Agriculture Reference
In-Depth Information
In detail, the band around 1600 cm
-1
, due to ν
as
(-COO
-
) and ν(C=C) of aromatic rings
vibrations [17-18], increased in the CM
30
sample compared to CR
30
and C
30
. We suggest that
the carboxylate groups provide a dominant contribution to this vibration in accordance with
the potentiometric data titration and
13
C NMR analysis (see below). Another important
modification appeared around 1660 cm
-1
, assigned to C=O stretching in aromatic compounds
or in amide I [40]. It appeared as a shoulder in C
30
and its relative intensity increased in CR
30
compared to the CM
30
spectrum. The variation of the relative intensity of this band observed
in CM
30
seems likely to be attributable to the presence of polar substituents. Moreover, the
CM
30
spectrum was characterized by two well resolved bands at 1640 and 1630 cm
-1
. The
region around 1500 cm
-1
, in both CR
30
and CM
30
, showed an important structural change
compared to the control (C
0
). The peaks at 1550 cm
-1
, 1535 cm
-1
and 1514 cm
-1
indicate a
large aromatic content from lignin derivates in HA extracted from the soil amended with CR
30
(data supported by NMR). In the C
30
sample this region was not well resolved. These
components are chiefly identified by the presence of the peak at 1514 cm
-1
, assigned to the
lignin component absorption produced by the stretching of
ortho, para
and 1,3,4-tri-
substituted aromatic rings of guaiacyl [41]. This is in agreement with the presence of bands
around 1660 cm
-1
, 1235 cm
-1
and 1080 cm
-1
, which can be attributed to the vibration of 2-
methoxy phenol originated from lignin degradation that appeared more intense in the CR
30
spectrum.
On the basis of these results we may infer that lignin residues from crop rotation
(wheat/corn) do not undergo a complete degradation in soil, but they can persist over time and
they can be included in the molecular structure of humic substances during the humification
process. However, considering that the δ
13
C value (Table 1) of HA from soil amended with
CR
30
is similar to that of HA from C
0
and is also closer to that of wheat, the relative
contribution of wheat-derived lignin to HA seems to be greater than that of corn-derived
lignin.
The relative intensity of bands in the 1460-1300 cm
-1
region increased particularly in
CM
30
and CR
30
than to the controls. Moreover over 30 years in all spectra appeared a broad
band at around 1230 cm
-1
assigned to ν(C-O) stretching motion of phenolic groups, and acid
dimers with electron-withdrawing groups and C-C (in condensed aromatic rings) [42]. This
band noteworthy increased in CR
30
whereas no vibration appeared in the C
0
spectrum.
Furthermore, the region between 1120 -1084 cm
-1
is characteristic of ν
s
(C-OH) and C-O-C in
sugar and ether group vibrations, respectively.
13
C NMR Spectroscopy
The spectra of samples after 30 yrs (Figure 2) significantly differed in all regions if
compared to the control (C
0
). The region corresponding to the alkyl-C
(50-0 ppm) showed a
decreasing trend from C
30
to CM
30
and CR
30
; compared to C
0
, this region increased in C
30
,
decreased in CR
30
, and remained unchanged in the CM
30
treatment. The increase in alkyl-C in
the HA, therefore, was observed only when the soil did not receive any amendment and it was
probably a result of crop rotation. In C
30
, this C fraction seems to be the main cause of the
small increase in humic C observed, in spite of the large decrease in TOC (Tab.1). The alkyl-
C component is actually involved in the long-term stabilization of SOM [23].
