Agriculture Reference
In-Depth Information
9.1
Introduction
Rice (
Oryza sativa
) is mostly cultivated under flooded conditions and consumes up
to 43 % of the world's irrigation resources (Bouman et al.
2007
). Water scarcity is
becoming a major problem for agriculture, and it is expected that by 2025, 15-20
million ha of irrigated rice will suffer some degree of water scarcity (Tuong and
Bouman
2003
). The aerobic rice is a water-saving rice system in which potentially
high yielding and fertilizer-responsive adapted rice varieties are grown without
standing water. Aerobic rice requires the same amount of nutrients as flooded rice,
but there is a problem of phosphorus (P) availability, due to its rapid fixation/
immobilization with other soil elements (Goldstein
1986
). And hence, P becomes
unavailable to plants, and it is estimated that about 70-90 % of externally applied
phosphatic fertilizers become fixed in soil (Holford
1997
). To obviate this, micro-
organisms, especially phosphate-solubilizing bacteria (PSB) and arbuscular mycor-
rhizal (AM) fungi, have been found to have the ability to solubilize P in soil and
could reduce fertilizers inputs (Khan et al.
2007
). As a result, PSB increases P
availability to plants and fulfills the metabolic demands of plant P (Panhwar
et al.
2011
; Tao et al.
2008
). A number of mechanisms involving organic acids
(solubilization) and enzyme production (mineralization), the release of H
+
, chela-
tion, and respiratory H
2
CO
3
production are the documented evidence for P trans-
formation in soils (Khan et al.
2010
). Of these, production of organic acids is the
main mechanism used to mineralize inorganic P (Khan et al.
2009
; Rodr´guez
et al.
2004
). The organic forms of P in contrast are mineralized into inorganic P
by some of the enzymes, for example, phosphatase and phytase. Different levels of
microbial phosphatase activity have been observed in various types of soils
(Kucharski et al.
1996
). In addition, enzymes perform a vital role in P release and
simultaneously improve crop yields (Wyszkowska and Wyszkowski
2010
).
Phytases are known to hydrolyze phytates to a series of lower phosphate esters of
myoinositol and phosphate which in turn contributes hugely to plant nutrient cycle.
In tropical soils, poor availability of soluble P is a common problem because
most of the soil P remains as a fraction of Fe or Al-P. The cheap source of phosphate
rock (PR) is relatively less soluble; nevertheless, it has been observed that the
bioavailability of PR can be increased by applying PSB (Zapata and Axmann
1995
). Here, PSB plays a significant role in solubilizing fixed soil P leading to
greater availability of P to plants and, concomitantly, the larger increase in crop
yields (Gull et al.
2004
). Release of P by PSB from insoluble and fixed/adsorbed
forms is, therefore, an important aspect of P availability in soils. Hence, the use of
PSB as inoculants simultaneously increases P uptake by the plant and crop yields.
