Agriculture Reference
In-Depth Information
The two of most important plant nutrients are nitrogen (N) and phosphorus (P), for which
microbial-based technologies could be developed to greatly reduce the need for off-farm
purchased fertilizers. Universally, N is typically the most limiting crop nutrient. Biological
N fixation, particularly the development of productive associations of N-fixing bacteria or
transgenic N-fixing nonlegume crops, has great potential to transform the productivity
and quality of crops produced by sub-Saharan Africa (SSA) and South Asia (SA) farmers.
Biofertilization accounts for approximately 65% of the N supply of crops worldwide, pri-
marily via
Rhizobiaceae-
legume symbiosis (Lugtenberg et al., 2002).
Rice, wheat, and maize are the three major staple food crops for the world population.
These crops can require as much as 100-200 kg fertilizer-N ha
−1
to produce optimal yields
under ideal moisture and temperature conditions. This is an expensive input, particularly
for farmers from developing countries. Thus, development of biological N-fixing systems
that they could use directly on the farm would transform their productivity and profitabil-
ity. Background levels of N fixation by free-living or endophytic bacteria are likely quite
low (~5-10 kg N ha
−1
y
−1
), which are inadequate for optimum crop growth for most crops.
However, it should be pointed out that the standard fertilization method of soil amend-
ments is inefficient, with only 40-60% of the N applied taken up plants because it is lost
through leaching or immobilized by soil microorganisms. Here, particularly endophytic
N-fixing organisms have potential to overcome this low efficiency because the N they
produce presumably would be taken up directly by plants and not be susceptible to losses,
such as leaching or immobilization of N, that happen when N fertilizers are applied to
soils. This lowers the amount that microbial N fixers would have to provide by about 50%
of what is needed when amending soils with N fertilizers.
It should also be pointed out that large areas of the developing world are semiarid,
where crops like millet and sorghum are grown under much lower yield potential and
thus have much lower N requirements than crops in higher-rainfall regimes. Thus, in this
environment optimizing N-fixing organisms would require much lower N requirements,
which potentially makes biological N fixation more feasible.
The other important nutrient is phosphorus, particularly in the tropics as many soils in
the region have inherent low P levels and high P adsorption capacity. This is exacerbated by
the lack of P replacement of P removed by crop harvesting and soil erosion. Unlike N defi-
ciency, which potentially can be corrected by biologically fixing atmospheric N
2
, P deficien-
cies need to be corrected by adding P sources to soils and increasing P uptake efficiency.
In the following sections, specific categories of biological nutrient enhancement oppor-
tunities are discussed. Besides biological N
2
fixation and increasing nutrient availability in
the rhizosphere, microorganisms can enhance nutrient efficiencies by increasing root surface
area, enhancing other beneficial symbioses of the host plant, and having combinations of
microbial interactions, which are further discussed in the rhizosphere bioengineering section.
Free-living diazotrophs require a chemical, nonphotosynthetic energy source, whereas
the photosynthetic diazotrophs utilize light energy. Associative diazotrophs inhabit the
rhizosphere soil or apoplastic space of roots, can obtain energy materials from the plants,
and have the greatest potential to be exploited for fixing N and making it directly avail-
able to plants (An et al., 2001). The most notable of these is the legume-
Rhizobia
symbiosis.
However, given the long research history of legume N fixation, our emphasis focuses on
developing biological N fixation for nonlegume crops because, if successful, this could
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