Influence of adjacent stand on spatial patterns of soil carbon and nitrogen in Eucalyptus and Albizia plantations

The chemistry and fertility of forest soils can be strongly influenced by tree species. Many studies have addressed the effects of monocultures on forest soil chemistry, but few have examined the effects of varying ratios of species within stands. In the absence of well-designed trials across a range of mixture ratios, we examined the spatial influence of adjacent stands on soil chemistry as a first approximation of the effect of mixed-species stands. We examined soil total carbon (C), nitrogen (N), and C/N along transects in adjacent, replicated, 12-year-old plantations of pure Eucalyptus saligna (Sm.) and pure N-2-fixing Albizia falcataria (L.) Fosberg. Soils from the center of the Eucalyptus stands had more C, less N, and higher C/N than soils from the center of the Albizia stands. The effects of the neighbor species were apparent for only about 5 m into the stands. This limited distance of the neighboring plot effect suggests that a species mixture ratio of 5:1 would be the highest ratio that would show any effect of the minor species on these soils.