Agriculture Reference
In-Depth Information
compared. This experimental setup allows the use of the 13 C abundance technique in order to
study the C transformations and dynamics since the wheat monoculture represents the
reference site that has the same soil properties, land use and environmental conditions of the
site with corn. In particular, in this study we could evaluate the effects of the two plant
species cropped in monosuccession for a long time period, on the level of SOC and of its
stable pool of humic acids (HA). Moreover, we could quantify the amount of corn-derived C
in total organic C and in the humic fraction after 30 years of continuous corn cropping, and
compare the effects of different treatments such as mineral fertilization and organic inputs, on
the amount of corn-derived C accumulated in soil.
M ATERIALS AND M ETHODS
The long-term field experiment located in Cadriano, (University of Bologna, Italy,
Department of Agricultural Sciences and Technologies) initiated in 1966, but soil samples
from 1966 to 1972 are no longer available. The soil is a Typic Udochrept with the following
chemical characteristics: pH (in water) 6.9; sand 56%; silt 16%; clay 28%; total organic C
(TOC) 8.5 g kg -1 ; total N 1.1 g kg -1 ; cation exchange capacity 16.5 cmol c kg -1 .
The experiment compares two continuous cropping system, wheat ( Triticum durum ) and
corn ( Zea maize ) that did not receive any mineral or organic fertilization (Control), or treated
with mineral fertilizers (Min) or with organic amendments (Org). In the Min treatment, wheat
received 200 kg N ha -1 and 150 kg P ha -1 , whereas corn received 300 kg N ha -1 and 150 kg P
ha -1 , in the Org treatment both monocultures received 20 q ha -1 of manure applied to soil at
the end of the summer at the ploughing. The crop yields and the whole above-ground biomass
were removed at harvest, whereas the below-ground biomass remained in soil.
Each treatment was conducted in triplicate using a randomized block design. Soil
samples from 1973 and from 2002 were taken at the depth of 25 cm, air dried and grinded,
then analyzed for soil total organic C (TOC), humic acids C (HA) and their isotopic ratio. All
analytical determinations were conducted in six replicates.
Humic acid isolation . Thirty g of dry soil finely grinded, were extracted with 300 mL of
0.1 M NaOH plus 0.1 M Na 4 P 2 O 7 at 65 °C by shaking for 24 hours. The suspension was then
centrifuged at 5,000 x g for 30 min and filtered through a 0.45 μ m filter (Millipore, Bedford
MA - USA). Twenty-five ml were separated from the rest of the filtered solution, acidified
with H 2 SO 4 to precipitate HA and analyzed according to Ciavatta and Govi, 1993 to
determine the amount of C from HA expressed on a dry soil basis. The rest of the filtered
solution was acidified with 5 M HCl to pH < 2 to precipitate the HA and centrifuged at 5,000
x g for 20 min. The precipitated HA were re-solubilized in 0.5 M NaOH and then dialyzed
against H 2 O until neutral pH before being freeze - dried. The measurement of the humic C
isotopic ratio was carried out on the freeze-dried HA samples.
Total organic C and δ 13 C . Soil total organic C was measured with an elemental analyzer
(Thermo Electron mod. EA 1110), since this soil does not contain inorganic C the
pretreatment with HCl was not necessary. The percentage of C in the sample was calculated
using acetanilide as a certified standard containing 71.09% of carbon. The C isotopic ratio of
soil total C and HA-C was measured with CF-IRMS (Continuous Flow-Isotope Ratio Mass
Spectrometry mod. Delta Plus Thermo Electron, Bremen, Germany). The C isotope values
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