Properties of humic acids of soil under different management regimes

Abstract

The aim of the work was to determine the elemental composition, hydrophilic and hydrophobic properties and polydispersity of humic acids of soils under different management regimes. The research involved forest and agricultural soil samples. Humic acids were exposed to the following analyses: elemental composition, division into hydrophilic and hydrophobic as well as low- and high-molecular fractions with chromatographic methods. The present results, confirmed by the cluster analysis, showed that the basic factor conditioning the properties of humic acids is the soil type, but also one can disregard neither the soil management method nor the agricultural practices applied.

Full text

Abstract. The aim of the work was to determine the elemental composition, hydrophilic and
hydrophobic properties and polydispersity of humic acids of soils under different management
regimes. The research involved forest and agricultural soil samples. Humic acids were exposed to the
following analyses: elemental composition, division into hydrophilic and hydrophobic as well as
low- and high-molecular fractions with chromatographic methods. The present results, confirmed by
the cluster analysis, showed that the basic factor conditioning the properties of humic acids is the soil
type, but also one can disregard neither the soil management method nor the agricultural practices
applied.
The content and quality of humus are directly and indirectly determined by
physical, chemical, biological and environmental soil properties. The content and
properties of organic matter of soil are conditioned not only by soil-and-climatic
factors but also by the soil management (use) methods. In forest soils one of the
basic factors determining the properties of the organic horizon is the tree stand
species composition [6, 7, 10]. The tree stand species composition is also affected
by the properties of deeper horizons as a result of mobility of nutrients from surface
layers. In the soils under agricultural use the soil humus properties are mostly
determinedbypost-harvestresidueleftaftertheharvestofcrops[1,8,9].
The parameters which are changing in the humification process, besides
elemental composition, are the degree of polydispersity of the molecules of humus
substances [1, 4] and the share of hydrophobic and hydrophilic fractions [2, 3]. The
above parameters are important from the ecological perspective since the
proportions of these fractions determine the solubility of humic acids and, as a
result,theirmigrationdownthesoilprofile.
POLISHJOURNALOFSOILSCIENCE
VOL.XLII/22009PLISSN0079-2985
SoilChemistry
BO¯ENADÊBSKA*,NORASZOMBATHOVA**,
MAGDALENABANACH-SZOTT*
PROPERTIESOFHUMICACIDSOFSOILUNDERDIFFERENT
MANAGEMENTREGIMES
Received December7,2009
**Prof. B. Dêbska, DSc., M. Banach-Szott, DSc.; Department of Environmental Chemistry,
University of Technology and Life Sciences, Bernardyñska 6, 85-029 Bydgoszcz, Poland.
**N. Szombathova, DSc.; Department of Geology and Pedology, Slovak University of
Agriculture,Tr.A.Hlinku2,949-76Nitra,Slovakia.

