Agriculture Reference
In-Depth Information
fraction while the more refractory C is decomposed at
higher temperatures ( 450°C) [31, 33].
The mass loss that occurs during heating can be used to compare the relative abundance
of
more and less labile C whilst the position of peaks reflects
the structure and chemical
composition of the sample.
Moreover, combining thermal analysis with the spectroscopic techniques can be obtained
important structural information on soil organic matter composition.
In this study, advanced analytical techniques such as δ
13
isotopic ratio-mass spectrometry
(IRMS), DRIFT, and
13
C NMR spectroscopies and thermal analysis (TG-DTA) were
exploited to characterise humic acids isolated from a soil amended with crop residues and
cow manure over a 30-year period. The objective was to verify how the amendment practices
may influence the HA structure.
M
ATERIALS AND
M
ETHODS
The soil samples analyzed (horizon Ap, 0-40 cm depth) were taken from plots of a field
experiment conducted for over 30-years at the agricultural farm of the University of
Bologna's Agricultural Faculty (Cadriano, Italy). This soil is classified (Soil Survey Staff -
USDA SCS 1989) as a
Typic
Udochrept.
The main physical-chemical characteristics of the
soil studied were the following: pH (in water) 6.9; texture: sand 56%; silt 16%; clay 28%;
total calcium carbonate (CaCO
3
) < 1%; total organic C (TOC) 7.5 g kg
-1
; total N 1.1 g kg
-1
;
cation exchange capacity 16.5 cmol
c
kg
-1
. A more detailed description of the field
experiments have previously been reported [34].
However, briefly, the experimental design included plots amended over a 30-year period
with cattle manure (CM) and crop residues (CR) constituted by wheat straw or corn-stalks
(each biomass followed in succession), using un-amended soil as the control (C). CM was
added to the plots at the following rate: 6 t ha
-1
of dry matter after wheat tillage and 7.5 t ha
-1
of dry matter after corn tillage. All the plots examined, including the control C, were fertilized
with 100 kg N ha
-1
yr
-1
added as ammonium nitrate [34]. Each treatment (CM, CR and C) was
conducted in triplicate using a randomized block design.
Analyses were carried out on soil samples taken from: i) un-amended soil in 1972 (C
0
),
and ii) after 30-years (C
30
) and iii) after a 30-year amendment period with CM
30
and CR
30
.
All analytical determinations were conducted in triplicate.
Humic Acid Isolation
10 g of air-dried soil previously crushed and sieved at 2 mm, were put into a 250 mL
flask and extracted under N
2
with 100 mL of 0.1 M NaOH plus 0.1 M Na
4
P
2
O
7
at 65 °C using
an end-over-end shaker at 100 rpm for 24 hours. The suspension was centrifuged at 5,000 x
g
for 30 min and then filtered through a 0.45
μ
m filter (Millipore, Bedford MA - USA). The
solution was acidified with 5 M HCl to pH < 2 to precipitate the HA and afterwards was
centrifuged at 5,000 x
g
for 20 min in order to eliminate the supernatant. The HA
solutions
were dialyzed against d-H
2
O until a neutral pH was achieved and finally freeze-dried and
