tannins alter soil organic matter extraction, solubility ... alter soil org… · tannins alter...
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Net WSC (0-5 cm)
Treatment
Water GA TA PGG
mg g-1 soil
-3
-2
-1
0
1
2
3
4
Forest
Pasture
Net WSN (0-5 cm)
Treatment
Water GA TA PGG
0.0
0.1
0.2
0.3
0.4
0.5
0
50
100
Rel
ativ
e R
oot L
engt
h (%
)
0
50
100
0 4 8 12 16
0
50
100
Al = 0 to 3 µM
Al = 6 µM
Al = 9 to 12 µM
a
c
b
Tannic acid (mg L−1)
Tannins Alter Soil Organic Matter Extraction, Solubility of Metals, and Root PhysiologyJonathan J. Halvorson*, Javier M. Gonzalez, and Thomas B. Kinraide
USDA-ARS, Beaver, WV 25813
Problem:Tannins are common plant-derived polyphenolic compounds that precipitate proteins and react with other biomolecules but knowledge of their effects on soil organic matter, metals, and root physiology is incomplete.
Goals: Determine if tannins or related phenolic compounds influence
1) solubility of organic matter in soils,
2) metal mobilization in soils,
3) root growth.
Why:This information is needed to determine the impacts of tannins and other phenolic compounds on soil organic matter formation, nutrient cycling and toxicity risk of some metals and to improve the management of silvopastures.
Conclusions
� Reactions between some tannins and soil organic matter might rapidly decrease the solubility of labile soil C and interact with soil N.
� Phenolic compounds solubilize and/or mobilize metals in soils and affect important in soil chemical processes.
� Roots incubated with TA and Al together are less intoxicated than roots incubated in TA or Al alone possibly signifying that TA-Al complexes on the root surface are not intoxicating.
Thanks to J. Harrah, T. Robertson, and B. Sweeney for excellent assistance in collection and analysis of data. *[email protected]
Understanding how tannins affect soil organic matter and nutrient cycling may be important in silvopastures.
Extraction studies:
Soils (0-5 cm) were from paired pasture and forestsites in West Virginia (USA). The average pH was 4.5 (in 1:1 soil/water) and total C was 5.6%.
Procedure: Tannin/phenol solutions (10 mg g-1 soil) were added to soil. After shaking for 1 hour, samples were centrifuged, decanted, and analyzed.
Analysis: Water Soluble-C and –N (WSC, WSN)were analyzed with a Shimadzu TOC-VCPN. Net values were calculated by subtracting the C or Nadded by the compounds. Elemental analysis of the supernatant was performed using a Spectro ICP spectrometer. Total “phenolic” content was determined by the Prussian blue method (Hagerman, 2002). Error bars are the standard error of the mean.
Not Preincubated Preincubated
Compounds used in these experiments were selected because of their postulated role on plant nutrient cycling and varying complexity.
O
OOO
O
GG
G
G
OG
β-pentagalloyl-D-glucose (PGG)
15
O
OH
OH
OH
OH
OH
O
OH
OH
OH
OH
OH
O
OH
OH
OH
OH
OH
Sorghum procyanidin
Catechin
O
OH
OH
OH
OH
OH
Tannic acid
G = gallic acid ester
O
OG
GO
G
G
O
GG
OG
G O
GG
OH
OH
OH
OH
O
Gallic Acid
OH
OH
OH
O
O
CH3
Methyl gallate
Interactions between TA and Al in root elongation.
Roots were pre-incubated or not in TA, rinsed, incubated in Al, and then stained with haematoxylin. This Al stain demonstrates that TA pretreated roots adsorb more Al than roots not pretreated. Measurements with microelectrodes indicate that TA treatment of roots enhances the surface negativity of the roots. This effect may be part of the reason for greater adsorption of the cationic Al.
Extractable Mn (0-5 cm, n=8)
Number of applications (10 mg g-1 soil)
0 2 4 6 8
0
200
400
600
800
Multiple applications of tannins resulted in an L-type sorption curve for WSC, suggesting that soils had a maximum sorption capacity. Hydrolyzable tannins reduced the losses of WSN compared to the condensed tannin or the H2O control. The Prussian Blue assay, a measure of the “total phenolic content”, indicates that tannins were no longer sorbed by soil after 5 applications.
Multiple applications of methyl gallate increased extraction of Al and Fe (Fe not shown). Similarly, gallic acid increased extraction of Mn and Ca (Ca not shown) compared to the other treatments.
A single application of tannic acid (TA) reduced net total extraction of WSC and WSN in West Virginia soils (n=5) suggesting that TA-C sorbed on soil and interacted with labile soil-N. A purified gallotannin, PGG, sorbed more strongly than TA while gallic acid (GA), a non-tannin phenolic, had less effect.
Tannic acid and Al are both intoxicating, but each will alleviate the toxicity of the other within appropriate concentration ranges. In the top figure (a) small concentrations of TA alleviate the intoxication by small Al concentrations, but continued additions of TA result in toxicity. In the next two figures (b and c) one can see that greater concentrations of TA are needed to alleviate the toxicity of the greater Al concentrations. Note too that Al alleviates TA toxicity, shown by increased root length at 8 mg L-1 TA in response to greater Al concentration. We estimate that each TA molecule may bind five Al3+ ions.
Pretreatment of wheat roots with tannic acid enhances the adsorption of Al.
At least two mechanisms are involved:
1. Complex formation.
Metals in soils bind to organic ligands through H-bonding, coordination (one donor group), and/or chelation. The last two mechanisms form stronger complexes with Al and Fe and also might affect the P cycle since P forms complexes with Al and Fe.
2. Redox reactions.
Phenolic compounds reduce the insoluble Mn(IV) to the soluble Mn(II) form:
Mn(IV)(s) + Phenolics ���� Mn2+(aq) + Quinones
Redox reactions are common and important in the formation and stabilization of soil organic matter. Quinones are reactive compounds that self-polymerize or co-polymerize with other compounds such as amino-containing compounds to form humic-like substances.
Extractable N (0-5 cm, n=8)
-1
0 2 4 6 8
0
50
100
150
200
Number of applications (10 mg g-1 soil)
Number of applications (10 mg g-1 soil)
H2O
Condensed Tannin
(Sorghum)
Tannic Acid
Hydrolyzable Tannin
(PGG from TA)
Gallic Acid
Methyl Gallate
Catechin
Prussian Blue Assayµmol Gallic Acid equivalents
Number of applications (10 mg g-1 soil)
0 2 4 6 8 10
% of ad
ded
0
20
40
60
80
100
120
Extractable C (0-5 cm, n=8)
Number of applications (10 mg g-1 soil)
0 2 4 6 8
Total net Extraction
(mg kg-
1 )
-12000
-10000
-8000
-6000
-4000
-2000
0
2000
4000
Extractable Al (0-5 cm, n=8)
0 2 4 6 8
Cum
ulative net Extraction
(mg kg-
1 )
0
100
200
300
400
500