Agriculture Reference
In-Depth Information
cells. The last group is frequently described as rhizo-deposits. The soil carbon pools are
oxidized by different groups of heterotrophic organisms for their metabolic needs. The most
important and active heterotrophs in soil are microorganisms: bacteria, fungi, actinomycetes
and protozoans. The activity of this heterotrophic community is affected by different soil
environmental factors: SOM quality, nutrient availability, temperature, moisture, redox
potential. The quality of SOM is one of the most important factors affecting the
decomposition since humified carbon slows down the process since the acquisition of energy
from such a substrate is slow. Microbial growth in soil is also limited by available nitrogen
(N), since they have to satisfy their stoichiometric requirement (C:N ratio) from the substrate
they feed on which has a different C:N ratio. If mineral nitrogen is available the competition
for nutrients stimulating SOM decomposition by microorganisms is less intense. In soil there
is an active competition between plant roots and microorganisms for mineral nitrogen, root
uptake of N raises the competition for nutrient and decreases the microbial growth and
metabolism, thereby depressing SOM decomposition (Schimel et al., 1989; Bottner et al.,
1999). N fertilization, soil temperature (affected in agricultural systems by spatial and
temporal reduction of plant canopy) and moisture (affected in some agricultural systems by
irrigation) strongly influence soil CO
2
emission, by modifying the relative contribution of the
two components (autotrophic and heterotrophic) (Wiseman & Seiler, 2004; Davidson &
Janssens, 2006).
The adoption of better management practices (BMPs) can improve soil organic carbon
(SOC) content, enhance soil quality, restore degraded ecosystems, increase biomass
production, improve crop yield, and encourage investment in soil resources for soil
restoration (Lal et al., 1998). The removal of crop residues does not necessarily induce a rapid
decrease of SOM content. For instance, Campbell et al. (1991) showed that the removal of
straw over a period of 30 years did not significantly affect the SOM content of an old wheat-
wheat-fallow rotation system.
The effect of N fertilization on SOM mineralization and resulting CO
2
emission is very
complex since many factors, both natural and/or linked to crop management, are involved in
affecting the biological activities responsible of the process. Moreover, experimental design
and measurement techniques present some difficulties. For example, it is difficult to
discriminate among the different biogenic sources of CO
2
; SOM-derived and plant-derived
CO
2
is essential to evaluate the real capacity of soil as source or sink of atmospheric CO
2
.
Khan et al. (2007) reported a promoted SOM decomposition by N fertilizer in a long-
term study (51-year, Illinois). Other experiments (long term studies or experiment with
isotopic signature) have shown that fertilizing crops with N results in higher levels of soil C
over time (Paustian et al., 1992; Liang et al., 1996; Paustian et al., 1997; Wilts et al., 2004;
Jagadamma et al., 2007). After these studies, the positive effect of N fertilization was
basically due to the high yield crop which increases the annual input of crop residue to soils,
mainly coming from roots and root exudates during growth. The same authors stated that an
adequate fertilization contributes to the increase of SOM and does not alter the turnover of
native SOM. This conclusion is in contrast with process called ―priming effect‖ (Bingeman et
al., 1953) considered as a stimulation of the mineralization of native SOM after the
incorporation of fresh organic matter such as green manure, straw, rhizodeposits. This
phenomenon has been clearly observed at the rhizosphere scale. For example, Liljeroth et al.
(1994) showed in laboratory conditions without mineral nutrient supply, that the
rhizodeposition by wheat and maize induced a two-fold increase in the mineralization rate of
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