Page 134 - Enzymes in Tropical Soils
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122 Enzymes In Tropical Soils
(Juma and Tabatabai, 1977; Pang and Kolenko, 1986; Fox and Comerford, 1992;
Sarapatka et al., 2004). The dissolution of P-fertilizers produces orthophosphates
as shown in Eq. 8.2 as follows. The high concentrations of ortophosphases may
then hinder the process of organic P decomposition. Sarapatka et al. (2004)
showed that the acid phosphatase activity in the rooting zones of various species
and cereal cultivars is negatively correlated with increasing pH and available
phosphorus level in the nutrient medium.
-
2+
-
+
Ca 3 (PO 4 ) 2 + 2 H 2 O + 2 H 3 Ca + 2 H 2 PO 4 + 2 OH ....... Eq. 8.2
The inhibition of acid and alkaline phosphatase activities by the presence of
orthophosphate ions is of great importance because these enzymes play important
roles in the hydrolysis of soil organic P. On the other hand, orthophosphate is the
end product of P mineralization or P fertilizers in soils.
The process of organic P decomposition may proceed whenever the
concentrations of orthophosphates in the soil water somehow decreases. The
concentration of orthophosphates may decrease through several mechanisms such
3+
4+
3+
as: (a) plant-root absorption, (b) precipitation by Al , Fe , or Mn in acid soils or
2+
by Ca in alkaline soils, and/or (c) specific adsorption by sesquioxides or
allophanes.
Several workers showed that application of P-fertilizers decreased the soil
phosphatase activities in rhizosperes (Juma and Tabatabai, 1977; Pang and
Kolenko, 1986; Fox and Commerford, 1992). Juma and Tabatabai (1977) reported
that phosphate ions lowered the activity of phosphatase, unlike other anions such
-
-
-
2-
as NO 2 , NO 3 , Cl , and SO 4 . As suggested by Salam et al. (1997d), this
phenomenon is easily understood, because the increase in the concentration of
orthophosphates due to the dissolution of P fertilizers may inhibit the
transformation of organic P to inorganic P. This relationship is depicted in Fig. 8.2.
However, it was reported that the soil total P contents showed significant
and positive relationships with the activities of soil enzymes including phosphatase,
urease, and protease (Nannipieri et al., 1980; Frankenberger and Dick, 1983;
Baruah and Mishra, 1984; Baligar et al., 1988).
8.3 Effects of S on the Activity of Arylsulfatase
2-
Sulfur or sulfuric fertilizers in dry-land soils may produce SO 4 and this sulfur
ion is the most available to plant-root absorption. Similar to orthophosphases, this
2-
ion will hinder the organic S decomposition in producing SO 4 as shown in Eq. 4.5.
Abdul Kadir Salam - 2014
