Principles of Soil Management in Neotropical Savannas - Principles of Sustainable Soil Management in Agroecosystems - page 75

Soil properties are affected differently by the use of conventional tillage or NT
in the cultivation of annual crops, and this theme constitutes the scope of a large
research program in Brazil and the Cerrado. However, the nature and intensity of
these differences vary with soil type and the indicator chosen. Twenty years of con-
tinuous cropping of a clayey Haplustox near Brasília increased bulk density from 1.0
to 1.2 g cm -3 to a depth of 20 cm, either under NT or disk and moldboard plowing
(Jantalia et al. 2007). Although soil compaction implies a considerable reduction of
macropore volume, it remains adequate and perhaps at better levels considering the
typically low water retention capacity in pristine Oxisols. It is also possible to visu-
alize that the loose granular aggregates under native Cerrado vegetation get closely
packed upon cultivation, forming granular clods resembling subangular blocks,
separated by planar pores resembling fissures ( Figure 3.4 ). This effect is especially
visible under disk plow and NT, where large cavity macropores also occur, and large
macropores (either as fissures or cavities) are rarer with moldboard or chisel plowing.
The dynamics and distribution of SOC are particularly sensitive to the choice of
tillage systems for annual crops. Cerrado soils under annual tillage typically show low
but homogeneous SOC and N concentrations throughout the 20-cm depth, whereas
soils under NT have similar values for the 10- to 20-cm layer but much higher values
in the top 10 cm, resulting in significantly higher total C and N stocks (Nunes et al.
2011). Such accumulation is a direct consequence of keeping crop residue inputs on
the soil surface instead of mixing them into a soil teeming with decomposer biotas.
Thus, adoption of NT leads to SOC sequestration when compared to conventional
annual tillage, especially in clayey (Jantalia et al. 2007; Nunes et al. 2011) but also in
coarse-textured Oxisols (Bayer et al. 2006). The average rate of SOC sequestration
by NT in Brazil for 0- to 20-cm depth is +0.37 Mg ha -1 year -1 (Bayer et al. 2006).
While the average rate may change for deeper layers (e.g., 0-40 cm), the data sug-
gest that soil quality can be improved by SOC sequestration. Increase in SOC con-
centration under NT occurs not only for bulk soil but also for SOC occluded within
aggregates, which are often depleted by annual tillage with disk plowing or heavy
harrowing (Resck et al. 2001). However, just as most particulate SOC in Cerrado
Haplustoxes is not occluded within aggregates but rather is free in bulk soil (Zinn et
al. 2007b), annual tillage in Oxisols affects much more organic residues free in soil
than SOC occluded within aggregates (Pes et al. 2011). Thus, conventional tillage
exacerbates SOC losses when plowing is done before planting and also after harvest-
ing (Jantalia et al. 2007).
The effect of tillage systems on fluxes of GHGs is being more widely studied in the
Cerrado. Siqueira Neto et al. (2011) reported no difference in CO 2 , N 2 O, and CH 4 fluxes
and microbial C after 12 years cropping under NT and conventional tillage in southern
Goiás, although SOC levels were 25% lower under annual tillage. Such a contradiction
suggests that, after a significant amount of SOC pool (most likely POM) is oxidized
by annual tillage, soil microorganisms keep a basal respiration by oxidizing different,
perhaps more resistant, SOC forms that are similar in pool size to the labile SOC frac-
tion under NT system. Also, hidden C costs of agricultural systems have seldom been
reported in the Cerrado, which can have a strong impact especially on NT systems
(Batlle-Bayer et al. 2010). Nitrate leaching under annual crops is not a major concern
even in sandy soils under soybeans, since nitrate levels are often an order of magnitude