4.1. Effect of trees on soil biochemical properties
Trees in the agroecosystems likely enhanced soil nutrient pools, as
evidenced by higher extractable-P (both sites), and higher total N and
mineralizableN(tree-based intercropping site) found in the agroforestry
compared to the conventional agricultural system. These differences
may reflect an increase in the quantity and diversity of organic matter
in agroforestry systems, which was due to additional deposition of tree
leaves and fine roots with lower decomposition rates than the residues
of associated crops (Mungai and Motavalli, 2006). Indeed, deeper tree
roots can act as nutrient pumps by drawing nutrients from deep horizons
or by scavenging nutrients that have leached belowthe agricultural
crop rooting zone and recycling them back into the system via litterfall
(e.g., Allen et al., 2004; Bergeron et al., 2011). Deciduous tree species
that were used in this study, especially hybrid poplar, offer a high N
and P recycling potential through the mineralization of tree litter in the
soil. For example, in a tree-based intercropping system in eastern
Canada, Thevathasan and Gordon (2004) estimated N release from hybrid
poplar annual litterfall to be equivalent to 7 kg N ha−1 yr−1.
Trees may also act as effective traps for atmospheric dust and they may
serve as focal points for attracting soil macro- and micro-fauna, which
can contribute to enhancing the local nutrient cycle (Scholes and
Archer, 1997). Accordingly, these positive feedback processes on soil
nutrient cycling imply that inorganic fertilizer needs may be reduced
in agroforestry compared to conventional agricultural systems. Lower
soil pH in the agroforestry than in the conventional agricultural system
at the tree-based intercropping sitewas likely the result of higher Nmineralization
rates and netH+excretion by plant roots following additional
uptake of cations by the trees (Tang and Rengel, 2002).