Soil Fertility as a Contingent Rather than Inherent Characteristic - Principles of Sustainable Soil Management in Agroecosystems - page 151

versus conventional practices (inorganic fertilization and continuous flooding)
reported significant increases in both microbial biomass C (MBC) and MBN under
SRI management, regardless of the sampling dates during the growing season (Zhao
et al. 2010).
Crop management practices, particularly those affecting soil C availability and
aerobic soil status, thus are associated with enhanced soil microbial populations.
The greater abundance of microbes such as diazotrophs and P-solubilizing bacteria
seen in Table 6.2 would be consistent with the increases observed in the populations
of Azospirillum in and on the roots of SRI-grown rice plants reported in Table 6.1 .
This invites more research on the effects of endophytic microbes in crop plant roots.
6.5 EVIDENCE ON THE BENEFICIAL EFFECTS
OF ROOT ENDOPHYTES
Independently of the research being conducted on the effects on rice plants of SRI
management in Madagascar and several rice-producing countries in Asia, research
as been undertaken on rice root endophytes in the Nile delta of Egypt. Rice has been
grown there for centuries in alternation with a leguminous fodder crop (berseem
clover), so that the soils there are well stocked with Rhizobia. It is well known that
Rhizobia inhabit root nodules in legume plants in a classic mutually symbiotic rela-
tionship, fixing N 2 for the benefit of plant growth and receiving organic acid photo-
synthate supplies from the plant in return. However, similar symbiotic relationships
of soil microbes with nonleguminous plants that had no root nodulation were not
well established.
That a variety of free-living diazotrophic bacteria found in the rhizosphere of
nonleguminous plants, particularly of gramineae (grass-family) species, could fix N 2
and provide other benefits to plants was documented (for an early summary on this,
see Döbereiner and Pedrosa 1987). However, their beneficial effects, which include
plant growth promotion stimulated by hormones and induced systemic resistance
that protects plants against pathogens, are still not well or fully understood.
Much of the early work on these beneficial microbes that live in the root zone
and are found also in plant roots as endophytes was done on rice and sugarcane
( Saccharum officinarum) ) in Brazil. Considerable research was carried out on this
subject in the 1990s (Peoples et al. 1995; Ladha et al. 1997), but interest waned there-
after. Perhaps this was because the attempts to induce and increase BNF, treating
bacterial inoculation of soil and plants as a kind of alternative to fertilizer, did not
produce the anticipated gains in crop productivity.
That status changed considerably when studies by Dazzo and Yanni with a large
number of colleagues (Yanni et al. 1997, 2001; Dazzo and Yanni 2006) showed that
the root interiors of field-grown rice plants were naturally inhabited by plant growth-
promoting rhizobial microbes as endophytes. After providing experimental proof
of the endophytic infecting ability of rhizobia in rice using the criteria of molecular
Koch's postulates, they found that certain endophytic strains of rhizobia significantly
benefited rice growth through the vegetative and reproductive stages of its life cycle,
and also enhanced the protein content of rice grains (Biswas et al. 2000b; Chi et al.
2005; Yanni et al. 2001).