Phosphorus

Question 1

Phospholipid cell membrane
Nucleic acids
Energy storage and release

Answer 1

The central roles of

phosphorus in organisms

Question 2

PHOSPHORUS NUTRITION

Overall, however, a much smaller proportion of total P is available to plants

Answer 2

In plants

N = 1.0-5.0% dry weight
P = 0.3-0.5% dry weight

Soil reserves of P ~ 50% of soil reserves of N

Question 3

Running out of phosphorus is natural- natural ecosystems progressively lose P and become increasingly P limited- and their productivity declines.

Answer 3

3 m tonnes P in pee and poo only 0.3 m tonnes returned to the land - global food security Cordell et al 2009

Question 4

Conventional sanitation systems lead to linear flows of nutrients from agriculture, via humans, to recipient water bodies.

Answer 4

The valuable nutrients are rarely re-channelled back into agriculture. Even when sewage sludge is used only a small fraction of the nutrients in excrement return to the soil, and those in urine are lost.

Question 5

Sustainable crop production depends on sustainable nutrient supplies

Sustainable Sanitation and Ecological Sanitation (EcoSan)

Human excrement, urine and food waste all are rich in nutrients that have come from the soil- if we do not return these nutrients to the soil but instead release them into rivers or landfills the soil becomes depleted- requiring other sources of nutrients to be added.

Answer 5

Sustainable development goals- sanitation and clean water

Question 6

In the case of some finite resources such as fossil fuels, alternatives can be found.
There are no substitutes for phosphates.

Answer 6

Are we approaching ‘Peak Phosphorus”? (see later)

What do we do then?

Question 7

This suggests 300 years supply left at current rates of use- but the population is set to rise and demand for P fertilizer is also set to rise.

Answer 7

Elser & Bennett (2011) Nature 478 29-31

Question 8

Use of ‘biosolids’ as a form of fertilizer

Potential to reduce extent of nutrient wash out of agricultural fields

Answer 8

(Esteller et al., 2009)

Waste Management 29: 1936-1944.

Question 9

Increased mortality and reduced growth rates with increasing amounts of plastic fragments in soil

Answer 9

Lwanga et al. 2016

Question 10

Integrated life cycle management

Waste is a resource, and its management should be holistic - recycling nutrients to source.

Answer 10

Animal manure makes a significant contribution to greenhouse gas emissions and environmental pollution
it could provide biogas and fertilizer through anaerobic digestion.
But what are the consequences for soil carbon?

Question 11

Integration of livestock production and waste management.

Cattle indoor housed in the winter.

Answer 11

Reduction in carbon return to soil, but digestate easier to apply to land to recycle nutrients without pollution.
Does not stop methane release from cattle!

Question 12

Phosphorus limitation to plant growth

 Most P is in insoluble chemicals unavailable to plants

 The concentration of plant-available P (PO4-) is very low in most soils

Answer 12

 Large stores of P occur in organic matter and non-exchangeable inorganic compounds

P is present as an anion in all soils:

participates 	in anion exchange reactions and in 	chemisorption forming insoluble precipitates  	with some of the most abundant soil elements 	(Ca, Fe, Al)

Question 13

The concentrations of Ca, Si, Fe and Al are all many order of magnitude higher than the concentration of P in most soils

Answer 13

Problems of low P availability and mobility in soil:
How do plants cope?

1. Root distribution in relation to profile distributions of nutrient

Root activity in uptake of P detected in shoots after injection of radioactive 32P into soil (peat) at various depths in the field.

2.	Root morphological adaptation
	Increased surface area & proliferation
	Root hairs
	Mycorrhizas
	Specialized roots 
                 - proteoid roots

(Drew and Saker, 1978)

(Marschner 1995)

Question 14

3. Root biochemical activity
   Phosphatases
   Rhizosphere pH changes
   Secretion of organic acids

Answer 14

Organic acids exuded by roots differ in their abilities to chelate and dissolve nutrient ions

Dependence upon P mineralization from organic matter: role of microbes

	Mineralization  > 0.2% P
	Immobilization < 0.2% P

Question 15

4. Temporal activity
   Storage of P
   Internal recycling

Plant adaptations to increase P uptake (not considering mycorrhizas)

Stones
provide
phosphorus…
for a time

Answer 15

Are we really going to run out of phosphorus in agriculture soon?

Phosphorus is present in small amounts in many primary (volcanic) rocks and is released slowly by weathering.

Question 16

What other geological resources other than high-grade rock P are available that have not been considered by Cordell et al. (2009), Gilbert (2009) and Elser & Bennet 2011)?

Answer 16

Can we use fast-weathering rock-based fertilizer with low P content to rebuild soil and provide P rather than depending on more limited reserves of high-grade rock phosphate (apatite) especially in tropical high rainfall areas with acidic soils?

Question 17

Taylor et al, (2015) identify abundant reserves of basalt rock on the continents in excess of 14,000,000,000,000 tonnes.
Basalt typically contains about 0.25% P2O5, so this contains over 35,000,000,000 tonnes of P2O5.

Current rates of P2O5 use as fertilizer is 45,000,000 tonnes per year- so the most accessible basalt resources could supply over 777 years of P (assuming equal availability to high grade rock P).

Answer 17

Taylor et al. (2015) suggest adding 100,000,000,000 tonnes of basalt rock grains over 20,000,000 Km2 of tropical land to increase the weathering flux rate of Ca and Mg from silicates into the oceans to sequester atmospheric CO2.

This would deliver 250,000,000 tonnes of P2O5.
This is 6 times current P fertilizer use in agriculture- but basalt-P is not as readily available to plants as fertilizer P.

Question 18

Remineralization is the utilization of natural broad elemental spectrum rock dust
materials for the purpose of renewing the mineral content of soils through weathering.

Borrelli et al., (2017) on soil erosion rates see Lecture 1.

Formation of new clay minerals, may also increase soil carbon storage and decrease P sorption by sesquioxides and make new soil.

Answer 18

Intensively weathered tropical soils like oxisols and ultisols are acidic, depleted of Ca, Mg and other base cations and silica and rich in residual iron and aluminium oxides –giving bright red colours and strong binding of the tiny remaining P reserves.

Question 19

Risks of large-scale rock dust fertilizer use

Energy, greenhouse gas and economic costs of rock mining and grinding.

Answer 19

Health risks of silicate dusts.
Possible risks from heavy metals and harmful elements and minerals.

Potentially irreversible effects on biogeochemistry, ecosystems and soils.

Public acceptability?

Question 20

CHECK SLIDES

Answer 20

CO-BENEFITS OF ENHANCED WEATHERING FROM ADDING BASALT TO SOIL IN AGRI-ECOSYSTEMS

Question 21

Phosphorus often limits plant growth in nature- especially on very ancient and deeply weathered soils (e.g. Tropical rainforest regions and Australia).

Global food production has become very dependent on P fertilizer use but the current source material (P-rich rocks) are a finite resource that is being rapidly and unsustainably depleted.

Answer 21

However, there are potentially alternative sources of P such as fast-weathering basalt which are much more abundant and may deliver co-benefits such as Si and trace elements, as well as generate clay.

Substantial losses of P fertilizer are occurring through soil erosion and through unidirectional flows in the food-supply system through to human sewage and urine.

Foliar N: P ratios can indicate which of these elements is limiting plant growth.

Plants show a range of adaptive features in response to P limitation- particularly in their roots.