution."

Scene eight

The screen zooms in to show the roots of the plants growing in the soil, and the four icons of dung beetles, earth worms, fungi and bacteria reappear. Small blue 'bubbles' of P are seen moving from the fertiliser being added into the soil by the tractor to the plant roots and to the icons of living organisms, demonstrating the uptake of Inorganic P by plant roots and living organisms.

The scene is accompanied with the text "Inorganic P in fertiliser is commonly applied to increase the availability of P to pastures and crops. Applied P moves from the fertiliser granule into the soil solution where it can be taken up directly by plant roots, or utilised by soil organisms. The majority of the Inorganic P, however, will be retained by the soil."

Scene nine

A zoomed area (10⁶) shows the layers of clay, and Inorganic P being weakly bound the clay as labile P.

The scene is accompanied with the text "Inorganic P retained by the soil can be labile, or non-labile. Inorganic P that is weakly bound to clay and soil organic matter is described as labile P. Labile P is readily transferred to the soil solution where it can be utilised by plants and microorganisms. Over time, labile P will form less soluble P compounds with other soil minerals and become non-labile P."

Scene ten

Non-labile P is shown with the same zoomed area (10⁶) with the layers of clay as in Scene 9. Blue 'bubbles' of P stop moving from the fertiliser into the soil, however, small blue 'bubbles' of P are shown slowly moving from the non-labile P stores in the clay back into the soil.

The scene is accompanied with the text "Non-labile (or fixed) P is strongly bound in soil and only slowly dissolves back into the soil solution. The pH of the soil partly controls the availability of P. In acid soils, P is often bound with aluminium (Al) and iron (Fe). In alkaline soils, P is often bound with calcium (Ca)."

Scene eleven

Animation moves further to the left, and a crop plant appears (wheat) showing plant mass above the ground and root mass below the ground. A zoom area over the root mass appears showing a plant root with long root hairs, and mycorrizal fungi.

The scene is accompanied with the text "Plants have adapted to cope with low levels of P in soils. Some plants form a partnership with mycorrizal fungi to substantially increase the surface area of the roots for P uptake. Others plants have modified their root structure by growing numerous fine root hairs."

Scene twelve

The animation zooms back out so all elements of the animation are visible on the screen at one time. The tractor and cow above the soil are shown, and within the soil profile we can see the Inorganic and Organic P pools, arrows moving P between the pools and into plant roots and living organisms. The addition of P to the soil is also shown by arrows from the cow's manure and tractor distributing P fertiliser into the soil. Arrows also demonstrate the movement of P as labile and non-labile P with soil clay particles.

The scene is accompanied with the text "The transformations of P in soil are just one part of a complex cycle of P through soil, water and living organisms. A better understanding of how P cycles in the broader environment can help us manage P fertiliser inputs and reduce excess P losses."

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