The aim of the present paper was to determine the elemental composition,
hydrophilic and hydrophobic properties and polydispersity of humic acids of soils
whichdifferinthemanagementmethodandthefarmingregimesapplied.
MATERIALANDMETHODS
For investigation, soil samples were used from depth 0.02 – 0.10 m: four
samples from Bab locality and three samples from Dolna Malanta locality
(Slovakia) [15, 16]. Soil samples from intensively cultivated soils were compared
withthecontrolsamplesfromadjoiningforestsoil.
SoilsamplesfromBabwerecomparedinpairs(1,2and3,4):
–sampleNo.1–forestsoil,classifiedasLuvicPhaeozems;
–sampleNo.2–cultivatedsoil,classifiedasHaplicChernozem;
–sampleNo.3–forestsoil,classifiedasOrthicLuvisols;
–sampleNo.4–cultivatedsoil,classifiedasOrthicLuvisols.
The second chosen locality was Dolna Malanta of the area of the Experiment
Station of the Slovak Agricultural University of Nitra. Soil samples were collected
from ecological and integrated farming systems and from the near castle park
(control sample). Parent material in this locality is proluvial sediment mixed with
loess and the main soil type is Orthic Luvisols [16]. Three soil samples were
characterisedinthislocality:
– sample No. 5 – ecological farming regime, where only farmyard manure (FYM)
wasused;
–sample No. 6 – integrated farming regime, where both FYM and calculated
doses of inorganic fertilizers were used. (The application rate of inorganic
fertilizer is calculated on the basis of the macroelement concentration in the soil
andplantrequirements);
–sampleNo.7–thenearbypark,approximately250mfromtheabovesites.
From all the soils there were extracted humic acids for which the following
analysesweremade:
–the elemental composition (Perkin Elmer 2400 CHN analyzer). On the basis of
the elemental composition the values of atomic ratios: H/C and w– internal
oxidationdegreewerecalculatedaccordingtotheformula:
w=(2O+3N-H)/C,where:O,N,H,C–contentinatomicpercentage;
–hydrophilic and hydrophobic properties were determined with the liquid
chromatograph HPLC Series 200 with DAD detector by Perkin-Elmer. The
separation involved the use of column X-Terra C18, 5 mm, 250 x 4.6 mm.
Solutions of humic acids were applied in 0.01 mol/L NaOH of the concentration
of 2 mg/mL; injection of the sample – 0.1 mL; solvent – acetonitril–water;
solvents flow in the gradient (ratio H2O : ACN (v/v) over 0-6 min – 99.5 : 0.5,
7-13 min – 70 : 30, 13-20 min – 10 : 90); detection – at the wavelength of 254 nm.
132 B.DÊBSKA etal.

Based on the areas determined under peaks (Fig. 1), the share of hydrophilic
(HIL) and hydrophobic (HOB=HOB-1 + HOB-2) fractions in humic acids
moleculesandtheparameter:HIL/HOB[2,3,12,13]weredetermined;
–polydispersity with the use of high performance size exclusion chromatography
(HPSEC) (Chromatograph HPLC Perkin Elmer Series 200, Diode Array UV
detector operating at 254 nm). Humic acids were separated with the TSK
G3000SW column (7.5 x 600 mm). Sodium acetate 0.01 mol/L at pH=7 was
used as eluent. The guard column was TSK G3000SW (7.5 x 75 mm). Solutions
of humic acids were applied in 0.01 M CH3COONa of the concentration of 0.05
mg/mL. A sample of 100 mL was injected into the column. The peak area was
determined: S1 – the first peak area, S2 – the second peak area as well as the
S1/S2ratio[4,11,14].
RESULTSANDDISCUSSION
The highest content of carbon was determined in the molecules of HAs of
Haplic Chernozem (sample No. 2) The content of carbon in HAs No. 2 was 4.52 pp
higher as compared with HAs of Luvic Phaeozems (1) and on average 3.63 pp
higher than in the molecules of humic acids of Orthic Luvisols (HAs marked with
symbols 3,4,5,6, Table 1).The content of nitrogen in HAs of the soils investigated
ranged from 2.37 to 2.86. The highest content of this element was reported for HAs
of soil of ecological and integrated farming regimes. A higher content of oxygen
was recorded, in general, for HAs of forest soils as compared with HAs of arable
soils. The HAs molecules of a lower content of hydrogen showed, in general,
higher values of the degree of internal oxidation (parameter w,Table 1). For HAs
isolated from the soils sampled in Bab there were noted higher values of parameter
win HAs molecules of arable than forest soils. The lowest values of the degree of
internal oxidation were reported for HAs of soil of an integrated farming regime.
PROPERTIESOFHUMICACIDSOFSOILUNDERDIFFERENTMANAGEMENTREGIMES 133
SampleNo. C H N O H/C w
1 33.75 43.18 2.79 20.29 1.28 0.171
2 38.27 39.42 2.68 19.64 1.03 0.206
3 34.47 42.37 2.46 20.70 1.23 0.186
4 34.40 41.96 2.37 21.27 1.22 0.223
5 34.95 42.44 2.86 19.74 1.21 0.161
6 34.73 44.09 2.84 18.34 1.27 0.033
7 36.24 40.02 2.40 21.33 1.10 0.272
TABLE1.ELEMENTALCOMPOSITIONOFHUMICACIDS(INATOMICPERCENTAGE)

