Agriculture Reference
In-Depth Information
fixation, increased soil organic matter, improved weed control, and increased crop yield (Pimentel
et al., 1992, 1995; Sainju and Singh, 1997; Williams et al., 1998; Altieri, 1999; Reddy et al., 2003;
Wortman et al., 2012). While cover crops have traditionally been used as a soil conservation tool
(Pimentel et al., 1995), there is increasing interest in using cover crops to enhance agronomic crop
performance. However, maximizing agronomic benefits associated with cover crops will depend on
appropriate species selection and residue management (Ashford and Reeves, 2003; Wortman et al.,
2012).
The use of legumes as cover crops has been shown to reduce synthetic N input demands by
50-100% depending on the species, the duration of cover crop growth, and the subsequent crop N
requirement (Biederbeck et al., 1996; Burket et al., 1997; Wortman et al., 2012). While legume spe-
cies have a potential for biological nitrogen fixation, faster-growing cover crop species (i.e., grasses
and mustard species) may be more useful in scavenging nitrates and nutrient cycling (Dabney et al.,
2001). A mixture of legume and nonlegume species may maximize the benefits of biological N
2
fixation and nutrient cycling, as legumes can increase N availability to other crop species in mix-
ture leading to increased productivity (Kuo and Sainju, 1998; Mulder et al., 2002). In addition,
termination method and residue management can influence N mineralization, soil availability, and
crop uptake (Sainju and Singh, 2001). Incorporation of a cover crop residue via a field disk or plow
often results in rapid N mineralization and plant availability, but management of the residue on the
soil surface has been shown to result in greater crop N uptake and yield (Sainju and Singh, 2001).
Hence, residue management on the soil surface with conservation tillage methods may be effec-
tive in syncing N mineralization and availability with crop demand and uptake (Parr et al., 2011;
Wortman et al., 2012).
8.2.3 I
mprovInG
s
oIl
m
ICroBIal
B
Iomass
Biological soil properties are related to many changes in the soil environment, which determine the
availability of nitrogen for plant growth and development. Microbial and biochemical soil proper-
ties have been suggested as early and sensitive indicators of soil quality as they manifest themselves
over shorter timescales and are central to the ecological function of a soil (Karlen et al., 1994;
Bandick and Dick, 1999; Fageria, 2002). Soil microbial and enzyme activities in particular are
increasingly used as indicators of soil quality because of their relationship to decomposition and
nutrient cycling, ease of measurement, and rapid response to change in soil management (Dick,
1984; Dilly et al., 2003; Geisseler and Horwath, 2009). In a long-term study, Kandeler et al. (1999)
found that enzyme activities were significantly increased in the top 10 cm of the profile after 2 years
of minimum and reduced tillage to conventional tillage.
One of the management practices that significantly affects microbial biomass in the soil is tillage.
Minimum tillage favors higher microbial biomass compared to conventional tillage. In addition,
seasonal fluctuations in soil moisture, temperature, and substrate availability can also have large
effects on microbial biomass and activity. Franzluebbers et al. (1994) found that the soil microbial
biomass changed significantly during the cropping season in all crop sequences and tillage regimes
under investigation. Bausenwein et al. (2008) also reported significant effects of sampling dates on
microbial biomass and enzyme activities under minimum tillage.
In general, conservation tillage practices leave a significant amount of plant residue on the soil
surface. This results in increased soil organic matter content in the topsoil, which in turn leads to
higher microbial biomass and activity (Geisseler and Horwath, 2009). Several studies have shown
that reduced tillage increases the organic matter content of the soil and results in physical and
chemical stratified soils, with more nutrients and organic matter localized near the surface (Logan
et al., 1991; Cannell and Hawes, 1994; West and Post, 2002).
Including grain legumes in crop rotations has other benefits, including breaking disease and pest
cycles in cereals (Stevenson and Van Kessel, 1996), improving soil structure (Chan and Heenan,
1991), and improving biological soil health through increased microbial biomass, diversity, and
