Agriculture Reference
In-Depth Information
depth of soil sampling must be considered when results from different experiments are
compared. Hooker et al., 2005, for instance, in a 28 years experiment comparing no-till or
conventional tillage with residues returned or residues removed from soil, measured the
contribution of corn-derived C at two different soil depths: 0-5 and 5-15 cm. The values they
found for the conventional tillage with mineral fertilization and residues removal that was the
same management practice of our experiment, were similar to ours if the whole depth of
sampling (0-15 cm) is considered.
The mineral fertilization and the manure addition caused an increase in the proportion of
corn-derived C in SOC as a consequence of the increase in biomass production both above
and below-ground. The higher crop yield obtained with mineral fertilization and manure
addition, also imply a higher production of roots and rhyzodeposition that can explain the
greater amount of corn-derived C in the treated plots compared to the control.
The treatments positively influenced the ability of soil to sequester C not only by
increasing the proportion of new C
4
-C entering the SOC, but also by slowing down the
turnover rate of the relic C
3
-C. In our experiment the values of C half-life we found ranged
between 55 and 86 years that fall within ranges presented by other authors (Balesdent et al.,
1990; Gregorich et al., 1994; Angers et al., 1995; Clapp et al., 2000). Lower values are also
reported in literature but these are usually referred to surface soil layers. Hooker et al., 2005
calculated for the surface layer of 0-5 cm shorter C
3
-C half-life ranging between 14 and 19
years, compared to the deeper layer of 5-15 cm. In our experiment the reason for the higher
values is that they were calculated for the whole sample without distinguish between surface
and deeper horizon.
The treatments had a different influence on the proportion of corn-derived C found in the
HA-C compared to what observed in SOC. The values ranged between 24% and 26% of the
HA-C and this implies a different turnover rate compared to SOC. In the control, in spite of
the net loss of SOC and HA-C over the experimental period, we observed an increase in the
proportion of new C
4
-C in this pool (26%) compared to the proportion measured for the SOC
(20%). Without addition of mineral nutrients over long time period, this pool was actually
able to sequester organic C, by including in its structure a great amount of new C
4
-C input
entering into the soil. We calculated that 40% of the total corn-derived C in soil was
recovered as HA-C. On the other hand, in the control, the HA fraction was less efficient in
storing the old C
3
-derived C compared to SOC, with a turnover time 25 years shorter than that
measured for SOC. It must be considered that in the control the only source of nutrients
derived from mineralization of soil organic matter and in this particular condition the humic
fraction could have been preferentially mineralized than other fractions such as the humin that
is reported to be more recalcitrant to degradation (Stevenson,1994). Moreover, as already
mentioned, the HA from non fertilized long-term fields seems to be less resistant to microbial
degradation than those from fertilized fields.
In the HA extracted from the Min treated plots we did not observe any preferential
immobilization of corn-derived C since the proportions measured in HA and SOC were
similar, meaning that the amount of corn-derived C entering the HA increased parallely to the
increase in HA content. With mineral fertilization, however, the HA were able to store a
greater amount of C compared to the control, mainly the old C
3
-C that with this treatment had
a much slower turnover rate. This, again, was probably related to the quality of HA because
those formed in long-term fields annually supplied with mineral fertilization are reported to
be more resistant to degradation (Filip and Kubàt, 2001).