A comparison of two farming methods: integrated and ecological showed that HAs
isolated from the soil samples of the integrated farming regime demonstrated a
higher content of hydrogen and, as a result, a higher value of the H/C ratio and
a lower content of oxygen and a lower value of the degree of internal oxidation, as
compared with HAs of soil where the ecological farming regime was applied. As
reported by Szombathova et al. [16], lower values of the H/C ratio recorded for
HAs of soil of ecological farming regime are probably due to a disappearance of
aliphatic chains by biological and/or chemical oxidation and relative increase in
aromatic groups. An increased content of aromatic parts of macromolecules is
connectedwithhigherstability,maturityandqualityofhumicacids.
One of the parameters affecting e.g. the sorption capacity of humic acids are
their hydrophilic and hydrophobic properties. The division into hydrophilic and
hydrophobic fractions of humic acids is made using the RP-HPLC method (Fig. 1).
Fractions of hydrophilic (HIL) character correspond to peaks within the range
from 2.50 to 6.10 min, while fractions within the range from 11.20 to 17.60 min are
hydrophobic fractions (HOB). Fraction HOB-1 is a hydrophobic fraction of a
shorter retention time from 11.20 to 15.60 and fraction HOB-2 ranges from 15.60
to 17.60 min. The share of hydrophobic fractions considerably exceeded the share
of hydrophilic fractions (Table 2). The share of hydrophilic fractions in the
molecules of HAs ranged from 32.1 to 42.2 %, and thus the fraction which is
dominant in the molecules of HAs is the fraction of hydrophobic properties. The
lowest number of hydrophilic fractions was contained in HAs of soils of ecological
and integrated farming regimes (Table 2). For the other soils, irrespective of their
management method, the share of hydrophilic fraction ranged from 40.6 to 42.2 %.
134 B.DÊBSKA etal.
0
100
200
300
0 5 10 15 20
2
mAU
time [min]
Fig.1.SelectedRP-HPLCchromatogramofhumicacids.

The differences in the share of hydrophilic and hydrophobic fractions resulted
in the differences in the calculated values of the HIL/SHOB ratio. The highest
values of this parameter were recorded for HAs of Orthic Luvisols – forest soil.
The lowest values of the HIL/SHOB ratio were reported for HAs of soils of
ecological and integrated farming regimes. As reported earlier [2, 3, 5], the value
of parameter HIL/SHOB is connected with the degree of maturity of HAs
molecules (the greater the degree of maturity, the higher the values of HIL/SHOB)
and it is also a character which is specific to the soil type formed in specific soil and
climatic conditions. The parameters reported defining the hydrophilic and
hydrophobic properties of HAs point clearly that they are determined by the soil
type as well as, to some extent, by the soil use method and the farming regimes
applied.
Sample HPSEC chromatograms are given in Fig. 2. The pattern of chromato-
grams suggests that the molecules of humic acids include two fractions. The first
peak, of a shorter retention time, falling from about 13.00 to 16.00 min,
corresponds to the fraction of humic acids the molecules of which are bigger in
size, while the other one the maximum of which occurred most frequently at the
retention time of about 19.00 min is found for the molecules smaller in size. The
share of high-molecular fraction in the molecules of humic acids of the soils
researched ranged from 13.7 to 21.5%, the fraction of smaller molecules was then
dominant. The greatest share of the high-molecular fraction was recorded for
humic acids of Orthic Luvisols under forest management (locality Dolna Malanta).
Of the HAs of arable soils, most high-molecular fraction was contained in HAs of
soil of ecological farming regime. The share of S1 fraction in the molecules of HAs
of soil of integrated farming regime was lower than in HAs of soil where the
ecological farming regime was applied but higher than in the molecules of HAs of
theotherarablesoils.
PROPERTIESOFHUMICACIDSOFSOILUNDERDIFFERENTMANAGEMENTREGIMES 135
SampleNo. HIL
(%)
HOB-1
(%)
SHOB HIL/SHOB S1
(%)
S1/S2
1 40.6 51.1 59.4 0.683 20.6 0.259
2 41.4 49.5 58.6 0.706 14.1 0.164
3 42.2 50.7 57.8 0.730 13.7 0.159
4 41.8 50.8 58.2 0.717 14.9 0.157
5 34.4 55.4 65.6 0.525 19.8 0.248
6 32.1 56.5 67.9 0.473 17.7 0.215
7 41.2 52.7 58.9 0.700 21.5 0.274
TABLE 2. HYDROPHILIC AND HYDROPHOBIC PROPERTIES AND POLYDISPERSITY OF
HUMICACIDS

