4 Discussion
The physical properties studied did not present great variation in the
profile, as demonstrated through a comparison of the three layers. L2
layer showed slightly lower values for porosity but higher values for BD
and PR than the other layers; however, this layer presented the normal
values for S index (Fig. 1). The PR values increased with increasing
soil depth, as demonstrated by a comparison of L2 with the surface layer
of Oxisols in the Brazilian Cerrado. Similar trends have also been
reported by Silva et al. (2012) and Cherubin et al. (2015). Further, de
Moraes et al. (2016) also reported the reduction in hydric properties
and porosity at soil depth of 0.10-0.20-m.
The subdivision of the layered profile (L1, L2 and L3) was important in
this study due to direct influence of the variables on soybean
productivity.. Nevertheless, variables only at L1 were significantly
related with the soybean yield (Table 1). However, the soil properties
at L2 indicated a lower quality compared with those of L1 and L3,
nonsignificant and negative influence of this lower quality was observed
on the soybean yield. These trends might be attributed to the strong
stratification of the chemical properties in the surface layer under
long-term NT (Caires et al., 2015; Martínez et al., 2016) because L1
contains most of the roots of the cultivated plants (Calonego et al.,
2017). Therefore, any fluctuation in variables atL1, which contains most
of the roots of soybean, would have greater effects on productivity than
changes in L2 and L3.
The development of the root system of plants depends on various factors,
including the physical conditions of the soil, such as BD, PR and
particle size (Rogers et al., 2000). The ability of the soybean’s root
system to provide a sufficient amount of water to the entire plant
depends on its abundance, diameter and root length. These
characteristics of the root system increase its area of absorption,
its transport capacity and the volume of soil exploited (He et al.,
2016). However, the benefits of favorable soil physical conditions, such
that the genetic potential is expressed and the root growth in the
profile is adequate, might not result in productivity gains in relation
to those found in lower-quality areas if the rains are well distributed
or if the chemical properties are stratified in the surface layer (Conte
et al., 2009). Although the architecture of the root system depends on
several factors, the distribution of nutrients in the profile is also
important; for example, phosphorus stimulates lateral growth of the root
system, and nitrogen stimulates an improved depth distribution. Thus,
the surface application of nutrients might restrict the depth
distribution of roots (He et al., 2016).
The direct positive relation of SOM and its fractions to the clay
content might be attributed to favorable conditions for the formation of
aggregates that arise from the combination of clay and SOM, which is
intensified in high-fertility soils due to the presence of divalent
cations (Denef and Six, 2005). The formation of aggregates increases the
chemical stabilization of SOM by cations (Kögel-Knabner et al., 2008),
increases the sorption of SOM on clay surfaces (Vogel et al., 2014) and
decreases the action of microorganisms due to physical insulation
(Baldock and Skjemstad, 2000). However, in this study, the opposite
trends were also observed in sandy soils (Fig.7), a negative
relationship shown.
The non-significant relationship of PAW and S index with SOM and its
fractions was expected because NT increases the AWC to plants with
greater efficiency (in cases with the same water content (PAW) and the
same pore distribution (S index)) compared with that obtained under
conventional plow-based systems (Le Quére et al., 2015). The significant
relationship of PR with OLF is important which should further be
investigated to potentially alleviate the soil compaction under NT
systems.
The PR and BD are strongly correlated (Mota et al., 2014), and both
exerted a negative direct effect on soybean yield. The values of PR and
BD for which the line of adjustment of the model crossed the line of the
productivity. The average values of PR and BD in studied plots were
approximately 1.6 MPa and 1.25 Mg m-3, respectively.
The value of PR below than 2.0 MPa was proposed by Taylor et al. (1966)
and the values of 2.5 MPa and 1.64 Mg m-3 proposed by
Tormena et al. (2017) for PR and BD, respectively. Furthermore, the PR
was measured at the soil moisture content corresponding to a potential
of 6 kPa. Both PR and BD might have adversely affected the root growth
at 0-0.07 m soil layer (Tormena et al., 2017), but an increment in soil
moisture can alleviate these negative effects.
