Environmental Engineering Reference
In-Depth Information
Our results showed that P release induced by organic acids in alkaline soil was a short
time reaction, however it was more complex in acidic soil. Jones and Darrah (1994) also
showed that the reaction between H-citrate and P containing minerals was complete within 1
hour [18]. However some research showed the release of P from soils is usually rapid in the
first few hours, but then slowly continued until the apparent equilibrium is approached [22-
25]. Strom et al. (2005) showed as reaction time extended, re-sorption and re-precipitate of P
was found in the soil solutions [17]. In this study, because there was no obvious change
detected in Ca and Mg releases over time in soil A, we presume that the dissolution of CaCO
3
and MgCO
3
may provide with Ca and Mg. Previous study also showed the dissolution of
CaCO
3
especially in high soils lead to additional cation gains after citrate addition [18]. In
soil B, re-precipitating as insoluble Ca-P and Mg-P were proved by the decrease of Ca and
Mg in soil solutions as time extended. Meanwhile, the dynamics change of P release may be
associated with that of organic acids. As time extended, organic acids can be mineralization
[26], which would re-increase sorption sites, and then lead to re-sorption of P. In alkaline
condition, as extraction period extended, the rate of oxalate biodegradation was slower than
that of the other organic acids due to precipitation of Ca-oxalate [26, 27], so the P release was
steadier.
Previous studies showed that proton and organic anion play a role in soil P, Ca and Mg
releases together [5, 18, 28]. Jones and Darrah (1994) showed that 25% of P could be
accounted for proton effect, 35% could be accounted for citrate, 40% may be attributable to
the interaction of the protons and citrate together in the neutral soil [18]. Strom et al. (2005)
studied that P is generally more efficiently extracted by citrate, oxalate and acetate (pH 7.5),
while, more Ca released when the extract solution pH < 3.5 in the alkaline soil [17].
However, the influence of pH-dependence of organic acid was different and resulted from
different concentrations, organic and soil type and so on. Our results showed that, in the
alkaline soil, the organic acid of initial pH at the concentration of 1 mmol/L has a higher
efficiency on P, Ca and Mg mobilization.
In the bulk soil, the concentrations of low molecular weight organic acids vary from 10
-2
to 10
2
μ
M
[14]. From our results, it appears that these organic acids have little effect on P, Ca
and Mg mobilization in alkaline soil while promote P release, but inhibit soil Ca and Mg
releases in acid soil. However organic acids were continuously discharged from plant roots
and further studies with the leaching experiment are required to prove their effects on P
release from soil. Near the rhizosphere, the concentration of low molecular weight organic
acid can reach to 1-10 mmol/L [29, 31]. These high concentrations of organic acid were
proved to mobilize significant quantities of P, Ca and Mg into soil solution. From this view,
these organic acids are of vital significance in P deficiency soils.
C
ONCLUSION
In conclusion, the results showed that P releases were highly dependent on the type,
concentration, reaction time and pH of organic acids and soil type. Generally, high
concentrations (≥1 mmol/L) of organic acids can significantly promote P, Ca and Mg
releases, except acetic acid for P and oxalic acid for Ca and Mg. However, when there is
rainfall or irrigation, it may aggravate the loss of soil phosphorus, which will induce the
