Agriculture Reference
In-Depth Information
activity (Lupwayi and Kennedy, 2007). The decline in pathogen populations with crop rotation
results from natural mortality (due to the absence of a susceptible host) and the antagonistic activi-
ties of microorganisms that coexist in the root zone (Peters et al., 2003). Lupwayi et al. (2012)
reported that the economic and environmental benefits of including grain legumes in crop rotations
may tempt farmers to grow them more frequently than recommended, resulting in potential changes
to the soil chemical, physical, and biological properties.
Crop diversity in rotations affects soil microbial communities because different plant species
produce different root exudates (Garbeva et al., 2008), and different crop residues returned to the
soil after harvest offer heterogeneous substrates to soil microorganisms during residue decomposi-
tion (Lupwayi et al., 2004). Monocultures have been shown to result in less soil microbial biomass,
activity, and diversity than diversified crop rotation (Acosta-Martinez et al., 2010; Li et al., 2010;
Lupwayi et al., 2012). These effects are implicated as major causes of yield decline in monocultures
because soil microorganisms are the main drives of nutrient cycling and biological pest control in
the soil (Lupwayi et al., 2012).
Lupwayi et al. (2012) reported that a field pea grown in monoculture had less microbial biomass
and bacterial functional diversity than a field pea rotated with wheat. Nayyar et al. (2009) also
reported less microbial biomass, enzyme (dehydrogenase, phosphatase, and urease) activities, and
arbuscular mycorrhizal colonization of the field pea in field pea monoculture than in the wheat-pea
rotation. Reductions in the soil microbial biomass in crops grown in monoculture relative to the
same crops grown in rotations have been reported in soybean in Argentina (Vargas Gil et al., 2011),
sugarcane in Australia (Holt and Mayer, 1998; Pankhurst et al., 2005), and cotton in the southern
United States (Acosta-Martinez et al., 2010). Low microbial biomass carbon in field pea mono-
culture relative to a rotation with wheat is probably related to the low crop biomass and organic C
returned to the soil with field pea residues under pea-pea rotation.
Soil invertebrates (i.e., earthworm) and functional groups (bacteria) contribute to a wide
range of soil services vital in agricultural soils, such as water and air movement and nutrient
cycling (Dominati et al., 2010) through feeding, excretion, burrowing, casting, and litter incor-
poration (Lavelle et al., 2006). Even in intensive systems, it would appear that invertebrates
still have the ability to contribute to soil quality, as Schon et al. (2011) found that invertebrates
improved plant uptake of N even in an artificial high-fertility, compacted pasture soil (Schon
et al., 2012). Similarly, several authors have reported that invertebrates contribute to a wide
variety of soil services and as such can be regarded as indicators of these services and soil con-
ditions (Bardgett, 2005; Lavelle et al., 2006). Improvement in soil physical conditions and the
addition of food resources available to the soil food web can increase soil invertebrates (Schon
et al., 2012).
8.3 NITROGEN FERTILIZER MANAGEMENT PRACTICES
Mineral nutrients in various forms have been applied to crops for thousands of years. However, dur-
ing the twentieth century, science has addressed the task of identifying the essential nutrients and
recommending the amounts, forms, and timing needed for optimal crop growth (Angus et al., 1993).
In this context, improving the efficiency of fertilizer N use is vital to achieve and sustain high crop
yields and reduce losses of N that can potentially deteriorate environmental quality. Fertilizer N is
being increasingly recognized as an expensive input and also a source of nitrate contamination in
the groundwater (Bijay-Singh and Yadvinder-Singh, 2004). Appropriate modification in fertilizer
source or management practices can lead to a reduction in losses of N and increased fertilizer N use
efficiency (Thind et al., 2010).
Culman et al. (2013) reported that the management choices that growers make can have dra-
matic impacts on the local ecosystem services. For example, the N fertilizer type, rate, and appli-
cation in relationship to crop demand are important regulators of N cycling efficiency and loss
pathways; this has direct and indirect consequences for water and soil quality (Robertson, 1997;
