3.2. Soil chemical indicators of soil under different land uses
Figure 2 shows the chemical properties associated with soil quality for
the two depths. The acidification process (evaluated through pH, acidity
and exchangeable aluminium) showed significant differences
(P<0.05) according to land use at both depths (Figure 2a, b
and c). Most land uses exhibited very acidic pH levels; they were below
5, ranging between 4.47 and 4.72, and the forest land-use type had the
lowest pH level (Figure 2a). The acidification process was more
accentuated on the surface layer, with higher concentrations of acidity
(>1.5 meq 100 s-1) and exchangeable
aluminium (>1 meq 100 s-1) levels that
are considered toxic but decreased with depth to acceptable levels
(Figures 2b and c).
Total organic carbon (TOC) was significantly higher (p<0.05)
on the surface layer (Figure 2d). The forest land-use type had the
highest TOC level, with values between 8 and 13%. However, the rest of
the land uses, especially in the surface layer, showed high
concentrations of TOC (> 5%), which is related to the
historical use of forest cover in the Ecuadorian Amazon Region, leading
to higher carbon storage (Nieto &
Caicedo, 2012).
Figure 2 .
The availability of some nutrients presented significant differences
(p≤0.05) at both depths (Figure 3). The available P varied significantly
with use and depth and Chakra_A (12.3 mg kg-1) and
Chakra_C (16.52 mg kg-1) had higher concentrations
(Figure 3a). The rest of the uses, particularly the forest land-use
type, showed values that were categorised as low, because they are below
the critical level of 10 mg kg-1(Bai et al., 2013). Of the exchangeable
bases (K+, Ca2+ and
Mg2+), only the exchangeable Ca2+content showed significant differences (p < 0.05; Figure 3c)
for the surface layer. In general, the assessed land uses showed low
levels of K+ (Figure 3b); they were below 0.22meq 100
s-1 (INIAP, 2012) and hence are considered low, except
for some livestock uses (Cattle_A and _B) that showed medium levels
(0.32meq 100 s-1) probably due to the contribution of
excreta. The calcium content in most land uses at both depths was low
(< 2 meq 100 s-1) and only soils in the
Chakra_A, Chakra_C and Cattle_B coverages reached average levels
(Figure 3c).