The value of the α parameter of the Van Genuchten model was strongly
affected by water loss at high potentials close to zero. The increase in
the value of α has a positive and direct relationship with the presence
of macropores with diameters close to the lower limit of the class, and
drainage starts at high potentials. The parameter n depends on the rate
of water loss from the point of inflection of the curve, and increases
the rate of water loss with increase in the value of n. Therefore, to
obtain an increase in the modulus of n, an equilibrium of the proportion
of pores with each diameter class is required. In this context, the
interventions that promote soil mobilization or the use of a seed drill
can lead to a strong disturbance in the seeding row, increases the
volume of macropores, and reduces the value of n, leading to advesre
effects on productivity.
The matric potential at the inflection point of the curve, as indicated
by I, reveales the continuity of the pore quality which is given by n,
and its value depends on the pore size distribution in the micropore
class. A lower loss of water in this category is associated with a lower
potential. The distribution of pores that increases the values of n
and I depends on the structural arrangement of the soil, which can only
occur in the absence of plowing, and the combined effects of the SOM,
plant root system, biological activity and other mineralogical and
chemical components that moderate soil aggregation.
The significant positive effect of total porosity and S index at L1 on
the soybean yield indicates the need to balance the capacity between
water and air (gas exchanges) to harvest high yields. Aeration, when
reduced by water in the pores, linearly affects the root growth
(Benjamin and Karlen, 2014). The soybean yield under Brazilian edaphic
conditions is strongly dependent on biological nitrogen fixation by
symbiotic bacteria. However, the fixation efficiency depends on
satisfactory aeration and gaseous exchange (Tewari and Arora, 2016). A
pore size distribution with balanced proportions in different diameter
classes ensures the desired combination of aeration and water
availability, as indicated by the S index (Dexter, 2004).
The non-significance of the relationship between the soybean yield and
PAW/AWC might be attributed to a favorable rainfall distribution in
quantities greater than the minimum soybean demand during the three
harvest cycles of soybean. In the case of frequent rains, the role of
soil capacity to supply water is minimal (Calonego et al., 2017). In
this study, the soils under NT management had moderate to high PAW
values.
For soils of sandy and clayey texture, NT management can affect the PAW
and the productivity. The factors influenced by NT to reduce the
difference in productive potential between sandy and clayey soils are
increase in the SOC concentration, soil fertility (Sá et al., 2015) and
water use efficiency by plants (Le Quére et al., 2015). The presence of
crop residue mulch on the soil surface reduces the water loss through
evaporation. The straw mulch also creates micro dams that delay water
runoff and increase infiltration into the soil (Zhao et al., 2013).
These effects add to the productive potential of sandy soils, which can
then approach that of clayey soils.
Minasny and McBratney (2018) reported a modest SOM effect from increases
in the PAW; specifically, only 1.4 and 1.9% increase in the PAW were
obtained for every 10 g.kg−1 (1%) increase in the SOC
in clayey and sandy soils, respectively. However, other benefits of
conservation agriculture are obtained through the indirect effects of
mulching, which can reduce evaporation and runoff and improve soil
aggregation and biogeochemical functioning. Nonetheless, the review was
primarily based on data from soils in temperate regions of the
north-central USA and Europe or in dry regions of Australia. Under these
conditions, predominant soils contain high-activity clays with strong
affinity for physiochemical interaction with water. In contrast, the
soils of the tropical Brazilian Cerrado region contain predominantly
low-activity clays and under these conditions, SOM could be most
critical for soil aggregation, structure (Vezzani and Mielniczuk, 2012),
and CEC (Ciotta et al., 2002) and replaces the effect of clays on
physiochemical interactions with water. In contrast, to the conclusions
drawn by Minasny and McBratney (2018) reported a strong and positive
effect of SOM on the PAW.
The data reported in this study regarding the relationship of SOM and
its fractions to the PAW, PR and S depended on the predominance of
clayey soils in the study area: 85% of the plots contained more than
40% clay, and the high levels of SOM masked the effects. Tavares Filho
et al. (2012) reported a negative relationship between the PR and SOM in
a red Oxisol, with a SOM content ranging from 12 to 40
g.kg-1. The S index, affected by the pore size
distribution, was positively related with the FLF, and this relationship
constitutes the basis for microbial activity, leading whose action is
critical to the formation and stabilization of aggregates (Six and
Paustian, 2014) and to increases in the S index.