Agriculture Reference
In-Depth Information
size distribution, aggregate strength, erodibili-
ty and susceptibility to crusting and compac-
tion. It increases plant available water cap-
acity and water infiltration rate, and decreases
surface runoff. It improves anion and cation
exchange capacity, thereby enhancing nutri-
ent retention and buffering against changes in
pH and dissolved elemental concentrations.
Furthermore, SOM enhances the activity and
diversity of soil organisms, including earth-
worms, which enrich soil structure, and
microbial biomass, which in turn affects C
turnover and nitrification/denitrification (Lal,
2012;
Fig. 19.1
). Managing soils to maintain
and enhance SOM stocks is therefore crucial
to safeguard and increase food production
(see Chapter
10,
this volume).
SOM consists of more than half its
weight in carbon, and is one of the main
pools of carbon on Earth (see Box 19.1). Al-
though the exact amounts of soil carbon are
uncertain, it is clear that the top metre of
soil holds about three times more carbon
than all the forest biomass of the world
combined, and around double the CO
2
con-
tent of the atmosphere (see Box 19.1; Joost-
enandCouwenberg,2008;Lal,2012).Small
losses from this large pool may thus have
significant impacts on atmospheric carbon
dioxide concentrations. Since the start of
agriculture, the soil and biotic carbon pools
have been decreasing, leading to soil deg-
radation and increased emissions of CO
2
to
the atmosphere.
Economic/aesthetic
quality
Ecological quality
•
Elemental cycling
Oxidation of CH
4
Nitrification/denitrification
Use efficiency of input
•
•
Land value
Aesthetic/cultural value
•
•
•
Chemical quality
Biological quality
Soil biodiversity
Bioturbation
Microbial biomass carbon
Carbon turnover
•
•
•
•
Charge properties
Buffering capacity
Nutrient retention
Elemental transformation
•
•
•
•
Soil
organic
matter
Physical-mechanical
quality
Hydrologic quality
•
Surface area
•
Infiltration rate
Surface runoff
Plant available water
capacity
•
•
•
•
•
Structure/tilth
Erodibility
Aggregate strength
Crusting
Compaction
•
•
Fig. 19.1.
Effects of soil organic matter on soil quality. (From Lal, 2012.)
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