Soil Fertility as a Contingent Rather than Inherent Characteristic - Principles of Sustainable Soil Management in Agroecosystems - page 148

• In the fourth set of treatments, it was seen that when compost was used with

SRI methods instead of NPK, there was a further yield increase with SRI
methods, to 10.2 Mg ha -1 , and the Azospirillum population density increased
to 1.4 million CFU mg -1 root tissue. In this case, the increase in grain yield
increase could be attributed to increased availability of N through biological
processes or to other effects from microbial-plant symbiosis. Unfortunately,
these processes and effects were not studied in this research as it was explor-
atory, and a few laboratory facilities were available in the country.

It should not be inferred that these effects were due just to variations in the popu-
lation density of Azospirillum in, on, and around the roots. This bacterial genus was
chosen for analysis because it was relatively easier and more reliable to count than
other microbes within the endophytic community. In the research, Azospirillum was
regarded as a proxy or indicator for overall levels of bacteria associated with plant
roots. As a diazotroph, Azospirillum could have been contributing fixed N to the soil
and to the plants because it has the ability to fix atmospheric N. However, as seen in
Section 6.4, why or how this and other endophytes enhance crop plant performance
appears more complex than just increasing the availability of N. BNF appears to be
just one of several mutually beneficial functions that symbiotic endophytes perform.
These results from the high plateau in central Madagascar fortunately directed
some other researchers' attention to the effects that soil microorganisms living not
just around but also within the roots could have on rice plants when managed with
SRI practices that create conditions favoring greater root growth. Young seedlings
transplanted singly and carefully, with wide spacing, in soil well-endowed with
organic matter and actively aerated by mechanical weeding, and having mostly aero-
bic soil conditions with no continuous flooding, produced larger and longer-lived
root systems (Barison and Uphoff 2011). These practices also favor larger popula-
tions of aerobic soil organisms, as seen in Section 6.4.

6.4 INCREASES OF BENEFICIAL MICROORGANISMS IN
RHIZOSPHERES IN RESPONSE TO MANAGEMENT PRACTICES
The findings reported from Madagascar prompted researchers in India and Indonesia
to undertake their own evaluations of the effects of SRI management on the soil bio-
ta.* Their findings are summarized in Table 6.2 . The increases in microbial popu-
lations seen there—both from direct counts and from the biochemical “footprints

* In 2002, two theses were done at Tamil Nadu Agricultural University (TNAU) in India, assessing
differences in soil microbial populations and activity in the rhizospheres of rice plants under SRI com-
pared with conventional management. Then, the soil biota in over 200 farmers' fields across 11 dis-
tricts of Andhra Pradesh state of India were evaluated over four seasons in 2005 and 2006 under a joint
project of the Worldwide Fund for Nature (WWF) and the International Crop Research Institute for the
Semi-Arid Tropics (ICRISAT) in Hyderabad, India. Similar research was conducted in 2009 by the
Soil Biotechnology Laboratory in the Department of Soil Sciences and Land Resources at the Institut
Pertanian Bogor (IPB) in Indonesia. Each study used standard methods for making soil biological
measurements, as reported in Uphoff et al. (2009) and Anas et al. (2011). Some similar research was
undertaken in China during this period by agronomists evaluating the effects of SRI management,
reported below.