Agriculture Reference
In-Depth Information
SOC by crop residues depends on crop species. For example, corn (
Zea mays
L.) residues can
provide about 1.7 times more C than barley (
Hordeum vulgare
L.), oat (
Avena sativa
L.), sorghu m
(
Sorghum bicolor
L. Moench), soybean (
Glycine max
. L. Merr.), sunflower (
Helianthus annuus
L.), and wheat (
Triticum aestivum
L.) residues based on production levels (Allmaras et al., 2000;
Wilhelm et al., 2004). Crop residues on the soil surface protect OM-rich topsoil (Gilley et al., 1986;
Gregorich et al., 1998; Nelson, 2002).
4.8.10 k
eepInG
l
and
under
p
asture
The lower total soil C and N observed in cultivated cropland soils compared with native rangeland
soils have been well documented (Bauer and Black, 1981; Woods and Schuman, 1988; Bronson
et al., 2004). Grasses in pasture are generally considered as maintaining or increasing SOM con-
tents (Dalal et al., 1995). Gebhart et al. (1994) reported that the conversion of cropland into peren-
nial grass cover through the Conservation Reserve Program in the United States increased SOC
in the Great Plains and potentially sequestrated Cerri et al. (1991) and Feigl et al. (1995) observed
increased C in surface soil after 1 and 2 years of pasture use in central Amazon of Brazil. Similarly,
increase in C and N has been reported in surface soils of eastern Amazon of Brazil after several
years of pasture (Feigl et al., 1995). Webb et al. (2003) also reported that SOC content increases
in a quadratic manner in many soils under grasslands. Wilcke and Lilienfein (2004) reported that
the pasture replaced the original C faster and to a greater depth in the Cerredo soils of Brazil than
the
Pinus
plantation. These authors explained that this could be attributed to the differences in root
architecture and the amount of root litter. Furthermore, lime and fertilizer applications to the pas-
tures increase the biomass production and thus the root litter input into the soil.
When animals are fed on pastureland, a large proportion of the nutrients ingested by animals is
returned to the soil in the form of urine and feces (West et al., 1989). Animals retain only a small
proportion, about 20%, of the nutrients they ingest, and the rest is returned to the soil through excreta
(Rao et al., 1992). The expected buildup in soil fertility in a grass-legume pasture under grazing
could result from a more rapid cycling and greater proportion of nutrients in a plant-available form.
The appropriate management of pastureland improves the soil biological activity and reduces soil
erosion. The increased biological activity is beneficial to the soil properties such as mineralization,
humification, texture, porosity, water infiltration, and retention. Rao et al. (1992) reported that the
contribution of legume residues to SOM quality and turnover, together with improved soil fertility,
soil structure, and biological activity, was associated with a 1 t ha
−1
yield increase in a rice crop fol-
lowing 10-year-old grass + legume plots that did not require any N fertilizer when compared with
rice following a grass-alone pasture of the same age.
4.9 OM VERSUS ENVIRONMENT
Maintaining an adequate level of OM in the arable lands is crucial to keep the soil quality high,
which is responsible for improved environmental quality. OM reduces soil erodibility, which is one
of the main factors in soil degradation and contamination of water bodies by pesticides and other
chemicals. OM also functions as a filter to retain heavy metals and nutrients, which reduces envi-
ronmental contamination.
4.9.1 r
eduCtIon
In
n
utrIent
/h
eavY
m
etals
l
eaChInG
The leaching of nutrients or heavy metals in groundwater is a serious environmental problem in
modern high-input agriculture. Sediment, N, and P in a runoff are the major sources of nonpoint-
source pollution (Blanco-Canqui et al., 2004). Several studies have shown that fine soil fractions
are often preferentially transported to surface water through a runoff, and nutrients and toxic heavy
metals or pesticides attached in the fine fractions are discharged along with the runoff (Ghadiri
