Agriculture Reference
In-Depth Information
4.7.5 m
ICroBIal
B
Iomass
OM is one of the essential components of soil quality which support soil microbial life. The micro-
bial biomass mediates many important functions in soils that include nutrient mineralization and
cycling, and decomposition and formation of SOM as they are the main sources of enzymes in soils
(Tabatabai, 1994; Acosta-Martinez et al., 2004). Transformation and storage of soil nutrients is
regulated by the microbial biomass present and the flow of nutrients through the soil microbial frac-
tion can be substantial (Martens, 1995; Prenger and Reddy, 2004). Enzymes are present in the soil
within various biotic and abiotic components (Burns, 1982). Microbial biomass C and N comprise
only 1-3% of total soil C and up to 5% of total N in soils, respectively, but are biologically the most
active fraction of SOM (Smith and Paul, 1990; Franzluebbers et al., 2001; Acosta-Martinez et al.,
2004). Several studies highlighted the role of microbial biomass in decomposition of substances
such as carbohydrates and lipids originating from plant and microbial activity in the improvement
of soil quality (Tisdall, 1994; Wright et al., 1999; Morse et al., 2000; Zaher et al., 2005).
Microorganisms play an important role in the acquisition and transfer of nutrients in soil. For
phosphorus, soil microorganisms are involved in a range of processes that affect P transformation
and thus influence subsequent availability to plant roots. In particular, microorganisms can solubi-
lize and mineralize P from inorganic and organic pools of total P (Richardson, 2001). In addition,
microorganisms may effectively increase the surface area of roots. The increase in surface area may
increase the availability of water and nutrients. Extensive ranges of soil bacteria and fungi that are
able to solubilize various forms of precipitated P have been reported (Rodriguez and Fraga, 1999).
Predominant among these organisms are
Bacillus, Pseudomonas, Penicillium,
and
Aspergillus
spp.
(Richardson, 2001). There is also a need for better management of P fertilizer in agricultural sys-
tems so as to minimize the adverse environmental effects of P loss (Tunney et al., 1997). Improving
the efficiency of P uptake through a microbial biomass is an important strategy both economically
and environmentally (Richardson, 2001).
Humic substances extracted from manures increase the efficiency of N-fixing organisms such
as
Rhizobium
and
Azotobacter
. OM serves as a source of energy for both macro- and microfaunal
organisms (Fageria and Gheyi, 1999). The numbers of bacteria, actinomycetes, and fungi in the
soil are related in a general way to the humus content. Earthworms and other faunal organisms are
strongly affected by the quantity of plant residue material returned to the soil (Stevenson, 1982). The
OM content of the soils also influences the pathogenic microorganisms. An adequate supply of OM
favors the growth of saprophytic organisms relative to parasitic organisms and thereby reduces the
population of the latter. Biologically active compounds in soils, such as antibiotic and certain phen-
otic acids, may enhance the ability of certain plants to resist attack by pathogens (Stevenson, 1982).
4.8 MANAGEMENT PRACTICES TO IMPROVE/STABILIZE OM IN SOIL
Pools of SOC account for about 68% of C in forest biomes (Kimble et al., 2003). Carbon may be
sourced from or stored in the soils of terrestrial ecosystems (Kirschbaum, 2000; Post and Kwon,
2000; Wets and Post, 2002; Clark and Johnson, 2011), and outcomes are determined by land man-
agement decisions, anthropogenic or natural disturbances, and environmental conditions (e.g., cli-
mate) (West and Post, 2002). Changes in SOC pools represent the balance between inputs from
litter, root turnover and exudates, faunal necrotic mass production, and losses via respiration by soil
organisms, erosion, and leaching (Clark and Johnson, 2011).
Adopting appropriate soil and crop management practices can improve and/or stabilize the SOM
content of soils. These practices include liming acid soils, use of organic manures including cover
crops, use of adequate fertilizer rates for annual crops, and use of appropriate crop rotation. In
addition, the use of conservation tillage can also improve the OM content of the soil. The major
management practices to improve or stabilize OM in the soil are summarized in Table 4.4 and their
detailed discussions are given in the succeeding sections.
