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).