Environmental Engineering Reference
In-Depth Information
1.
I
NTRODUCTION
Nutrient elements (such as P and Ca, etc.) were important to plant growth and play a
major role in agricultural production [1-3]. However, compared to other essential nutrients,
their availability in soils are relatively low [4, 5]. For example, P deficits is an important
limiting factor in agricultural production because P was easily bound to soil minerals and
charged sites or fixed into organic forms that are inaccessible to plants [6, 7]. In order to
adapt P deficiency, plant roots can exude organic acids to mobilize poorly soluble P [8]. It has
now been demonstrated that some dicotyledonous plant roots, and especially non-mycorrhizal
plants such as
Lupinus albus
(but not
L. angustifolia
) and
Brassica napus
, are capable of
releasing large amounts of organic acids into rhizosphere in response to P deficiency [9-11].
In addition, it is likely that Ca deficiency, which increases membrane permeability of root
cells, will also cause the increase in organic acid efflux [12, 13].
Organic acids such as acetic, citric, malic and oxalic acids are ubiquitous found in soil
solution [1, 14]. For example, of the total organic acids exuded from
white lupin
proteoid
roots, citrate usually comprises of 80 to 90%, malate 5 to 10%, and small amounts of oxalate
[1]. These organic acids can promote soil P release by complexation with metal cations (e.g.
Ca
2+
, Mg
2+
, Fe
3+
or Al
3+
) and dissolution of sparsely P [9, 13]. Previous studies showed that
malate, citrate and oxalate could lead to 10-1000-fold higher soil solution P concentrations
depending on the soil type and speciation and concentration of organic acids [9, 15, 16].
Meanwhile, a significant mobilization of Ca and Mg was observed when citrate and malate
were added into soils [17]. Jones and Darrah (1994) showed that the release of organic acids
in response to P deficiency can be of significance in soils where contained a large amount of
acid soluble Ca-P component [18]. To better understand the soil P mobilization mechanism of
organic acid, we synchronically studied the P, Ca and Mg releases. At the same time, factors
such as organic acids type, the concentration, pH, reaction time and soil conditions were also
investigated to get a better understand about organic acids effects on P, Ca and Mg releases
from soil.
2.
M
ATERIALS AND
M
ETHODS
2.1. Soil
Surface soil samples (0-15 cm) were collected from Shandong Province based on their
pH, organic-C content, total-P content and clay particle content. The samples were air-dried,
and then 2 mm sieved. Characteristics of the soils are presented in Table 1.
2.2. Soil Nutrient Extractions
The influences of different organic acids (citric acid, malic acid, oxalic acid and acetic
acid) at different concentrations (0, 0.05, 0.1, 0.5, 1, 2, 5, 10, 20 mmol/L) on P, Ca and Mg
releases from soils were determined. Triplicated soils (2 g) were extracted with 20 mL of
organic acid solutions for 24 h at 25°C centrifuged 4390g for 10 min, and the supernatant
