Agriculture Reference
In-Depth Information
this kind of C component. However, differences can be found between the two treatments. In
CR
30
the high anomeric C signal, associated with carbohydrates, is related to the presence of
polymers such as hemi-cellulose or cellulose often encrusted with lignin. In this condition,
these substances are not readily accessible to microorganisms and therefore, their
decomposition slowly occurs, allowing their preservation in soil. In addition, the resonance at
105 ppm assigned to unsubstituted aromatic rings is likely to derive from non-degraded
lignin, indicating a crop-residue preservation with little or no alteration in soil. This is again
supported by DRIFT which showed stronger signal in the 1500-1570 cm
-1
region due to
lignin moieties. The aromatic C content, also increased in CM
30
, but no increase in anomeric
C signal was observed. This would probably suggest a microbiologically-mediated
breakdown of resistant material favoured by the amendment and synthesis of new organic
molecules. This hypothesis is further supported by the more intense resonance corresponding
to N or O substituted aromatic rings observed in CM
30
as compared to CR
30
and C
30
. This was
particularly evident in the phenolic C (150-170 ppm) that showed a significant increase in
CM
30
. The considerable amount of aromatic components substituted, (i.e. hydroxtyl,
methoxyl or carboxyl groups) suggests that residue transformation in soil is aimed towards in
situ-synthesis of new HS. Additional support is given by the region between 70-60 ppm,
characteristic of compounds with hydroxyl groups on adjacent carbon atoms [44]. Among
these groups, inositols and sugar alcohols might be considered the most common natural
compounds present in this sample. The intensities of the signals at 73 ppm (C
α
in units with
β-O-linkages) and 53 ppm (methoxyl groups) also provide evidence of the different effects of
the treatments: O-CH
3
groups are more intense in CM
30
than CR
30
spectra, whereas these
groups disappear in C
30
. Finally, the resonance areas of the carboxylic C (195-165 ppm)
showed an increase in carboxylic groups only in CM
30
, consistent with the results obtained
using the other techniques. Manure originally containing components with more oxygenate
groups, such as carboxylic groups [45], of course, influences the genesis of HA towards an
enrichment of oxygenate functional groups in their structures.
Thermal Analysis
The TG-DTA curves of HA from control and amended plots are shown in Figure 3. Each
curve was characterized by two intense exothermic reactions at 300°C and at 460°C,
respectively. The strong thermal combustion of the first peak might be influenced by the
accumulation or formation of recent organic carbon. The increase in carboxylic groups in the
plot amended with CM, as supported by titration and NMR analyses, might mainly be
responsible of the strong exothermic reaction observed in this sample. The second exothermic
peak remarkabily changed in CM
30
whereas no significant loss weight appeared in CR
30
with
respect to the control (C
0
). In addition the position of the second exothermic peak is shifted
towards higher temperature in both treatments (Figure 3). This suggests that high resistant
temperature substances such as aromatic structures and cleavage of C-C bonds [16, 31] are
involved in exothermic reaction. The different thermal behaviour of CR
30
and CM
30
might be
the consequence of structural and chemical modification of HA undergone to continuous
application with manure and crop residues.
