Soil - the foundations of sustainable agro-ecosystems Flashcards
How do we perceive soil?
Do we understand our connections to it?
The universe:
13.7 ± 0.13 billion years
Our solar system:
4.5 billion years
Virtually all elements heavier than C have originated from supernovae –exploding super-stars from before our sun existed
Soil is a major reservoir of microbial biodiversity with which humans have evolved.
The microbial world influences human wellbeing directly, and indirectly via effects on food
quality, climate and the environment.
Urban populations are so psychologically distanced from soil and food production that
they don’t recognize themselves as dependent on world’s ecosystems.
The average ‘ecological footprint’ of a human in high income countries like ours is 5-10 ha
but there are only 1.9 ha of productive land per person in the world (in 2003)
The supply of water and nutrients from soil to crops in a 15.6 million km2 area (approximately 7800 km3 of topsoil to 0.5 m) is what now largely sustains 7.2 billion humans
Soil functions: food, fibre and fuel carbon sequestration water purification climate regulation nutrient cycling habitat for organisms flood regulation
More than 70% of available freshwater is used in agriculture according to UNESCO 2001, It is estimated that it
takes 5 million litres of water to grow a hectare of maize (Pimental et al., 2004).
Soil ecosystem services-
Food production: crops and livestock Fresh water: filtration and storage Fibre: cotton, linen, wool Wood: construction, paper Fuel: wood, peat, straw, biomass Carbon and nutrient storage Climate regulation Waste detoxification and pollution control Host biodiversity and support the biodiversity of above-ground ecosystems.
Climate change effects
mean we are losing earth at an increasing rate
Food production and consumption and waste accounts for ~25-33% of global greenhouse gas emissions
Soil ecosystems are the critical foundations of human food supplies – but we are destroying this life support system!
When we lose soil, we are losing a resource that is, for practical purposes and human timespans, essentially non-renewable.
Most of the organic matter and fertility of soil is concentrated in the upper 20 cm of soil- the topsoil.
A cm of topsoil typically takes >200 years to form, yet it can be swept away in a few seasons.
How we sustainably manage agro-ecosystems now, and in the immediate future, will determine the fate of humanity- determining our quality of life and Earth’s carrying capacity for future generations.
“Each year, we lose 15 billion trees and 24 billion tonnes of fertile soil”
For every person on the planet two trees are lost and more than 3 tonnes of soil is eroded every year.
At rates of soil formation in agricultural fields it would require at least 2 ha of land per person to balance this loss- there is less than 1.9 ha of land per person in the world (Rees 2003), of which only 0.2 ha is arable land.
Estimate 35.9 Pg yr−1 of soil eroded globally in 2012 (> 4.5 tonnes per person per year)
(Compare this to the Global Land Outlook estimate of 24 bn tonnes)
There appears to have been an overall increase in global soil erosion driven by cropland expansion and human caused land use change from 2001-2012.
Large areas of global croplands and deforested regions used for grazing livestock
suffer unsustainable rates of soil erosion
South America, sub-Saharan Africa and SE Asia are major areas in which soil erosion has been getting much worse.
However, even the UK has showed a worrying increase.
Nearly 33% of the worlds arable topsoil lost to erosion or pollution (especially with salt) in the last 40 years.
Erosion rates from ploughed fields average 10-100 times greater than rates of soil formation.
Erosion leads to preferential removal of organic matter and clay - removing nutrients and releasing CO2.
Globally soil erosion is increased
60% by human activity. From Yang et al., (2003).
Revised Universal Soil Loss Equation
A = R x LS x K x C x P
A = potential annual soil erosion (1000’s of Kg ha-1 y-1)
R = rainfall erosivity
LS = slope length and steepness factors
K = soil erodibility- controlled by soil texture (% sand, silt, clay),
organic matter content, structure and permeability.
C = cropping and management
P = soil conservation practices to reduce erosion losses
Main anthropogenic causes of soil erosion
Cultivation: tillage, bare soil, crops with small roots, reduced soil organic matter, compaction, reduced water storage and infiltration. Soil vulnerability to wind and water erosion.
Over-grazing
Deforestation
Salinization- salt pollution from poor irrigation- leading to very poor vegetation.
In Europe soil formation rates including dust deposition range from 0.3-1.4 Mg (tonnes) ha-1 yr-1.
This defines the maximum tolerable rates of soil erosion if soil stocks are not to be degraded.
“Actual soil erosion rates for tilled, arable land in Europe are, on average, 3 to 40 times greater than the upper limit of tolerable soil erosion”.
The UK is currently about 52% self-sufficient for food- enough to feed us from 1st January to 10 days into July each year.
Nearly half our food now comes from international sources.
India: about 45% the total land area is affected by serious soil erosion.
The average rate of soil erosion is estimated here as 16.4 tonnes per hectare causing a loss of 5.334 billion tonnes annually (about 4 km3).
In 2010 the average rate of soil erosion in China was 9.4 tonnes per ha per year, giving a total soil loss of 8.9 billion tonnes (about 7 km3).
Croplands are losing soil at an average rate of nearly 28 tonnes per ha per year!
