3.3.2 Bulk density (Bd)
Figure 4 shows Bd results in the six SUs for the three analyzed depths.
In natural vegetation SUs, Bd increased with depth, which is consistent
with results from Alvarado and Forsythe (2005). An average increase of
0.11 g cm-3 is from the 0–5 cm level to the 20–25 cm
level. In contrast, in agricultural uses, Bd was approximately constant
in the studied profile. The higher variation of Bd between natural and
agricultural uses was at the surface in F (0.16 g
cm-3) and followed by crops (<0.05 g
cm-3). Pore size redistribution and its temporary
increase as result of ploughing (I. Benavides et al., 2018) could cause
the observed Bd pattern in agricultural SUs.
[Insert Figure 4]
At the surface, four statistically significant different groups in Bd
were observed: the first with natural grasslands (Ls and Ss; 0.89 g
cm-3), the second for Ds (0.78 g
cm-3), the third for F (1.02 g
cm-3), and the fourth for crops (Oc and Pc; 0.84 g
cm-3). The differences between these groups result
from the vegetation characteristics (plant root density associated with
porosity) (Aggarwal, Choudhary, Singh, & Chakraborty, 2006) and soil
management, which directly influence Bd. At 20–25 cm depth, two
statistically dissimilar groups appeared: the first with Ls, Ss, Ds and
F; and the second with crops. The linkage in the uses for the first
group is soil genesis at this depth (<20 cm), and for the
second group is the distance of ploughing from the surface (30 cm).
The natural vegetation had a Bd in the assessed profile between 0.86 and
1.09 g cm-3, in agreement with other values reported
for dry páramo natural vegetation (Buytaert et al., 2005) and with those
observed by Farley, Kelly, and Hofstede (2004) (1.17 g
cm-3) whose annual rainfall (1,130 mm) was much higher
than that of the present research. F had critical Bd values, with an
increase of up to 19.5% compared to natural vegetation (values between
1.03 and 1.17 g cm-3), consistent with other studies
(Daza-Torres et al., 2014). Likewise, the results are in agreement with
Estupiñán, Gómez, Barrantes, and Limas (2009), who found an increase in
Bd of up to 35.0% in fallow SUs compared to natural vegetation Sus.
Podwojewski et al. (2002) found that overgrazing can increase Bd up to
0.80 g cm-3.
Bd conditions in this study were related to soil management. In cropped
areas, ploughing initially favors soil conditions to increase crop
productivity, by boosting aeration and nutrient availability, breaking
the natural soil structure and reducing Bd (Strudley, Green, & Ascough
Ii, 2008). However, the rapid oxidation of SOM (without physical
protection) and the low concentration of interchangeable bases force to
deep ploughing (Hofstede, 2001), which in the long term can provoke an
undesirable effect: soil depletion and Bd increase (Daza-Torres et al.,
2014; Podwojewski et al., 2002). This phenomenon, however, was not yet
evident in Oc and Pc in the study area, due to the recent transformation
of natural vegetation to these uses. On the other hand, F had more
critical conditions because of soil abandonment, possibly due to the
previously explained cultural practice in the area. Results found for Bd
are similar to those from Quichimbo et al. (2012) comparing natural
vegetation and fallow.