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.