Do legumes and non-legumes tree species affect soil properties in unmanaged forests and plantations in Costa Rican dry forests?

Legume tree species, which are abundant in tropical dry forests, may be a critical regulator of soil nutrient dynamics because of their high foliar nitrogen (N) and potential for symbiotic N fixation. We investigated whether three legume tree species (Acosmium panamense, Dalbergia retusa, and Gliricidia sepium) have distinct soil chemistry under their crowns compared to nearby non-legume species (Rehdera trinervis, Swietenia macrophylla, and Quercus oleoides) when grown in two habitats: a diverse secondary forest or in 18-yr old monoculture plantations in northwestern Costa Rica. We quantified soil moisture, pH, labile carbon (C), inorganic N (NH₄⁺ and NO₃^-), net N mineralization rates, total C and N, δ¹⁵N signature and enzyme activity. We predicted that legumes would have higher soil nutrient availability under their crowns, but that this effect would be more pronounced in plantations, where tree species diversity is low. In the forest, soils under Dalbergia had the highest values of total C and N, and extractable nitrate, whereas soils under Acosmium the highest N mineralization rates. The activity of acid phosphatase enzymes varied among the soils under different species in both habitats, with the highest activity in the soils under the legume Acosmium at the forest site. In the plantations, Acosmium had the highest values for total soil C and N, labile C, and potential N mineralization rates. We conclude that 1) the legume species did not have consistent effects as a functional group, possibly due to different amounts of nodulation in individuals within species, and 2) as hypothesized, the magnitude of the species effect was more pronounced in the plantations than in the diverse secondary forest.