Agriculture Reference
In-Depth Information
proportion ranging from 35% (Org) to 40% (control) of the total corn-derived C in soil
was recovered as humic C, confirming the important role of this pool as a C reservoir in
soil.
Keywords:
Soil orgnic C, Humic Acids, δ
13
C, manure, mineral fertilization
I
NTRODUCTION
Preservation of soil organic C (SOC) in agricultural lands is of primary importance for
maintaining soil fertility and productivity as well as environmental quality. Intensive
cultivation often causes reductions in C content, contributing to the increase in atmospheric
CO
2
concentration. This negative effect can be mitigated by sustainable management
practices such as crop residues input, organic amendments or reduced tillage, that can
increase organic matter input to the soil, or decrease decomposition of soil organic matter and
oxidation of SOC (Follett, 2001; Paustian et al., 2000).
To evaluate the effects of these practices on SOC storage, however, requires a better
knowledge of SOC dynamics and long-term field experiments. The measure of the
transformations of SOC and its pools from a quantitative point of view can only be obtained
in experiments where a change in vegetation from C
3
to C
4
plants or vice versa has occurred.
In this case the SOC turnover can be followed by measuring the isotopic composition of C in
soil after the vegetation change (Balesdent et al., 1988; Flessa et al., 2000; Clapp et al., 2000;
Wilts et al., 2004). The different photosynthetic pathway of C
3
and C
4
plant species is
responsible for their different isotopic composition that is reflected in that of the soil. This
natural label allows to quantify the new C input. When a C
4
plant, usually, in agroecosystems,
corn or sorghum, is grown on a soil that has always carried C
3
plants, the δ
13
C of SOC
progressively changes, and the shift is directly related to the proportion of new C
4
-derived C
(Balesdent et al., 1987). The longer is the period of cultivation of the C
4
plant, the more the
δ
13
C value of the SOC approaches that of the plant community (Nadelhoffer and Fry, 1988)
because only slight isotope fractionation may occur during early stages of soil organic matter
decomposition in well-drained, mineral soils (Boutton, 1996). Since these unique δ
13
C values
persist during decomposition and soil organic matter formation, the SOC turnover rate can be
determined by the rate at which its δ
13
C value changes to approach that of the new plant
community (Balesdent et al., 1987; Boutton et al., 1998).
Based on the same assumption, we can calculate the turnover rate for particular pools of
SOC by measuring the change of their δ
13
C values. One of the most important pools of SOC
is that of the humic substances and in particular of humic acids (HA) that are complex
macromolecules modified from plant compounds or newly synthesized during decomposition,
that accumulate in soil (Stevenson, 1994). Because of their resistance to microbial
degradation (Quails, 2004), they represent a crucial component of SOC where C tends to be
stored for long periods and an important reservoir of nutrients.
In the long-term field experiment in Cadriano, at the University of Bologna, Italy, in
1966, there was a change of vegetation from C
3
to C
4
species, since a monosuccession of corn
started on a soil previously cropped with C
3
plants. Plots with continuous wheat cropping
were also included in the experiment and for both monocultures different treatments were
