Soil fertility response to land-use changes in agroforestry and silvopastoral systems of the Ecuadorian Amazon: A multivariate and stratification ratio approach

Conversion of Amazonian forests to pastures and crops threatens soil fertility, but whether tree-based land uses conserve it remains poorly quantified in the Ecuadorian Amazon. We compared a secondary forest (SF, reference) with a cacao agroforestry system (AFC), two silvopastoral systems (Axonopus scoparius and Paspalum dilatatum pastures with trees; GGWT, DGWT) and the same pastures without trees (GGWIT, DGWIT) in Napo and Pastaza provinces. Each land use comprised three plots (5 – 7 sampling points each) sampled at 0 – 10 and 10 – 30 cm (184 samples). Land-use effects were tested with linear mixed models with plot as a random effect. In the topsoil, land use affected aeration porosity (AP; 11.4% in AFC to 19.2% in SF), soil organic matter (SOM; 8.5% in GGWIT to 17.9% in GGWT) and exchangeable K+ (0.09 cmolc/kg in AFC vs. 0.22 – 0.28 cmolc/kg in GGWT, DGWT and DGWIT), whereas bulk density, pH, total N, available P, Ca2+ and Mg2+ did not differ significantly. Relative to SF, AP decreased by 15% – 41% in all land uses, SOM by 47% in GGWIT and K+ by 43% in AFC. Most nutrients were concentrated in the topsoil. Stocks in the 0 – 30 cm layer of exchangeable K+ (53 – 102 kg/ha) and, except in DGWT, of available P (6.5 – 22.2 kg/ha) were below forest-maintenance thresholds, whereas NH4+ stocks were highest in treeless pastures and lowest in AFC. Multivariate analyses isolated AFC and grouped the silvopastoral systems with SF. Retaining trees in pastures and replenishing K and P in cacao agroforestry systems are recommended.