Phosphorus Flashcards
Phospholipid cell membrane
Nucleic acids
Energy storage and release
The central roles of
phosphorus in organisms
PHOSPHORUS NUTRITION
Overall, however, a much smaller proportion of total P is available to plants
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
Running out of phosphorus is natural- natural ecosystems progressively lose P and become increasingly P limited- and their productivity declines.
3 m tonnes P in pee and poo only 0.3 m tonnes returned to the land - global food security Cordell et al 2009
Conventional sanitation systems lead to linear flows of nutrients from agriculture, via humans, to recipient water bodies.
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.
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.
Sustainable development goals- sanitation and clean water
In the case of some finite resources such as fossil fuels, alternatives can be found.
There are no substitutes for phosphates.
Are we approaching ‘Peak Phosphorus”? (see later)
What do we do then?
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.
Elser & Bennett (2011) Nature 478 29-31
Use of ‘biosolids’ as a form of fertilizer
Potential to reduce extent of nutrient wash out of agricultural fields
(Esteller et al., 2009)
Waste Management 29: 1936-1944.
Increased mortality and reduced growth rates with increasing amounts of plastic fragments in soil
Lwanga et al. 2016
Integrated life cycle management
Waste is a resource, and its management should be holistic - recycling nutrients to source.
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?
Integration of livestock production and waste management.
Cattle indoor housed in the winter.
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!
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
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)
The concentrations of Ca, Si, Fe and Al are all many order of magnitude higher than the concentration of P in most soils
Problems of low P availability and mobility in soil:
How do plants cope?
- 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)
- Root biochemical activity
Phosphatases
Rhizosphere pH changes
Secretion of organic acids
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
- Temporal activity
Storage of P
Internal recycling
Plant adaptations to increase P uptake (not considering mycorrhizas)
Stones
provide
phosphorus…
for a time
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.
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)?
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?
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).
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.
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.
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.
Risks of large-scale rock dust fertilizer use
Energy, greenhouse gas and economic costs of rock mining and grinding.
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?
CHECK SLIDES
CO-BENEFITS OF ENHANCED WEATHERING FROM ADDING BASALT TO SOIL IN AGRI-ECOSYSTEMS
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.
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.
