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| Titel |
Transformations in soil organic matter and aggregate stability after conversion of Mediterranean forest to agriculture |
| VerfasserIn |
Lorena Recio Vázquez, Gonzalo Almendros, Pilar Carral, Heike Knicker, José Antonio González Pérez, Francisco Javier González Vila |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250072112
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| Zusammenfassung |
| Conversion of forest ecosystems into croplands often leads to severe decrease of the soil
organic matter (SOM) levels with the concomitant deterioration of soil structure. The present
research focuses on the effects of cultivation on the stability of soil macroaggregates, as well
as on the total quantity and quality of SOM.
Three representative soils from central Spain (i.e., Petric Calcisol, Cutanic Luvisol and
Calcic Vertisol) were sampled. Each site had natural vegetation (NV) dominated
either by characteristic Mediterranean forest (dehesa) or cereal crops (CC) under
conventional tillage. For each site, three spatial replicates of the NV and CC were
sampled. Soil aggregate stability was measured by the wet sieving method. The
structural stability index was then calculated as the mass of aggregated soil (>250 μm)
remaining after wet sieving, as a percent of total aggregate weight. The analytical
characterization of the SOM was carried out after chemical fractionation for quantifying
the different organic pools: free organic matter (FOM), humic acids (HA), fulvic
acids (FA) and humin (H). Furthermore, whole soil samples pretreated with 10 %
HF solution were analyzed by CP-MAS 13C NMR and the purified HA fraction
was characterized by elementary analysis, visible and infrared spectroscopies and
Py-GC/MS.
A marked reduction in the proportion of stable aggregates when the natural ecosystem
was converted to agriculture was observed. Values of the structural stability index (%)
changed over from 96.2 to 38.1, 95.1 to 83.7 and 98.5 to 60.6 for the Calcisol, Luvisol and
Vertisol respectively. Comparatively higher contents of SOM were found in the soils under
NV (11.69 to 0.93, 3.29 to 2.72 and 9.51 to 0.79 g C-
100 g-1soil) even though a quantitative
rearrangement of the SOM pools was noticed. In all sites, the relative contribution of the
labile C (FOM) to the total SOM content decreased when the forest soils were converted into
croplands, whereas the proportion of both HA and FA increased in the cultivated soils.
Considering the differences in molecular characteristics of the HAs, cultivation increased
aromaticity and humification degree, reflected in the reduction of the H/C atomic
ratio and the increase of the E465 nm optical density of the HAs. The 13C NMR
spectra of the whole soils showed increased signal intensity in the alkyl and O-alkyl
regions in NV sites compared to agricultural systems. Infrared spectroscopy displayed
a less conspicuous pattern in HAs from CC sites. Moreover, the major aromatic
pyrolytic products in CC soils were alkylphenols, naphthalenes, benzenes, pyrenes and
N-compounds (pyrroles, indoles...), with lower abundance of methoxyphenols regarding NV
sites.
Cultivation reduced SOM concentration and macroaggregate stability in the studied soils.
The loss of organic C mainly affected labile pools of SOM, which could be partly explained
as the organic debris (fungal hyphae, fine roots, polysaccharides) are the main binding agents,
so the breakdown of macroaggregates with the tillage exposes the fresh organic materials to
microbial degradation. The final consequence is an enrichment on recalcitrant C fractions in
the cultivated soils. |
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