Parameter S1/S2 (the ratio of the share of high-molecular fraction to the share
of low-molecular fraction) is assumed as the index of polydispersity of the
molecules of humic acids. The higher the value of this parameter, the higher the
polydispersity of HAs [1, 3, 5]. The highest values of parameter S1/S2 were
reported for humic acids of forest soils: Luvic Phaeozems and Orthic Luvisols. Of
the arable soils, higher values of ratio S1/S2 were reported for HAs of soil located
inDolnaMalantathantheHAsofsoilssampledinBab.
In order to determine the similarities (differences) of the objects investigated, a
cluster analysis was carried out. The objects of similar properties are located on
dendrograms in homogenous groups. The analysis was based on all the parameters
defined and the results are presented in Fig. 3. The dendrogram differentiates
136 B.DÊBSKA etal.
0
2
4
6
0 5 10 15 20 25 30
2
mAU
time [min]
Fig.2.SelectedHPSECchromatogramofhumicacids
Ward m et hod
Distance
0
5
10
15
20
25
30
6 5 4 3 2 7 1
Fig.3.Clusteranalysisbasedonparametersdetermininghumicacidsproperties.
Distance

between the two main groups: one was made up of humic acids isolated from the
soil sampled from the experimental plots in Dolna Malanta where integrated and
ecological farming regimes were applied. The other group defines two subgroups:
one with HAs of forest soils (Luvic Phaeozems – locality: Bab, Orthic Luvisols –
locality: Dolna Malanta), the other one: HAs of Orthic Luvisols – locality Bab
(forest and arable soil); that group included also HAs of Haplic Chernozem. The
results, verified with the cluster analysis, demonstrated that the basic factor
conditioning the properties of humic acids is the soil type, however, one can
disregardneithertheirmanagementmethodnortheagriculturalpractices.
CONCLUSIONS
1. The factors determining the properties of humic acids included the soil type,
the management method, the agricultural practices applied as well as the condi-
tionsthesoilswereformedin.
2. Humic acids of soils under agricultural management located in Bab showed
a lower content of hydrogen and higher values of the degree of internal oxidation as
comparedwithHAsofforestsoils.
3. Humic acids of soils of an integrated and ecological farming regimes
demonstrated lower values of the degree of internal oxidation and ratio
HIL/SHOBascomparedwiththeotherHAs.
4. Humic acids of an integrated farming regime, as compared with HAs of soil
where the ecological farming regime was applied, revealed higher values of ratio
H/C and lower values of HIL/SHOB, S1/S2 and the degree of internal oxidation.
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W£AŒCIWOŒCIPRÓCHNICYGLEBOZRÓ¯NICOWANYM
ZAGOSPODAROWANIU
Celem niniejszej pracy by³o okreœlenie sk³adu pierwiastkowego, w³aœciwoœci hydrofilowo-
hydrofobowych oraz polidyspersyjnoœci kwasów huminowych gleb ró¿ni¹cych siê sposobem
zagospodarowania i stosowanymi systemami uprawy. Do badañ wykorzystano próbki gleb leœnych i
u¿ytkowanych rolniczo. Kwasy huminowe wyizolowane ogólnie przyjêt¹ metod¹ Schnitzera
poddano analizom: sk³adu pierwiastkowego, rozdzia³u na frakcje hydrofilowe i hydrofobowe oraz
nisko-iwysokocz¹steczkowemetodamichromatograficznymi.
Otrzymane wyniki badañ, potwierdzone analiz¹ skupieñ wykaza³y, ¿e podstawowym
czynnikiem warunkuj¹cym w³aœciwoœci kwasów huminowych jest typ gleby, ale równie¿ nie mo¿na
pomin¹æsposobuzagospodarowaniagleb,czyte¿stosowanychzabiegówuprawowych.
138 B.DÊBSKA etal.