Agriculture Reference
In-Depth Information
Copper (Cu) also readily forms complexes with
soil organic matter and becomes unavailable
(Inaba and Takenaka, 2005). Amendment with
organic manure is considered an effective
countermeasure for detoxification of Cu in
Cu-contaminated soils (Paradelo et al ., 2011).
The molecular weight of organic matter plays
a role in the toxicity and bioavailability of
heavy metals to plants: organic moieties with
larger molecular weight form more stable
complexes, and have greater detoxification
ability, than those with smaller molecular
weight.
On the other hand, since the mobility of
cationic nutrients is enhanced by the forma-
tion of metal-organic complexes, the com-
plexed metals may be lost through leaching,
leading to plant deficiency. For instance,
peatland soils are usually deficient in Cu
and cobalt (Co). Cu deficiency can also occur
in soils with a high content of total Cu and
organic matter, because of the low bioavail-
ability of Cu-organic complexes (Edwards
et al ., 2012).
practices used as well as by the type of manure.
Key management factors are timing of appli-
cation, the quantity applied (ensuring it is in
line with crop requirements) and attempting
to predict the supply of crop-available N from
manures so that fertilizer applications are
reduced accordingly (Salazar et al ., 2005).
Zhou et al . (2010) gives examples of very
large accumulations of nitrate in the soil pro-
file, leading to a high risk of nitrate leaching
to water, when this was not done in the case
of vegetable crops grown under plastic in
north-west China. Nitrate accumulations to a
depth of 1 m were frequently in the range
100- 500 kg nitrate-N ha 1 . Similarly, P re-
leased from manures can also accumulate in
soil and become a water pollutant. The rap-
idly increasing number of 'confined animal
feeding operations' (CAFOs) around the world
means that there is a trend towards large
quantities of manure to be concentrated at a
limited number of locations. This manure is
a resource for increasing soil organic C con-
tent and as a source of nutrients for crops,
but its management presents a major chal-
lenge if pollution from the sites of applica-
tion is to be avoided.
Under flooded conditions, such as low-
land rice cultivation, excess labile organic C
input leads to extreme anaerobic condi-
tions, which stress the growth of rice plants
and soil microbial diversity and activities.
It  also leads to increased emissions of me-
thane, a greenhouse gas about 25 times more
powerful than CO 2 .
Problems from Excess
Organic Inputs
Although increased content of C is generally
beneficial for the functioning of agricultural
soils, an excess can be deleterious. The main
example is the situation where large amounts
of animal manure, often from intensive live-
stock enterprises, are applied to a limited area
of land. If much of the organic matter in the
manure is in readily mineralizable forms, the
released N can become a pollution hazard to
water (as nitrate or dissolved organic N) and
air as ammonia or nitrous oxide. For example,
surface-applied manures, especially those
with a high concentration of readily decom-
posable N compounds, leads to large losses
of N to the atmosphere as ammonia. This can
be reduced by either rapid incorporation or
injection, in the case of liquid slurries, but
this can lead to increased production of ni-
trate in soil and a greater risk of nitrate leach-
ing (Sagoo et al ., 2007). All N loss processes
from manure, whether ammonia volatiliza-
tion, nitrous oxide emission or nitrate leach-
ing, are influenced greatly by the management
Interactions between N and C in
Natural Ecosystems
In addition to the importance of C for nutri-
ent cycling in agricultural soils, and hence
for food security (one of the provisioning
roles of ecosystems), soil C and nutrients
interact within natural and semi-natural
ecosystems. Excess N is released from agri-
cultural systems as various gases (oxidized
and reduced forms of N) and transferred
elsewhere. One negative impact of this is
soil acidification, as demonstrated in China
(Guo et al ., 2010). This can affect both agri-
cultural and non-agricultural soils. N deposited
 
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