Agriculture Reference
In-Depth Information
NMR Spectroscopy
The NMR spectra were obtained by dissolving the HA (70-80 mg) in 0.5 mL of 0.5 M
NaOD. The spectra were recorded with a Bruker ACF 250 spectrometer (Karslruhe,
Germany), using a 5 mm multinuclear probe. Proton decoupled 62.890 MHZ
13
C spectra were
accumulated with a 32 K data point, gated decoupling sequence, 30° pulse angle, 12.8 KHz
spectral width, an acquisition time of 1.3 s and a relaxation delay of 3 s. Generally 100,000
scans were accumulated. A line broadening (20-30 Hz) was used prior to Fourier
transformation. Chemical shifts are relative to tetramethylsilane (TMS).
The spectra were divided into the following four chemical regions [28]: to the aliphatic
carbons (0-40 ppm), to the peptidic and oxidized carbons (40-110 ppm), to the aromatic and
phenolic carbons (110-160 ppm) and to the carboxylic (160-190 ppm) carbons [26, 36]. In
particular we considered an additional subdivision in the region at 115-99 ppm that might be
assigned to anomeric C in polysaccharides, acetal or ketal.
TG-DTA analysis
. Thermogravimetric analysis (TG) and differential thermal analysis
(DTA) were carried out simultaneously using a TG-DTA92 instrument (SETARAM, France)
as described in previous paper [20].
The Statgraphics version 5 plus (Statistical graphics system by statistical graphics
corporation) was used for statistical calculations.
R
ESULTS AND
D
ISCUSSION
The amount of SOC after 30-years farming is shown in Table 1. Compared to the
beginning of the experiment C
0
(1972), a significant decrease (P<0.05) in TOC of about 23%
was observed in the control after 30-yrs (C
30
), whereas with the CR treatment the C loss was
around 10%. The soil treated with manure, on the contrary, did not show any significant
change with respect to the control (C
0
).
The δ
13
C of the TOC over the 30-years of farming became progressively less negative
(Tab. 1) in all the treatments but the greatest variation was seen in the CR
30
plot. The changes
in all treatments are due to the presence of corn in the crop-rotation, while additionally in CR,
to the amendment practice of adding corn residues after the corn harvest. Comparing the
isotopic signature of TOC to that of humic C after 30-years, a much smaller variation is seen
and this would suggest a preferential utilization of C
3
-derived C in the humification process.
The δ
13
C change of TOC, therefore, would mainly be due to the presence of C
4
-derived C in
the non-humic fraction. In contrast to soil TOC, the percentage of humic C significantly
(
P
<0.05) increased in both CM
30
and CR
30
treatments. These changes in C content reflect the
effect of long-term application of manure and crop-residues on HA. In spite of the substantial
decrease in TOC, even the control after 30 yrs (C
30
) measured a small increase in humic C
(approximately 6%). Since the control did not receive any addition of organic material, the C
sources used to maintain the level of humic C throughout the 30 year period, were from both
rhyzodeposition during plant growth and C derived from mineralization of the root residues
remaining in the soil after the harvest [37]. The fact that the two treatments (CM and CR)
both show an increase in humic C and a decrease in TOC indicates that HA are able to store C
