Agriculture Reference
In-Depth Information
increase in HA-C in both monocultures that was again more pronounced with the Org one
that showed the highest content of HA.
Table 1. Soil total organic C (TOC) and humic C (HA-C) content in the control and in
the plots of both monocoltures treated consecutively over 30 years with mineral
fertilizers (Min) and manure (Org)
Wheat
Corn
TOC g kg
-1
Treatments
8.03 ± 0.06
†
Control
7.23 ± 0.29
8.80 ± 0.17
a
Min
8.25 ± 0.04
9.12 ± 0.07
a
Org
9.65 ± 0.13
HA-C g kg
-1
Control
3.03 ± 0.05
2.08 ± 0.03
Min
4.00 ± 0.03
3.35 ± 0.03
Org
4.35 ± 0.05
3.76 ± 0.02
†
Mean
± standard error, n=6 replicates
a
Within column, means followed by the same letter are not statistically different at 0.05 level of
probability.
Corn-Derived C in Soil and HA
After 30 years of continuous corn cropping the δ
13
C of the TOC became significantly
(P<0.05) less negative as a result of the corn-derived C entered into the soil (Figure 1a). This
shift in isotopic signature was significantly greater in the treated plots than in the control. The
mass balance equation indicated that, whereas in the control about 20% of corn-derived C
was measured in 2002, both treatments induced an increase in the proportion of corn-derived
C in SOC, so the C
4
-C input at the end of the experiment was about 26% and 30% with Min
and Org treatment respectively (Table 2). The amount of new corn-derived C, on a dry soil
basis, present in SOC after 30 years is shown in Figure 2a, the highest input was measured
with the Org treatment, then with the Min treatment whereas in the control we found the
lowest amount of corn-derived C.
The δ
13
C of the HA-C in the corn plots also showed a shift towards higher values due to
new C
4
-C that entered into this C pool (Figure 2a). However, unlike what observed in SOC,
the difference in δ
13
C between the wheat reference plot and the corn plot, was similar for all
the treatments and of about 4‰ δ units (Figure 2b). As a consequence the proportion of corn-
derived C in HA turned out to be similar for all the treatments, ranging from 24% in the Min,
to 26% in both control and Org treatment (Table 2). The amount of corn-derived C present in
the humic fraction, and expressed on a dry soil basis, however was different and, in the
treatments significantly greater than in the control (Figure 1b).
By comparing the proportion of corn-derived C in HA and SOC, we observed that in the
control the value in HA (26%) was higher than that found in the SOC (20%), whereas in the
Org treatment it was lower than the 30% measured in SOC. It seems that, without any
treatment, the new corn-derived C entering the soil, would be preferentially immobilized.
