Root activity patterns in an Amazonian agroforest with fruit trees determined by P-32, P-33 and N-15 applications

In a multi-strata agroforestry system in the central Amazon near Manaus, we studied the root activity distribution of different fruit trees and a legume cover crop in comparison to monocultures and a secondary forest site. Uptake of applied P-32, P-33 and N-15 from 0.1, 0.6 and 1.5 m depth was compared in both the dry and wet season. The results obtained with P-32 were similar to those with N-15 but showed a higher variability, probably due to the lower mobility of P than N in soil and thus the labeling of a smaller soil volume with P-32. During the dry season, topsoil root activity measured with N-15 was around 80% for all species with the exception of the palm tree Bactris gasipaes Kunth., which had a higher uptake from 0.6 m (50%) than from 0.1 m (30%). The subsoil (1.5 m) root activity was higher, when Bactris was not regularly cut for heart of palm harvest but grown for fruit production. Additionally, relative subsoil root activity of Theobroma increased and topsoil root activity of both Bactris and Theobroma decreased when intercropped in comparison to the monoculture. During the rainy season, the topsoil tree root activity slightly increased attributable to increasing water availability near the soil surface. The lowest isotope enrichment was noted for the secondary forest trees despite their low above ground biomass. The magnitude of the isotope enrichment was related to the foliar P and less pronounced to the foliar N contents, indicating higher nutrient cycling for nutrient-rich plant species. Despite the significant subsoil root activity (1.5 m) there was little evidence that large amounts of nutrients below 1 m depth can be recycled by the investigated tree species. More important may be a rapid recycling of nutrients from 0-1 m depth.