Agriculture Reference
In-Depth Information
Where the mineral surface charge is a func-
tion of soil pH, soils have the ability to ad-
sorb/desorb anions and cations. These two
reactions are occurring simultaneously in
the soil, and these processes are hard to sep-
arate. Ion exchange reduces the leaching of
calcium (Ca 2+ ), magnesium (Mg 2+ ), potas-
sium (K + ) and sodium (Na + ) from the soil,
making them bioavailable to the plants, serv-
ing as macronutrients necessary for plant
growth. On the other hand, adsorption con-
trols the mobility of anions, cations and heavy
metals in the subsurface. Chemical weather-
ing dissolves the minerals and transforms
their chemical composition into new, sec-
ondary minerals (Drever, 1997; Langmuir,
1997). Chemical weathering is a kinetic phe-
nomenon. There are two types of chemical
weathering: one that results in complete dis-
solution without further precipitation (con-
gruent dissolution), and the weathering that
results in dissolution, where new products are
formed and dissolved ions partially precipi-
tate out of solution (incongruent dissolution);
for instance, the chemical weathering of kao-
linite results in the production of gibbsite.
Surface soil structure also develops
through aggregate formation (Banwart et al .,
2012; Nikolaidis and Bidoglio, 2013). Clay
minerals and oxide coatings bind with soil
organic matter and microbial biomass, creat-
ing aggregate particles that are larger than
the original components. Humic and fulvic
substances bind strongly to clay minerals
and oxide coatings, forming organic-metal
surface complexes. Negatively charged clay
surfaces of minerals bind organic compounds
through cation bridges, and iron and alumi-
num oxide surfaces bind organic compounds
by electrostatic forces (Nikolaidis and Bidoglio,
2013). pH, redox status, clay content and oxide
coating content affect organic sorption and
surface complexation to surfaces. Silt-clay
size particles (< 53  μm) form microaggregate
size particles ( 53-250   μm), while the com-
bination of the two fractions form macroag-
gregate size particles and a well-graded soil.
The organic matter sequestered within the
macroaggregates is less susceptible to deg-
radation. Stamati et al . (2012b) have shown
that the mean particle diameter of the soil
can increase significantly in a set-aside
(uncultivated) soil where there is active car-
bon addition compared to a cultivated soil,
and it can be an order of magnitude higher
than the mean particle diameter of the ac-
tual minerals within the soil. Soil aggregates
sequester carbon and nutrients for plant
growth and increase soil moisture within
the aggregate micropores for plant and mi-
crobial growth (Stamati et al ., 2012a). The
developed soil structure allows oxygen dif-
fusion from the atmosphere and water
drainage necessary to prevent waterlogging.
Microbial action degrades the sequestered
organic material, decreasing the cohesion of
the aggregate, which eventually breaks up,
allowing for faster organic matter decom-
position and associated depletion of nutri-
ents and the release of CO 2 to the atmosphere.
In addition to soil biota, aboveground plant
communities drive carbon sequestration and
nutrient turnover through organic matter in-
put to the soil. How much carbon and nutri-
ents are stored in the soil depends on the
balance between microbial decomposition
and plant fixation rates.
Plant roots play a significant role in the
soil aggregation process by entangling soil
particles with the mycorrhizal system and
through root exudates that can prime micro-
bial activity, change soil pH and impact
local mineral weathering activity. Other soil
terrestrial fauna, and especially earthworms,
play a significant role in soil aggregation,
due to bioturbation, excretion of casts and
deposition of mucus on the walls of the bur-
row (Nikolaidis and Bidoglio, 2013).
Human activities have impacted soil
ecosystem services significantly (Nikolaidis,
2011), and in particular the soil function to
filter and transform water. The main ecosys-
tem drivers that affect soil functions and ser-
vices are climate change, land-use change and
aboveground biodiversity change (Nikolaidis,
2011). In relatively undisturbed forest and
grassland soil ecosystems, nutrient cycling
(C, N, P, K) is regulated by the plant-soil-
microorganism system, and nutrient leach-
ing is controlled tightly by the hydrologic
pathway and the reactivity of the host min-
erals (Stamati et al ., 2011). Riparian forests
have been used as natural and engineered
systems to improve the water quality of
 
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