Agriculture Reference
In-Depth Information
observed in the corn plots. Other long-term studies highlighted the positive effect of manure
addition on C sequestration as a result of increased root yields, higher humification rate
constant and the direct application of organic matter through manure (Kundu et al., 2007;
Bhattacharyya et al. 2007). In particular the authors measured in a 30 years rotation with
wheat and soybean an increase in above and below-ground biomass, with the application of
farmyard manure together with mineral N alone or NPK that was probably associated, apart
from N, P, and K supply, to other benefits induced by manure addition, such as improvements
in microbial activity and soil physical conditions.
The HA-C content measured in the corn plots at the first sampling in 1973 was
significantly lower than the values found in the wheat plots. This difference was probably due
to the previous soil exploitation by the corn that had already been cropped for 7 years. Such
exploitation continued with corn over the 30 years of the experiment since in the control the
level of HA-C in 2002 was significantly lower than in 1973. This trend was not observed with
wheat that at the end of the experiment did not show any variation in the HA-C level.
The treatments significantly increased (P<0.05) the HA-C compared to the control and
this positive effect was more evident in the corn plots. The greater increase in HA-C induced
by the Org treatment was probably related to the direct application of organic matter through
manure and to a higher humification rate constant (Bhattacharyya et al., 2007). The repeated
addition of mineral fertilizers over years also maintained a high HA-C content. Similar results
were obtained by Galantini and Rosell, 2006 that after 8 years of continuous wheat found a
significant higher content of HA-C when the wheat was annually fertilized compared to the
control without fertilization.
The capability of soil to maintain a high content of HA-C or even to increase this C pool
is dependent on the balance between humification rate and degradation. The humification rate
is positively affected by C and nutrients availability that both are factors stimulating the
microbial activity. Degradation is the counterbalanced mechanism that breaks down the
formed humic fraction and that is itself mediated by microorganisms. It has been reported that
HA-C from long-term fertilized soils are more resistant to degradation than those from non
fertilized soils and that the HA's were better utilized by microorganisms when they serve as
sole source of C or N than under more favourable nutrients conditions (Filip and Kubàt,
2001).
Corn-Derived C in TOC and Humic Acids
In SOC after 30 years, a proportion ranging from 20 to 30% was C originating from corn.
In the Halle experiment Flessa et al., 2000, found that after 37 years of continuous corn, 15%
of SOC accounted for corn-derived C. In other studies higher proportions of corn-derived C
are reported (Puget et al., 1995; Balesdent et al., 1990), but these differences are mainly
related to different management practices. The amount of corn-derived C in soil usually
increases with no-till management compared to conventional tillage and with the return of
above-ground biomass to soil compared to the removal. In our experiment the above-ground
biomass was removed so the contribution of corn-derived C to the SOC only originated from
roots system and rhyzodeposition. Soil characteristics also affect the amount of new C
4
-C in
soil because in sandy soil the organic matter is hardly protected by the formation of stable
aggregates or through adsorption to mineral surfaces (Sollins et al., 1996). Moreover the
