The content and properties of organic matter (OM) are important factors affecting the rate of water uptake by aggregates and, in turn, the stability of soil structure. To investigate the dynamics of these interactive processes, a sequential Low Temperature Ashing (LTA) treatment, based on the activation of oxygen plasma by radiofrequencies, was used to progressively oxidize OM on 3-4 mm aggregates of different types of soils. The characteristics of residual and oxidized OM after the treatments, were respectively assessed by: i) fractionation at different molecular size (>100, 100-50, 50-30, 30-8, 8-5, <5 kDa) of humic substances extracted from aggregates by 0.5 M NaOH, using Centricon plus-20 centrifugal filter device (Millipore) and by ii) differential analysis of soil samples. Total organic carbon (TOC) and total extractable carbon (TEC) in soil samples and in humic fractions were measured by CHN elemental analyser. The rate of water uptake by aggregates exposed to oxygen plasma for different times was determined by a simple apparatus based on the capillary adsorption of water. Sequential exposure to oxygen plasma caused, in most cases, a significant loss,10-15%, of TOC, just after lhr, and about 50% after 24 hr of treatments. The extractability of the humic fractions of OM, with respect to TOC, was enhanced and also the distribution of the fractions of different molecular weight in the extracts was affected. Specifically, 1 hr of treatment produced a significant reduction of the >100 kDa fraction with a concomitant increase in the fractions of lower nominal molecular weight limit. In contrast, after 24 and 48 hrs of exposure, the percentage of the humic fraction > 100kDa greatly increased with respect to TOC content while the fractions of lower molecular weight decreased. The <5 kDa fraction was an exception as it remained almost constant. This fact may be due to the oxidation of hydrophobic compounds (such as fatty acids and long chain hydrocarbons), with a consequent enhancement of the solubility of molecular structures greater than 100 kDa. This hypothesis seems confirmed by the distinctive feature revealed by the differential IR spectra of OM of soil aggregates, i.e. the disappearance, just after 24 hrs of treatment, of the strong CH stretching bands in the range 2800¬2900 cm-1, assigned to long chain hydrophobic compounds. The rate of water uptake shows a dramatic increase, up to threefolds after 24 hrs of LTA treatment, and results in a close relationship with the loss of OM and with the amount of extracted humic substances. These findings demonstrate the role of the hydrophobic fractions of soil OM in affecting the water affinity of the aggregates and, consequently, the stability of soil structure. From these results it can be hypothesized that these hydrophobic constituents are mostly located at the interfaces of stable natural aggregates. When the environmental conditions cause the oxidation and breakdown of these substances, the uptake of water greatly increases and the aggregates become more prone to be dispersed.
The effect of organic matter oxidation by Low Temperature Ashing (LTA) on the rate of water uptake by soil aggregates
D'Acqui LP;
1998
Abstract
The content and properties of organic matter (OM) are important factors affecting the rate of water uptake by aggregates and, in turn, the stability of soil structure. To investigate the dynamics of these interactive processes, a sequential Low Temperature Ashing (LTA) treatment, based on the activation of oxygen plasma by radiofrequencies, was used to progressively oxidize OM on 3-4 mm aggregates of different types of soils. The characteristics of residual and oxidized OM after the treatments, were respectively assessed by: i) fractionation at different molecular size (>100, 100-50, 50-30, 30-8, 8-5, <5 kDa) of humic substances extracted from aggregates by 0.5 M NaOH, using Centricon plus-20 centrifugal filter device (Millipore) and by ii) differential analysis of soil samples. Total organic carbon (TOC) and total extractable carbon (TEC) in soil samples and in humic fractions were measured by CHN elemental analyser. The rate of water uptake by aggregates exposed to oxygen plasma for different times was determined by a simple apparatus based on the capillary adsorption of water. Sequential exposure to oxygen plasma caused, in most cases, a significant loss,10-15%, of TOC, just after lhr, and about 50% after 24 hr of treatments. The extractability of the humic fractions of OM, with respect to TOC, was enhanced and also the distribution of the fractions of different molecular weight in the extracts was affected. Specifically, 1 hr of treatment produced a significant reduction of the >100 kDa fraction with a concomitant increase in the fractions of lower nominal molecular weight limit. In contrast, after 24 and 48 hrs of exposure, the percentage of the humic fraction > 100kDa greatly increased with respect to TOC content while the fractions of lower molecular weight decreased. The <5 kDa fraction was an exception as it remained almost constant. This fact may be due to the oxidation of hydrophobic compounds (such as fatty acids and long chain hydrocarbons), with a consequent enhancement of the solubility of molecular structures greater than 100 kDa. This hypothesis seems confirmed by the distinctive feature revealed by the differential IR spectra of OM of soil aggregates, i.e. the disappearance, just after 24 hrs of treatment, of the strong CH stretching bands in the range 2800¬2900 cm-1, assigned to long chain hydrophobic compounds. The rate of water uptake shows a dramatic increase, up to threefolds after 24 hrs of LTA treatment, and results in a close relationship with the loss of OM and with the amount of extracted humic substances. These findings demonstrate the role of the hydrophobic fractions of soil OM in affecting the water affinity of the aggregates and, consequently, the stability of soil structure. From these results it can be hypothesized that these hydrophobic constituents are mostly located at the interfaces of stable natural aggregates. When the environmental conditions cause the oxidation and breakdown of these substances, the uptake of water greatly increases and the aggregates become more prone to be dispersed.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
