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with Brazilian no-till regimes. In principle, both the decreased erosion losses of
SOM-rich topsoil (Lal 2002; Rasmussen and Collins 1991) and the slower SOM
mineralization rates in zero-till soil compared to plowed soil suggest that no-till
provides more favorable conditions for SOM buildup than conventional tillage. Not
turning the soil, for example, means the following: (1) less soil macroaggregates are
disrupted, consequently leading to the increased formation of stable microaggre-
gates that occlude and protect particulate organic matter (POM) from microbial
attack (Amado et al. 2006; Feller and Beare 1997; Lal et al. 1999; Six et al. 1998,
1999, 2000; Fabrizzi et al. 2009); (2) there is less stimulation of sharp increase
in microbial activity and concomitant release of CO
2
in response to enhanced soil
aeration (Bayer et al. 2000a,b; Bernoux et al. 2006; Kladivko 2001); and (3) there
is less mixing of residues deeper into the soil where conditions for decomposition
are often more favorable than on the soil surface (Blevins and Frye 1993; Karlen
and Cambardella 1996). In this context, Mielniczuk (2003) estimated the rate of
SOM mineralization under conventional tillage regimes in Southern Brazil to be,
on average, 5% to 6% per year compared to an average of about 3% per year in
no-till soils. Although the actual amount of SOM storage potential in a given soil is
in turn largely determined by climate and the capability of soils to stabilize and pro-
tect SOM, this in itself generally is largely determined by soil texture, soil mineral
surface area, and soil mineralogy, with soil parameters such as water-holding capac-
ity, pH, and porosity acting as rate modifiers (Six et al. 2000). The large majority
of Brazilian literature does indeed suggest that SOM accumulation in no-till soils
exceeds that of plowed soils and that this is the case over a range of soil textures,
from sandy loams (Amado et al. 1999, 2000, 2001, 2002, 2006; Bayer et al. 2000a,b,
2002) to heavy clay (>60% clay) soils (Amado et al. 2006; De Maria et al. 1999;
Perrin 2003), both in Southern Brazil (Muzilli 1983; Sá et al. 2001a,b; Zotarelli et
al. 2003) as well as in the degraded savanna cerrado region further north (Corazza
et al. 1999; Freitas et al. 1999; Resck et al. 1991, 2000; Scopel et al. 2003). Bernoux
et al. (2006) reviewed some 25 published and unpublished data sets on the rate of C
accumulation in Brazilian no-till soils and observed that reported C accumulation
rates in excess of those found in comparable plowed soils vary from around 0.4 to
1.7 t C ha
-1
year
-1
for the 0- to 40-cm soil layer in the cerrado region and between
0.5 and 0.9 t C ha
-1
year
-1
in Southern Brazil, with an overall average accumulation
of 0.6 to 0.7 t C ha
-1
year
-1
.
Brazilian research data also indicate that the composition and quality of SOM in
no-till soils differ from those of plowed soils. Various studies have also found that
the relative amount of free labile or more recent (e.g., POM) rather than humified
and occluded SOM fractions is higher in no-till soils compared to plowed soils,
which in turn has important ramifications for soil structure and nutrient cycling and
as a source of energy for soil microbial biomass. Other studies suggest that SOM
responds linearly to increasing rates of residue input over a variety of soils and cli-
mates (Bayer 1996; Black 1973; Burle et al. 1997; Rasmussen and Collins 1991;
Testa et al. 1992; Teixeira et al. 1994). For example Burle et al. (1997) obtained a
close relationship between SOC in the 0- to 17.5-cm soil layer and residue quantity
added by 10 different no-till cropping systems. Sisti et al. (2004) and Amado et
al. (2006) further studied the role of N additions in SOM buildup under no-till in
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