Effects of afforestation of a paramo grassland on soil nutrient status

Plantations of radiata pine (Pinus radiata) cover more than 4,000,000 ha worldwide [P.B. Lavery, D.J. Mead, Pinus radiata: a narrow endemic from North America takes on the world, in: D.M. Richardson (Ed.), Ecology and Biogeography of Pinus, Cambridge University Press, Cambridge, 1998, pp. 432–449]. In many areas, including the Ecuadorian Andes, these plantations have been established on former grasslands. Although this land use has grown over the past four decades in Ecuador, little is known about the effect of the change in vegetation cover on nutrient cycling in the high-altitude grassland systems where the plantations are frequently established. We examined changes in soil nitrogen, phosphorus, and acidity with stand age, using a chronosequence of stands ranging from 0 to 25 years of age. The effects of planting the grasslands with pine were most notable near the soil surface (0–10 cm), although in some cases changes deeper in the A horizon were also large. Total nitrogen concentrations became increasingly depleted along the chronosequence at intermediate depth (P=0.04), while N was gained in the litter (P=0.02) and upper-A horizon (P=0.001) until the plantations reached 20 years, at which point it again declined. In the top 10 cm, concentrations of NO3- increased dramatically under pine (P<0.001), while NH4+ was lower than in grasslands (P=0.04). Unlike nitrogen, neither total nor available phosphorus was significantly altered by the change in vegetation. Soil pH was higher in the grassland soils (5.5) than under pine stands of any age (P<0.01), all of which had a mean pH of 5.2. Acidification under pine occurred only in the top 10 cm, with no differences in pH at other depths, indicating that it is being driven by soil processes that predominate in the near-surface environment. These results demonstrate that the change of vegetation can affect soil properties on a decadal time scale, with implications for long-term site productivity.