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| Titel |
Chernozem aggregate waterstability loss investigation in a long-term bare fallow experiment |
| VerfasserIn |
N. A. Vasilyeva, E. Y. Milanovskiy |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250027803
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| Zusammenfassung |
| The research is focused on mechanisms of aggregate waterstability controlled by soil organic
matter (SOM). The objects of the research are two contrast variants of typical chernozem –
under native grassland and under a 60-year bare fallow experimental plot (100 m2) on the
territory of Central Chernozem Biosphere Reserve, Russia. Seasonal plowing and deficiency
of fresh plant residues (due to weeding out) resulted in a rapid mineralization of SOM. The
Corg content in the 0-20 cm topsoil under native grassland is 6-4.5 %. For the last two
decades Corg content under bare fallow has stabilized on the 2.6% level and is therefore
assumed to represent stable SOM pool. However excellent aggregate waterstability of
chernozem is completely lost under bare fallow. Therefore the aim of our study is to reveal
the role of different SOM pools spatial and functional organization in aggregate waterstability
formation.
Bulk soil samples were collected from 2 m grassland profile and 1.5 m bare fallow
profile with 10 cm interval and simultaneous measurements of soil field density and
moisture. Following samples were analysed: bulk samples, dry and wet-sieving
aggregates, undisturbed and pulverized aggregates, granule-densimetric fractions
obtained by sedimentation of bulk samples (clay 5 mkm) with following densimetric fractionation in bromoform (light Ï 2.4 g/cm3), and above mentioned
samples after removal of SOM by hydrogen peroxide. Isolation of aggregates and
granule-densimetric fractionation were carried out for bulk soils at 0-20, 40-50 and 80-90 cm
depth.
We use elemental analysis (C, H, N), size exclusion and hydrophobic interaction
chromatography of humic substances (HS), laser diffraction particle size analysis, specific
surface area (SSA) measurements by nitrogen adsorption and micromorphological
examination of thin sections.
Detailed characteristics obtained for aggregates and granule-densimetric fractions from a
typical chernozem soil under native grassland and under 60-year bare fallow show that
overall decrease in Corg under bare fallow is about 50% due to loss of light fractions, 30%
due to loss in clay fraction and 15% due to loss in coarse silt medium fraction. Light fraction
has predominant particle sizes of D10-D90=(2-20) mkm in fine silt and (7-70 mkm) in coarse
silt. Light fraction particles are mostly hydrophobic in nature and may function as high
contact seals between soil particles, enhancing hydrophobic interactions within
aggregate in the presence of water, and preventing its rapid entrance into the aggregate.
Reduction of light fractions fivefold and the increase in SSA (opening of clogged
pores) due to overall loss in Corg content allow more rapid water move into the
aggregate. Moreover, stable residual of light fraction under bare fallow becomes
extremely water-repellent which should make the aggregate system unstable upon water
percolation.
We observe mineralization and washing out of low-molecular hydrophilic HS from bare
fallow soil components. Hydrophilic HS may function as bonding mediators between mineral
particles and high-molecular hydrophobic HS. Therefore we propose that disruption of
aggregate waterstability results from SOM hydrophilic-hydrophobic disbalance.
Molecular parameters of HS hydrophobic and hydrophilic components are substantially
different. The enhancement in hydrophobic interaction ability of HS components is
accompanied by increase in molecular weight and Corg content, and decrease in nitrogen
content.
Waterstable aggregates are shown to contain HS with stronger hydrophobic properties
than that of dry-sieving ones and it results in a more pronounced reduction of SSA upon
waterstable aggregation.
The experimental results give evidence and characteristics of at least four SOM pools in a
typical chernozem soil: degradable sorbed SOM (accounts for 0.9% of Corg content), which
probably includes dissolvable SOM, degradable particulate organic matter (POM) (1.1%),
stable sorbed SOM (2%) and stable POM (0.5%).
In analytical aspect of the research possibility to describe soil components as a whole
through parameters of soil particles and SOM characteristics can be realized:
a) profile distribution of the average Corg content over soil layer mass is closely fitted by
the sum (exp1+exp2+C), indicating particular OM pools;
b) Corg content (~2%) of stable to long-term degradation sorbed SOM is set
in the power parameter of experimental curve SSA (Corg) for chernozem under
grassland.
c) fractal dimension discontinuities correspond to classification boundaries of colloidal,
clay, silt and sand particles (0.5; 2; 20 mkm);
d) qualitative change of hydrophobic properties is described by one parameter
presumably related to structure of high molecular HS. |
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