Agriculture Reference
In-Depth Information
2005). Tillage promotes SOM loss through crop residue incorporation into soil, physical break-
down of residues, and disruption of macroaggregates (Beare et al., 1994; Paustian et al., 2000;
Six et al., 2000a,b; Wright and Hons, 2004). In contrast, conservation or NT reduces soil mixing
and soil disturbance, which allows SOM accumulation (Blevins and Frye, 1993). Many studies
have shown that conservation tillage improves soil aggregation and aggregate stability (Beare
et al., 1994; Six et al., 1999). Conservation or minimum tillage promotes soil aggregation through
enhanced binding of soil particles as a result of greater SOM content (Jastrow, 1996; Paustian
et al., 2000; Six et al., 2002). Microaggregates often form around particles of undecomposed SOM,
providing protection from decomposition (Gupta and Germida, 1988; Gregorich et al., 1989; Six
et al., 2002; Wright and Hons, 2004). Microaggregates are more stable than macroaggregates, and
thus tillage is more disruptive of large aggregates than smaller aggregates, making SOM from
large aggregates more susceptible to mineralization (Cambardella and Elloitt, 1993; Six et al.,
2002; Wright and Hons, 2004). Since tillage often increases the proportion of microaggregates to
macroaggregates, there may be less crop-derived SOM in CT than conservation or NT (Six et al.,
2000a; Wright and Hons, 2004). Fungal growth and mycorrhizal fungi, which are promoted by
NT, contribute to the formation and stabilization of macroaggregates (Tisdall and Oades, 1982;
Beare and Bruce, 1993).
The larger SOM accumulation in conservation tillage had been observed in intensive cropping
systems, where multiple crops are grown yearly (Ortega et al., 2002; Wright and Hons, 2004). The
use of conservation tillage, including no till, is being considered as part of a strategy to reduce
C loss from agricultural soils (Kern and Johnson, 1993; Paustian et al., 1997; Denef et al., 2004).
Crop species also influence SOM accumulation in the soil. Residue quality often plays an impor-
tant role in regulating long-term SOM storage (Lynch and Bragg, 1985). Crop residues having a
low N concentration, such as wheat, generally decompose at slower rates than residues with higher
N, such as sorghum and soybean (Franzluebbers et al., 1995; Wright and Hons, 2004), since wheat
residues often persist longer and increase SOM more than does sorghum or soybean (Wright and
Hons, 2005).
4.8.2 a
doptInG
a
pproprIate
C
rop
r
otatIon
Conventional monoculture agriculture systems can reduce the quality of soils by loss of OM and
structure because of the low level of organic inputs and regular disturbance from tillage practices
(Acosta-Martinez et al., 2004). Crop rotation may have many positive effects on soil quality and
consequently on crop production. Crop rotation is defined as a planned sequence of crops growing
in a regularly recurring succession on the same area of land, as contrasted to the continuous cultur-
ing of one crop or growing of a variable sequence of crops (Soil Science Society of America, 2008).
Bullock (1992) defined crop rotation as a system of growing different types of crops in a recurrent
succession and in an advantageous sequence on the same land. Crop rotations are a key component
of successful organic arable systems (Robson et al., 2002). Rotations can be optimized to conserve
and recycle nutrients and minimize pest, disease, and weed problems (Lampkin, 1990; Robson
et al., 2002). Appropriate crop rotation has a significant influence on the SOM content of soils. The
results of long-term field trials in Illinois (USA) showed that crop rotation influenced the content of
SOM (Odell et al., 1984). The level of soil C and N was highest in the rotation of maize-oats-clover,
and lowest in the permanent corn rotation (Mengel et al., 2001). The fundamental prerequisite of an
appropriate crop rotation is listed in Table 4.5.
Crop rotations under CT that provide residues with low C/N ratios stimulate the decomposition
of native SOM to a greater extent than do rotations providing residues with high C/N ratios (Ghidey
and Alberts, 1993; Sisti et al., 2004; Wright and Hons, 2004). Under NT, crop rotations have been
shown to have a minimal effect on native SOM decomposition (Sisti et al., 2004). Wright and Hons
(2004) also reported that greater differences in SOM between crop species occurred under CT
rather than NT, especially in subsurface soil. Wani et al. (1994) reported that green manures and
