Agriculture Reference
In-Depth Information
I
NTRODUCTION
Long-term field experiments have demonstrated how the chemical characteristics of soil
organic matter (SOM) can be influenced by management and amendment practices [1,2]
Usually, intensive cultivations characterized by frequent soil tillage cause a mineralization of
soil organic carbon (SOC) while organic fertilisation (manure and crop-residues) improves
the quantity and quality of SOC [3]. However, the chemical changes in SOM
are complex
because no well defined chemical structure has been discovered. The key is to find reliable
analytical methods to determine the qualitative and quantitative changes in SOM and finally
to identify a category of molecules that are representative of those changes that taking place.
Recently, emphasis has been placed on the refractory fraction of SOC, in the dynamics of
organic C and C sequestration. This fraction is an heterogeneous mixture of organic
compounds, consisting of aromatic, aliphatic, phenolic, and quinonic functional groups with
varying molecular sizes and molecular weights, called “humic” substances (HS) [4,5]. The
main characteristic of these macromolecules is their extreme stability to biochemical or
chemical degradation over time, whether in soil [6] or in coals [7] or composts [8]. In spite of
the indefinite nature of HS structure, some materials, such as cellulose or lignin components,
may be accumulated and preserved during the humification process [9]. Their presence in HS
is considered as an early humification process [10]. In particular the early humic fraction is
that more important for agronomic practices [11] with respect to that associated to mineral
complexes (insoluble in alkali). Although numerous studies have investigated the dynamic
fraction of SOC [12,13] few have examined how the molecular structure of humic acids
(HA), the main fraction of the HS, is influenced by long-term field amendment experiments.
A better understanding of the changes occurring in HS might give additional information on
the effects of management and fertilisation practices. Since the molecular structure of HA is
complex, different analytical techniques are an essential precondition for more accurate
structural interpretations to predict the ability of HS to sequester C in different cropping and
soil management systems. Carbon isotopic composition has been successfully used in studies
of SOM dynamics [14]. The δ
13
C value of both total organic C and HS depends on the δ
13
C
value of soil vegetation [15-16]. Due to the different isotopic fractionation in C
3
and C
4
plants
during the photosynthetic pathways, δ
13
C can be used as a natural tracer to follow the
dynamics of C
3
and C
4
-derived C in soil [14].
A rapid characterisation of SOM can be obtained by using spectroscopic techniques and
thermal analysis. In particular among spectroscopic techniques the diffuse reflectance infrared
Fourier transform (DRIFT) has often been applied to investigate HS from different origins
[17-20], to follow organic matter degradation in soil [21] and in forest [22], and to study as
management influence the SOM characteristics [3, 23-25]. Another spectroscopic technique
which has arguably provided the most interpretable information on the nature and relative
abundance of chemical groups present in HS is liquid-state
13
C NMR spectroscopy [26-30].
The application of liquid-state
13
C NMR and DRIFT spectroscopies resulted useful tools to
analyze the influence of agricultural management on structural changes of SOM [23-25].
In addition thermal analysis (TA) represents a rapid and accurate approach in the
investigation of HS [16, 20, 31-33]. This technique involves a continuous and simultaneous
measurement of weight loss (TG) and energy change (DSC) during
heating of the sample.
During heating of HS a first mass loss (
300°C)
is produced by decomposition of the labile C
